Planetary astronomers using the Hubble Space Telescope have discovered a giant crater on Vesta, one of the largest asteroids in the solar system, which may explain a class of small asteroids previously discovered.
     The crater on Vesta is about 460 km (285 mi.) across, nearly as large as the diameter of Vesta, about 530 km (330 mi.). The crater is about 13 km (8 mi.) deep, enough to go into the mantle of the asteroid.
     Astronomers had expected some kind of large crater on the asteroid, but in the words of team member Peter Thomas of Cornell University, "it's still a surprise when it's staring you in the face."
     Vesta had been linked to a class of small asteroids that appeared to be compositionally similar to Vesta but no other class of asteroids. The similarities, found several years ago by MIT professor Richard Binzel and then-graduate student Shui Xu, also linked the asteroids to a class of meteorites which also appeared similar to Vesta.
     The crater on Vesta was caused by a large impact, which would have ejected smaller parts of the asteroids -- chips off Vesta -- into their own orbits. The orbits go back to a gravitational resonance in the asteroid belt caused by Jupiter, which can eject the asteroids into the inner solar system, including Earth.
     The images show the crater is at the south pole of the asteroid, which the astronomers say is no coincidence. The cataclysmic impact likely redistributed enough mass to cause the asteroid's rotation axis to shift so that it passed through the crater, as a way of preserving angular momentum.
     The Hubble observations of Vesta, coordinated by team leader Ben Zellner of Georgia Southern University, consisted of 78 images of Vesta taken in May 1996, when the asteroid was 177 million kilometers (110 million miles) from Earth. Hubble previously observed Vesta in 1994.
     The results of the Hubble observations were reported in the September 5 issue of Science.

European astronomers have found a new class of black holes surrounded by a warped disk of dust and gas emitting intense ultraviolet light.
     Astronomers Philippe Crane and Joel Vernet of the European Southern Observatory used the Hubble Space Telescope's Faint Object Camera to observe the core of the active galaxy NGC 6251, about 300 million light years from the Earth. There they found a bright beam of ultraviolet light, likely coming from gas near the black hole.
     However, unlike other black holes, the disk of dust that surrounds the black hole exists only at a distance, leaving the black hole "bare." This allows the beam of light to illuminate the surrounding disk, allowing the astronomers to see the disk was warped, not unlike the brim of a hat.
     "This is a completely new phenomenon which has never before been seen. It blew my mind away," said Crane. The disk shows up as a finger-like extension extending at right angles from the main jet from the black hole.
     The warped disk may be caused by the gravitational pull of the mass of the galaxy which contains the black hole, or it may be caused by precession as the hole and disk spin around.