=============================================================================
DIDSY DATA ARCHIVE 27th August 1993
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This archive is in 3 sections:
1) Description and DIDSY results from the GEM encounter.
2) How the analysis was performed.
3) Sensitivity maps of the bumper shield for DID 2, 3 and 4 sensors.
[NOTE: The sensitivity maps have been pulled out into separate files during
PDS ingest. See the "did*map.tab" files.]
Archive compiled by Dr Neil McBride
Unit for Space Sciences
Physics Laboratory
University of Kent at Canterbury
Canterbury CT2 7NR, UK
Tel: 0227 764000 ext 3242
Fax: 0227 762616
Email: nm1@ukc.ac.uk
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SECTION 1
=============================================================================
The follow text gives a description of the experiment and the results
obtained at the GEM encounter. The text essentially makes up the paper
published in Nature (Vol 362 No 6422 22 April 1993).
Dust particle impacts during the Giotto encounter with comet Grigg-Skjellerup
-----------------------------------------------------------------------------
By:
J.A.M. McDonnell (1)
N. McBride (1)
R. Beard (1)
E. Bussoletti (2)
L. Colangeli (3)
P. Eberhardt (4)
J.G. Firth (5)
R. Grard (6)
S.F. Green (1)
J.M. Greenberg (7)
E. Grun (8)
D.W. Hughes (9)
H.U. Keller (10)
J. Kissel (8)
B.A. Lindblad (11)
J.-C. Mandeville (12)
C.H. Perry (5)
K. Rembor (15)
H. Rickman (13)
G.H. Schwehm (6)
R.F. Turner (5)
M.K. Wallis (14)
J.C. Zarnecki (1)
(1) Unit for Space Sciences, Physics Laboratory, University of Kent,
Canterbury, Kent CT2 7NR, UK.
(2) Instituto di Fisica Sperimentale, Istituo Universitario Navale, Via A.
De Gasperi 5, 80133 Napoli, Italy.
(3) University of Cassino, Via Zamosch 43, 03043 Cassino (Fr), Italy.
(4) Physikalisches Institut, Universitat Bern, CH-3012 Bern,
Sidlerstrasse 5, Switzerland.
(5) Space Science Dept., Rutherford Appleton Laboratory, Chilton, Didcot,
Oxon OX11 OQX, UK.
(6) PSS, ESTEC, Postbus 299, 2200 AG Noordwijk, Netherlands.
(7) Rijksuniversiteit te Leiden, Huygens Laboratorium, Postbus 9504, 2300 RA
Leiden, Netherlands.
(8) Max Planck Institut fur Kernphysik, D-6900 Heidelberg, Germany.
(9) Department of Physics, University of Sheffield, Sheffield, S3 7RH, UK.
(10) Max Planck Institut fur Aeronomie, D-3411 Katlenburg-Lindau,
Germany.
(11) Lund Observatory, Box 43, S-221 00 Lund, Sweden.
(12) ONERA-CERT-DERTS, BP No. 4025, 31055 Toulouse Cedex, France.
(13) Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden.
(14) School of Mathematics, University of Wales, Senghennydd Road, Cardiff
CF2 4AG, UK.
(15) Fakultat fur Physik, Universitat Karlsruhe, D-7500
Karlsruhe, Germany.
Summary
-------
In the European Space Agency's 1992 Giotto Extended Mission (GEM)
to comet Grigg-Skjellerup, the Dust Impact Detection System (DIDSY)
was successfully operated during a fly-by approaching within some
200~km from the nucleus, and gives in situ detections of pristine
cometary meteoroids. Three meteoroid impacts on the spacecraft's
front shield were detected and the particle masses have been
established as 100 (+105/-50) ug, 2 (+4/-1) ug and 20 (+25/-10) ug.
The cumulative mass index over this mass range is found to be
alpha=-[0.27 (+.13/-.20)] (1 sigma errors), with the upper limit
being -0.74 (3 sigma). The slope is consistent with the 1986 Halley
data over this mass range, and is supported by data from the GRE
experiment which indicates the impact of a very large meteoroid of
mass 14 (+40/-4) mg. The low value of alpha signifies a higher rate
of mass loss than previously thought and hence a higher (by mass)
dust to gas ratio. The value alpha=-0.27 gives the total mass that
impacted the spacecraft as approximately 22 mg, this being
dominated by the largest particle.
The Dust Impact Detection System (DIDSY) was designed to measure the flux of
dust particles in the coma of Comet Halley in the mass range 10^-19 kg to
>10^-6 kg (see refs 1 and 2). Giotto is protected from hypervelocity dust
impacts by two annular meteoroid bumper shields. The front shield consists
of 1 mm thick aluminium and is situated 250 mm above a rear Kevlar shield.
