NASA JPL: Orbit for Hermes Dynamically Linked from 1937 to 2003

Steven R. Chesley

Paul W. Chodas

Jet Propulsion Laboratory

October 16, 2003

Using sophisticated orbit determination tools, the difficult problem of finding
a precise orbit for the long-lost and recently rediscovered asteroid Hermes has
been solved.

The recovery of Hermes was announced on October 15, 2003 by the Minor Planet
Center (MPC) in Cambridge Massachusetts. The object was initially noted by
Brian Skiff of the LONEOS asteroid search program at the Lowell Observatory in
Arizona, and key follow-up measurements were provided by James Young of JPL’s
Table Mountain Observatory in California. Tim Spahr of the MPC located
prediscovery observations from the last 7 weeks and computed the new object’s
orbit. Noticing that the orbit was very similar to that of Hermes, last seen
during its close approach in 1937, Spahr concluded that the new object was
almost certainly Hermes. Definitive proof of the object’s identity was still
lacking, however, because an orbit linking the known positions in 1937 to those
in 2003 could not be found.

Finding the precise orbit of Hermes is difficult because its trajectory is
very chaotic. In the 66 years since it was last seen, the asteroid has made
numerous close approaches to both the Earth and Venus. Since the orbital changes
at each approach depend highly on the circumstances of the encounter, finding
an orbit with the precise sequence of encounter conditions that links positions
in 2003 to those in 1937 is a challenging problem in orbit determination.

We have now solved this problem by using the JPL Sentry impact monitoring
software in a novel way. Starting from the 2003 positions, Sentry found twelve
distinct dynamical pathways that produced encounters in 1937, each with a
different sequence of intervening close approach circumstances. Comparing these
predicted 1937 encounters with the one determined directly from the 1937
observations, we were able to identify the most consistent candidate, and then
zero in on the precise orbit that best matches the positions in both 1937 and
2003. We now know that since it was last seen, Hermes has made eight close
approaches to the Earth and Venus to within 0.06 AU, including an Earth approach
to within about 1.6 lunar distances in 1942. The new orbit solution allows us
to predict future close approaches with great accuracy; we can now predict
that Hermes will not approach the Earth any closer than about 0.02 AU (8 lunar
distances) within the next hundred years.