Scientists continue to analyze the data received from Galileo's historic encounter with an asteroid. Although Galileo flew past asteroid Gaspra (see photo) on October 29, 1991, because of the limited data transmission rate, the Project only received the bulk of the Gaspra information in November 1992. The images reveal Gaspra to have a cratered surface and irregularly shaped body about 19 by 12 by 11Êkilometers. The spacecraft passed about 1600 kilometers from Gaspra at a relative speed of about 8 kilometers per second.
The recent data from Gaspra suggested that the asteroid may have a magnetic field. This evidence is based on the behavior of the solar wind, an ionized stream of gas that flows out from the Sun at two million kilometers an hour and carries a magnetic field. The solar wind field changed direction a few hundred kilometers from the asteroid, as if it had slammed into a magnetic region. This change in direction is known as a field rotation. Galileo again detected a field rotation shortly after closest approach. "At the present time, we cannot be sure these field rotations were produced by the interaction with Gaspra," emphasized Dr. Margaret Kivelson, of U.C.L.A. and the Principal Investigator for the Magnetometer. "Field rotations are a common feature of the solar wind and their association with Gaspra may be fortuitous. Our conclusions are highly speculative, but extremely plausible.
"One of the big puzzles is understanding the processes that account for the properties of the diverse planet-like asteroids that appear between Mars and Jupiter (in the asteroid belt)," Kivelson continued. "Their composition is thought to be related to that of meteorites. If Gaspra has a permanent magnetic moment, the finding would have bearing on its thermal history, have implications for the history of the magnetic field of the early solar system, and give us more reason to believe some asteroids are very rich in iron or iron-nickel alloys of considerable economic value."
To have caused these field rotations, scientists believe Galileo passed through a magnetic wake, like the wake caused by a ship or a bullet in flight. The angle that this wake sweeps back depends not only on the size of the obstacle (the asteroid), but also on the properties of the medium (interplanetary plasma) and the kind of waves that can propagate in it. For plasma physicists, one aspect of interest here is that the Gaspra wake may present a rare example of waves carried by a "whistler wing." This phenomenon has never been observed outside the laboratory, but the actual results closely follow what was predicted from laboratory results.
Scientists are intrigued that Gaspra's magnetic field appears to resemble the fields measured in some meteorites. Galileo's AugustÊ28, 1993, encounter with asteroid Ida may help to understand these results. Like Gaspra, IdaÕs composition is believed to be representative of a major class of main-belt asteroids. However, the two asteroids' compositions should differ somewhat.
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