Impacts are detected by three separate subsystems. The first subsystem
comprises 4 piezoelectric sensors. Three of these sensors (DID2, DID3 and
DID4) are mounted on the rear of the front shield and are sensitive to the
momentum imparted to the shield. DID2 and DID3 can operate independently, or
measure in coincidence. DID4 operates independently and is located on a 30
degree section of the shield isolated by acoustically insulating joints.
Another piezoelectric sensor (DID5) is situated on the rear shield behind
the DID4 section. The impact plasma and momentum subsystem (IPM, DID1)
comprises two subunits, located behind an aperture in the front shield. The
plasma subunit (IPM-P) determines particle mass by measuring the plasma
produced when a particle impacts on a metal target plate. The momentum
subunit (IPM-M) uses a piezoelectric detector situated on the rear of the
metal target plate. IPM-P and IPM-M can measure in coincidence. The
capacitor impact sensor subsystem (CIS, DID7) comprises a thin-film,
dielectric-filled capacitor fixed to the front shield. Impacts are indicated
by brief discharging (a simple schematic of the sensor positions is given
in section 3).
The DID1 IPM-P sensor suffered impact damage at the Halley encounter and at
Grigg-Skjellerup, the instrument was noisy and gave no interpretable
results. The DID1 IPM-M sensor however, was fully operational. The DID7 CIS
sensor also suffered impact degradation at Halley, and was largely non-
operational. The piezoelectric momentum sensors (DID2 to 5) were fully
operational.
The 1986 Halley encounter geometry was such that particles impacted the
front shield at normal incidence with relative velocity 68.4 km/s. A
hypervelocity particle having momentum mv will transfer momentum epsilon*mv
to the shield (due to target ejecta) where epsilon is the momentum
enhancement factor. The value taken at Halley (ref 3) was epsilon=11. At
Grigg-Skjellerup the impacting particle trajectories made a 21.2 degrees
angle with the shield at a relative velocity of 13.99 km/s. These factors
reduced epsilon significantly. Experiments using the 2 MV dust accelerator
at the Unit for Space Sciences, Kent, yield a value of epsilon=3.1 +/- 1.0.
This reduced value of epsilon raises the piezoelectric mass detection
threshold. Also, the reduced resolved area of the front shield and the fact
that the activity of Grigg-Skjellerup was estimated to be approximately 1%
that of Halley, meant that few detections were expected. Pre-encounter
modelling indicated total fluences between 0 and 40 particles for miss
distances ranging between 300 and 50 km respectively. The data gathered were
wholly consistent with these predictions. DIDSY was operated for
approximately 35 hours from 18:42UT 9th July, with an impact time resolution
of 2.83s. All times given here are ground receive times (GRT) UT (where GRT
= spacecraft time + 11m53s).
Three particle impacts were detected, by sensors DID2, DID3 and DID4,
compared with 2400 at the Halley encounter. The DIDSY telemetry is
transferred in data frames of 64 bytes, and a full `DIDSY data block' (DDB)
is constructed from 8 data frames. Each DDB contains information on
particle impacts in the last data gathering interval (2.83s at the Grigg-
Skjellerup encounter). At 15:30:43 GRT (coinciding with nutation particle,
see below) 4 data frames were lost or corrupted, then a good data frame was
received, then 4 more were lost. These were the only frames lost or
corrupted during the whole time the experiment was active. The data drop-
outs meant that some information was lost from 2 consecutive DDBs (DDB
numbers 56963 and 56964). However, some reconstruction of the DDBs could be
performed and it was found that no particle impacts could have occurred
during the data drop-outs. The first particles detected by DIDSY were
recorded in DDB number 56966.
Due to the impact degradation of CIS, the bias voltage was 0.22 V as opposed
to 2.3 V at Halley. This voltage was stable for 21 hours up to closest
approach, and then at 15:31:31/32, changed to 0.20 V and remained stable for
the rest of the operating period. Significantly, this change coincided with
the impact interval of the third particle, and indicates an impact directly
on CIS. The CIS voltage was monitored approximately once a second giving a
better time resolution on event 3. The times and the masses of the 3
meteoroids are given in Table 1 The first 2 events were near closest
approach when the spacecraft-comet distance (ref 6) was likely to have been
<200 km and the third event was in excess of 40 sec after closest approach
at >500 km from the nucleus.
