Asteroid Hermes, lost for 66 years, is found by Arecibo's telescope

An asteroid that has eluded astronomers for decades turns out to be an unusual pair of objects traveling together in space, a planetary scientist using the National Science Foundation's Arecibo Observatory radio telescope and his colleagues report.

The asteroid Hermes was re-discovered Oct. 15 after being lost for 66 years. Now Jean-Luc Margot, a former research associate at the observatory and now a researcher at the University of California-Los Angeles, has determined that the asteroid is in fact two objects orbiting each other. The two objects together would cover an area approximately the size of Disneyland. The binary object was some 19 million miles out at the time of rediscovery.

Margot and colleagues are analyzing new radar measurements from the Arecibo Observatory, Puerto Rico, operated by the National Astronomy and Ionosphere Center (NAIC) at Cornell for the NSF.

Hermes makes frequent close approaches to Earth, Venus, Mars, as well as Vesta, the third largest asteroid in the main asteroid belt between Mars and Jupiter.

While several other asteroids have satellites, the other known binaries with trajectories that cross the orbit of the Earth consist of a large primary asteroid orbited by a much smaller one.

"Hermes is the first asteroid ever discovered in the near-Earth population where the two components are essentially equal in size," Margot said. "It's a very unusual binary, a puzzle. It may have formed when it swung so close to a planet that it was ripped apart by gravitational forces, but we don't know for sure. One of our goals is to learn more about the two components and how they rotate about each other in the hopes that we may be able to deduce how Hermes became a double asteroid."

Hermes was first observed in 1937 as a fast-moving bright object 460,000 miles from Earth. It then disappeared from view, making an unobserved swing past Earth in 1942 at a distance of 378,000 miles -- which, in astronomical terms, is quite close, about 1.6 times the distance between Earth and the moon.

Since 1937 it has circled the sun almost exactly 31 times, said Brian Marsden, of the Minor Planet Center in Cambridge, Mass. Its orbits can change appreciably over time due to gravitational influences of the planets, noted Michael Nolan, an Arecibo Observatory astronomer.

On Oct. 15, Brian Skiff of the Lowell Observatory Near-Earth-Object Search sighted a mysterious object; Timothy Spahr at the Minor Planet Center in Cambridge identified similarities with the 1937 observations; and Steven Chesley and Paul Chodas at NASA's Jet Propulsion Laboratory (JPL) linked the observations to Hermes.

The same day, Margot and his team proposed to observe the asteroid with the Arecibo Observatory's high-powered radar system -- a proposal that was accepted within hours.

The team, which is funded by NASA, has been given five sessions at Arecibo and five sessions at the Goldstone radar telescope in California to observe Hermes. Due to the urgent nature of the proposal, Margot observed from his home computer while his associates, Nolan, Victor Negron, Alice Hine and Don Campbell, Cornell professor of astronomy and associate director of NAIC, were at the Arecibo telescope.

Hermes travels on an elliptical orbit and reaches deep into the inner solar system, crossing Venus' orbit. The new research has made it possible to extend the time interval over which the trajectory can be computed reliably, said Jon Giorgini, a senior engineer at JPL and member of the team.

"As far as impact risk, there is no cause for worry in our lifetimes," Giorgini said. "Over hundreds of thousands or millions of years, Hermes could impact the Earth, but only if it doesn't hit Venus first."

This article is adapted from a report by Stuart Wolpert, University of California-Los Angeles.