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
(NSF) Arecibo Observatory radio telescope and his colleagues report.

The asteroid Hermes was re-discovered last week after being lost for
66 years. Now Jean-Luc Margot, a researcher in the Department of
Earth and Space Sciences 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.

Margot and colleagues are analyzing new radar measurements from the
Arecibo Observatory, Puerto Rico, the world’s largest single-dish
radio telescope, operated by the National Astronomy and Ionosphere
Center (NAIC) at Cornell University, Ithaca, N.Y., for the NSF. The
astronomers are scheduled to obtain additional measurements using the
Arecibo telescope this weekend (Oct. 25-26).

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.

“Because the components are close to each other, they raise
appreciable tides in each other and each has slowed down the other’s
spin significantly. They are now likely in a doubly synchronous
state, where their spin period is equal to their orbital period.
This means they constantly present the same face to each other, just
like Pluto and its satellite, Charon.”

Hermes, was first observed in 1937 as a fast-moving bright object and
then went undetected until last week, although it had circled the sun
almost exactly 31 times since then, said Brian Marsden, of the Minor
Planet Center in Cambridge, Mass.

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 goals of the proposal were to measure precisely the distance and
velocity of this object, to improve the knowledge of its trajectory
and help trace back its history, to characterize Hermes’ physical
properties, and to search for satellites.

Margot and collaborators have 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, Mike Nolan,
Victor Negron, Alice Hine, and Don Campbell, associate director of
NAIC, were at the Arecibo telescope.

Hermes gets as close as 378 000 miles from Earth — which, in
astronomical terms, is quite close, about 1.6 times the distance
between Earth and the moon. Orbits can change appreciably over time
due to gravitational influences of the planets, noted Nolan, an
Arecibo Observatory scientist.

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.”

Margot and colleagues described their observations and data
in an International Astronomical Union Circular this week. Margot’s
research is funded by NASA. His co-authors are Nolan, Negron, Hine,
Campbell, and Ellen Howell at NAIC; Lance Benner, Steven Ostro, and
Giorgini at JPL; and Marsden at the Minor Planet Center.