Idiosyncrasies of Ida.

Imagine dropping a rock from eye level and having to wait 30 seconds for it to hit your feet. That's what would happen if you were standing on the surface of asteroid 243 Ida. Gently toss another rock straight ahead, in the same direction that Ida rotates, and it would land in front of you, as expected. But throw the same rock forward a bit harder, and it would actually double back over your head and strike the surface behind you.

This surprising behavior stems from a combination of Ida's low surface gravity, rapid rotation, and especially its peanutlike shape, which makes the asteroid's gravitational field decidedly irregular. These strange dynamics may explain the placement of a cluster of house-sized boulders on the asteroid, as well as the varied color and reflectivity of its surface, assert Paul Geissler, Jean-Marc Petit, Richard Greenberg, and their colleagues at the University of Arizona in Tucson.

At first glance, the location of the boulders, which the Galileo spacecraft spied when it flew past Ida in August 1993 (SN: 8/6/94, p.93), would seem to pose a puzzle. Scientists believe that the boulders, known as ejecta blocks, are large pieces of Ida's surface shaken loose when a chunk of debris --probably a fragment of another asteroid -- slammed into Ida and dug a crater. Because they make easy targets for any other fragment that might strike the asteroid, the boulders won't stay intact long and therefore must be young. However, the only two sizable craters nearby are too old to be the boulders' sources.

In contrast, the impact basin Azzurra, one of the youngest large craters detected on Ida, appears a far more likely candidate and could easily supply the raw material for the boulders. But this crater lies on the opposite face of the asteroid from the boulder concentration. Galileo's flyby only allowed the craft to view one half of the elongated asteroid at high resolution, where the craft saw boulders clustered near the body's leading edge as it tumbles end over end. Azzurra resides along the trailing rotational edge of the other half of the asteroid.

Thanks to Ida's weird dynamics, however, this youthful crater may actually reside in the perfect spot to account for the boulders. Using a computer model, Geissler and his collaborators tracked the path of debris thrown upwards by the impact that created Azzurra. They found that many of the higher velocity fragments would loop back, and ultimately settle on the opposite half of the asteroid, closely matching the observed distribution of house-sized boulders.

The simulations also show that a similar distribution of boulders should reside on the asteroid segment that Galileo didn't observe closely.

In contrast to Galileo's brief encounter with Ida, the NEAR craft, scheduled for launch next February, will orbit the near- Earth asteroid 433 Eros for an entire year, beginning in December 1998. Intriguingly, this misshapen asteroid rotates almost as rapidly as Ida, which makes a complete revolution every 4.6 hours. NEAR should determine whether Eros has ejecta blocks on its leading edges, Geissler says.

In their simulation, Geissler and his team also found that finer material excavated from Azzurra would settle closer to the crater, accounting for the brighter, slightly bluer patches in these regions.