To Photo of Ida Galileo's second encounter with an asteroid was a great success and has added much to our understanding of these bodies in our solar system. On August 28, 1993, Galileo swept within 2400 kilometers of Ida, at a relative velocity of 12.4 kilometers/second, capturing many images of the asteroid.
Ida is 56 kilometers long, and is heavily cratered. This extensive cratering seems to contradict earlier theories that Ida is a relatively young asteroid, although views of other portions of the asteroid (to be returned this spring) may shed more light on this.
Previously, many scientists did not believe that asteroids had magnetic fields, because they are too small to have a dense metallic core. However, if the current asteroids are pieces of larger "parent bodies" that were broken apart, they could have magnetic fields. Another possibility is that strong fields in the solar wind were imposed on the asteroids at a time early in the solar system when they were heated more than now.
Dr. Margaret Kivelson, Principal Investigator for the Magnetometer and a physicist at UCLA, heads up the Galileo investigation into magnetic fields. As at asteroid Gaspra in October 1991, Galileo again detected changes in the interplanetary magnetic field as it passed Ida. Both times the instrument measured several swings or shifts in the direction of the magnetic field, called field rotations, which Dr. Kivelson believes may be produced by the asteroid's interaction with the solar wind. If so, the observations do not necessarily show that Ida has a magnetic field. However, coupled with the same findings at asteroid Gaspra, indications are strong that such magnetic fields exist.
Galileo may have a unique opportunity to view a comet impacting a planet. In July 1994, Comet Shoemaker-Levy is expected to collide with Jupiter, and Galileo should be able to view the impact. Detailed planning for these observations will begin in March.
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