Number 21: 05/08/2003
A scientific publication by SGF and NEODyS

NEOs' Spacemissions : Galileo

The Galileo Spacecraft's Studies of Asteroids and Comets
by Clark R. Chapman - Southwest Research Institute (Boulder CO USA)

On Sunday, September 21st, the Galileo spacecraft will plunge into the planet Jupiter, finally ending a long, productive scientific investigation of that planet, its moons and rings, and its immense magnetosphere. But that is not all that Galileo accomplished since the project commenced in the late 1970s and the spacecraft was finally launched on a Space Shuttle from Cape Canaveral, Florida, in 1989.
The spacecraft took a long, circuitous tour of the solar system before finally arriving at Jupiter in late1995, where it has been performing its prime and extended missions eversince.
Of course, the asteroid belt is between Earth and Jupiter, so it was inevitable that the spacecraft would pass through the belt. With no asteroid missions yet approved by the time Galileo was on its way, NASA was persuaded to support an asteroid flyby by Galileo as a "poor man's" asteroid mission. Indeed, Galileo actually had its trajectory nudged twice so that it would fly by two asteroids, Gaspra and Ida.
Carrying instruments designed to study Jupiter's moons, Galileo was well equipped to study important issues about asteroids. The most important instruments were the camera and the near-infrared mapping spectrometer.

figure 1. This is a false-color image of Ida, greatly enhancing the normal slightly reddish color of many asteroids, but showing the slightly bluer patches that are more recent deposits. These color trends demonstrate that space weathering is gradually altering Ida's colors. Ida's little moon, Dactyl, is to the right.

figure 2. This close-up of Ida's tiny moonlet, Dactyl, shows that it has impact craters on it, too. Possibly there is even a chain of craters, extending to the lower right of the most prominent crater.

figure 3. This enhanced-color image of angular Gaspra shows hints of color differences across the asteroid. Also note how Gaspra's surface is sprinkled with small craters, but there are no obvious large craters.

figure 4. Gaspra and Ida are shown together to compare their relative sizes. One can also see that while Gaspra is sprinkled with little craters, Ida is saturated with medium-sized craters as well as those that would be too big for Gaspra.

Observing asteroids

Galileo's flybys of Gaspra and Ida happened in October 1991 and August 1993, respectively. They remained by far the best data on asteroids until the NEAR Shoemaker spacecraft went into orbit around the Earth-approaching asteroid Eros in early 2000 (see issue number 18 of "Tumbling Stone"). Since asteroids are small and very far away from Earth, the most significant thing that Galileo did was "get up close". Essentially, apart from some radar observations of small asteroids close to the Earth, no asteroid had been seen as much more than a moving, star-like point-of-light. Now, on bodies just tens of km in size, we had images with just 54 m/pixel for Gaspra and twice as sharp for Ida.
In some ways, the asteroids looked somewhat as had been imagined: they were both oddly shaped bodies apparently covered with craters and not much else. But more careful examination revealed quite an array of interesting features. First, the crater populations were actually not at all similar. Gaspra's surface is peppered with tiny craters but it has very few moderate-sized craters, and none that are large (except for some planar areas, termed "facets", which may be remnants of some ancient impacts). Ida, on the other hand, is saturated with craters of all sizes, looking like close-up images of the lunar surface. Why should these asteroids, both orbiting in similar parts of the asteroid belt, have such different impact histories? One idea is that Gaspra was created more recently than Ida and hasn't existed long enough to get its full share of larger craters. Another idea is that Gaspra may be metallic, and it is very difficult to gouge out large craters from the surface of such a strong object.
Among the other features noticed in closer studies of the images are linear features ("grooves") and, on Ida, blocks (boulders). The largest block on Ida is about 150 m across, which would be marginally resolved in the Gaspra images, which may be why Gaspra has no blocks.
Alternatively, since the blocks on Ida are believed to be ejected from large cratering impacts on Ida, the lack of blocks on Gaspra may simply reflect the absence of large craters on that body.
Spectral studies of both bodies (both with the infrared spectrometer and with the camera, using its color filters) proved to be very interesting. Unlike what was subsequently found about Eros by NEAR Shoemaker, Galileo data revealed significant color variations across the surfaces of Gaspra and, especially, Ida. Patches of geologically more recent materials (either material ejected from a large, recent crater and spread around the asteroid, or localities where a small, recent impact penetrated the surficial soils, exposing "bedrock") appear to be less red in color and show stronger mineral absorption bands than most of the asteroid. On Gaspra, it appears that a thin, surficial soil (or "regolith") has gradually moved downhill, exposing ridges, which also are less red in color.
Evidently there is a process, termed "space weathering" that gradually reddens colors, and suppresses absorption bands, as time progresses. Most likely, space weathering is caused by solar wind particles and/or micrometeoroids striking the surficial grains, vaporizing material, and recondensing, causing a deposit rich in sub-microscopic iron particles that redden reflected light.

