The interest in Hermes is not limited to its history or hazard. In 2002 we observed an asteroid, 2002 SY50, that bears a striking resemblance to Hermes in terms of size and orbital parameters, and it is possible that the two objects are dynamically related. A close planetary encounter might have tidally disrupted a large object, leaving Hermes and 2002 SY50 on similar orbits.
A Doppler spectrum of the radar echo obtained on Oct 17 shows two clearly separated components.
The set of radar images below show the relative motion of the
components over the course of about one hour on Oct 19. Motion is
counter-clockwise. In each panel, the component at positive Doppler
frequencies (right), moves toward the observer, while the component at
negative Doppler frequencies (left), moves away from the observer.
Radar illumination is from the top. The range resolution is 75 m.
Doppler broadening of the Hermes primary and secondary based on CW data obtained 2003 Oct 18, Nov 04, Nov 08, and Nov 11. Echo widths were measured at a power level equivalent to three standard deviations of the noise, were assigned error bars corresponding to the frequency resolution, and were converted to a common 12.6-cm wavelength. The solid lines represent the expected Doppler broadening of bodies with 315-m and 280-m radii if their spin axes are perpendicular to our orbital plane solution. The orbit determination process does not incorporate any constraints related to the Doppler broadening of the individual bodies. The good agreement suggests that the orbital solution is accurate, that the primary and secondary are not appreciably elongated, and that their obliquities have been driven to zero by tides.
"Using radar, the position and velocity of Hermes has been measured to within 300 meters and 50 mm/s; much more accurately than with optical telescopes. Including those measurements in a new orbit solution suddenly allowed us to accurately predict Hermes motion over many centuries, from 1561 to 2103.
During those 542 years, it makes 23 close approaches to the Earth, 29 to Venus, 7 to Mars, and 7 to a large asteroid called Vesta. Hermes closest approach was in the skies over the Earth in 1942, as World War II was fought underneath, when it passed about 1.8 times further away than the Moon. However, no one noticed it at the time.
Eventually, the gravitational forces from all these encounters with the planets blurs out our knowledge of where Hermes will be, requiring more measurements in the future to make specific longer predictions. Although technically classified as a "Potentially Hazardous Asteroid", this only means it has an orbit such that, over hundreds of thousands or millions of years, Hermes could potentially come very near the Earth. However, there is no risk now."
Jon Giorgini also provided a close approach table.
Jean-Luc Margot Assistant Professor Department of Astronomy Cornell University 304 Space Sciences Bldg Ithaca, NY 14853 607 255 1810http://www.astro.cornell.edu/~jlm