Event Names Time (GRT) Mass
----- ----- ---------- ----
Expected closest - 15:30:36 (+/-14s) -
approach (ref 4)
Nutation particle Whopper 15:30:43 14 (+40/-4) mg
GRE (ref 5)
Maximum optical - 15:30:43 (+/-3s) -
brightness OPE (ref 6)
DIDSY Event 1 BigMac 15:30:48-51 100 (+105/-60) ug
(DID 2 and DID 4)
DIDSY Event 2 Barley 15:30:51-54 2 (+4/-1) ug
(DID 3)
DIDSY Event 3 Bretzel 15:31:31-32 20 (+25/-10) ug
(DID 3 and CIS)
Table 1 GEM encounter events. The errors quoted (to 1 sigma)
------- on the 3 DIDSY particle masses incorporate both the
statistical and systematic errors and represent the
absolute uncertainty in the particle masses.
The fluence, being the time integrated particle flux, is shown in Table 2.
The wide distribution of masses yielded a most significant and unexpected
mass distribution.
log10 Cumulative Fluence (per m^2) log10 Particle Mass (kg)
---------------------------------- -------------------------
0.640 -8.80
0.427 -7.68
0.125 -7.00
Table 2 The cumulative fluence for the three particles
------- detected by DIDSY. The slope gives the cumulative
mass distribution index alpha=-0.27.
A most probable upper mass cut-off at 10^-4.23 kg (59 mg) was calculated for
1 detected particle hitting the entire resolved spacecraft area
(approximately 3.6 m^2). The cumulative mass distribution index alpha, was
found to have a value of alpha=-0.27. The error in this gradient is
dominated by the statistical number uncertainty as opposed to the particle
mass uncertainty. An analysis based on the null detection of a 4th DIDSY
particle indicates that alpha=-[0.27 (+.13/-.20)] (1 sigma errors) and could
be as high as -0.74 (3 sigma limit).
Additional support for a low slope mass distribution was given by results
from the Giotto Radio Science Experiment (ref 5) (GRE) which indicated that
a particularly large meteoroid impacted at the rear of the spacecraft (hence
no DIDSY detection). The effective mass (where m_eff=epsilon x m_real) was
calculated to be between 30 and 54 mg. Taking epsilon=3.1 as before, gave
the most likely value of the real mass of the particle as approximately 14
mg with the lower limit being 10 mg. It is possible for a penetrating impact
to have little or no momentum enhancement, and so a reasonable upper limit
is 54 mg. This particle was not used to determine alpha, but signifies that
the slope of -0.27 is entirely consistent with the detection of this
particle, and indicates the slope may hold over four decades of mass. An
integration between mass limits 10^-9.35 and 10^-4.23 kg gives the total
mass that impacted Giotto as approximately 22 mg. With such a low slope,
this mass is dominated by the largest impacting particle and is consistent
with the GRE meteoroid. Assuming rho=1 g/cm^3, the cumulative cross-
sectional area of the particles between these mass limits is 12 mm^2.
The low slope found from the 3 DIDSY particles also agrees well with Halley
data over the same mass range of 10^-9 to 10^-7 kg, where the Halley time
averaged mass distribution index was found to be -0.2. From 10^-9 to 10^-15
kg, the Halley index increased to -0.9. Although the Grigg-Skjellerup data
presented here says nothing about masses <10^-9 kg as such, it is likely
that the index also increases at smaller mass to account for the
significance presence of optical scattering particles (ref 6).
DIDSY can give no information as to the composition of impacting particles
(ie, solid versus aggregate) and the number of detections were not high
enough to give information about the variation of the mass distribution
index with time. However the very low mass distribution slope agrees with
the Halley data and IRAS dust trail data. The domination of the large
particles is important for both the modelling of nucleus structure and
composition, and in modelling the contribution and evolution of cometary
particles to the interplanetary dust flux. It also signifies a higher rate
of mass loss than previously thought and hence a higher (by mass) dust to
gas ratio.
Acknowledgements
----------------
We acknowledge the UK Science and Engineering Research Council for financial
support and thank Norbert Schmidt for his help during the encounter project.
We especially acknowledge the efforts of project scientist Gerhard Schwehm
and project manager Manfred Grensemann for achieving the `mission
impossible'.
References
----------
(1) McDonnell, J.A.M., W.M. Alexander, W.M. Burton, E. Bussoletti, D.H.
Clark, G.C. Evans, S.T. Evans, J.G. Firth, R.J.L. Grard, E. Grun, M.S.
Hanner, D.W. Hughes, E. Igenbergs, H. Kuczera, B.A. Lindblad, J.-C.
Mandeville, A. Minafra, D. Reading, A. Ridgeley, G.H. Schwehm, T.J.
