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                 GALILEO IDA ENCOUNTER NOTEBOOK
                           August 1993


1.0  INTRODUCTION

The Galileo spacecraft's Venus-Earth-Earth gravity assist (VEEGA) trajectory
provided two opportunities to make close observations of asteroids.  On
October 29, 1991, Galileo encountered the first of these asteroids, Gaspra, at
a distance of 1,601 km.  Most of the data gathered at Gaspra were recorded for
playback in November 1992 when the spacecraft was again in the vicinity of
Earth.

The opportunity to visit Gaspra and Ida was not the result of celestial good
fortune but was in fact the result of careful trajectory design.  A NASA
policy put into effect after the start of the Galileo program required that
all missions which pass through the asteroid belt make a close observation of
an asteroid if at all possible.  This policy was applied retroactively to the
Galileo program.  The asteroid belt (defined here as that region lying between
the orbits of Mars and Jupiter where the vast majority of discovered asteroids
reside) extends from approximately 2 to 3.5 astronomical units (AU) from the
Sun (one AU equals Earth's mean distance from the Sun).  Galileo would pass
through this region twice -- during the Earth-Earth leg when it encountered
Gaspra, and during the Earth-Jupiter leg when it will encounter Ida.

The orbits of more than 4,000 asteroids were checked for possible candidates
which would allow for a successful VEEGA transfer to Jupiter with acceptable
propellant consumption to achieve a close asteroid flyby.  Few of these
candidates survived the first look, however, due to the strict timing
requirements for a successful VEEGA trajectory.  Remaining candidate asteroids
were then processed with trajectory optimization software to determine which
would be the best targets.  Gaspra and Ida were chosen due to their
accessibility and the low propellant cost required to divert Galileo to them.


1.1  Organization of Document

The basics of the Ida encounter, not including the details of experiment
design and science objectives, are provided in Sections 1.0 through 3.0.
Section 4.0 provides a summary of the science objectives and a comprehensive
description of each instrument.

1.2  Background

Galileo was launched from the Kennedy Space Center aboard the space shuttle
Atlantis on October 18, 1989.  Correct alignment of all the planets involved
required that Galileo be launched between October 12 and November 21, 1989.
In the sometimes backwards and upside-down world of orbital mechanics, it is
often necessary to slow down in order to go faster.  The Inertial Upper Stage
(IUS) actually expended most of its energy to slow Galileo down from the
Earth's orbital velocity which it shared so that it would fall in towards the
Sun.  The spacecraft made its closest approach to Venus on February 10, 1990
at which time the planet's orbital energy provided a net increase in speed
relative to the Sun of 2.2 km/sec or approximately 5,000 mph.  This sent
Galileo on a trajectory that carried it around the Sun and back to Earth.
Galileo passed 960 km (596 mi) from Earth's surface on December 8, 1990.
Earth provided an additional speed increase of 5.2 km/sec or approximately
11,600 mph.  During its Earth-Earth leg, Galileo passed 1,601 km (995 mi) from
the center of Gaspra on October 19, 1991. Galileo continued on its Earth-Earth
trajectory and returned to Earth for the last time on December 8, 1992 passing
303 km (188 mi) over the South Atlantic.  At that time, Galileo received an
additional boost of 3.7 km/sec (8,300 mph), enough energy to reach Jupiter as
planned on December 7th, 1995.  Ida will be Galileo's final detour before
reaching the veils of the largest planet in our solar system.

1.3  The Study of Asteroids

Asteroids have fascinated astronomers since their discovery almost 200 years
ago.  There are many questions which scientists hope to answer through
studying asteroids, questions which extend far beyond the esoteric realm of
pure science and answers that may help us understand the forces that shaped
the solar system and life on Earth.

One of the most important reasons scientists are so interested in asteroids is
that they are believed to represent "leftovers" from early planetary
formation.  Current theory holds that asteroids are, or are remnants of,
planetesimals, the small accumulations of material from which the planets were
formed.  Four and a half billion years ago, processes of accretion led to
fewer but larger planetesimals.  In time, some of these became massive enough
that their gravity began to draw other planetesimals to them.  Once this point
was reached, they quickly gathered up most of the remaining planetesimals and
nebular material to become the planets we now know.

In the region between Mars and Jupiter, however, the planetesimals never
became large enough to form a planet.  This is most likely due to the strong
gravitational influence of Jupiter.  Asteroids, therefore, are composed of
some of the oldest material left in the solar system and can provide an
invaluable tool for understanding the processes that went into planetary
formation, as well as the composition and physical state of the young solar
system.  They are believed to be a major source of meteorites, from which we
have received much of our knowledge about the age of the solar system and
chemical composition of the early solar nebula.

Scientists are interested in the question of why no planet formed between Mars
and Jupiter, and how the asteroids and their region of the solar system have
changed.  Asteroids are continually being bombarded and in many cases broken
up into smaller asteroids through collisions.  Their surfaces have recorded
aeons of this disruption and reaccretion.  By analyzing this record,
scientists can learn much about the distribution of asteroids and comets in
the past.

