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The International Cometary                                                                                                        
Explorer (ICE) Mission to Comets                                                                                                  
Giacobini-Zinner and Halley                                                                                                       
                                                                                                                                  
  J.C. Brandt                                                                                                                     
  Laboratory for Astronomy and Solar Physics, NASA /Goddard Space                                                                 
 Flight Center, Greenbelt, Maryland, USA                                                                                          
                                                                                                                                  
1. History and scientific                                                                                                         
mission objectives                                                                                                                
                                                                                                                                  
  The Third International Sun-Earth Explorer (ISEE-3) was launched on 12 August                                                   
1978 as one element of a three-spacecraft mission that began in 1977. The original                                                
purpose was to study the solar-wind interaction with the Earth's magnetosphere. The                                               
spacecraft was maintained in a 'halo orbit' about the libration point, L1, where it                                               
monitored the solar-wind input. It completed four years of uninterrupted operation at                                             
that location.                                                                                                                    
  Several opportunities to use ISEE-3 in an extended mission phase were avallable.                                                
Among the most attractive scientifically were exploration of the distant geotail and an                                           
intercept of periodic Comet Giacobini-Zinner. Either or both of these options were                                                
available. The comet option was constrained to an intercept of Giacobini-Zinner in                                                
September 1985; specifically, an intercept of Comet Halley was not possible.                                                      
  In order to explore the comet option, a subcommittee of the ISEE Science Working                                                
Team (SWT) was formed at the request of the ISEE-3 Project Scientist, T. von Rosen-                                               
vinge, and chaired by E. Smith. The subcommittee and the SWT as a whole found                                                     
the option to be of considerable scientific interest, and a report entitled 'Intercept of                                         
Giacobini-Zinner by ISEE' was issued in June 1982, and revised in May 1983.                                                       
  An ad hoc subcommittee of the Space Science Board recommended that NASA pro-                                                    
ceed with both the geotail option and the comet intercept. After an in-house review                                               
and a review of the readiness of the spacecraft to perform the intercept by the Goddard                                           
Space Flight Center, NASA approved the intercept with Comet Giacobini-Zinner.                                                     
  The manoeuvres necessary to achieve the trajectory that would send the spacecraft                                               
into the distant geomagnetic tail and to an intercept of Comet Giacobini-Zinner are                                               
not simple. They are the brainchild of R. Farquhar, Flight Director for the Project.                                              
Basically, five gravitational encounters with the Moon were required to change the                                                
spacecraft's orbit (Fig. 1). The last encounter (also see Fig. 6) was on 22 December                                              
1983 when the spacecraft made a close swingby, passing only 120 km above the lunar                                                
surface. This manoeuvre effectively 'launched' the spacecraft from the Earth-Moon                                                 
system. At the same time, the spacecraft was renamed the Internationai Cometary Ex-                                               
plorer (ICE) to correspond to its new mission.                                                                                    
                                                                                                                                  
Figure 1. A colour-coded schematic showing                                                                                        
the trajectory of the ISEE-3/ICE spacecraft.                                                                                      
ICE returns to the vicinity of Earth in the year                                                                                  
2013 and, if retrievable, might provide comet                                                                                     
dust for analysis                                                                                                                 
                                                                                                                                  
  The first extensive survey of the distant geotail was accomplished by ISEE-3 in                                                 
1983. This survey achieved distances as great as 237 Earth radii in the tail. Aside from                                          
the scientific significance of the results obtained, the study of particle, field, and wave                                       
phenomena in the geotail provided an excellent rehearsal and scientific baseline for                                              
experiment operations during the cometary encounter. Results from the geotail excur-                                              
sion were presented in the October 1984 issue of Geophysical Research Letters (Vol.                                               
11, No. 10).                                                                                                                      
   The primary ICE scientific objective is to study the interaction between the solar                                             
wind and a cometary atmosphere by passing through the plasma tail. This will be ac-                                               
complished by the intercept with Comet Giacobini-Zinner on 11 September 1985.                                                     
Details of the targeting strategy are discussed below. Additional scientific objectives                                           
following the tail intercept of Comet Giacobini-Zinner include the support of Comet                                               
Halley studies through the measurement of solar-wind conditions upstream of                                                       
P/Halley. Such opportunities occur in October 1985 and March 1986.                                                                
                                                                                                                                  
                                                                                                                                  
2. Comet Giacobini-Zinner                                                                                                         
(G/Z)                                                                                                                             
                                                                                                                                  
  This short-period comet was discovered by M. Giacobini at Nice in 1900 and                                                      
rediscovered in 1913 by E. Zinner at Bamberg. The discovery and recovery history                                                  
of the comet is summarized in Table 1. The perihelion distance of 1.03 AU occurs                                                  
near the ecliptic plane, making G/Z well suited to spacecraft encounters during its in-                                           
tervals of maximum nuclear and atmospheric activity. The orbital elements are listed                                              
in Table 2.                                                                                                                       
                                                                                                                                  
  Typically, the plasma tail of G/Z is observed to begin developing at a heliocentric                                             
distance of about 1.7 AU. Conventional photographic observations at previous appari-                                              
tions show that the plasma tail can exceed 500000 km in length. The coma, also                                                    
observed photographically, reaches a typical diameter of about 50000 km (Fig. 2).                                                 
G/Z is associated with the Draconid (Giacobinid) meteor showers. An analysis by                                                   
D.K. Yeomans indicates that there is a limited possibility that a Giacobinid shower                                               
will occur on 8.5 October 1985, although it is unlikely to rival the great displays of                                            
this shower in 1933 and 1946. Ground-based observers and interplanetary-dust-                                                     
particle experiments should attempt to observe this shower. Details are given in. the                                             
Comet Giacobini-Zinner Handbook.                                                                                                  
                                                                                                                                  
  The early recovery of Comet G/Z by ground-based observers was of special interest                                               
to the ICE-mission flight dynamicists. In the event that the observed orbit had differed                                          
significantly from the expected orbit of Comet G/Z, early detection of Comet G/Z                                                  
would have been crucial to carefully schedule operation of the onboard propulsion                                                 
capability in order to effect the flyby successfully. Fortunately, the first recovery                                             
photographs (Fig. 2) obtained on 3 April 1984, by S. Djorgovski, H. Spinrad, G.                                                   
Will, and M. Belton with the 4 m Mayall telescope at Kitt Peak National Observatory,                                              
revealed that G/Z was within 4 arcsec of the anticipated position. Current projections                                            
indicate that further use of the propulsion system involves changes well within the                                               
available fuel capacity. The celestial mechanics of the encounter produces an intercept                                           
with the spacecraft travelling from south to north in the rest frame of the comet.                                                
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Table 1. Discovery and recovery of periodic Comet Giacobini-Zinner                                                                
                                                                                                                                  
                                                                                                 Approximate                      
Apparition  Designation        First observation  Observer/Institution                           magnitude  Reference             
                                                                                                                                  
