Online Astronomy eText: Asteroids, Comets, and Interplanetary Debris
Hermes Link for sharing this page on Facebook
(also see Science at NASA, Oct 31, 2003)
      The asteroid Hermes (1937 UB) was recently rediscovered after having been lost for sixty-six years. Hermes is a Near-Earth Object (an NEO) -- an object which passes very close to the orbit of the Earth -- and, as it turns out, one of a number of asteroids which actually consist of two bodies, orbiting around each other.
Hermes, as observed with a 24-inch Schmidt telescope on Oct 15, 2003. Viewing from left to right and top to bottom, Hermes is seen at 20 minute intervals, moving a little over a tenth of a degree between exposures, or a little more than 0.8 degrees per day. (Brian Skiff, LONEOS, Lowell Observatory)

      On November 4, 2003, Hermes was about 0.048 AUs, or 4.5 million miles from the Earth. At its closest approach its 777 day long orbit passes within 350,000 miles of the Earth, but the asteroid is rarely that close, as the Earth is usually somewhere else in its orbit at the time that Hermes passes it. When it was discovered (and lost) in 1937, Hermes passed only 460,000 miles from the Earth, but at its October 2005 "approach" it was never less than 76 million miles from the Earth, as the Earth was well ahead of the asteroid when it reached our orbit. The next time that Hermes will be relatively close to us will be in 2040, when it will pass 2,600,000 miles from the Earth.


The orbit of Hermes
(Click on the image for a list of close approaches to various objects.)
      Hermes was found and lost so quickly at the time of its discovery that its orbit was somewhat uncertain, and it took quite a bit of high-powered calculating to work backwards from the current observations through all of the perturbations caused by the Earth, Venus and Vesta (which it occasionally passes), to find that it was the "lost" asteroid.
      Radar images by the Arecibo radio telescope show that Hermes consists of two small bodies orbiting each other, probably with synchronous rotation. In the radar images below the asteroids are moving counter-clockwise around each other. Each of the bodies is about 1200 feet in diameter.

Radar images showing the two separate bodies and their relative motion. (Jean-Luc Margot)