The study of asteroids has evolved rapidly in the last two centuries due
largely to the advancement of observing techniques.  The first ground-based
observations of an asteroid were made in 1801 when Ceres, the largest
asteroid, was discovered.  (The following year, William Herschel, the
discoverer of Uranus, christened this type of planetesimal an "asteroid.")
Seventy-five years later, the first asteroid-sized satellites, Phobos and
Deimos, were discovered orbiting Mars.  This was followed in 1892 with the
discovery of a third asteroid-sized satellite, Amalthea, orbiting Jupiter.  In
1983, the study of asteroids made significant strides when the Infrared
Astronomical Satellite (IRAS) was launched into Earth orbit.  For the
following year, the spacecraft measured nearly 2,000 asteroids as part of its
asteroid-comet survey.  In 1989, the first radar images of an asteroid, 4769
Castalia, were made.  (An asteroid is assigned a number as soon as its orbit
is determined accurately enough for its position to be predicted and verified.
Thus, Castalia was the 4,769th asteroid with a known orbit.)  In December
1992, further advances were made when JPL scientists obtained the highest
resolution images of an Earth-approaching asteroid by beaming a radio
transmission from Goldstone's 70m antenna to the asteroid, 4179 Toutatis.  At
the time, the asteroid was 4 million kilometers (2.5 million miles) from
Earth.  Echoes reflected back to Goldstone's 34m antenna from Toutatis were
relayed to the 70m station where they were decoded and processed into images.
The images revealed that Toutatis is a "contact binary" asteroid consisting of
two irregularly shaped objects with an average diameter of about 4 and 2.5
kilometers (2.5 and 1.6 miles).  Toutatis is the most irregularly-shaped solar
system object yet seen.

Interest in, and the study of, asteroids is no longer limited to astronomers
and planetary geologists but now includes paleontologists and biologists.
There is considerable geologic evidence that impacts by asteroids and comets
played an important role in the evolution of life on Earth and may have had a
hand in determining which species survived, and which perished.  In the last
two decades, geologists have discovered that our world has been struck by
asteroids or comets many times over during its recent past (within the last
500 million years or so).  These impacts have been recorded in layers of clay
containing high concentrations of iridium, an element not commonly found on
the Earth's surface but relatively abundant in asteroids and comets.  The
iridium layer coincides closely with the extinction approximately 65 million
years ago of the dinosaurs and a large number (perhaps 75 percent) of other
life forms.  The catastrophe apparently indiscriminately affected all parts of
Earth -- sparing no continent or climatic belt.  Indeed, asteroids may have
even played a role in the rise of the dinosaurs:  Some scientists believe that
another mass extinction of many of Earth's species occurred 215 million years
ago due to the impact of a massive asteroid.   Dinosaurs, somehow spared from
this catastrophe,  may have rapidly evolved to fill the resulting ecological
void (only to suffer the same fate 150 million years later).

Galileo may have the unique opportunity to actually witness a series of
planetary impacts during its voyage to Jupiter.  In July 1994, comet
Shoemaker-Levy is expected to collide with the giant planet.  The comet was
shattered into a dozen or so chunks last year by Jupiter's gravitational tidal
forces when it passed nearby the planet.  The impact could be as powerful as
the one scientists believe occurred 65 million years ago on Earth.  The force
of the impact could create an unbelievable light show for a few days as each
remnant of the comet collides with Jupiter.  Galileo will be in a fortuitous
position to watch these fireworks -- the event is expected to happen on the
limb of Jupiter as seen by Galileo (but on the far side as viewed from Earth)
at that time.  At this time, no firm plan exists to use Galileo to record this
event.

By teaching us about the orbital interactions and periodic bombardments which
asteroids undergo, Galileo's experiments may be able to shed some light on
many critical questions pertaining to the origin, evolution and extinction of
life.  Eventually, the study of asteroids may lead to the use of asteroidal
materials in a variety of endeavors which may prove to be of economic benefit.

1.4  What we learned at Gaspra

On October 29, 1991, a small and highly elongated asteroid 2.20 AU from the
Sun made history.  On that day, the asteroid known as Gaspra became the first
to be imaged at close range by a spacecraft.  Galileo shuttered a total of 16
images of Gaspra during its encounter.  By November 1992, all of the data
acquired at Gaspra had been returned to Earth.

Gaspra's highly irregular shape -- 19 by 12 by 11 kilometers (12 by 7.5 by 7
mi) -- indicates that it is a fragment of a parent body (most likely a larger
asteroid) which suffered a catastrophic collision.  Its asymmetry is not
surprising since small celestial bodies tend to be less symmetric than large
ones.  Until Toutatis was imaged by ground-based radar in December 1992,
Gaspra held the distinction of being the most irregularly-shaped object yet
observed.   (Comparisons of shape are measured by how large the object's limb
profile deviates from the best elliptical fit.)