 1          1900III   (1900c)  20 Dec 1900        Giacobini/Observatoire de Nice                  11          A.N. 154, 161       
 2          l913V     (1913e)  23 Oct 1913        Zinner/Remeis-Sternwarte                      9-10          A.N. 196, 167 & 353 
 3          1926VI    (l926e)  l6 Oct 1926        Schwassmann/Hamburger Sternwarte                14          A.N. 229, 122       
 4          1933III   (l933c)  23 Apr 1933        Schorr/Hamburger Sternwarte                     15          IAU Circ. 435       
 5          1940I     (19391)  15 Oct 1939        Van Biesbroeck/Yerkes Observatory ,             15          IAU Circ. 797       
 6          1946V     (1946c)  29 May 1946        Jeffers/Lick Observatory                        17          IAU Circ. 1046      
 7          1959VIII  (1959b)  8 May 1959         Roemer/US Naval Observatory                     20          IAU Circ. 1677      
 8          1966I     (1965g)  17 Sept 1965       Roemer & Lloyd/US Naval Observatory             20          IAU Circ. 1923      
 9          l972VI    (1972d)  11 Mar 1972        Roemer & McCallister/University of Arizona      19          IAU Circ. 2390      
10          l979III   (1978h)  30 Apr 1978        Shao & Schwartz/Harvard College Observatory  20-21          IAU Circ. 32l6      
11          l985      (1984c)  03 Apr 1984        Djorgovski & Spinrad/Univ. Calif. Berkeley      23          IAU Circ. 3937      
                                                  Will & Belton/Kitt Peak National Observatory                                    
General references:                                                                                                               
    l. Baldet M F 1950, Liste Generale Des Cometes De L'Origine A l948                                                            
    2. Marsden B G 1982, Catalogue of Cometary Orbits, 4th Edition, Smithsonian Astrophysical Observatory                         
                                                                                                                                  
                                                                                                                                  
Table 2. Orbit parameters for Comet                                                                                               
Giacobini-Zinner                                                                                                                  
                                                                                                                                  
q      = perihelion distance          1 .03 AU                                                                                    
                                                                                                                                  
Q      = aphelion distance            6.00 AU                                                                                     
                                                                                                                                  
e      = eccentricity                 0.708                                                                                       
                                                                                                                                  
i      = inclination                  31.9deg                                                                                     
                                                                                                                                  
OMEGA  = longitude of ascending node  194.7deg                                                                                    
                                                                                                                                  
omega  = argument of perihelion       172.5deg                                                                                    
                                                                                                                                  
P      = period                       6.5 y                                                                                       
                                                                                                                                  
T      = time of perihelion           1985 Sept                                                                                   
                                      5.25(ET)                                                                                    
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Figure 2.  Recovery photograph of Comet                                                                                           
Giacobini-Zinner obtained on 3 April 1984                                                                                         
with a charge-coupled device at the prime                                                                                         
focus of the 4 m telescope at the Kitt Peak                                                                                       
National Observatory. The observers were                                                                                          
S. Djorgovski and H. Spinrad of the                                                                                               
Astronomy Department, University of                                                                                               
California, Berkeley and G. Will and                                                                                              
M. Belton of the Kitt Peak National                                                                                               
Observatory                                                                                                                       
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
  In order to develop the targeting strategy, a baseline tail model (Fig. 3) was                                                  
developed. The starting point was a gas production rate of 2.3 X 10**28 molecules/s at                                            
perihelion (derived by N. Divine from the measured brightness of Comet G/Z during                                                 
previous apparitions). The model suggests a plasma tail width of about 5000 km from                                               
the nucleus, consistent with the measured size on a photograph of Comet G/Z obtained                                              
by E. Roemer on 26 October 1959 (Fig. 4). Features of the model include a bow                                                     
shock, a two-lobed magnetic tail, and neutral sheet separating the two magnetic lobes                                             
in the tail. The model-dependent nature of these features should be kept in mind.                                                 
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Figure 3. Schematic of the baseline plasma-                                                                                       
physics model for Comet Giacobini-Zinner                                                                                          
near perihelion                                                                                                                   
                                                                                                                                  
Figure 4. Photograph of Comet Giacobini-                                                                                          
Zinner, obtained by E. Roemer on 26 October                                                                                       
1959 (Official US Navy photograph). Linear                                                                                        
and angular scales have been added                                                                                                
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
  The targeting strategy is based on the desire to penetrate the central, denser, fully-                                          
formed plasma tail of Comet Giacobini-Zinner. Thus, the following elements must be                                                
considered in the formulation of a targeting strategy:                                                                            
                                                                                                                                  
1.  the dust hazard                                                                                                               
2.  full formation of the tail; directly in the wake of the nucleus conditions may be                                             
    chaotic and it is thought that the tail may not be fully organized closer than a few                                          
    thousand kilometres to the nucleus                                                                                            
3.  the cross-section of the plasma tails tends to increase or 'flare' with increasing                                            
    distance from the nucleus (Fig. 3)                                                                                            
4.  the plasma tail tends to wag back and forth in response to variations in the solar-                                           
    wind velocity.                                                                                                                
                                                                                                                                  
Elements 1 and 2 set a lower bound to the distance from the nucleus at which the in-                                              
tercept occurs. Elements 3 and 4 conflict. The increasing cross-section 3 indicates that                                          
targeting away from the nucleus increases the probability of intersection the tail for                                            
a given targeting accuracy. The tail wagging 4 indicates that targeting away from the                                             
nucleus decreases the probability of a successful tail intercept. A probabilistic model                                           
has been developed for elements 3 and 4 and a solution sought for a 0.99 probability                                              
of successful intercepts for a range of reasonable targeting accuracies. A solution con-                                          
sistent with all presently available information and elements l through 4 indicates 3                                             
tailward targeting distance of 10000 km from the nucleus.                                                                         
   The relative geometry of the intercept is shown in Figure 5. The spacecraft will pass                                          
essentially south to north through the tail of the comet. Clearly, we expect to pass                                              
through the bow shock, the contact surface separating the mixed solar-wind and com-                                               
etary plasma, and the central plasma tail. As shown in Figure 5, the path of the ICE                                              
spacecraft is nearly parallel to the expected orientation of the neutral sheet separating                                         
the magnetic tail lobes. For the anticipated targeting accuracies, we cannot reasonably                                           
expect to intersect the neutral sheet. However, we may be able to detect the  neutral                                             
sheet from the signatures of electromagnetic waves and plasma waves generated in the                                              
neutral sheet and detected by ICE in the plasma tail.                                                                             
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Figure 5. The geometry of the cometary                                                                                            
encounter, as seen looking towards the Sun                                                                                        
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
  Clearly, the maximum scientific return from the intercept depends on extensive,                                                 
supporting ground-based observations of the comet. The appropriate observations                                                   
have been requested through the International Halley Watch and a Comet Giacobini-                                                 
Zinner Handbook has been prepared (Yeomans & Brandt, 1985). Because the                                                           
'Giacobini-Zinner Watch', with a focus on the time period of September 1985, comes                                                
before the time period of maximum effort for Halley, the Giacobini-Zinner Watch can                                               
serve as a valuable dress rehearsal for Halley's Comet.                                                                           
  The coverage requested can be divided into four levels of activity:                                                             
1. Light coverage: This time period would be approximately one year centred on the                                                
   encounter date. The comet's development is to be monitored as it passes through                                                
   perihelion. Observations would emphasize astrometry and photometry.                                                            
2. Moderate coverage: This level has the goal of obtaining one good-quality observa-                                              
   tion per observer per day during the time periods 28 August to 4 September and                                                 
   18 to 25 September 1985. These weeks, together with the periods of heavy and                                                   
   intense coverage (see below), provide good coverage for one month or a full solar                                              
   rotation, which is important for establishing the comet's plasma environment. The                                              
   comet's heliocentric distance is almost constant (1.03 <r< 1.07 AU), and the                                                   
   ecliptic latitude separation of the comet and the ICE spacecraft is less than 10deg                                            
   for this month. Thus, ICE can serve as an excellent upstream monitor during this                                               
   period, which is also one of excellent comet visibility from Earth. Therefore, we                                              
   expect the following International Halley Watch networks - Astrometry, Radio                                                   
   Studies, Infrared Spectroscopy and Radiometry, Spectrophotometry and Spec-                                                     
   troscopy, Photometry and Polarimetry, Near-Nuclear Studies, and Large-Scale                                                    
   Phenomena - to be actively involved.                                                                                           
3. Heavy coverage: This level has the goal of obtaining at least two observations per                                             
   observer per day where appropriate, e.g. for time-sequence photographs. The                                                    
   period covers 4 September to 18 September 1985, and the same networks as in                                                    
   2 would be involved.                                                                                                           
4. Intense coverage: This period covers the night of the encounter, which is schedul-                                             
   ed for 11.00 UT on ll September 1985. We have requested maximnm effort from                                                    
   all observers during this night.                                                                                               
                                                                                                                                  
