RBBS in a Box Volume 1 #3.1/RBBSIABOX31.cdr/inst/ice.001

Text File | 1985-10-02 | 11.6 KB | 195 lines

[850929.001]

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            First Comet Probe Reveals Structure of Great Complexity

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                              By Craig Corvault

GODDARD SPACE FLIGHT CENTER, MD. -- The first direct exploration of a comet
carried out Sept. 11 by the U.S. International Cometary Explorer (ICE)
satellite has shown that the thousands of comets roaming the solar system are
likely to be far more complex and dynamic than Earth-based observation and
analysis have indicated.
  The seven-year-old ICE vehicle, which was diverted from its previous
mission across 1 billion miles of space, spent 20 min. within comet Giacobini-
Zinner, penetrating the object 4,886 mi. behind its nucleus.  The penetration
was at a point in the comet's coma where its ion and dust tails have formed
but have not yet emerged from the spherical gas and dust cloud making up the
coma head, according to Robert W. Farquhar, the Goddard astrodynamicist who
conceived the mission (AW&ST Mar. 22, 1982, p. 22).

U.S. FLYBY
   The U.S. flyby 44 million miles from Earth took place six months before
Soviet, European and Japanese spacecraft will reach Halley's Comet in March,
1986.  ICE will now be retargeted to measure the solar wind ahead of Halley.
"From the point of view of the history of the U.S. space program, I think it's
another one of those firsts that this country enjoys making."  NASA
Administrator James M. Beggs said.
   Goddard Space Flight Center and Bendix controllers here commanding the
vehicle, built by Fairchild Space Co., detected no change in the satellite's
system status during the encounter.  They had feared cometary dust could coat
its solar cells and reduce available electrical power or kill the spacecraft
entirely.  The satellite has wire antennas spanning 300 ft. tip-to-tip, and
managers would not have been surprised to see dust impacts either shear them
off or bend them back around the 5 x 6-ft. satellite, but no such damage
occured.  The lack of cometary dust was one of the big surprises of the
mission.
   "It is easy for us to say now that things went fine, but going in it was an
extremely hazardous mission," Farquhar said about the dust hazard, which is
likely to be far more serious when other spacecraft visit comets such as
Halley.
   The 930-lb. ICE satellite had a closing speed with the comet of 13 mi./sec.
as it entered the body at 6:53 a.m. EDT, then emerged 14,000-15,000 mi. on the
other side of the coma about 20 min. later.
   During its approach and exit from the area of the comet, the ICE satellite
instrumentation, designed originally for an entirely different mission,
provided an unexpectedly detailed characterization of multiple cometary
features - many of them unexplained.  The spacecraft is not equipped with an
imaging system, so pictures of the comet were not an objective.  The comet's
interaction with the solar wind, dynamics within the tail and identification
of what constituents make up the plasma in the tail were to be measured,
however.
   The size of the comet's coma/tail area where ICE penetrated was not known
accurately before the encounter and the satellite found that feature about
three times wider than some models had predicted.  Other data on Giacobini-
Zinner obtained as recently as 24 hr. prior to flyby indicated that the
coma/tail area to be penetrated would be as large as it proved to be as ICE
flew through it with 45,000 mph. spacecraft velocity.
   Optical observatories around the world and the Goddard International
Ultraviolet Explorer spacecraft in geosynchronous orbit provided images of the
comet to adjust ICE targeting and aid data analysis.
   Another unusual finding made by the satellite's 10 functioning instruments
was that cometary effects seen starting only 1 hr. away from penetration were
present for more than 2 hr. on the other side of the comet as ICE exited the
vicinity.  These data indicate that the environment created by the comet in
the solar wind was asymmetrical - an observation that puzzled scientists here.
   "Just from the cursory looks we have had at the data I think a lot of
people are inclined to believe they do not show the kinds of effects we
expected to see and will cause us to rethink what kinds of things are going on
in comets," Edward Smith, principal investigator for the satellite's magnetic
fields experiment, said.
   The Jet Propulsion Laboratory scientist also headed the working group that
initially studied whether the ICE spacecraft would be suitable for returning
data on a comet, since it was launched in 1978 as a solar wind monitoring
vehicle not outfitted for a comet encounter.
   The spacecraft's entry point was more than 1,000 mi. closer to the 1.5 mi.-
dia. solid nucleus than had been planned until four days before the encounter.
   Updated cometary tracking data, however, then showed an ICE course
correction would be needed to cancel a targeting error that would place ICE
500 mi. off the comet tail's axis.  Penetration of the tail, not the coma, had
always been the ICE objective.
   "That's why we wanted to be out a little farther, because we were afraid we
would still be in the comet's ionosphere at this distance - that perhap's it
wasn't safe to retarget," Steven P. Maran, senior scientist in Goddard's
Laboratory for Astronomy and Solar Physics, said.
   The retargeting began at 8 a.m. EDT Sept. 8, when the spacecraft's 4-lb.-
thrust TRW hydrazine thrusters were fired 299 times to change course and
adjust vehicle attitude.  ICE moved to within 2 million miles of the comet
Sept. 9, then closed the distance to 1 million miles Sept. 10.
   As the spacecraft closed in on the comet early Sept. 11, the Japanese
attempted to image Giacobini-Zinner with the ultra-violet camera on their
Planet-A spacecraft headed for Halley's comet, while NASA also attempted to
obtain comet data with the Lyman alpha spectrometer mounted on the U.S.
Pioneer Venus orbiter spacecraft circling Venus.
   NASA's Deep Space Tracking Network mobilized one of the most extensive
tracking efforts of the U.S. space program to follow ICE.