Gaspra's mean radius of about 7 km (4.2 mi) places its size between those of
the Martian satellites, Deimos and Phobos.  It has fewer craters per unit area
than most planetary satellites and no intermediate or large craters (2 to 6 km
in diameter).  In addition to craters, linear features 200 to 400 m wide and
up to several kilometers long were discovered.  If similar to grooves as seen
on Phobos, they are likely evidence of nearly catastrophic impacts.  Other
possibilities are that the linear features are trough-like or coalescing
elongated depressions.

Gaspra's age is estimated to be 200 million years based on the assumptions
that it has a primarily rocky composition and that a certain number of
projectiles, as evidenced by the number of visible craters, would have
impacted it during its lifetime.  Most celestial objects Gaspra's size have an
expected lifetime of half a billion years; thus, Gaspra has not yet reached
midlife if the assumptions used in calculating its age are correct.  However,
if the asteroid's interior is metallic rather than rocky, its age could be
significantly greater -- several billions of years; or, if the projectile
impact rate was underestimated, Gaspra could be younger.

Measurements made by Galileo's magnetometer during the encounter revealed that
the interplanetary magnetic field was distorted around Gaspra from 1 minute
before the flyby until 2 minutes afterward.  The readings support a surprising
(although still speculative) conclusion -- Gaspra is magnetized!  Two
processes which may have enabled Gaspra to achieve this state are:  (1) A
molten core could have been created in Gaspra or its parent body by heat from
the decay of radioactive isotopes within the parent body, and magnetization
could have resulted from the creation of a dynamo due to convection within the
core; or (2) a strong magnetic field (as existed early in the Sun's history)
could have magnetized Gaspra or its parent body if either solidified or was
abruptly shocked while within the field's reach.

One of the primary objectives of the Gaspra and Ida encounters is to determine
if S-class asteroids, the class to which both Gaspra and Ida belong, are the
parent bodies of either the ordinary chondrite meteorites or the stony-iron
meteorites.  This is known as the "S-asteroid debate."  Ordinary chondrite
meteorites are considered primitive in comparison with the stony-iron
meteorites because the stony-irons are rich in metal and other compounds due
to the extensive melting and geochemical fractionation they underwent within
their respective parent bodies.  S-class asteroids are one of the most common
type of asteroids.  Their defining feature is their surface composition --
varying proportions of olivine and pyroxene and iron-nickel metal.
Unfortunately, a key source of data to resolve this debate -- the mass of
Gaspra -- could not be determined because mission safety precluded the flyby
distance at Gaspra being small enough to make accurate mass measurements.
However, further study of data from Galileo's near-infrared mapping
spectrometer taken at Gaspra may help resolve this debate since it will permit
searches for marked compositional heterogeneity across the asteroid's surface.
And, since Ida is also an S-class asteroid, the likelihood of resolving this
debate grows as the Ida encounter approaches.


2.0  WHAT WE KNOW ABOUT IDA

The asteroid 243 Ida was discovered on September 29, 1884 by J. Palisa in
Vienna.  The asteroid was named by a Viennese, Herr von Kuffner, presumably
due to the mythological association between Ida and Jupiter -- Ida was a nymph
who cared for the infant Jupiter while Jupiter was in hiding from his father,
Saturn, who had threatened to eat him.

Ida, like Gaspra, is an S-type asteroid.  Approximately one-sixth of the
asteroids fall within this classification.  S-type asteroids are reddish
objects with moderate albedos implying that they are composed of a mixture of
pyroxene, olivine and iron.  Ida is slightly pyroxene-dominated whereas Gaspra
is richer in olivine.  The significance of the pyroxene-olivine ratio is that
olivine-rich asteroids are not ordinary chondrites.  This is an important
factor in the S-asteroid debate as discussed earlier.  By mapping composition
units on Ida with Galileo's cameras and its near-infrared mapping spectrometer
and comparing them with similar data from Gaspra, the S-asteroid debate could
be resolved.

Ida is a member of the Koronis family of asteroids.  This implies that some
time ago a large asteroid, Koronis, suffered a catastrophic collision and
broke into many remnants (children) of which Ida is one.  Ida and the other
members of this "family" share nearly identical orbital elements -- thus
meeting the qualification of being an asteroid family.  The benefit to
studying families is that by studying the offspring one is provided with a
glimpse of the interior of the parent body.  Models of the Koronis family
suggest that the collision giving rise to Ida happened only tens of millions
of years ago.

During the time of closest approach, Ida will appear from Earth to be located
at right ascension 196.7 degrees, declination -8.0 degrees toward the
constellation Virgo.  Ida will be 3 to 4 degrees northwest of Spica, Virgo's
brightest star.