Comet Giacobini-Zinner is well situated for viewing during the month of September                                                 
1985 for Northern-Hemisphere observers. An abbreviated ephemeris is included as                                                   
Appendix 1. For a few days in September, Comets Giacobini-Zinner and Halley will                                                  
be close together in the morning sky; on 14 September, they will be separated by less                                             
than 2deg. Comets Giacobini-Zinner and Halley should be roughly 8th and 12th                                                      
magnitude, respectively, and present a historic opportunity for wide-field celestial                                              
photography.                                                                                                                      
                                                                                                                                  
For the possible Giacobini meteor shower on 8.5 October 1985, we will rely on the                                                 
newly-formed IHW network for Halley Meteor Studies for coverage.                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
3. The ICE instruments                                                                                                            
                                                                                                                                  
  The spacecraft supports 13 scientific investigations, which are listed in Table 3. The                                          
line separates those experiments that will probably contribute useful measurements at                                             
a comet and those that probably will not. The latter are principally experiments that                                             
measure high-energy cosmic-ray nuclei and electrons. This division is, of course,                                                 
somewhat arbitrary. All experiments are candidates for operation during the cometary                                              
encounter to the extent permitted by available spacecraft power and a suitable safety                                             
margin.                                                                                                                           
                                                                                                                                  
l. Solar-Wind Plasma                                                                                                              
  The plasma electron analyzer is a 90deg, spherical-section electrostatic analyzer. Elec-                                        
trons from secondary emitters are coupled into a discrete-dynode electron multiplier                                              
to provide spatial resolution in polar and azimuth angles. Electrons having                                                       
5<E(e)< 1500 eV are analyzed in 16 steps. A complete two-dimensional distribution                                                 
is obtained in one spacecraft revolution and is read out every 24 s at 1024 bit/s.                                                
                                                                                                                                  
_________________________________________________________________________________________________________________________________ 
Table 3. ICE experiments and status                                                                                               
                                                                                                                                  
                          Principal                                Experiment                                                     
Title                     Investigator       Affiliation           status                                                         
--------------------------------------------------------------------------------------------------------------------------------- 
 1.   Solar-Wind Plasma   S. Bame            Los Alamos            Electrons only                                                 
                                             National Laboratory   (ion portion falled)                                           
 2.   Plasma Composition  K. Ogilvie         NASA/Goddard          Operational                                                    
                                             Space Flight Center                                                                  
 3.   Energetic Protons   R. Hynds           Imperial College      Operational                                                    
                                             London                                                                               
 4.   X-Ray, Low-         K. Anderson        Unlversity of         X-rays and E(e)>200 keV                                        
      Energy Electrons                       California, Berkeley  (low-energy electron                                           
                                                                   portion failed)                                                
 5.   Low-Energy          D. Hovestadt       Max-Planck-           Partial failure                                                
      Cosmic Rays                            Institut fur Physik                                                                  
                                             und Astrophysik                                                                      
 6.   Magnetometer        E. Smith           Jet Propulsion        Operational                                                    
                                             Laboratory                                                                           
 7.   Plasma Waves        F. Scarf           TRW Systems           Operational                                                    
 8.   Radio Waves         J.-L. Steinberg    Paris Observatory,    Operational                                                    
                                             Meudon                                                                               
--------------------------------------------------------------------------------------------------------------------------------- 
 9.   Medium-Energy       T. von Rosenvinge  NASA/Goddard          Operational                                                    
      Cosmic Rays                            Space Flight Center                                                                  
10.   High-Energy         E. Stone           California Institute  Partial failure                                                
      Cosmic Rays                            of Technology         (isotope portion)                                              
11.   High-Energy         M. Wiedenbeck      Unlversity of         Partial failure                                                
      Cosmic Rays                            Chicago               (drift chamber)                                                
12.   Cosmic-Ray          P. Meyer           Ualversity of         Operational                                                    
      Electrons                              Chicago                                                                              
13.   Gamma-Ray Bursts    B. Teegarden       NASA/Goddard          Partial failure                                                
                                             Space Flight Center   (PHA memory)                                                   
_________________________________________________________________________________________________________________________________ 
                                                                                                                                  
                                                                                                                                  
2. Plasma Composition                                                                                                             
                                                                                                                                  
  The ion-composition experiment consists of a Wien filter, whose magnetic and elec-                                              
tric fields provide velocity selection, followed by an electrostatic analyzer which                                               
transmits ions based on the ratio of their energy to their charge. The range covered                                              
by the dual analyzer is normally 470 to 10500 V. In the solar wind, the corresponding                                             
mass/charge range extends from 1.0 to 5.6 AMU for singly ionized particles.                                                       
  The mode of operation of the experiment is controlled by a microprocessor and will                                              
be adjusted to optimize the experiment's performance for the comet flythrough. A                                                  
suitable measurement programme could cover the mass range from 4 to 50 in 25 steps                                                
and a velocity range from 20 to 200 km/s in 25 steps. Normally, 20 min would be                                                   
required for each readout of this portion of parameter space.                                                                     
  The performance of this instrument may be seriously degraded due to aberration                                                  
caused by the encounter trajectory; for ion velocities believed to be reasonable, the                                             
bulk flow is not in the acceptance cone of the instrument. Nevertheless, the                                                      
measurements are important and the microprocessor will be reprogrammed to an op-                                                  
timum cometary approach.                                                                                                          
                                                                                                                                  
3. Energetic Protons                                                                                                              
  Energetic protons and heavy nucleons are detected by three identical charged-                                                   
particle telescopes aligned at different angles (30deg, 60deg and 135deg) to the spin axis.                                       
Each telescope consists of a 1 cm**2 solid-state detector of 30 micro m thickness im-                                             
mediately followed by a 2 cm**2 detector of 150 micro m thickness in anticoincidence.                                             
Pulse-height analysis resolves the particle energies into eight channels between 35 keV                                           
and 1.6 MeV. The count rate from each telescope and each channel is resolved into                                                 
eight sectors in the equatorial plane to measure the anisotropy of the particles.                                                 
Shielding, a mechanical collimator, and a magnetic broom prevent electrons from                                                   
entering the telescope. To sample the three telescopes, eight energies and eight sectors                                          
requires 32 s at 1024 bit/s.                                                                                                      
                                                                                                                                  