Stevenson, Z. Sekanina, R.F. Turner, M.K. Wallis & J.C. Zarnecki. ESA SP
1077, 85-108 (1986).
(2) McDonnell, J.A.M.. J. Phys. E:Sci. Instrum., 20, 741-758 (1987).
(3) McDonnell, J.A.M., M. Alexander, D. Lyons, W. Tanner, P. Anz, T. Hyde,
A.-L. Chen, T.J. Stevenson & S.T. Evans. Adv. Space Res., 4, 297-301
(1984).
(4). Morley, T., ESOC, private communication, 9th July (1992).
(5). Patzold, M., P. Edenhofer, M.K. Bird & H. Volland. Astron. Astrophys.,
in press (1993), and private communication.
(6). Levasseur-Regourd, A.C., B. Goidet, T. Le Duin, C. Malique, J.B.
Renard & J.L. Bertaux. Planet.Space Sci., 41, No 2, 167-169 (1993).
=============================================================================
SECTION 2
=============================================================================
The follow text gives a description of how the data analysis was performed
for the three particle detections.
When a particle impact is detected by a DIDSY piezoelectric detector, the
detector produces a voltage V volts which is stored by the onboard
electronics as a digital value 0 to 255. The conversion between V and
digital value D(V) for the DIDSY experiment is given by
D(V) = (A*log10(V)+B)*(1-(C/V)^D) (1)
where A, B, C and D are constants and have been determined for sensors DID2,
DID3 and DID4 as
Sensor | A | B | C | D
------------------------------------------------
DID2: | 28.004 | 176.66 | 0.03 | 1.760
DID3: | 47.145 | 155.41 | 0.04 | 7.322
DID4: | 30.718 | 174.51 | 0.03 | 2.112
The three DIDSY events gave digital values given below. V is obtained by
solving the equation above.
Event | Sensor | D(V) | V
---------------------------------------
Event 1 | DID2 | 197 | 5.335
| DID4 | 109 | 0.063
Event 2 | DID3 | 102 | 0.077
Event 3 | DID3 | 127 | 0.250
Note that event 1 triggered two sensors, DID2 and DID4. This gives the
useful result that the ratio of voltages produced by DID2 and DID4 is 84.7,
and any mass-position solution for this impact must satisfy this voltage
ratio.
For a non-penetrating impact (and it is very unlikely that any of the
detected particles penetrated the shield) by a particle which gives rise to
a sensor voltage V volts, the particle mass m_p kg is given by
V
m_p = ------------ * (Pi/Ps)^-1 (2)
e*v*cos(t)*S
where v is the relative impact velocity (m/s), t is the angle the impact
makes with the shield normal (0 for Halley and 68.8 degrees for Grigg-
Skjellerup), e is the momentum enhancement factor (epsilon in section 1) and
was experimentally determined to be 3.1 for the Grigg-Skjellerup encounter,
and S is the absolute sensitivity of the piezoelectric detectors and is
approximately 50 kV/N/s. The uncertainty in this value is of order 2. The
value Pi/Ps is the relative signal attenuation for a particular impact site
compared to a `direct hit' on the sensor (ie, Pi is signal from impact at
site i, and Ps is the signal obtained if the impact was on the sensor site,
all other things being equal. The signal attenuation for an impact on a
particular area element of the front shield is known, due to laser shot
mapping carried out at the Rutherford Appleton Laboratory (see ref 2 in
section 1). The values of Pi relative to Ps are given in section 3 for the
three sensors DID2, DID3 and DID4.
A particle mass probability distribution is obtained by identifying the mass
required to produce the observed sensor signal from each area element. In
examining all possible impact sites, a cumulative mass distribution
N(m)=k*m^alpha is assumed, where N(m) is the cumulative fluence, k is a
constant and alpha is the cumulative mass distribution index. A probability
of the particle being between mass m and m+dm can be calculated thus
P(m) = [ sum(i=1,shield) dN(m)*a(i)*f(m) ] * (3)
[ sum(m=0,infinity) sum(i=1,shield) dN(m)*a(i)*f(m) ]^-1
where dN(m) is the differential mass distribution function (the number of
particles of mass m to m+dm) and a(i) is the area of the ith shield element
(these areas are given in section3). The function f(m)=1 if the mass m gives
the observed signal for this area element, and f(m)=0 if it does not. (The
nomenclature here is such that sum(i=1,shield) means the sum from i=1 to
i=whole shield) This is an iterative process where particle mass is
determined using an initial chosen value of alpha. Additionally, possible
impact sites of events 1 and 3 can be constrained due to event 1 registering
on DID2 and DID4 giving the voltage ratio 84.7 (see above), and event 3
being in the region of the CIS sensor. The mass distribution is then
calculated using the effective shield areas (see below) so yielding a new
value of alpha. The process is repeated until alpha converges. The
determined particle mass however is not strongly dependent on alpha.