Ida's known characteristics are summarized in the following table (Gaspra's
characteristics are provided as a comparison):

IDA vs. GASPRA
A Brief Comparison

-- Ida is almost twice as large in diameter and eight times as large in volume
   (Ida can be represented as a triaxial ellipsoid with the following
   dimensions:  53 km by 23 km by 18 km with a mean diameter of 28 km (17.4
   mi) vs. 19 km by 12 km by 11 km with a mean diameter of 14 km (8.4 mi)) for
   Gaspra)

-- Ida is in the middle of the asteroid belt (Gaspra is located in the inner
   edge)

-- Ida has a more rapid spin rate (Ida's spin period = 4.63 hours; Gaspra's
   spin period = 7.04 hours)

-- Ida is thought to have a more irregular shape

-- Ida is a member of the Koronis family of asteroids (Gaspra is in the Flora
   family)

-- Ida may be much younger than Gaspra (tens of millions of years as opposed
   to 200 million years for Gaspra)

-- Ida is possibly an ordinary chondrite (most common meteorite) parent body
   (Gaspra is possibly a stony-iron meteorite parent body)


3.0  PREPARING FOR THE ENCOUNTER

The three main challenges in planning for a successful encounter at Ida were
(1) to identify Ida's position with respect to the Galileo spacecraft with as
high a degree of certainty as possible; (2) to prepare the spacecraft to point
the instruments on target; and (3) to develop a data return strategy which
will optimize the science return.  The first two objectives were especially
challenging due to Ida's small size and ephemeris uncertainty.  The third
objective, an optimized data return strategy, was also challenging due to data
rate limitations and constraints on the use of the non-redundant onboard tape
recorder.

3.1  Navigating to Ida

Precise knowledge of Ida's orbit only became of interest to astronomers after
it was selected as a flyby target for Galileo.  Thus, a long-term record of
regular Ida observations which the navigation team could rely on to assist in
the determination of Ida's orbit did not exist.  Instead, ground-based
observations of Ida combined with Doppler and range measurements of the
spacecraft are being used to narrow the position uncertainty of both Ida and
Galileo.  Additionally, star-position data acquired by the Hipparcos
spacecraft was used to update Ida's apriori ephemeris.  The uncertainty in the
position of Galileo relative to Ida can be represented by an ellipsoidal
volume of space no larger than 530 km by 420 km by 210 km (330 by 260 by 130
mi) (95-percent probability).  Further reductions in Ida's position
uncertainty will be made by the Galileo Navigation Team prior to closest
approach through the use of optical navigation and a technique known as
single-frame mosaicking which was first employed at Gaspra.

Optical Navigation

Optical navigation (OPNAV) consists of a series of photographs taken by the
spacecraft's imaging system of a target body against a star background.  OPNAV
pictures give information on the apparent position of the target body when
compared to the known positions of the background stars.  OPNAV pictures were
used by Galileo for orbit determination purposes for the first time at Gaspra.

Prior to the encounter, Galileo will take a total of four optical navigation
images of Ida on July 22, August 12, August 17 and August 21 (all dates UTC)
beginning with OPNAV 2.  (Originally, five OPNAV pictures were planned;
however, OPNAV 1 was cancelled due to a spacecraft safing event.)  After all
of the data from the preceding OPNAV has been played back, the next OPNAV
image will be shuttered.  As of this document's deadline date, OPNAV 2 has
already been processed.

OPNAV 5 originally was scheduled to occur 5 days prior to encounter but was
moved back 2 days (to -7 days) in order to accommodate the Mars Observer orbit
insertion which required the Deep Space Network 70m antennas at the same time.
As a result, OPNAV 5 will be executed and played back before the final Ida-
encounter trajectory correction maneuver (TCM 21).  Thus, data from OPNAV 5
(and OPNAV 4) will be used to adjust Galileo's trajectory through design
revisions of TCM 21.

Single-Frame Mosaicking

In order to ensure that adequate data would be returned in each OPNAV image to
allow extraction of position measurements, the single-frame mosaic technique
was conceived.  This technique, first used at Gaspra, involves performing
several small scan platform slews while the solid-state imaging instrument's
camera shutter is open.  It enables several sets of Ida and star images to be
acquired in one frame.  Single-frame mosaicking has 3 main advantages over the
routine OPNAV method in that it (1) decreases the sensitivity to data outages
because multiple sets of Ida and star images can be distributed over different
lines of the same picture; (2) provides the capability to use different
exposure times for each mosaic position since the stop times between the
mosaic slews can be varied; and (3) allows an area larger than the size of the
camera field-of-view to be covered by scan platform motions.


3.2  Instrument Pointing

Galileo will fly by Ida in the southern hemisphere (75 degrees south ecliptic
latitude) on the asteroid's dark side, passing approximately 2,400 km (1,500
mi) from the center of the asteroid while traveling at a speed relative to the
asteroid of 12.4 km/sec (27,700 mph).  The volume of space corresponding to
Ida's position uncertainty, when projected onto the plane normal to the
pointing direction of the scan platform, represents an ellipse-shaped area
within which the asteroid may be found at some level of probability.  For
science planning purposes, the elliptical region of sky searched was chosen so
that there would be a 95-percent probability of capturing the asteroid within
the boundaries of the ellipse.  The scan platform instruments, which would
normally be aimed directly at an object of interest, instead must scan the
entire area of position uncertainty and record data for the entire region in
order to be certain of capturing Ida.  This is a difficult task given that the
target is moving at such a high relative velocity.