4. X-Rays, Low-Energy Electrons                                                                                                   
  Photons in the energy range 5-1250 keV are measured with a proportional counter                                                 
and a scintillation detector. A burst memory allows capture of fine timing detail both                                            
for solar flares and for cosmic-gamma-ray bursts. Electrons with energies greater than                                            
200 keV are also detectable by the scintillation detector.                                                                        
                                                                                                                                  
5. Low-Energy Cosmic Rays                                                                                                         
  The instrument now consists of two different sensor systems: (i) an electrostatic                                               
deflection analyzer system; and (ii) a thin-window flow-through proportional-                                                     
counter/solid-state-detector telescope. The combination measures the elemental abun-                                              
dances, charge-state composition, energy spectra and angular distributions of ions                                                
over parts of the range 2 keV/charge to 80 MeV/nucleon and of electrons between 75                                                
and 1300 keV.                                                                                                                     
                                                                                                                                  
6. Magnetometer                                                                                                                   
  The vector helium magnetometer is mounted on the end of a 3 m boom (Fig. 6) and                                                 
makes in-situ measurements of the ambient magnetic field vector in the frequency                                                  
range 0-3 Hz. The instrument automatically chooses one of eight ranges in which to                                                
operate, the four lowest full-scale ranges being +/-4, +/- 14, +/-42, and +/- 146 nT. The                                         
precision of measurement is 1/256 of the full-scale field values (0.016, 0.055 nT,                                                
etc.). The absolute accuracy is limited by knowledge of the spacecraft magnetic field,                                            
which is presently known to better than 0.1 nT. The time resolution corresponds to                                                
three triaxial samples per second at 1024 bit/s . The magnetometer contains a three-                                              
channel spectrum analyzer which is connected to the search-coil sensor of the plasma-                                             
wave investigation (see No. 7).                                                                                                   
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Figure 6. The ISEE-3/ICI spacecraft                                                                                               
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
7. Plasma Waves                                                                                                                   
                                                                                                                                  
  The plasma-wave investigation measures fluctuating electric and magnetic fields                                                 
associated with waves above the ion cyclotron frequency (1 - 100000 Hz). Spectral                                                 
analysis is performed on signals from three detectors: a pair of long electric-field                                              
antennas, a short electric-field sensor and a search coil, both of which are mounted                                              
on one of the experiment booms (Fig. 6). Electric-field fluctuations between 20 and                                               
100000 Hz are measured in 16 frequency channels once per second at 1024 bit/s.                                                    
  Cometary dust particles hitting the spacecraft or the electric antennas are expected                                            
to cause detectable outputs similar to those detected when Voyager passed through                                                 
Saturn's ring-plane.                                                                                                              
                                                                                                                                  
8. Radio Waves                                                                                                                    
  Fluctuating electric fields are detected in three pairs of orthogonal antennas, one                                             
aligned with the spacecraft spin axis and the other two in the equatorial plane (Fig.                                             
6). The antenna voltages are preamplified and then analyzed by a radiometer con-                                                  
sisting of four superheterodyne receivers, which step through the frequency range                                                 
from 30 kHz to 2 MHz. Two channels are narrow-band with a filter bandwidth of                                                     
3 kHz, and two have a filter bandwidth of 10 kHz. The narrow-band filters sweep bet-                                              
ween 30 kHz and 1 MHz in 12 steps, while the broadband filters cover frequencies                                                  
between 40 kHz and 2 MHz, also in 12 steps. The frequencies are scanned in a                                                      
nonlinear manner so that the highest frequencies are sampled 12 times as often as the                                             
low frequencies. The complete scan sequence consists of 72 steps; 56 s are required                                               
to read out all channels at 1024 bit/s.                                                                                           
                                                                                                                                  
9. Medium-Energy Cosmic Rays                                                                                                      
  The medium-energy cosmic-ray experiment measures the elemental composition of                                                   
nuclear energetic particles over wide ranges in energy (~ 1-500 MeV/nucleon) and                                                  
in charge (Z= 1-28). Electrons of ~0.2-10 MeV are measured as well. These                                                         
measurements are accomplished using all-solid-state detector telescopes with detectors                                            
ranging in thickness from 15 to 3000 micro m. This experiment has been designed to study                                          
solar-flare particles, galactic cosmic rays, and particles accelerated by the in-                                                 
terplanetary medium.                                                                                                              
                                                                                                                                  
10. High-Energy Cosmic Rays                                                                                                       
  This experiment measures the isotopic composition of solar and galactic cosmic rays                                             
with 5-250 MeV/nucleon and for all elements with charge Z<= 30. It consists of two                                                
thin (50 micro m) solid-state detectors, which determine the entering particle trajectory,                                        
followed by solid-state detectors that measure the particle energy.                                                               
                                                                                                                                  
11. High-Energy Cosmic Rays                                                                                                       
  Similar to the preceding experiment, this instrument is a telescope constructed from                                            
trajectory-defining drift chambers, a large stack of solid-state detectors and a surround-                                        
ing scintillator anti-coincidence detector. The experiment has been designed to                                                   
measure the isotopic abundances of galactic cosmic rays from ~ 20 to ~500                                                         
MeV/nucleon for isotopes with masses from A = 1 to 64.                                                                            
                                                                                                                                  
12. Cosmic-Ray Electrons                                                                                                          
  Eight active detectors are used to determine the energy spectrum of electrons in the                                            
range 5-400 MeV. The experiment also determines the energy spectrum and relative                                                  
abundances of nuclei from protons to the iron group in the range ~ 30 MeV/nucleon                                                 
to 15 GeV/nucleon. It may also be able to count cometary dust particles impinging                                                 
on the front detector.                                                                                                            
                                                                                                                                  
13. Gamma-Ray Bursts                                                                                                              
  The gamma-ray-burst experiment is designed to detect cosmic gamma-ray bursts                                                    
with high spectral resolution (<3.5 keV at l MeV) and high temporal resolution (as                                                
fast as 4 ms). The primary sensor, an intrinsic germanium solid-state detector cooled                                             
to ~ l00 K by a two-stage radiative cooler, spans the energy range 0.05-6.5 MeV                                                   
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Figure 7.  The geometry of 'launch' and the                                                                                       
encounter on 11 September 1985.  The range at                                                                                     
encounter is 0.47 AU and the Sun-Earth Comet angle                                                                                
is 79.6deg.                                                                                                                       
                                                                                                                                  
Figure 8.  ICE shortly before encountering                                                                                        
Comet Giocobini-Zinner in September 1985.                                                                                         
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
These experiments, in particular, experiments 1 through 8, although not chosen with                                               
cometary research in mind, are well-suited for probing the plasma-tail environment.                                               
The ICE spacecraft has no imaging capability nor any experiments designed to study                                                
dust particles (but see descriptions for Nos. 7 and 12). For additional details of the                                            
instruments as launched, see the July 1978 issue of IEEE Transactions of Geoscience                                               
Electronics (Vol. GE-16, No. 3).                                                                                                  
                                                                                                                                  
