Hermes Is Double!

Astronomers are making the most of the recovery of Hermes after 66 years, even though the long-lost minor planet will not reach its minimum distance from Earth (7,157,000 kilometers) until November 4th. And already there’s a surprise: Hermes is double!

On October 18th and 20th, Jean-Luc Margot (University of California, Los Angeles) and nine colleagues beamed high-frequency radio pulses toward Hermes using the 305-meter dish at Arecibo Observatory in Puerto Rico. Double radar echoes came back, a clear sign of reflection by something more intriguing than a solitary minor planet. The astronomers measured a difference in range of 150 meters on October 18th and 600 meters on the 20th. They conclude that Hermes consists of two separate bodies, each 300 to 450 meters across, circling one another in 13 to 21 hours.

Meanwhile, two campaigns involving amateur and professional astronomers have established the rotation period of at least one of Hermes’ components, or perhaps the complete pair. On October 24th, Petr Pravec (Ondrejov Observatory, Czech Republic) reported to the Minor Planet Mailing List that brightness fluctuations detected by Brian Warner, Peter Kusnirak, and David Higgins yield "a clear and unique period" of 13.89 hours.

Independently, Raoul Behrend (Geneva Observatory) has analyzed a light curve obtained by observers in France, Belgium, and Switzerland and finds nearly the same period: 13.96 hours. Because the components are close, perhaps almost touching, their rotation rates probably match their period of revolution around one another, but that conclusion awaits further data.

According to Pravec’s list, there are now 18 confirmed binaries among the near-Earth asteroids.