Without any further improvement on the apriori knowledge of Ida's position, it
would be highly improbable to capture an image of the asteroid greater than
several dozen pixels.  Fortunately, position data obtained from OPNAV pictures
can be used to decrease significantly the position uncertainty of Ida.  As
mentioned previously, the final two OPNAV pictures (OPNAV #4 and OPNAV #5)
will be used to update the design of the final pre-Ida trajectory correction
maneuver (TCM-21) in order to achieve the best possible trajectory delivery
accuracy.  TCM-21 will be executed two days prior to closest approach.

3.3  Data Return Strategy at Ida

The data return strategy developed for the Ida encounter was designed so as to
ultimately return the key science observations at Ida -- primarily solid-state
imaging and near-infrared mapping spectrometer observations of Ida, the
magnetometer search for Ida's interaction with the solar wind, and a minimal
set of calibrations.  The strategy reflects the key constraint of limiting the
tape recorder start/stop cycles required to play back recorded data.  The
other Ida experiments have been designed to use only the data that will be
embedded in the return of the solid-state imaging, near-infrared mapping
spectrometer and magnetometer observations, including the ultraviolet
spectrometer and the photopolarimeter-radiometer experiments.

There are three ways to return data collected during the Ida encounter.  The
first two, unloading data which have been transferred to the command and data
subsystem from the tape recorder (data memory subsystem memory readouts) and
real-time memory readouts of certain instruments, have been used previously
during the Galileo mission.  The third method, known as the command and data
subsystem buffering technique, will be used to acquire data from the
magnetometer instrument only.  This technique, conceived of for the Ida
encounter, was first employed earlier this year for engineering purposes.  It
involves immediately transferring data acquired from 1 hour before to 1 hour
after closest approach to the command and data subsystem buffer for later
playback.  Due to this technique, it will be possible to obtain the
magnetometer data since otherwise it would have had to have been extracted
from the solid-state imaging data stream -- a prohibitive task.

Return of the Ida data must be apportioned between the month-long period
beginning one day after closest approach and a four-month long period (March-
June) in 1994 when the Earth is again between the Sun and Galileo.  The
challenge of the data return activity is in determining the location of data
on the tape recorder primarily due to uncertainties in both Ida's location and
in scan platform pointing.  A special survey technique, known as the "jailbar
search," has been developed for Ida which allows for sampling 2 camera lines
out of every 330.  Upon inspection, packets which contain Ida data, as opposed
to "black sky" data, will be identified.  Based upon the known tape recorder
location of each packet, the spacecraft will be instructed to download the
selected frames.  This technique guarantees locating Ida in the highest
resolution images; thus, it will greatly reduce the playback time needed to
download the highest priority encounter data.  The total science data return
at Ida is expected to be comparable to that achieved at Gaspra.


4.0  SCIENCE OBJECTIVES AT IDA

There are four primary science objectives for the Ida encounter.  The first is
to characterize global properties such as size and shape.  The second
objective is to characterize surface morphology and particle size, search for
geologic and evolutionary processes and obtain crater frequency distributions
for collisional history and relative ages.  The third objective is to
characterize compositional properties such as surface composition, chemical
composition and surface mineralogy.  The fourth objective is to characterize
possible magnetic field effects by conducting a magnetometer search for field
perturbations, especially solar wind whistler wing effects, such as apparently
detected at Gaspra.  Achievement of these objectives will give scientists a
basis for comparative analyses of other asteroids and small bodies throughout
the solar system.

The following is a description of the Galileo instruments to be used during
the Ida encounter.

4.1  SSI

The solid-state imaging instrument (SSI) uses a 176.5mm aperture Cassegrain
telescope to focus incident light from an object or body of interest onto a
solid-state image-detector array known as a charge-coupled device (CCD).  The
focal array has a resolution of 800 lines by 800 elements.  By comparison,
this is approximately twice the resolution of conventional television.