                                                                                                                                  
4. The spacecraft                                                                                                                 
  The spacecraft (Fig. 6) is a developed version of the earlier Interplanetary Monitor-                                           
ing Platform (IMP). Spacecraft weights are listed in Figure 6.                                                                    
  The spacecraft body is a 16-sided cylinder, 1.74 m in diameter and 1.6 m high.                                                  
Solar arrays mounted on the faces of the cylinder provide 160 W of primary power                                                  
at 28 V. A distinctive feature is a superstructure, or tower, which elevates the                                                  
telemetry antenna above the spacecraft body and provides a clear field of view for                                                
several cosmic-ray detectors.                                                                                                     
  The spacecraft body supports a total of ten appendages. Two equatorial experiment                                               
booms (3 m long) support the magnetometer and plasma-wave sensors. Four wire                                                      
antennas (each 49 m long) are deployed in the spin plane as part of the radio-wave                                                
and plasma-wave investigations. Two axial antennas (7 m each) extend above and                                                    
below the spacecraft parallel to the spin axis to render the radio-wave measurements                                              
three dimensional. Finally, two short inertial booms provide stability.                                                           
  The radio system consists of two redundant S-band transponders operating at 2217                                                
and 2270 MHz. There are two low-gain antennas and a telemetry antenna having                                                      
medium gain (7 dB) and a fan beam of +/-6deg. The radiated power is 5W. The standard                                              
telemetry rate in halo orbit was 2048 and 512 bit/s.                                                                              
  The spacecraft is spin-stabilized at 19.75 rpm. Attitude information and control is                                             
provided by two fine Sun sensors and a panoramic attitude sensor. The spin axis is                                                
maintained perpendicular to the ecliptic plane to within <1deg.                                                                   
  Propulsion and attitude control use monopropellant hydrazine. The system contains                                               
eight propellant tanks and 12 thrusters, including redundancy. A large amount of pro-                                             
pellant was carried at launch (94 kg) as a safeguard against large errors associated with                                         
the launch vehicle. As a result of a nominal launch, gas usage has been much lower                                                
than anticipated, resulting in a substantial amount of excess gas still being available.                                          
                                                                                                                                  
5. Mission description                                                                                                            
  The 'effective launch' of ICE took place on 22 December 1983. As the spacecraft                                                 
distance from Earth increased, tracking and data acquisition involved the NASA                                                    
Deep Space Network (DSN). The necessity for this involvement is apparent if one                                                   
considers that the ISEE-3 radio system was designed to transmit from the halo orbit                                               
at a geocentric distance of 0.01 AU, whereas the distance to the spacecraft at cometary                                           
encounter will be 0.47 AU. A major effort required for the ICE mission was the outfit-                                            
ting of antennas in the DSN to operate at the ICE frequencies.                                                                    
  Current plans are to utilize the 64 m DSN (Goldstone, Madrid, Canberra) and the                                                 
300 m dish at Arecibo as the prime station. The anticipated data rate at encounter will                                           
be 1024 bit/s, although 512 bit/s may be used at other times. The acceptable perfor-                                              
mance is based on a bit error rate of 10**-4. There will be additional coverage by the                                            
64 m station at Usuda, Japan. Operations outside the month centred on the encounter                                               
date of 11 September 1985 are basically cruise-science measurements, which will be                                                
discussed in the next section.                                                                                                    
  On 11 September 1985, Comet Giacobini-Zinner will be at the location shown in                                                   
Figure 7. The ICE spacecraft will be approaching the aim point on the main plasma                                                 
tail axis 10000 km from the nucleus, as shown schematically in Figure 8.                                                          
Some idea of the spatial scales associated with key instruments and their sampling                                                
times is given in Table 4 for the expected data rate of 1024 bit/s. For the sampling                                              
period indicated, the 'spatial resolution' is the distance travelled at the relative en-                                          
counter speed of 20.7 km/s. These dimensions should be compared with the estimated                                                
distance between bow-shock crossings of about 175 000 km and a measured main tail                                                 
diameter of 5000 km. Expressed in terms of time, we expect the spacecraft to be inside                                            
the bow shock for about 2 h 20 min and in the maln plasma tail for about 4 min.                                                   
  The magnetometer will produce many measurements during the encounter period                                                     
and we use it to illustrate possible scientific product. Current models of comets do not                                          
indicate a major amplification of the cometary magnetic field over the solar-wind                                                 
value. However, major changes in field-line direction are expected. Well away from                                                
the comet the magnetic field should, on average, show the Archimedian spiral angle                                                
of 135deg or 315deg to the radial appropriate for normal solar-wind fiow. Interior to the                                         
bow shock we expect a different, possibly somewhat chaotic, orientation tending to                                                
the ordered two-lobed configuration along the axis of the plasma tail. If the neutral                                             
sheet is encountered, a magnetic reversal should be recorded. The model-dependent                                                 
nature of this description must be stressed. For example, the bow shock may or may                                                
not exist. We should know after 11 September 1985, and the model will be tested in                                                
this and in other respects. Obviously, we need data from as many different ex-                                                    
periments as possible to complete our model testing.                                                                              
  To enhance the science return from the encounter, a Guest Investigator Program has                                              
been established by NASA.                                                                                                         
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Table 4. Spatial resolution of the scientific meaurements (assuming 1024 bit/s)                                                   
                                                                                                                                  
