PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT NOTE = "INTRODOC.TXT file is a digitized version of a 1994 COSPAR report 2011-05-04 A.Raugh Removed illegal tab characters" PUBLICATION_DATE = 1994-07-31 END_OBJECT END *****File INTRODOC.TXT Giotto's Encounter with Comet P/Grigg-Skjellerup M. G. Grensemann and G. Schwehm European Space Agency, Directorate of Scientific Programmes, European Space Research and Technology Centre, Noordwijk, Netherlands Friday 10 July, 1992 marked the second high point in the career of the Giotto spacecraft which was originally designed to have a life-span of no more than eight months while performing its main mission: the exploration in situ of the comet Halley. Not only did Giotto survive the harsh environment around Halley but it was also able, after more than 7 years in orbit around the Sun, to pass within about 200 km of a second comet, P/Grigg-Skjellerup, and pro- vide exciting scientific data concerning the environment in the vicinity of that comet. History Giotto was launched on 2 July, 1985, at 11:23:16 UTC aboard an Ariane-1. The mission reached fruition 8 months later during the night of 13-14 March, 1986, when Giotto passed comet Halley at a distance of less than 596 km of the nucleus, making spectacular images of the nucleus. Giotto was not expected to survive the passage through the cometary dust surrounding Halley, where particles would strike the spacecraft with an explosive force 50 times that of a bullet. However, after passing Halley, the spacecraft still appeared to be functioning quite well. It was decided to continue payload operation until the early morning of 14 March, and to make an additional science pass during the night of 14/15 March. It was found that the magnetometer (MAG), the optical probe experi- ment (OPE), the energetic particles analyzer (EPA), and the particulate impact analyzer (PIA) were working nominally; the Johnstone plasma analyzer (JPA) and the dust impact detector system had lost some of their measuring capabilities but could still provide more than 60% of their scientific data. The ion mass spectrometer (IMS) had lost one measurement channel (HERS, the high energy range spectrometer, optimized for measurements in the outer coma), but the high intensity spectrometer (HIS) optimized for measurements in the inner coma had survived the encounter. The Reme plasma analyzer (RPA) seemed to be severely damaged; only its electron electrostatic analyzer (EESA) could still provide sufficient scientific data to permit determin- ation of electron spatial density (see Table 1). The big unknown remained the Halley multicolor camera (HMC). It seemed likely that the camera's outer baffle had been lost during the encounter and that the unit was no longer correctly balanced. In February 1990 the long process of reactivation was started, as Giotto made its approach towards Earth. The spacecraft had been dormant for 4 years, had been subjected to cyclic extremes of temperature and was in an unknown attitude, one, more over that precluded communication via its high gain antenna (HGA). There was only one way to communicate with Giotto and that was via the omni-directional low gain antenna (LGA) mounted on the top of the spacecraft. Two hours after the start of the reactivation activities, the NASA Deep Space Network Station at Madrid reported acquisition of a weak signal (-166 dBm), and about 150 hours after starting, full control of the spacecraft was established through the HGA and Giotto again performed beyond its design envelope. From the spacecraft telemetry data received and analyzed, it was evident that Giotto had survived its odyssey through space extremely well. A check of the payload concluded, with a high degree of confidence, that the hibernation period had not caused any further degradation in instrument performances and that a viable payload remained to support another cometary encounter. Unfortunately, however, the HMC was no longer operational (Table 2). Once experiment checkout had been completed, the Control Centre made preparations for directing the spacecraft towards comet Grigg-Skjellerup. This was no ordinary maneuver, as Earth's gravitational field was to provide the kick needed to bring Giotto into the otherwise unreachable orbit. At 10:00 UT on 2 July, 1990, exactly 5 years after its launch, and with the MAG and EPA activated, Giotto flew over Earth at an altitude of 22000 km and made space history once again. After this activity, Giotto was put into its second hibernation which would last until the spacecraft was reactivated again on 4 May, 1992 for its mission to comet P/Grigg-Skjellerup. At that time the spacecraft to Earth range was 219 x 10^6 km. The operations with Giotto, who's health showed no further degradation since 1990 continued without problems, concentrating on guiding the spacecraft as close as possible toward his new target (Table 3). Two orbit-control maneuvers were performed: the first on 22 May, reducing the predicted flyby distance from 167400 km to 510 km. The final maneuver was performed on July 8, reducing the expected flyby distance to near zero. Encounter operations started some 24 hours before closest approach, scheduled for 15:30:36 UT on 10 July. There were a number of features of the comet Grigg-Skjellerup encounter that were most unlike those at encounter with Halley: (1) Grigg-Skjellerup would approach Giotto at an angle of 68 degrees instead of head-on as at Halley: hence the bumper shield would afford no protection. (2) The relative velocity was 14 km/s, with the comet meeting Giotto from below and behind, whereas Halley had met Giotto head-on at 68 km/s. (3) The Earth range at encounter was 214 x 10^6 km leading to a round- trip light time of 24 min. At Halley, the range had been 140 x 10^0 km and the round-trip light time was 16 min. During actual encounter phase on 10 July, 1992 there were no problems. The science data processed in real time at European Space Operations Centre (ESOC) provided immediate access to the cometary environment. Already at 03:00 UTC, more than 12 hours before the closest approach, the JPA detected the presence of cometary ions. During the last hour before closest approach, after JPA confirmed the increase in the density of the ions, RPA, the EPA, and the IMS also reported detection of ions. Meanwhile, periodic bit errors in telemetry were noticed at ESOC, causing occasional loss of data. At 15:20 the Optical Probe Experiment (OPE) could see cometary