KEY OBJECTIVE
   The first key science objective of the encounter was to see if the comet
had a bow shock - a massive shock wave pushed thousands of miles ahead and to
the side of the comet in the solar wind much like an aircraft flying
supersonically forms a shock wave in Earth's atmosphere.
   Scientists had different opinions about whether or not a comet should have
a bow shock because of the unique cometary features, including the lack of any
significant gravity to hold in place the massive ball of gas that is generated
around a comet as the Sun acts upon its icy surface.
   Whether Giacobini-Zinner, or other comets, have a bow shock remained
unresolved last week.  There is some activity in the area where the comet's
bow shock should be, but the ICE instruments found it much different from
planetary bow shocks observed around Venus or the Earth, for example.

INITIAL EVIDENCE
   "We first saw evidence that there might be a bow shock with the comet about
7.5 hr. before we got there," Samuel Bame, investigator for the Los Alamos
plasma electrons instrument, said.
   "As we came closer we saw hot [and therefore faster] electrons coming back
more and more frequently," he said.  The instrument then provided strong
evidence of crossing some sort of shock wave about 80 min. prior to
penetrating the comet, but Bane is not sure what kind of shock wave it was
because the data are so unusual.  ICE was then about 75,000 mi. away from
penetration.
   Bane said that once this feature was crossed, "the conditions were very,
very turbulent and you really could not get a very good sample on what the
situation was because things were changing so rapidly."
   He said that as the vehicle continued on through this region approaching
the comet, the turbulence subsided and the instrument began detecting hot,
fast plasma up to about one-half million degrees Kelvin.  As ICE crossed into
the comet itself, however, the plasma temperatures dropped to less than
100,000K, indicating the plasma in the comet's tail was moving much more
slowly.  "Inside this region of very cold plasma we saw the flow speed of the
plasma drop way down, very close to zero.  Then we came out through the ion
tail and did a mirror image of the entry," he said.
  Smith's JPL megnetic fields instrument was also key to finding a bow shock.
"We see some kind of phenomenon that looks like it could be associated with a
shock, but we have difficulty identifying it as a shock," he said.  Smith said
that from the time ICE started detecting detailed cometary phenomena on one
side to the time it stopped such detections on the other, the vehicle covered
200,000 mi.

FIELD LINES
   ICE data transmitted from directly inside the comet also elated
researchers.  As the solar wind moves away from the sun it pulls giant
magnetic field lines with it that deflect when they meet an obstacle, such as
a comet.  Theories had postulated that the magnetic field lines would drape
over the comet but continue to flow in the same direction, forming two
polarized lobes around the cometary body.  Viewed from above, the magnetic
field lines could strike the left side of the comet, flow up one side of the
tail and down the opposite side, with an extremely narrow "neutral sheet"
forming in between.
   Theory turned to reality as JPL researchers here watched transmissions from
ICE that first showed the magnetic field flowing one way, then the field
intensity dropping suddenly to near zero as the vehicle crossed the neutral
sheet that had been predicted.  After a few seconds in the neutral sheet, ICE
transmitted data showing the magnetic field lines going the opposite direction
- precisely what the theory had predicted.
   Frederick Scarf, principal investigator for the TRW plasma waves
instrument, said his system also made unexpected discoveries.
   "We detected a great many different phenomena in the comet with very strong
interactions producing great intensities," Scarf said.
   "When we were about a million kilometers from the comet and already
detecting some of the upstream phenomena, we saw things we had not detected
since we left the vicinity of Earth in December, 1983, so we knew we were near
an object interacting very strongly with the solar wind," he said.  "The thing
we first detected when we came in closer was a very strong bow shock at
188,000 km.
   "It is remarkable that it looked so typical and strong to us but does not
appear that way in the other instruments," he said.
   "As we came farther in we detected increasing levels of plasma wave
turbulence that suggest that the plasma was breaking up into beams and the
filaments were all colliding with each other," Scarf said.
   "At closest approach we detected the tail of the comet.  There are certain
radio waves that are a signature of a high-density region - and we went
through one about 10 min. before closest approach, and then again about 10
min. after that," Scarf said.
   Scarf said his instrument detected plasma intensities a full order of
magnitude higher than anything the ICE spacecraft had detected when it was
flying downstream of Earth - its second mission prior to the comet flyby.
   "It's really a remarkable measurement and we are at a loss to describe what
is going on," Scarf said.  "We thought a comet might be a benign object, but
it appears to be extremely active.

- from Aviation Week & Space Technology / September 16, 1985


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remely active.

- from Aviation Week & Space Technolo