Instrument                      Sampling period (s)   Spatial resolution (km)                                                     
Magnetometer                        1/3                         7                                                                 
Plasma Waves                                                                                                                      
  16 channel E                        1                        21                                                                 
  8 channel E, B                     16                       330                                                                 
Plasma Electrons                                                                                                                  
 Two-dimensional distribution        24                       500                                                                 
Energetic Protons                    32                       660                                                                 
Radio Waves                          56                      1200                                                                 
Plasma Ions                        1200                     25000                                                                 
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
6. ICE and Comet Halley                                                                                                           
  The ICE mission should be of direct benefit to the missions to explore Halley's                                                 
Comet in two general ways. The first is through the spin-off and synergism of the en-                                             
counter, and the second is through the use of ICE as an upstream monitor of solar-                                                
wind conditions.                                                                                                                  
  The ICE experience at encounter will give the first indications of spacecraft pro-                                              
blems in a cometary environment. For example, the dust emitted by Comet Giacobini-                                                
Zinner is thought to be extremely fluffy and possibly one hundred times less dense                                                
than the dust associated with Halley's Comet. Dust fluences from Comet Giacobini-                                                 
Zinner are estimated to be lower than for Halley's Comet. Thus, a relatively benign                                               
dust environment is expected for the ICE encounter. If spacecraft problems are en-                                                
countered and ascribed to dust, some revision of strategy for the Halley encounters                                               
may be indicated.                                                                                                                 
  On the scientific side, the Comet Giacobini-Zinner encounter is highly complemen-                                               
tary to the Halley encounters. In-situ data will have been obtained for two comets                                                
within a very short time, permitting intercomparisons. All the Halley probes are                                                  
targeted for the sunward side of the comet versus the tailward targeting for ICE. Both                                            
data sets are probably necessary to sketch a moderately complete picture of a comet's                                             
plasma and magnetic field.                                                                                                        
  Finally, experience with the 'Giacobini-Zinner Watch' carried out by the Interna-                                               
tional Halley Watch in September 1985 could lead to more efficient coverage of                                                    
Halley's Comet in March of 1986. The opportunity to observe both comets                                                           
simultaneously around 14 September 1985 has already been mentioned.                                                               
  The second role of ICE in Halley studies involves its use as an upstream monitor                                                
of solar-wind conditions. In late March 1986, the ICE spacecraft can be considered                                                
a 'fairly distant' probe to Halley's Comet (Table 5). A bipolar plot of the ICE trajec-                                           
tory showing the alignments is given in Figure 9.                                                                                 
  Two near-radial lineups are available, and relevant facts are assembled in Table 5.                                             
  Tracking of ICE at these times should not be a problem because the range from                                                   
Earth is approximately the same as the range at Comet Giacobini-Zinner encounter.                                                 
Furthermore, when ICE is in the cruise mode making solar-wind measurements, the                                                   
lower data rate of 512 bit/s would be acceptable.                                                                                 
  Investigations of cometary phenomena utilizing input parameters from solar-wind                                                 
measurements have been carried out successfully for some years. However, we note                                                  
that three-dimensional plasma ion speeds have been quite important in these investiga-                                            
tions. The challenge will be to fully exploit the ICE data.                                                                       
  With the caveat mentioned just above, ICE could provide important solar-wind data                                               
during cruise-mode operations, which would include the 'week' of the five spacecraft                                              
encounters with Halley's Comet in March 1986. The time delay between ICE and                                                      
Halley's Comet on 13/14 March 1986, calculated according to standard formulas, is                                                 
only two days, considered quite reasonable by scientists working in this field. If                                                
suitable ion-velocity coverage is obtained from ICE or another spacecraft, solar-wind                                             
data for the Halley encounters could be reasonably complete.                                                                      
  Finally, there is the near-radial alignment of ICE, Pioneer Venus Orbiter, Vega-1                                               
and 2, and MS-T5 in the interval 15 May to 15 June 1985, which presents an oppor-                                                 
tunity for multispacecraft investigation of comet-like phenomena over large scale                                                 
lengths (described in Galeev & Scarf, 1985).                                                                                      
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Table 5. Radial alignments with Comet Halley                                                                                      
                       Radial           Angular           Solar-wind                                                              
Date                   separation (AU)  separation (deg)  travel time (d)  Range (AU)                                             
                                                                                                                                  
31 October 1985      0.93               l                 4                0.41                                                   
28 March 1986        0.21               6                 1                0.56                                                   
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
Figure 9.  A bipolar plot of the ICE trajectory                                                                                   
showing the radial alignments with Halley's                                                                                       
Comet                                                                                                                             
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
               Appendix 1. Ephemeris for Comet Giacobini-Zinner                                                                   
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
                       EXPLANATI0N OF EPHEMERIS ENTRIES *                                                                         
                                                                                                                                  
                                                                                                                                  
J.D. = JULIAN DATE (TIMES IN THIS EPHEMERIS ARE EPHEMERIS TIMES)                                                                  
R.A.  1950.0 DEC. = GEOCENTRIC RIGHT ASCENSION AND DECLINATION REFERRED TO                                                        
   THE MEAN EQUATOR AND EQUINOX OF 1950.0 - LIGHT TIME CORRECTI0NS HAVE                                                           
   BEEN APPLIED.                                                                                                                  
R.A. APPN DEC. = APPARENT RIGHT ASCENSION AND DECLINATION - LIGHT TIME,                                                           
   ANNUAL ABERRATION, AND NUTATION CORRECTIONS HAVE BEEN APPLIED AND R.A.                                                         
   AND DEC. HAVE BEEN PRECESSED TO THE EPHEMERIS DATE.                                                                            
DELTA = GEOCENTRIC DISTANCE OF OBJECT IN AU.                                                                                      
DELDOT = GEOCENTRIC VELOCITY OF OBJECT IN KM/SEC.                                                                                 
R = HELIOCENTRIC DISTANCE OF OBJECT IN AU.                                                                                        
RDOT = HELIOCENTRIC VELOCITY OF OBJECT IN KM/SEC.                                                                                 
                                                                                                                                  
TMAG = TOTAL MAGNITUDE ESTIMATES AFTER THE ANALYSIS BY C.S. MORRIS.                                                               
   PRE-PERIHELION                                                                                                                 
     TMAG = 8.90 + 5*LOG(DELTA) + 11.00*LOG(R)    FOR R > 1.3 AU                                                                  
     TMAG = 9.60 + 5*LOG(DELTA) +  4.73*LOG(R)    FOR R = 1.3 TO 0.99 AU                                                          
   POST-PERIHELION                                                                                                                
     TMAG = 9.65 + 5*LOG(DELTA) + 10.48*LOG(R)    FOR R = 0.99 TO 1.71 AU                                                         
   IN CASES WHERE TMAG IS NOT COMPUTED, IT IS SET EQUAL TO ZERO.                                                                  
NMAG = NUCLEAR MAGNITUDE = 16.5 + 5*LOG(DELTA) + 5*LOG(R) + 0.03*BETA                                                             
                                                                                                                                  
THETA = SUN-EARTH-OBJECT ANGLE IN DEGREES.                                                                                        
BETA = SUN-OBJECT-EARTH ANGLE IN DEGREES.                                                                                         
MOON = OBJECT-EARTH-MOON ANGLE IN DEGREES.                                                                                        
                                                                                                                                  
THE FOLLOWING OSCULATING ORBITAL ELEMENTS ARE C0NSISTENT WITH THE EPHEMERIS:                                                      
                                                                                                                                  
EPOCH                           2446320.5            1985   SEPT. 12.0 (E.T.)                                                     
PERIHELION PASSAGE              2446313.74907        1985   SEPT.  5.24907 (E.T.)                                                 
PERIHELION DISTANCE             1.0282614 AU                                                                                      
ECCENTRICITY                    0.7075300                                                                                         
ARG. OF PERIHELION              172.48887                                                                                         
LONG. OF ASCENDING NODE         194.70595                                                                                         
INCLINATION                     31.87829                                                                                          
                                                                                                                                  
IN THE ABOUE ORBITAL ELEMENTS, THE ANGLES ARE IN DEGREES AND REFERRED TO THE                                                      
ECLIPTIC AND EQUINOX OF 1950.0.                                                                                                   
                                                                                                                                  
THE N0NGRAVITATIONAL PARAMETERS ARE AS FOLLOWS:                                                                                   
   A1 = -0.0543                                                                                                                   
   A2 = -0.0465                                                                                                                   
FOR THE DEFINITION OF A1 AND A2 SEE: MARSDEN ET AL, ASTRONOMICAL JOURNAL,                                                         
1973, VOL. 78, PP. 211-225.                                                                                                       
                                                                                                                                  
COMPUTATIONS BY D.K. YEOMANS - JPL                                                                                                
                                                                                                                                  
                                                                                                                                  
* For an expanded ephemeris. see The Comet Giacobini-Zinner Handbook Yeomans & Brandt, 1985.                                      
                                                                                                                                  