dust. At 15:31:02, when the first housekeeping telemetry format was received after the major impact, the data showed the HGA to be oscillating slightly around its nominal value. An increase in the spin rate by 0.003 RPM was also observed while the solar aspect angle readings were fluctuating between 89.26 degrees and 89.45 degrees, indicating a nutation of about 0.1 degrees. The small nutation of the spacecraft, experienced around closest approach, can be explained by the spacecraft being hit by one large dust particle of at least 30 mg effective mass, assuming that this particle struck on the upper end of the solar cell array. The good navigation of Giotto was confirmed by the data obtained by the OPE, based on an analysis of the brightness increase of the sunlight scattered by the dust particles in the inner coma. The closest approach was inferred to have occurred at a distance smaller than 200 km, with the brightness peak recorded at 15:30:43 with an uncertainty of only 3 seconds. At 03:00 on 11 July, 1992 all experiments were switched-off. Mission termination At 03:00 on 11 July, 1992 all experiments were switched-off. After a period of seven years in orbit around the Sun, Giotto operations were officially terminated on 23 July 1992, after the completion of final orbit adjustments and after configuring the spacecraft for its third hibernation. In its present trajectory course, Giotto will pass 219000 km above the Earth's surface on 1 July 1999, 14 years after the spacecraft's launch. Future operation of the spacecraft is considered doubtful, partly because the fuel remaining (4 +/- 3 kg) is insufficient for anything more than an Earth or Moon fly-by in 1999, and partly because of the age by then of both the spacecraft itself and the ground systems used to support the GEM mission. That Giotto survived its travels is a tribute to the quality of the spacecraft design and build and to the dedication of all concerned. The success of the Giotto Extended Mission has been due to the excellent joint efforts of the teams of ESOC, the Experimenters, and the small Project Team at ESTEC. It should also be remembered that without the unfailing support provided by NASA's Deep Space Network and Jet Propulsion Laboratory, the Giotto Extended Mission would not have been possible. TABLE 1. The Giotto Experiment Complement (Original Specifications) =============================================================================== Exper- Measurement Technique iment =============================================================================== HMC Colour imaging of cometary nucleus Narrow-angle CCD camera with and inner coma Ritchey-Chretien telescope NMS Energy and mass of neutrals Ionization by electron beam M-analyzer: 1-36 amu Electrostatic energy and E-analyzer: 10-1410 ev (1-57 amu) magnetic sector 210-2180 ev (9-89 amu) Field momentum analyzer Parallel plate electrostatic analyzer IMS Energy and mass of ions Sector magnet and electrostatic HERS: 20-8000 eV and 1-35 amu/q deflector HIS: 100-1400 eV and 12-57 amu/q Two quadri-spherical analyzers with magnetic deflection PIA Mass (3x10^-16 - 5x10^-10g) and Impact ionization, time-of- composition (1-110 amu) of individual flight tube dust particles (1 m) in which the ions are separated according to their masses DID Determination of mass spectrum of dust MSM/RSM: 3+1 piezoelectric particles from 10^-17 to 10^-3 g with elements for large masses three different detectors CIS: capacitor for medium masses IPM: impact charge measurements for small masses JPA FIS: solar wind and cometary ions from Hemispherical electrostatic 10 eV to 20 keV analyzer with subsequent IIS: cometary ions from 90 eV to 70 keV quadri-sperical sector and 1-45 amu Five electrostatic analyzers, each followed by a time-of- flight tube RPA EESA: solar wind and cometary electrons Quadri-spherical electrostatic from 10 eV to 30 eV analyzer with 4 pi viewing PICCA: cometary ions from 1 to 200 amu Hemispherical electrostatic analyzer EPA 3-D measurement of protons from 15 keV Three telescopes, each with two to 20 MeV, solid-state detectors electrons from 15 keV to 140 keV, alpha particles from 140 keV to 12.5 MeV MAG Interplanetary and cometary magnetic Outboard sensor: triaxial ring- field core fluxgate magnetometer Experiment range: 0.004-65536 nT Inboard sensor: biaxial OPE Coma brightness in four continuum Rearward-looking photo- (dust) bands polarimeter with eight and at four discrete wavelength interference filters (gaseous emissions of OH, CN, CO+, C2) GRE Cometary electron content and mass Phase differences between S- fluence and X- band RF signals =============================================================================== TABLE 2. Status of the Giotto Payload for the Comet Grigg-Skjellerup Encounter =============================================================================== Instrument Status Active During G-S Encounter =============================================================================== Halley Multicolor Camera aperture blocked; no baffle missing; blind Neutral Mass Spectrometer detectors dead no Ion Mass Spectrometer HERS: high-voltage no damage, HIS: no damage yes Particulate Impact Analyzer mass spectrum slightly no degraded Dust Impact Detection System some detectors showing yes increased noise Optical Probe Experiment no damage yes Magnetometer no damage yes Johnstone Plasma Analyzer high-voltage problems yes on one sensor Reme Plasma Analyzer cold ion composition: no high-voltage damage damage to electron yes electrostatic analyzer Energetic Particle Analyzer no damage yes Giotto Radioscience Experiment not applicable yes =============================================================================== TABLE 3. Key Mission Parameters for the Comet Grigg-Skjellerup Flyby (Halley Flyby Data Included for Comparison) =============================================================================== Grigg-Skjellerup Halley =============================================================================== Encounter data July 10, 1992 March 14, 1986 Relative flyby velocity 13.99 km s^-1 68.37 km s^-1 Flyby distance, km ~200 km 596 km Heliocentric distance 1.01 AU 0.90 AU Geocentric distance 1.43 AU 0.96 AU Distance above/below ecliptic 0.10 AU 0.02 AU Angle between spin axis and relative velocity 68.8 degrees 0 degrees vector (at 89.6 degrees SAA*) (at 107.2 degrees SAA*) =============================================================================== *SAA, solar aspect angle.