Earth and an Asteroid Play “Orbital Cat and Mouse”

The first asteroid discovered to orbit the Sun in
nearly the same path as Earth will make its closest approach
to our planet this month before scurrying away for 95 years.

The space rock, measuring about 60 meters (approximately 200
feet) across, is like a mouse teasing a cat. According to an
international team of astronomers, including a researcher
from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena,
Calif., the asteroid approaches the Earth, first on one side
and then on the other. The team’s report appeared in the
October 2002 issue of the journal Meteoritics & Planetary
Science.

The asteroid, named 2002 AA29, traces an unusual horseshoe
pattern relative to Earth. The asteroid alternately leads
and follows Earth around the Sun without ever passing it.
“In some ways, the Earth and this asteroid are like two
racecars on a circular track,” said JPL’s Dr. Paul Chodas,
who discovered the object’s unusual motion. “Right now the
asteroid is on a slightly slower track just outside Earth’s,
and our planet is catching up,” he said.

On Jan. 8, 2003, the tiny body will come within
approximately 5.9 million kilometers (3.7 million miles) of
Earth, its closest approach for almost a century. “Unlike
racecars, the two bodies will not pass when they approach
each other,” Chodas said. “Instead, the combined
gravitational effects of the Earth and Sun will nudge the
asteroid onto a slightly faster track just inside Earth’s,
and it will begin to pull ahead,” he said.

In 95 years, the asteroid will have advanced all the way
around to where it is catching up to the Earth from behind.
A similar interaction with gravity from both the Earth and
Sun will then push the asteroid back onto a slower outside
track, and the pattern will repeat. To an observer moving
with the Earth, the asteroid appears to trace out a
horseshoe pattern.

“There’s no possibility that this asteroid could hit Earth,
because Earth’s gravity rebuffs its periodic advances and
keeps it at bay,” said Dr. Don Yeomans, JPL manager of
NASA’s Near Earth Objects Program Office. “The asteroid and
Earth take turns sneaking up on each other, but they never
get too close,” he said.

The team’s calculations show that in about 600 years, the
asteroid may begin looping around Earth like a tiny, distant
quasi-moon. “The asteroid will appear to orbit the Earth at
that time, but in fact it will be too far away to be
considered a true satellite of our planet,” Chodas said.
“Our calculations indicate the space rock will circle the
Earth as a quasi-satellite for about 40 years before
resuming its horseshoe orbital pattern.”

Other members of the team investigating this object include
Dr. Martin Connors, Athabasca University, Canada; Dr. Seppo
Mikkola, University of Turku, Finland; Dr. Paul Wiegert,
Queen’s University, Canada; Dr. Christian Veillet, Canada-
France-Hawaii Telescope, Hawaii; and Dr. Kim A. Innanen,
York University, Canada.

JPL is a division of the California Institute of Technology
in Pasadena, Calif.