             EPHEMERIS (WITH PERTURBATIONS) FOR P/GIACOBINI-ZINNER                                                                
                                                                                                                                  
  YR  MN DY HR    J.D.       R.A. 1950.0   DEC.      R.A.  APPN   DEC. DELTA  DELDOT    R    RDOT  TMAG  NMAG THETA  BETA MOON    
1985   3 15 .0 2446139.5 18  35.523  + 4    8.24 18  37.251  + 4  9.82  2.38  -32.19  2.34 -14.88  14.8  21.0  75.7  24.3   31    
1985   3 25 .0 2446149.5 18  51.870  + 6   29.50 18  53.571  + 6 31.89  2.19  -31.48  2.25 -15.11  14.5  20.7  80.4  25.9  113    
1985   4  4 .0 2446159.5 19   8.273  + 9   13.69 19   9.943  + 9 16.88  2.01  -30.40  2.16 -15.33  14.1  20.5  84.7  27.4  110    
1985   4 14 .0 2446169.5 19  24.783  +12   22.08 19  26.417  +12 26.06  1.84  -29.04  2.08 -15.52  13.7  20.3  88.6  28.9   43    
1985   4 24 .0 2446179.5 19  41.454  +15   55.70 19  43.050  +16   .47  1.68  -27.41  1.99 -15.69  13.3  20.0  91.9  30.4  121    
1985   5  4 .0 2446189.5 19  58.413  +19   54.61 19  59.968  +20   .14  1.53  -25.56  1.89 -15.82  12.9  19.8  94.5  32.0    3    
1985   5 14 .0 2446199.5 20  15.905  +24   18.27 20  17.416  +24 24.57  1.38  -23.61  1.80 -15.89  12.4  19.5  96.4  33.9   56    
1985   5 24 .0 2446209.5 20  34.262  +29    4.80 20  35.729  +29 11.87  1.25  -21.60  1.71 -15.89  12.0  19.2  97.5  35.9  121    
1985   6  3 .0 2446219.5 20  54.078  +34   10.19 20  55.504  +34 18.04  1.13  -19.66  1.62 -15.79  11.5  19.0  97.7  38.4   85    
1985   6 13 .0 2446229.5 21  16.342  +39   29.04 21  17.734  +39 37.69  1.03  -17.88  1.53 -15.55  11.0  18.7  97.0  41.3   63    
1985   6 23 .0 2446239.5 21  42.588  +44   52.86 21  43.966  +45  2.35   .93  -16.33  1.44 -15.12  10.5  18.5  95.5  44.6  116    
1985   7  3 .0 2446249.5 22  15.443  +50    8.48 22  16.851  +50 18.84   .84  -15.03  1.35 -14.44  10.0  18.2  93.4  48.5   86    
1985   7 13 .0 2446259.5 22  59.057  +54   55.08 23    .586  +55  6.24   .75  -13.95  1.27 -13.44   9.4  18.0  90.8  52.9   60    
1985   7 18 .0 2446264.5 23  26.582  +56   56.06 23  28.231  +57  7.52   .71  -13.44  1.24 -12.79   9.3  17.9  89.4  55.3   85    
1985   7 23 .0 2446269.5 23  58.803  +58   32.21  0    .620  +58 43.80   .68  -12.90  1.20 -12.03   9.1  17.8  88.0  57.8    9    
1985   7 28 .0 2446274.5  0  35.984  +59   33.08  0  38.017  +59 44.53   .64  -12.30  1.17 -11.14   8.9  17.7  86.5  60.3  124    
1985   8  2 .0 2446279.5  1  17.576  +59   46.31  1  19.847  +59 57.25   .60  -11.59  1.14 -10.12   8.8  17.6  85.1  62.9   89    
1985   8  7 .0 2446284.5  2   1.895  +58   59.35  2   4.378  +59  9.34   .57  -10.71  1.11 - 8.97   8.6  17.5  83.7  65.4   51    
1985   8 12 .0 2446289.5  2  46.356  +57    2.41  2  48.979  +57 11.07   .54  - 9.62  1.08 - 7.69   8.4  17.4  82.4  67.9   42    
1985   8 17 .0 2446294.5  3  28.340  +53   51.22  3  31.006  +53 58.33   .52  - 8.24  1.06 - 6.27   8.3  17.3  81.2  70.1   84    
1985   8 22 .0 2446299.5  4   6.077  +49   28.17  4   8.700  +49 33.70   .50  - 6.53  1.05 - 4.75   8.2  17.2  80.2  72.0  141    
1985   8 27 .0 2446304.5  4  38.927  +44    1.87  4  41.454  +44  5.89   .48  - 4.52  1.04 - 3.13   8.1  17.2  79.5  73.4  140    
1985   8 28 .0 2446305.5  4  44.918  +42   50.15  4  47.423  +42 53.89   .48  - 4.09  1.03 - 2.80   8.1  17.2  79.4  73.6  129    
1985   8 29 .0 2446306.5  4  50.724  +41   36.57  4  53.205  +41 40.03   .47  - 3.65  1.03 - 2.47   8.1  17.2  79.3  73.8  118    
1985   8 30 .0 2446307.5  4  56.349  +40   21.26  4  58.806  +40 24.45   .47  - 3.20  1.03 - 2.13   8.0  17.2  79.3  74.0  107    
1985   8 31 .0 2446308.5  5   1.799  +39    4.35  5   4.231  +39  7.29   .47  - 2.74  1.03 - 1.79   8.0  17.2  79.2  74.1   95    
1985   9  1 .0 2446309.5  5   7.078  +37   46.00  5   9.485  +37 48.68   .47  - 2.28  1.03 - 1.45   8.0  17.1  79.2  74.2   84    
1985   9  2 .0 2446310.5  5  12.191  +36   26.35  5  14.573  +36 28.78   .47  - 1.81  1.03 - 1.11   8.0  17.1  79.1   74.3  73    
1985   9  3 .0 2446311.5  5  17.144  +35    5.54  5  19.501  +35  7.73   .47  - 1.34  1.03  - .77   8.0  17.1  79.1   74.3  61    
1985   9  4 .0 2446312.5  5  21.943  +33   43.73  5  24.274  +33 45.69   .47  -  .87  1.03  - .43   8.0  17.1  79.1   74.4  50    
1985   9  5 .0 2446313.5  5  26.591  +32   21.07  5  28.898  +32 22.80   .47  -  .40  1.03  - .09   8.0  17.1  79.2   74.4  39    
1985   9  6 .0 2446314.5  5  31.095  +30   57.70  5  33.377  +30 59.21   .47     .08  1.03    .26   8.1  17.1  79.2   74.3  28    
1985   9  7 .0 2446315.5  5  35.460  +29   33.79  5  37.718  +29 35.08   .47     .55  1.03    .60   8.1  17.1  79.3   74.3  17    
1985   9  8 .0 2446316.5  5  39.690  +28    9.46  5  41.924  +28 10.54   .47    1.02  1.03    .94   8.1  17.1  79.3   74.2   5    
1985   9  9 .0 2446317.5  5  43.790  +26   44.88  5  46.000  +26 45.76   .47    1.49  1.03   1.28   8.1  17.1  79.4   74.1   5    
1985   9 10 .0 2446318.5  5  47.765  +25   20.17  5  49.952  +25 20.85   .47    1.95  1.03   1.62   8.1  17.1  79.5   73.9  17    
1985   9 11 .0 2446319.5  5  51.620  +23   55.48  5  53.783  +23 55.97   .47    2.40  1.03   1.96   8.2  17.1  79.6   73.7  29    
1985   9 12 .0 2446320.5  5  55.357  +22   30.93  5  57.499  +22 31.24   .47    2.85  1.03   2.30   8.2  17.1  79.8   73.5  42    
1985   9 13 .0 2446321.5  5  58.983  +21    6.66  6   1.102  +21  6.79   .47    3.29  1.03   2.64   8.2  17.1  79.9   73.3  55    
1985   9 14 .0 2446322.5  6 2.499    +19   42.77  6   4.596  +19 42.74   .48    3.72  1.04   2.97   8.2  17.2  80.1   73.1  69    
1985   9 15 .0 2446323.5  6 5.911    +18   19.39  6   7.987  +18 19.19   .48    4.14  1.04   3.30   8.2  17.2  80.2   72.8  82    
1985   9 16 .0 2446324.5  6 9.221    +16   56.62  6  11.276  +16 56.26   .48    4.55  1.04   3.63   8.2  17.2  80.4   72.5  96    
1985   9 17 .0 2446325.5  6 12.433   +15   34.56  6  14.468  +15 34.04   .48    4.95  1.04   3.95   8.3  17.2  80.6   72.2 110    
1985   9 18 .0 2446326.5  6 15.551   +14   13.30  6  17.565  +14 12.62   .49    5.34  1.04   4.27   8.3  17.2  80.8   71.8 123    
1985   9 19 .0 2446327.5  6 18.576   +12   52.91  6  20.571  +12 52.08   .49    5.71  1.05   4.59   8.3  17.2  81.1   71.4 136    
1985   9 20 .0 2446328.5  6 21.513   +11   33.48  6  23.489  +11 32.51   .49    6.08  1.05   4.91   8.3  17.2  81.3   71.1 148    
1985   9 21 .0 2446329.5  6 24.365   +10   15.07  6  26.322  +10 13.96   .50    6.42  1.05   5.22   8.4  17.2  81.5   70.7 157    
1985   9 22 .0 2446330.5  6 27.133   + 8   57.74  6  29.072  + 8 56.49   .50    6.76  1.06   5.53   8.4  17.2  81.8   70.3 161    
1985   9 23 .0 2446331.5  6 29.820   + 7   41.54  6  31.742  + 7 40.16   .50    7.08  1.06   5.83   8.4  17.2  82.1   69.8 157    
1985   9 24 .0 2446332.5  6 32.429   + 6   26.51  6  34.334  + 6 25.01   .51    7.38  1.06   6.13   8.5  17.2  82.3   69.4  48    
1985   9 25 .0 2446333.5  6 34.961   + 5  12.69   6  36.849  + 5 11.07   .51    7.67  1.07   6.42   8.5  17.3  82.6   68.9 138    
1985   9 30 .0 2446338.5  6 46.544   - 0  37.46   6  48.354  - 0 39.64   .54    8.88  1.09   7.82   8.7  17.3  84.1   66.5  85    
1985  10  5 .0 2446343.5  6 56.454   - 5  55.38   6  58.194  - 5 58.03   .56    9.74  1.11   9.09   8.9  17.4  85.8   63.9  42    
1985  10 10 .0 2446348.5  7  4.808   -10  41.94   7   6.485  -10 44.98   .59   10.30  1.14  10.23   9.1  17.5  87.5   61.2  47    
1985  10 15 .0 2446353.5  7 11.668   -14  59.21   7  13.289  -15  2.58   .62   10.61  1.17  11.23   9.3  17.6  89.4   58.5  98    
1985  10 20 .0 2446358.5  7 17.073   -18  49.57   7  18.642  -18 53.20   .65   10.75  1.20  12.11   9.6  17.7  91.3   55.8 133    
1985  10 25 .0 2446363.5  7 21.055   -22  15.35   7  22.578  -22 19.16   .68   10.76  1.24  12.86   9.8  17.7  93.3   53.2 112    
                                                                                                                                  