SSI will obtain many images during the encounter, including at least one high-
resolution image of the asteroid.  Galileo's closing speed and uncertainties
remaining in the orbital parameters will make it necessary to mosaic an area
that is significantly larger than Ida itself in order to capture the
asteroid's image.  While Galileo is still far enough away to capture the
entire error ellipse (at 95-percent confidence) in one frame, SSI will take a
series of 75 individual images covering 1.08 of Ida's rotation (known as Ida's
"rotation movie").  The first 33 images will be comprised of 5 clear-filter
images, three 6-filter sets, and two 5-filter sets that are spaced to record
every 30 degrees of longitude.  The remaining 42 images are contained in seven
6-filter sets shuttered every 15 degrees of longitude.  These images will be
most useful in characterizing Ida's size and shape.  Following completion of
the Ida rotation movie, SSI will take a series of four mosaics representing
the highest priority observations.  The first mosaic includes a 6-color single
image and a 2x2 series through 4 filters.  This will provide the highest
resolution multispectral imaging of Ida and will abe used to study detailed
compositional variations across the surface of the asteroid.  The second
mosaic is comprised of SSI clear-filter images shuttered while the near-
infrared mapping spectrometer obtains a chemical map of Ida.  This mosaic will
be used in conjunction with the following high-resolution image to provide
stereo coverage of Ida which will assist greatly in determining its shape and
size.  The third mosaic, also clear filter, will provide the highest
resolution imagery of Ida for which capture of the entire asteroid is
guaranteed.  Its 30 frames will cover the entire 95-percent error ellipse and
will be acquired between 5.5 and 1 minute(s) before closest approach.
Depending upon which frames in the mosaic actually capture the asteroid, the
resolution may lie anywhere between 26 and 48 m/pixel, with a most probable
resolution of about 40 m/pixel.  The highest resolution possible in this
mosaic (26 m/pixel) is twice that achieved at Gaspra (54 m/pixel).  The final
mosaic, a 15-frame clear-filter, will cover only the center region of the
error ellipse giving a 50-percent probability that the center of the asteroid
will be captured.  It will start at 1 minute prior to closest approach and
finish at 1 minute after closest approach.  If acquired, it will be the
highest resolution SSI data obtained at Ida (24 to 26 m/pixel).  Out of all 15
images in this mosaic, Ida may appear in as many as six.  At the range the
final SSI mosaic is shuttered, approximately 2,400 km, Ida will stretch across
almost three SSI fields of view.  These final two mosaics were designed so
that an additional coverage of the region of most probable capture occurs at
closest approach.  These images will reveal details of Ida's surface
morphology, crater size and distribution, as well as  any surface processes
which may be at work.  In all, SSI will record 150 frames from which 21
individual views of Ida could be acquired.  All of the mosaics will be taken
in the last 15 minutes before closest approach.

The imaging at Ida will provide great improvements in spatial resolution as
well as spectral coverage as compared with the Gaspra encounter imaging.  The
highest possible multispectral resolution at Ida will be twice that achieved
at Gaspra (87 m/pixel at Ida compared to 160 m/pixel at Gaspra).  Even more
impressive is the fact that at Ida it will be possible to achieve more than
twice the highest clear-filter resolution obtained at Gaspra (24 m/pixel vs.
54 m/pixel).  This is because at Ida imaging will continue through periapsis
until 1 minute after closest approach; whereas the final image at Gaspra was
taken 8 minutes before closest approach.  Also, two additional filters have
been included in the color sequences.  There are many 6-color sets at Ida as
compared to 4-color sequences at Gaspra.  This will give additional insight
into the understanding of the surface composition on Ida.

4.2   NIMS

The near-infrared mapping spectrometer (NIMS) provides imaging and
spectroscopic data for the infrared region of the spectrum (0.7 to 5.2
micrometers wavelength).  This portion of the spectrum is important because it
provides information regarding composition, temperature and geology.  The NIMS
is a "push-broom imager" (so called for the way it scans images).  Its
principal components are a 9-inch Ritchey-Chretien telescope, a scanning
mirror assembly, a diffraction spectrometer with a scanning grating, and a 17-
detector focal plane array.  It has wavelength resolution capabilities of
0.027 micrometers for wavelengths greater than 1 micrometer, and 0.014
micrometers for 0.7 to 1.0 micrometers.  The NIMS is a scanning-type
instrument which creates an image by repeatedly sampling what amounts to a
three-dimensional array in X, Y and wavelength.  The combination of a
diffraction grating, scan mirror and scan platform motion allow NIMS to
measure and generate images in up to 408 separate wavelengths.

NIMS will "sweep out" mosaics to ensure imaging of the asteroid near closest
approach.  During its closest approach and high resolution observations of
Ida, NIMS will scan back and forth across the region where the asteroid is
located to be certain to capture the asteroid.  (During its other
observations, NIMS will "stop and shoot.")  NIMS will be performing several
different types of measurements, all of which cover the spectral range of 0.7
to 5.2 micrometers.  During approach, NIMS will collect a spectral lightcurve
of Ida.  The instrument will sample the Ida surface every 90 degrees of
rotation in 204 wavelengths, every 30 degrees in 102 wavelengths, and every 15
degrees in 102 wavelengths.  With the combined wavelength samples it is
possible to compose a hemispherically-resolved chemical heterogeneity map for
one full rotation of the asteroid.  NIMS's highest-priority is to obtain the
best possible spatial resolution of Ida's surface with 17 wavelength samples
while the instrument is in its fixed grating mode.  This is done during a
collaborative SSI/NIMS mosaic occurring at 5 minutes prior to closest
approach.

NIMS data will provide information about what minerals are present on Ida's
surface, one of the clues to the asteroid's origin.  Infrared images are
frequently able to discern features which have no contrast in pictures taken
in the visible spectrum.  NIMS will be able to provide more complete spectral
information on the asteroid than possible from Earth because of the
atmospheric absorption characteristics and limited spatial resolution
available.  NIMS will be able to extend the range of wavelengths sampled at
Ida including measurements in the thermal region of the spectrum.  All of this
will provide a better understanding of the regolith characteristics, surface
composition and chemical heterogeneity of Ida.


