Astronomers have used NASA's Hubble Space Telescope to discover a giant impact crater on the asteroid 4 Vesta. The crater is a link in a chain of events thought responsible for forming a distinctive class of tiny asteroids as well as some meteorites that have reached the Earth.

The giant crater is 285 miles across, which is nearly equal to Vesta's 330 mile diameter. If Earth had a crater of proportional size, it would fill the Pacific Ocean basin. Astronomers had predicted the existence of one or more large craters, reasoning that if Vesta is the true "parent body" of some smaller asteroids, then it should have the wound of a major impact that was catastrophic enough to knock off big chunks. The observations are described in the Sept. 5 issue of Science Magazine.

"In hindsight we should have expected finding such a large crater on Vesta," says Peter Thomas of Cornell University, Ithaca, NY. "But it's still a surprise when it's staring you in the face." Another surprising finding is that such a large crater, relative to Vesta's size, might have been expected to cause more damage to the rest of the minor planet.

"This is a unique opportunity to study the effects of a large impact on a small object," says Michael Gaffey of Rensselaer Polytechnic Institute, Troy, NY. "This suggests that more asteroids from the early days of the solar system may still be intact."

The collision gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space. This tore out an eight-mile deep hole that may go almost all the way through the crust to expose the asteroid's mantle (Vesta is large enough to be differentiated like Earth -- with a volcanic crust, core and mantle, making it a sort of "mini-planet".)

Because of the asteroid's small diameter and low gravity, the crater resembles smaller craters on the Moon that have a distinctive central peak. Towering eight miles, this cone-shaped feature formed when molten rock "sloshed" back to the bullseye center after the impact.

One clue for a giant crater came in 1994 when Hubble pictures showed that one side of Vesta's football shape appeared flattened. "We knew then there was something on Vesta that was unusual," says Thomas.

The astronomers had to wait for a better view from Hubble when Vesta made its closest approach to Earth in a decade, in May 1996, when the asteroid was 110 million miles away.

A total of 78 Wide Field Planetary Camera 2 pictures were taken. The team then created a topographic model of the asteroid's surface by noting surface irregularities along the limb and at the terminator (day/night boundary) where shadows are enhanced by the low Sun angle.

The immense crater lies near the asteroid's south pole. This is probably more than coincidental, say researchers. The excavation of so much material from one side of the asteroid would have shifted its rotation axis so that it settled with the crater near one pole.

Unlike some other large asteroids that have jumbled surfaces due to the asteroids' breakup and recollapse, the rest of Vesta's surface is largely intact, despite the cataclysm. This is based on previous measurements showing it has a surface of basaltic rock -- frozen lava -- which oozed out of the asteroid's presumably hot interior shortly after its formation 4.5 billion years ago, and has remained largely intact ever since.

Approximately six percent of the meteorites that fall to Earth are similar to Vesta's mineralogical signature, as indicated by their spectral characteristics. Vesta's spectrum is unique among all the larger asteroids. The crater may be the ultimate source of many of these meteorites.

Most meteorites are believed to come from other asteroids, but their specific objects of origin cannot be determined in most cases. Thus the distinctive mineralogical makeup of these meteorites means that Vesta is the only world other than the Earth, the Moon and Mars, for which scientists have samples of specifically known origin.

A mystery has been that the meteorites could not have traveled directly from Vesta because at Vesta's location in the asteroid belt, there are no perturbing gravitational forces that would cause pieces to fall into orbits intersecting the inner planets like apples shaken out of a tree. However, Vesta's "daughter" asteroids -- literally "chips off the block" which have color characteristics similar to Vesta -- are near a "chaotic zone" in the asteroid belt where Jupiter's gravitational tug can redirect fragments into orbits that intersect Earth's orbit.

A good determination of the shape of Vesta was necessary for the next step in interpretation, which will use multi-color images of Vesta obtained with Hubble to study the detailed mineralogy of surface regions, including the region of the giant crater. Also, a team led by Don McCarthy of the University of Arizona plans to obtain additional images of Vesta at longer wavelengths this fall using the new Near Infrared and Multi-Object Spectrometer science instrument onboard Hubble.

Members of the Vesta research team are Principal Investigator Ben Zellner of Georgia Southern University; the Co- Investigators are Richard Binzel, MIT; Michael Gaffey, Rensselaer Polytechnic Institute; Alex Storrs, Space Telescope Science Institute; Peter Thomas, Cornell University, and Dr. Ed Wells, Computer Sciences Corporation.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

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