Hermes != 2002 SY50

Three teams of dynamicists report that asteroid 2002 SY50, discovered
earlier this year, is not asteroid Hermes, which has been missing
since 1937.  The new object was found on an orbit very similar to the
one traced by the long-lost object, leading astronomers to suspect
that Hermes had returned.  By using highly accurate ranging data
obtained at the NASA/JPL Goldstone radar in California, however,
scientists have concluded that the two objects are not the same.  The
radar data will be used to examine if the two asteroids are
dynamically related.

For 65 years astronomers have searched for Hermes, a kilometer-sized
asteroid on an Earth-crossing orbit.  The asteroid was observed for
four days in 1937 and has been completely lost since then.  It is the
largest asteroid ever observed to make a close approach to Earth
within 750 000 kilometers (470,000 miles), less than twice the
distance to the Moon.  The Hermes case story emphasizes the importance
of keeping track of near-Earth asteroids, some of which could
potentially hit the Earth with devastating consequences.

Earlier this year astronomers were puzzled when the discovery of 2002
SY50 was announced by the Minor Planet Center (MPC).  This asteroid
has a size and trajectory so similar to those of the long-lost Hermes
that it seemed very likely they were the same object.  Yet, as more
optical measurements of the object's position were made and its orbit
was computed with increasing precision, the possibility that it was
Hermes could not be established satisfactorily or ruled out
conclusively.  The trajectory of 2002 SY50 could not be predicted
reliably more than a decade or so into the past or future.  "The
uncertainty is due to Earth and Venus close approaches that affect the
path of the asteroid and that make the identification with Hermes very
difficult", said Tim Spahr of the MPC, based in Cambridge,
Massachusetts.  "Prior to the radar data, close Earth approaches could
be predicted within +/- 10 days only for 1996 and 2002 encounters"
explained Jon Giorgini of the Jet Propulsion Laboratory (JPL) in
Pasadena, California.

Progress was made possible when astronomers reported a different kind
of measurements obtained with the powerful radar facility at
Goldstone.  The distance and velocity information obtained with the
radar refined the knowledge of 2002 SY50's orbit considerably.  This
allowed the dynamicists to compute close Earth approaches reliably
over a window spanning 1896-2071, and to conclude that 2002 SY50
cannot be the same asteroid as the long-lost Hermes.

Paul Chodas and Steve Chesley of JPL ran a "Monte Carlo" analysis
which derives its name from the famous casino city in the
Mediterranean.  During the month since discovery, they have simulated
millions of possible trajectories for 2002 SY50 with initial
conditions chosen in a random manner in order to represent the
uncertainty about the actual trajectory of the asteroid.  The radar
data has enabled them to show convincingly that 2002 SY50 was more
than 300 million kilometers (200 million miles) away from Earth in
October 1937, during the time when Hermes was within 10 million
kilometers (6 million miles) of our planet.

A group of experts in Europe reached similar conclusions.  Andrea
Milani, Maria Eugenia Sansaturio, and Giovanni Valsecchi reported that
their orbit computations using the radar data were incompatible with
the hypothesis that 2002 SY50 and Hermes were the same object.  In
addition to numerical simulations, their team used an analytical
technique to explore all possible encounters with Earth that could
have led 2002 SY50 to be in Earth's vicinity at the time of the Hermes
encounter.  They concluded that these scenarios were inconsistent with
the observations.

Gareth Williams of the MPC used the radar data to refine the orbit of
2002 SY50 and to attempt an identification with Hermes.  He used both
sets of 1937 and 2002 observations in an attempt to arrive at a
consistent solution.  He found it impossible to get an orbit to match
both sets of observations, and concluded that 2002 SY50 and Hermes
must be different objects.

Despite the conclusion by the three teams of dynamicists that 2002
SY50 is not Hermes, the similarities between their orbits remains
striking, and the question of whether or not these two objects are in
some way related remains open.  The new radar data will enable
dynamicists to more thoroughly explore the fascinating history of 2002
SY50.  Meanwhile, the search for Hermes continues.

The radar observations were obtained by a team led by Jean-Luc Margot
of the California Institute of Technology in Pasadena, California, and
Steve Ostro, Jon Giorgini, Lance Benner, Ray Jurgens, Martin Slade, of
JPL, and Michael Nolan of the Arecibo Observatory.