4.3  UVS and EUV

The ultraviolet spectrometer (UVS) measures wavelengths between 113 and 432
nanometers.  The UVS is a narrow field-of-view scanning-type instrument like
the NIMS and the photopolarimeter-radiometer (PPR) and is mounted on the scan
platform.  The instrument uses a 250-mm aperture Cassegrainian telescope
coupled with an Ebert-Fastie monochromator using three photomultipliers as
detectors.  The instrument can operate in a single wavelength monitoring mode
or can record the entire spectrum between 113 and 432 nanometers.
The extreme-ultraviolet spectrometer (EUV) is a modification to the
ultraviolet spectrometer instrument flown on the Voyager spacecraft.  The
modifications allow the instrument to gather spectral data in the 54 to 128
nanometer range.  The EUV is a concave objective grating spectrograph mounted
on the spun section of the spacecraft.  As Galileo spins, the EUV observes a
narrow annular ribbon of space.

During the encounter, the UVS will "piggyback" with the SSI and NIMS
observations.  Approximately five hours before closest approach, the UVS will
begin obtaining ultraviolet spectra over the range 162-323 nanometers,
piggybacked on fixed SSI pointing.  Two additional measurement cycles with
fixed pointing, when Ida is much smaller than the UVS field of view, follow at
about -4 hours and -3 hours.  The final four measurements made in the 1.5
hours before closest approach piggyback on SSI and/or NIMS mosaics covering
the Ida error ellipse.  The last three of these measurements span 113-323
nanometers.  These UVS measurements obtain data on the asteroid and its near-
space environment.

The EUV will be recording data on the region near Ida as Galileo passes by it.
It cannot be predicted that Ida itself will be captured within the field of
view of the EUV.  EUV will be looking at the hydrogen, helium and oxygen
spectrum of the interplanetary background in the vicinity of Ida, while UVS
will be looking at the asteroid and its vicinity.

The UVS will attempt to measure more accurately the albedo, color and
scattering properties of Ida.  The EUV and UVS will be used to determine the
presence and amount of atomic and ionic emission from the asteroid and any
associated atmosphere.  An atmosphere of a sort could be created on a body as
small as Ida through interactions with the solar wind and cosmic rays which
knock surface atoms free.  Limited outgassing could also generate an extremely
thin atmosphere.

4.4  PPR

The photopolarimeter-radiometer (PPR) is a descendent of an instrument flown
on the Pioneer Venus spacecraft.  It uses a 10-cm Cassegrain Dall-Kirkham
telescope to focus incident light from the object of interest through a filter
wheel.  The filter wheel determines which wavelengths are passed and which are
blocked.  For polarimetry, light must pass through both a half-wave retarder
plate and a spectral filter, while for photometry the light is passed through
only the spectral filter.  A second optics path obtains background light.
Light from this path is not passed unless the instrument is performing
radiometry.  When the instrument is performing photopolarimetry, the incident
light passes through the filter to a prism which splits the light into its two
separate polarization components and directs these separate beams to two
silicon photodiodes.  During radiometry, an optical chopper operating at 30 Hz
alternately directs flux from the scene-view and the space-view telescopes
onto a lithium-tantalate detector.  Photometry is measured in bands centered
at 618, 633, 646, 789, 830, 841 and 891 nanometers, while polarimetry measures
410, 679 and 945 nanometers.  The lithium-tantalate radiometry detector
measures incident (scene-view) and reference (space-view) infrared radiation
at 17, 21, 27, 37 and greater than 42 micrometers.

At Ida, the PPR will measure the intensity and polarization of reflected
sunlight in the visible region of the spectrum.  The PPR will also measure
thermal infrared radiation.  PPR will attempt to make observations very near
closest approach (beginning at +1 minute).  To do this, it will scan the
center of the error ellipse at high resolution, then the whole ellipse at
lower resolution.  On the outbound side, heading away from Ida, PPR will map
the entire 95-percent confidence ellipse as a polarimetry study in two filter
wheel positions.

Radiometric observations will result in brightness temperatures which can be
converted into thermal inertias for the regolith.  This, in turn, provides
information on the size of the particles which make up this surface layer.
Polarimetry analysis of how the particles which cover Ida polarize reflected
light will provide further information on the physical properties of the
regolith.

4.5  DDS

The dust detection subsystem (DDS) is designed to measure the mass, electric
charge and velocity of incoming particles ranging in size from 10^-16 to 10^-7
grams (less than a ten-millionth of a pound) and speeds between 1 and 70
km/sec.  The instrument has a fairly wide field of view, approximately 140
degrees, and is mounted on the spun portion of the orbiter to allow it to
sample direction as well as density.

Since the DDS is mounted on the spinning portion of Galileo, it has no
specific pointing requirements for making observations during the encounter.
While Galileo is in the vicinity of Ida, DDS will be recording impact rate,
particle mass, velocity, charge and impact direction.  This data will be used
to determine particle orbits and distributions.