           EPHEMERIS (WITH PERTURBATIONS) FOR P/GIACOBINI-ZINNER                                                                  
                                                                                                                                  
 YR   MN DY   HR   J.D.  R.A.  1950.0  DEC.     R.A.  APPN  DEC.  DELTA DELDOT   R  ROOT   TMAG  NMAG THETA BETA MOON             
                                                                                                                                  
1985  10   .0  2446368.5  7 23.635 -25 18.68   7  25.114  -25 22.61  .72 10.68 1.28 13.50  10.0  17.8  95.4 50.7  76              
1985  11   .0  2446373.5  7 24.816 -28  1.26   7  26.225  -28  5.25  .75 10.55 1.32 14.04  10.3  17.9  97.6 48.3  55              
1985  11   .0  2446378.5  7 24.599 -30 24.19   7  26.000  -30 28.17  .78 10.42 1.36 14.48  10.5  18.0  99.8 45.9  72              
1985  11   .0  2446388.5  7 20.084 -34 12.51   7  21.419  -34 16.30  .84 10.30 1.44 15.15  10.9  18.2 104.4 41.5 116              
1985  11   .0  2446398.5  7 10.764 -36 43.67   7  12.048  -36 47.04  .90 10.49 1.53 15.56  11.4  18.3 109.0 37.5  68              
1985  12   .0  2446408.5  6 57.943 -37 56.51   6  59.196  -37 59.29  .96 11.11 1.62 15.80  11.8  18.5 113.4 33.8  95              
1985  12   .0  2446418.5  6 43.530 -37 51.12   6  44.782  -37 53.24 1.03 12.26 1.72 15.89  12.2  18.6 117.2 30.7  97              
1986  12   .0  2446428.5  6 29.752 -36 34.16   6  31.028  -36 35.64 1.10 13.88 1.81 15.89  12.6  18.8 120.2 28.1  65              
1986   1   .0  2446438.5  6 18.353 -34 19.16   6  19.672  -34 20.10 1.19 15.92 1.90 15.81  12.9  19.0 121.8 26.1 116              
                                                                                                                                  
1986   1   .0  2446448.5  6 10.369 -31 22.94   6  11.745  -31 23.52 1.29 18.26 1.99 15.68  13.3  19.3 122.0 24.8  76              
1986   1   .0  2446458.5  6  6.128 -28  3.72   6   7.562  -28  4.12 1.40 20.72 2.08 15.51  13.7  19.5 120.6 24.0  74              
1986   2   .0  2446468.5  6  5.389 -24 36.99   6   6.881  -24 37.37 1.52 23.20 2.17 15.32  14.1  19.8 117.9 23.7 124              
1986   2   .0  2446478.5  6  7.733 -21 14.28   6   9.279  -21 14.80 1.67 25.54 2.26 15.10  14.5  20.1 114.1 23.6  55              
1986   2   .0  2446488.5  6 12.653 -18  3.90   6  14.246  -18  4.68 1.82 27.64 2.34 14.87  14.8  20.4 109.5 23.5  94              
1986   3   .0  2446498.5  6 19.638 -15 10.49   6  21.273  -15 11.64 1.98 29.46 2.43 14.63  15.2  20.6 104.3 23.3 115              
1986   3   .0  2446508.5  6 28.283 -12 36.43   6  29.955  -12 38.02 2.16 30.93 2.51 14.39  15.5  20.9  98.8 23.1  41              
                                                                                                                                  
Acknowledgement                                                                                                                   
  I would like to thank Drs. T. von Rosenvinge and M. Niedner for their generous                                                  
assistance in preparing this paper.                                                                                               
                                                                                                                                  
                                                                                                                                  
References                                                                                                                        
                                                                                                                                  
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