NASA scientists have for the first time detected a tiny but
theoretically important force acting on asteroids by measuring an
extremely subtle change in a near-Earth asteroid’s orbital path. This
force, called the Yarkovsky Effect, is produced by the way an asteroid
absorbs energy from the sun and re-radiates it into space as heat. The
research will impact how scientists understand and track asteroids in
the future.

Asteroid 6489 "Golevka" is relatively inconspicuous by near-Earth
asteroid standards. It is only one half-kilometer (.33 mile) across,
although it weighs in at about 210 billion kilograms (460 billion
pounds). But as unremarkable as Golevka is on a celestial scale it is
also relatively well characterized, having been observed via radar in
1991, 1995, 1999 and this past May. An international team of
astronomers, including researchers from NASA’s Jet Propulsion
Laboratory in Pasadena, Calif., have used this comprehensive data set
to make a detailed analysis of the asteroid’s orbital path. The team’s
report appears in the December 5 issue of "Science."

"For the first time we have proven that asteroids can literally propel
themselves through space, albeit very slowly," said Dr. Steven
Chesley, a scientist at NASA’s Jet Propulsion Laboratory and leader of
the study.

The idea behind the Yarkovsky Effect is the simple notion that an
asteroid’s surface is heated by the sun during the day and then cools
off during the night. Because of this the asteroid tends to emit more
heat from its afternoon side, just as the evening twilight on Earth is
warmer than the morning twilight. This unbalanced thermal radiation
produces a tiny acceleration that has until now gone unmeasured.

"The amount of force exerted by the Yarkovsky Effect, about an ounce
in the case of Golevka, is incredibly small, especially considering
the asteroid’s overall mass," said Chesley. "But over the 12 years
that Golevka has been observed, that small force has caused a shift of
15 kilometers (9.4 miles). Apply that same force over tens of millions
of years and it can have a huge effect on an asteroid’s orbit.
Asteroids that orbit the Sun between Mars and Jupiter can actually
become near-Earth asteroids."

The Yarkovsky Effect has become an essential tool for understanding
several aspects of asteroid dynamics. Theoreticians have used it to
explain such phenomena as the rate of asteroid transport from the main
belt to the inner solar system, the ages of meteorite samples, and the
characteristics of so-called "asteroid families" that are formed when
a larger asteroid is disrupted by collision. And yet, despite its
profound theoretical significance, the force has never been detected,
much less measured, for any asteroid until now.

"Once a near-Earth asteroid is discovered, radar is the most powerful
astronomical technique for measuring its physical characteristics and
determining its exact orbit," said Dr. Steven Ostro, a JPL scientist
and a contributor to the paper. "To give you an idea of just how
powerful – our radar observation was like pinpointing to within a half
inch the distance of a basketball in New York using a softball-sized
radar dish in Los Angeles."

To obtain their landmark findings, the scientists utilized an advanced
model of the Yarkovsky Effect developed by Dr. David Vokrouhlick” of
Charles University, Prague. Vokrouhlick” led a 2000 study that
predicted the possibility of detecting the subtle force acting on
Golevka during its 2003 approach to Earth.

"We predicted that the acceleration should be detectable, but we were
not at all certain how strong it would be," said Vokrouhlick´´. "With
the radar data we have been able to answer that question."

Using the measurement of the Yarkovsky acceleration the team has for
the first time determined the mass and density of a small solitary
asteroid using ground-based observations. This opens up a whole new
avenue of study for near-Earth asteroids, and it is only a matter of
time before many more asteroids are "weighed" in this manner.

In addition to Chesley, Ostro and Vokrouhlick´´, authors of the report
include Jon Giorgini, Dr. Alan Chamberlin and Dr. Lance Brenner of
JPL; David Capek, Charles University, Prague, Dr. Michael Nolan,
Arecibo Observatory, Puerto Rico, Dr. Jean-Luc Margot, University of
California, Los Angeles, and Alice Hine, Arecibo Observatory, Puerto

Arecibo Observatory is operated by Cornell University under a
cooperative agreement with the National Science Foundation and with
support from NASA. NASA’s Office of Space Science, Washington, DC
supported the radar observations. JPL is managed for NASA by the
California Institute of Technology in Pasdena.

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