Data obtained by the DDS will provide additional insight on the near-asteroid
environment.  Scientists hope to discover if asteroids are accompanied by
accumulations of small particles, and whether these particles are captured by
the asteroid, or are coming from the asteroid.  Because data obtained by the
DDS is stored in the instrument's memory, it will be much easier to return to
Earth.  Even at the transmission rate available at Ida's distance from Earth,
return of DDS data by memory readout will be completed within 34 hours after
the encounter, making it some of the first Ida data available for analysis.

4.6  MAG

The magnetometer (MAG) is composed of two sets of sensors mounted on the 11-
meter boom.  One set is mounted at the end of the boom and the other about 6.9
meters from the spacecraft spin axis.  This boom is located on the spinning
portion of the Galileo orbiter.  The outer sensor will be used at Ida and will
be operated in the approximately nano-Tesla range.  It is capable of sensing
changes in the magnetic field to hundredths of a nano-Tesla.  By way of
example, the Earth's magnetic field is approximately 50,000 nano-Teslas
measured at sea level.

The MAG need not be pointed at a particular target for its observations.
Instead it sweeps through space sampling magnetic field strength and
orientation as Galileo spins.  Throughout Galileo's recent interplanetary
cruise, the magnetometer has been sampling the magnetic fields at two-hour
averages.  As the Ida encounter approaches, the sampling rate will be
increased.  At approximately 16 hours before closest approach, the MAG will
increase its sampling rate to sample at 1-minute averages.  This data will be
stored in the instrument's memory and returned to Earth via memory readout.
For approximately 2 hours in the immediate vicinity of Ida, MAG will operate
in a special mode with 1.33-second resolution.  This data will be stored in a
command and data subsystem buffer and will be returned by memory readouts
starting 19 hours after the encounter.  An additional 29 minutes of normal
high-resolution data (0.22-sec resolution) will be recorded starting 1 minute
after closest approach.  This data will be played back in 1994.

The MAG will provide data on how Ida interacts with the solar wind.  The
presence and strength of any magnetic field, as seen at Gaspra, would provide
additional information on Ida's history and internal structure.

4.7  PLS

The plasma science (PLS) instrument uses electrostatic analysis to measure the
directional intensities of ions and electrons with energy per unit charge
between 0.8 and 52 keV.   The instrument employs two spherical section
electrostatic analyzers to measure the energy per charge and three sets of
miniature magnets provide the mass per unit charge of the plasma.  The PLS is
mounted on the spun section of the spacecraft near the base of the
magnetometer boom.  As with all fields and particles instruments such as the
DDS and MAG, the PLS sweeps through space as the spacecraft spins, sampling in
all directions successively.  This provides almost complete coverage of
charged particle velocity vectors at the spacecraft position.

PLS measurements will start at about closest approach and will run until
approximately 30 minutes past closest approach.  The PLS will analyze the
plasma for a range of radial distances from Ida.

PLS will provide measurements of the solar wind and search for evidence of
solar wind and asteroid interactions.  This information, combined with that
from the other fields and particles instruments, will provide additional
understanding of the space physics of the solar environment around Ida.  Also,
the PLS will test and calibrate modes intended for the Jupiter mission.

4.8  EPD

The energetic particle detector (EPD) can detect electrons and ions from
hydrogen to iron.  It can detect ions with energies between 20 keV and 55 MeV,
electrons with energies between 15 KeV and 11 MeV, and can determine elemental
species with energies between 10 keV and 15 MeV.  The EPD consists of two
separate, bi-directional telescopes, the composition measuring system and the
low-energy magnetospheric measuring system.  The instrument is mounted on a
stepping platform which is in turn located on the spun portion of the
spacecraft near the base of the magnetometer boom.  This allows the EPD to
view the entire region of space around Galileo.

The EPD will be activated approximately 20 hours prior to Ida closest
approach.  The stepping platform will be activated approximately 30 minutes
before closest approach.  The EPD will scan until about 30 minutes after
closest approach.  The EPD will search for energetic particles produced in the
interaction between the solar wind and the asteroid.

4.9.  PWS

The plasma-wave subsystem (PWS) is used to study electric and magnetic fields.
It uses an electric dipole antenna mounted at the end of the magnetometer boom
to measure electric fields and two search coil magnetic antennas mounted on
Galileo's high-gain antenna central post to measure magnetic fields.  Spectral
characteristics of electric fields between 5 Hz and 5.6 MHz and magnetic
fields between 5 Hz and 160 kHz can be measured.  High time-resolution
measurements are provided by a wideband receiver capable of waveform
measurements over bandwidths of 1 kHz, 10 kHz and 80 kHz.  Both portions of
the PWS are mounted on the spun portion of the spacecraft to more effectively
measure field strength and orientation.

PWS measurements will begin approximately at closest approach and will
continue until approximately 30 minutes after.  The PWS, as with the MAG, PLS
and EPD, will look for evidence of an Ida/solar wind interaction.


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