Hubble Telescope Finds Vesta Crater

September 03, 1997

ITHACA, N.Y. -- Proving that even minor planets can survive cosmic fender-benders, astronomers using the Hubble Space Telescope have discovered a large crater -- with an estimated diameter of 285 miles and about 8 miles deep -- on the asteroid Vesta, according to an article published in today's issue of the journal Science (Sept. 5).

Even in cosmic terms, the scientists admit it's a humdinger of a dinger.

"When I saw the raw pictures from the Hubble Space Telescope, it was like, 'Wow!' " said Peter C. Thomas, Cornell senior research associate in the Department of Radiophysics and Space Research. He explained the crater is big enough to have spewn enough impact debris to be a source of special types of meteorites and other Vesta-like objects.

"Clearly, when we got the pictures, the southern area of Vesta was mangled. These photos confirmed it was just one crater," he added.

The crater, roughly the diameter of Ohio, was found using the Hubble Space Telescope's Wide Field Planetary Camera 2. While not the biggest asteroid, Vesta is still the brightest. It was discovered by Wilhelm Olbers, a German astronomer, on March 29, 1807.

News of the dent's discovery is published in an article, "Impact Excavation On Asteroid 4 Vesta: Hubble Space Telescope Results," in today's issue of Science, (Sept. 5). Co-authoring the article with Thomas was Richard P. Binzel, Massachusetts Institute of Technology, Cambridge, Mass.; Michael J. Gaffey, Rensselaer Polytechnic Institute, Troy, N.Y.; Alex D. Storrs and Eddie N. Wells, Space Telescope Science Institute, Baltimore; and Benjamin H. Zellner, Georgia Southern University, Statesboro, Ga.

Tailing Vesta through space is a family of smaller asteroids with similar geochemical make-up which had previously puzzled astronomers. But with this discovery, astronomers can now deduce that the smaller objects -- some small fragments of which eventually reach Earth as meteorites -- ultimately originated from this blast of the ancient past.

Researchers believe these chips off the old asteroid account for a low percentage of the volume carved out. However, the minor planet is believed to be the origin of basaltic achondrite HED meteorites, which have evidence of a variety of volcanic materials. These materials are similar to the basalts that cover the ocean floors of the Earth and many surface volcanoes such as on Hawaii. These distinctive meteorites comprise six percent of all meteorites that fall to the Earth; thus it is possible that this one giant crater has indirectly supplied a noticeable fraction of the meteoritic material reaching earth in recent times.

Up to last year, getting a good portrait of Vesta was difficult. The research team using the space telescope managed to get the best images ever of Vesta in May 1996 because of its close proximity to Earth. While the Hubble Space Telescope travels with the Earth around the Sun, Vesta also rotates and moves around the Sun, making it tricky to obtain images of the minor planet's southern side, Thomas said.

Measuring about 460 kilometers in diameter -- or 285 miles -- the dent is considerable, in relation to the size of the potato-shaped Vesta, which is about 330 miles in diameter. Obtaining approximate dimensions of Vesta and the crater from images is calculated by knowing the distance from the Earth to Vesta, and factoring in the camera's characteristics, such as focal length. However, deducing the topography within the crater required using all 78 images and developing techniques to match all the images. These detailed measures occupied most of the research time and required new computer software.

Astronomers also learned the impact crater has a central peak, which is about eight miles high and looks like a raised bull's-eye. "The Moon has craters with raised peaks, too. But Vesta has one-seventh the gravity of the Moon," said Thomas. "Now we have tangible, physical evidence that to get craters with raised, central peaks on low-gravity objects like Vesta, we need large objects to make big collisions."

Cornell University

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