Ida's moon

An amazing discovery, first noticed by Ann Harch in partial images of Ida, was the first satellite orbiting around an asteroid ever discovered. (Using modern adaptive optics techniques on the world's largest telescopes, as well as radar and other modern techniques, astronomers have begun to find satellites orbiting around quite a few other asteroids. But Ida's moon was the first.) The little world is only about 1.5 km across and is named Dactyl. In all probability, Dactyl was formed in the catastrophic disruption of a very large asteroid long ago that created the Koronis family of asteroids (all orbiting the sun in similar orbits), of which Ida is a member. In the commotion immediately following that catastrophic event, evidently one little fragment got captured into orbit by the larger Ida. It is likely that little Dactyl has been broken up by subsequent impacts by other tiny asteroids a number of times; but the debris, remaining in orbit around Ida, are expected to rapidly reaccrete, which is probably why Dactyl is still with us.
Measurements of the moonlet's orbit has helped determine, via Kepler's Third Law, the mass and hence the bulk density of Ida. Both Ida's density and spectral attributes suggest that it, and probably the whole Koronis family, are made of material like the ordinary chondrite meteorites that are so common in our museums.

figure 5. A computer simulated image of the fragments of Comet Shoemaker Levy impacting Jupiter

figure 6. A time sequence of Galileo images of Jupiter, taken a few seconds apart, shows the Shoemaker-Levy 9 comet fragment "W" striking the dark side of the planet, becoming brilliant for a few seconds, then beginning to fade away.

figure 7. The drift-scan multiple-exposure image of Jupiter showing Comet Shoemaker-Levy 9 fragment "K" brightening rapidly, then fading more slowly, adjacent to the planet. The planet is smeared from top to bottom, then shifted to the right and smeared again. The K fragment impact and its aftermath were visible to Galileo's camera for more than 50 seconds. (The wiggles in the images are artifacts.)

figure 8. An enhanced image by the Hubble Space Telescope of the comet made in January 1994

Observing the comet Shoemaker Levy

The Galileo spacecraft had one more unexpected task to accomplish involving the solar system's smaller bodies. While the Galileo scientists and engineers were preparing for the encounter with Ida, a new comet was found in the skies by Carolyn and Eugene Shoemaker and David Levy. Termed Shoemaker-Levy 9, this comet was actually a series of nearly two dozen comets -- fragments of one comet that had broken up when it passed very close to the giant planet the previous year. It was soon predicted that the comet fragments would all crash into the planet itself in July 1994, perhaps causing the biggest interplanetary collisions to be observed since the invention of the telescope. As the world's astronomers prepared to study these remarkable phenomena, it was realized that the impacts of all the comet fragments would actually take place just around the edge of Jupiter, on its back side as viewed from Earth. (It would turn out that the devastation wrought in Jupiter's atmosphere was so immense that the consequences were readily visible in Earthbased telescopes when the impact sites rotated into view during the hour following each impact.)
Nevertheless, while the astronomers were temporarily disappointed that the fireworks would not be directly visible from Earth, it was realized that Galileo might save the day. It turned out that Galileo, enroute to Jupiter, was in an exceptional position to view the impacts directly. Galileo was off to the side (the correct side!) so that its camera and other instruments could witness the brilliant flashes of light as the comet fragments crashed into the planet's upper atmosphere at 60 km/sec and then exploded. I was fortunate to lead the scientific effort to acquire images of several of the impacts, mostly in a kind of "drift-scan" mode so that we could record the history of a single impact on a single image.
These data provide the "initial conditions" of the amazing phenomena unleased on Jupiter by these extraordinarily explosive impacts, some remnants of which still exist on Jupiter after all these years.

Indeed, not only the Shoemaker-Levy 9 impacts but also the legacy of Galileo's historic, first-ever close-up studies of asteroids will live on, even as the remarkable spacecraft that studied these small-but- important solar system bodies burns up in Jupiter's atmosphere, like a very tiny comet, in late September.

Images: copyright NASA