| MISSION_DESCRIPTION |
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 experiment (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 determination
of electron spatial density.
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.
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 22 000 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 its new target.
Two orbit-control maneuvers were performed: the first on 22 May,
reducing the predicted flyby distance from 167 400 km to 510 km. The
final maneuver was performed on July 8, reducing the expected flyby
distance to near zero.
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.
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 219 000 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.
Spacecraft ID : GIO
Target name : GRIGG-SKJELLERUP
Spacecraft Operations Type : FLYBY
Mission Phases
==============
Launch
------
The Giotto spacecraft was launched on July 2,
1985 onboard an Ariane-1 rocket from Kourou, French Guyana.
Mission phase start time: 1985-07-02
Mission phase stop time: 1985-07-02
Reactivation
------------
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.
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 22 000 km and made space
history once again.
After this activity, Giotto was put into its second hibernation.
The second reactivation of the Giotto spacecraft had been scheduled
for 4 May 1992. At 16:55 h local time, the first commands were sent
from the European Space Operations Centre (ESOC) in Darmstadt, Germany,
via NASA's Jet Propulsion Laboratory in Pasadena (Calif.), to the 70 m
Deep-Space Network (DSN) ground station in Madrid, Spain, for uplinking
to Giotto. At that time, the space probe was 219 million kilometres
from Earth.
The commands sent were the first in an extensive series designed to
awaken Giotto from its hibernation configuration. To ensure reception
of the signals by the tiny omnidirectional antenna on the top of the
spacecraft, the DSN's most powerful 95 kW transmitter had to be used.
The spacecraft was configured to allow the transmitter connected to
the Low-Gain Antenna to be turned on. At 18:14 h local time, the Madrid
ground station reported the reception of a downlink carrier with a
signal strength of -171 dBm, as predicted by the ESOC Operations Team.
Giotto was back, again at the first attempt, just as in 1990 after its
first period of hibernation! Once again, Giotto had lived up to the
confidence that the Project and Operations Teams had placed in it.
Only the waiting required was a little nerve-racking, because the
remoteness of the spacecraft from Earth meant that one had to wait
nearly half an hour for the spacecraft's response to a command to be
seen on the ground!
A series of 'blind' maneuvers were subsequently performed to orient
Giotto so that its High-Gain Antenna (HGA) pointed towards Earth and
telemetry could be received. This was accomplished on 7 May 1992,
when good telemetry was received continuously. At this point, the
Giotto reactivation effort could be truly declared a success!
Mission phase start time: 1990-02-19
Mission phase stop time: 1992-07-09
Flyby
-----
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.
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.
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.
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 219 000 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.
Mission phase start time: 1986-07-09
Mission phase stop time: 1986-07-11
|
| MISSION_OBJECTIVES_SUMMARY |
The specific scientific objectives of the Giotto Extended Mission can
be summarised as follows:
- characterisation of the changing features of the solar-wind flow
and observations of cometary pick-up ions and anomalous accelerati
- determination of electron densities
- observation of upstream waves, and determination of the locations
the various boundaries (bow shock, ionopause, etc.)
- observation of the magnetic pile-up region and cavity
- determination of dust spatial density and size distribution, and t
optical properties of the dust grains
- discrete gaseous emissions
- combined dust and gas densities.
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
=====================================================================
The functioning payload complement listed in the table above shows tw
main scientific areas that can be addressed during the Grigg-Skjeller
encounter: the particle and field instruments have the potential to
provide significant new results pertaining to the field of cometary
plasma physics. The Optical Probe Experiment and the Dust-Impact
Detection System (DID) will complement our knowledge of dust-producti
rates and size distributions for a low-activity comet.
The dust measurements can also provide important data to support futu
European and international space programmes. Future missions to comet
for example, will need engineering models of the cometary dust
environment for low-activity comets, which the GEM mission should hel
to improve.
Cometary plasma physics is part of a broader domain of space plasma
physics dealing with the interaction between a planetary atmosphere
and a flow of magnetised plasma. Examples include comets, Venus, the
Saturnian satellite Titan in the solar wind, and Io, Titan and Triton
in their planetary magnetospheres. Physically, comets are distinguish
from the other planetary bodies by their characteristic of possessing
non-gravitationally-bound atmosphere.
Three instruments that will be especially useful for the
Grigg-Skjellerup encounter survived the dust impacts unscathed during
Giotto's encounter with Comet Halley. Of these, the Implanted Ion
Sensor (IIS) of the Johnstone Plasma Analyser (JPA) provides the full
pitch-angle distribution for each chemical species in a range of
energies from 100 eV/q to 86 keV/q. Its time resolution is rather goo
with just 128 s required for a full set of observations. This
corresponds to a spatial resolution of approx. 14 km/s x 128s =
+/-1700 km in the cometary frame of reference.
The Magnetometer (MAG) is also fully operational with 35.4 ms
resolution, corresponding to 0.5 km spatial resolution. The
Magnetometer results can be processed and evaluated much more easily
for Comet Grigg-Skjellerup than for Halley because of the absence of
magnetic disturbances produced by the motors of the Halley Multicolou
Camera (HMC), which is no longer functioning.
In addition, the fully operational Energetic Particle Analyser (EPA)
contains three semiconductor telescopes, each incorporating two total
depleted silicon surface barrier detectors. Electrons, protons, alph
particles and heavier ions can be measured in eight channels covering
an energy range extending from approximately 30 keV to several tens o
MeV. High spatial and temporal (0.5 s) resolution is available.
Finally, although the Reme Plasma Analyser (RPA) is severely crippled
by dust damage, it may still provide some information on low-energy
electrons.
|
.png)
.png)