There are 2 sorts of error in the particle mass determination. Firstly, that
described by the standard deviation of the pseudo-Gaussian mass probability
function due to uncertainties in impact position, and secondly the
systematic errors due to uncertainty in the absolute sensor sensitivity and
momentum enhancement factor. The probability errors affect the determination
of alpha. The systematic errors do not affect the `relative' positions of
the mass probability functions but has the effect of moving the mass scale
either way by a factor of approximately 2. Hence this error does not affect
the determined mass distribution index.
The fluence is determined from the observed number of impacting particles of
mass >= m, and the `effective' shield area which is sensitive to these
particles, which, as we are calculating a cumulative fluence, is also a
function of N(m). The effective sensitive area does not necessarily equal
actual geometric sensitive area as small particles impacting a long way from
a sensor may not be detected, whereas a very large particle will be detected
at all places on the shield. Thus the actual sensitive shield area has to be
corrected for small particle masses to obtain a realistice estimate of the
particle fluence (number per m^2). The geometric area of the front shield
is 2.09 m^2 whereas the effective shield sensing areas are 2.09, 1.92 and
2.09 m^2 for events 1, 2 and 3 respectively. The effective sensitive shield
area for a particle of mass m_p is given by
a(eff) = (sum(i=1,shield) a(i)*N(m_pt))/N(m_p) (5)
where m_pt is either the m_p or m_t whichever is greater, and m_t is is the
detection threshold mass for the shield area element a(i). The value of m_t
is calculated by using equation (2), the minimum sensor voltage that would
trigger a detection (D(V)=1) being 0.03 V.
So now the cumulative fluence N(m) (number per m^2) can be calculated by
N(m) = n(m)/(a(eff)*cos(t)) (6)
where n(m) is the number of particles detected with mass greater than or
equal to m kg. The relevant figures are
Event | n(m) | log10(m kg) | a(eff) m^2 | log10(N(m))
---------------------------------------------------------
Event 1 | 1 | -7.00 | 2.09 | 0.125
Event 2 | 3 | -8.80 | 1.92 | 0.640
Event 3 | 2 | -7.68 | 2.09 | 0.427
The cumulative mass distribution index alpha, is found by plotting
log10(N(m)) against log10(m) and taking the gradient. A weighted linear
regression (each point weighted with root n(m)) gives alpha=-0.27. An
uncertainty in alpha can be determined by considering the effect of tilting
the straight line fit about the error weighted mean in this graph, and
considering when the 3 detections would not be consistent with the gradient,
within statistical errors. It is found that values of alpha between -0.07
and -0.40 are consistent with the detection of 3 particles within one sigma
statistical errors. A 3 sigma limit corresponds to alpha=-0.74. This
statistical uncertainty is dominant over the mass probability error. The
systematic errors do not affect this error analysis.
=============================================================================
SECTION 3
=============================================================================
The following tables give the values of Pi, the relative signal strength for
an impact on the ith area element of the shield compared to the signal
strength the same impact would give if it were directly on the sensor site
(signal Ps). In calculating the ratio Pi/Ps, use the value Ps=1E+05 (all 3
sensor values of Ps have been normalised to this value). The table also
gives a(i), the area of the ith element of the shield.
In conducting the laser shot mapping, the shield was divided into 1032
elements (not necessarily of equally area or shape). The position of each
element is given by the values theta and d in the tables. The distance d is
in m, and is measured radially from the centre of the shield. The angle
theta is in degrees and is defined thus: set up axes x and y, where the
origin is at shield-centre and the x axis passes through DID4 sensor (which
is situated centrally in the 30 degree acoustically isolated section of the
shield). The y axis is 90 degrees clockwise from the x axis (viewing from
above the shield). The angle theta is measured clockwise from the y axis,
about the origin. See the schematic below.
o o
o o + = piezoelectric sensor
o o
o o
o o o o
o o o o
o o o o + o o DID2
o o o o o o
o o x o o
o o o------- o + o DID4
o o | o o
o o | o o o
o o y | o + o o DID3
o o o o
o o o --- o
o --------- |IPM| o
o | CIS | --- o
o --------- o
o o
Simple schematic of the front meteor bumper shield
--------------------------------------------------
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[NOTE: The sheld map tables have been pulled out into separate files with
their own labels during PDS ingest. These files are:
did2map.tab DID2 shield map
did3map.tab DID3 shield map
did4map.tab DID4 shield map
]