[meteorite-list] Radar Reveals Five Double Asteroid Systems Orbiting Each Other Near Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:27 2004
Message-ID: <200204151606.JAA22017_at_zagami.jpl.nasa.gov>

http://www.news.cornell.edu/releases/April02/Asteroids.Margot.deb.html

Radar reveals five double asteroid systems orbiting each other near Earth,
likely formed in close encounters with planet

EMBARGOED FOR RELEASE: 2 p.m. Eastern Time, April 11, 2002

Contact: David Brand
         Cornell University
Office: 607-255-3651
E-Mail: deb27_at_cornell.edu

ITHACA, N.Y. -- Binary asteroids -- two rocky objects orbiting about one
another -- appear to be common in Earth-crossing orbits, astronomers using
the world's two most powerful astronomical radar telescopes report. And it
is probable, they say, that these double asteroid systems have been formed
as a result of gravitational effects during close encounters with at least
two of the inner planets, including Earth.

Writing in a report published by the journalScience on its Science Express
web site (April 11, 2002), the researchers estimate that about 16 percent of
so-called near-Earth asteroids (NEAs) larger than 200 meters (219 yards) in
diameter are likely to be binary systems, with about a three-to-one relative
size of the two encircling bodies. To date, five such binary systems have
been identified by radar, says lead researcher Jean-Luc Margot, an O.K. Earl
postdoctoral fellow in the Division of Geological and Planetary Sciences at
the California Institute of Technology.

Margot, who at the time of the observations was a research associate in the
planetary studies/radar group at the National Science Foundation's (NSF)
Arecibo Observatory in Puerto Rico (managed at Cornell University), says
that theoretical and modeling results show the binary asteroids appear to be
formed extremely close to Earth -- within a distance equal to a few times
the planet's radius (6,378 kilometers or 3,963 miles). "The fact that one
out of every six large NEAs is a binary and that they typically survive on
the order of 10 million years, implies that these close encounters must
happen frequently compared to the lifetime of the binary asteroids," says
Margot.

The Science article, "Binary Asteroids in the Near-Earth Object Population,"
is coauthored by Michael Nolan, research associate at Arecibo; Lance Benner,
Steven Ostro, Raymond Jurgens, Jon Giorgini and Martin Slade at the Jet
Propulsion Laboratory (JPL); and Donald Campbell, professor of astronomy at
Cornell. The observations were made at the 70-meter Goldstone NASA tracking
telescope in California and at Arecibo Observatory.

NEAs are formed in the asteroid belt, between the orbits of Mars and
Jupiter, and nudged by the gravitational attraction of nearby planets,
largely Jupiter, into orbits that allow them to enter the Earth's
neighborhood. Most of the asteroids are the remnants of the initial
agglomeration of the inner planets.

Astronomers have long speculated about the existence of binary NEAs, based
in part on impact craters on Earth. Of about 28 known terrestrial impact
craters with diameters greater than 20 kilometers, at least three are double
craters formed by impacts of objects about the same size as the newly
discovered binaries. Astronomers also have noted the changes in brightness
of reflected sunlight for some NEAs, indicating a double system was causing
an eclipse or occultation of one by the other.

In 2000, Margot and his co-researchers, using measurements from the
Goldstone radar, found that a small, roughly 800-meter-diameter
(half-a-mile) asteroid, 2000 DP107 (discovered only months before by a team
from the Massachusetts Institute of Technology), was a binary system.
Observations over eight days last October with the much more sensitive
Arecibo telescope clearly established the physical characteristics of
DP107's two asteroids as well as their orbit about each other. The smaller
object called the secondary, it was found, is about 300 meters (1,000 feet)
in diameter and is orbiting the larger asteroid, the primary, every 42 hours
at a distance of 2.6 kilometers (1.6 miles). The two asteroids appear to be
locked in synchronous rotation, with the smaller always with the same face
oriented to the larger.

Since that observation, says Margot, four more binary NEAs have been
discovered, all in Earth-crossing orbits and each with a main asteroid
significantly larger than the smaller body. "The primary is rotating much
faster than most NEAs in all five binaries that have been discovered," says
Cornell's Campbell. The Science Express article speculates that the most
likely way the binaries are created is by close encounters of asteroids with
the inner planets Earth or Mars. Of the five binary NEAs discovered to date,
none has an orbit that brings it as close to the sun as Venus or Mercury.

NEAs, basically piles of rubble held together by gravity, are on
trajectories that bring them within a few thousand miles of the planets,
where tidal forces ---- essentially the pull of gravity -- can increase the
spin rate of the asteroid, causing it to fly apart. The ejected rubble then
reforms in orbit around the larger asteroid.

"The asteroid is already rotating very quickly as it approaches the planet.
A little extra boost from tidal forces can be enough to exceed its breakup
limits, and it sheds mass. This mass can end up forming another object in
orbit around the asteroid. Right now this seems the most likely
explanation," says Margot.

There is an important reason for studying binary asteroids, says JPL's
Ostro: their potential for colliding with Earth. Knowing the density of
so-called PHAs (for potentially hazardous asteroids), he observes, "is an
extremely important input to any mitigation plans." He says, "Getting NEA
densities from radar is dirt cheap compared with getting a density with a
spacecraft. Of course, the most important thing to know about any PHA is
whether it is two objects or one, and this is why we want to observe these
binaries with radar whenever possible."

Margot notes, "Radar gives us very precise measurements of the size of the
objects and their shape. The radar measurements of the distance and velocity
of each component allows us to obtain precise information on their orbits.
>From this we can obtain the mass of each of the objects allowing, for the
Ūrst time, measurements of NEA densities, a very important indicator of
their composition and internal structure."

Arecibo Observatory is operated by the National Astronomy and Ionosphere
Center at Cornell under a cooperative agreement with the NSF. The research
was supported by the NSF, with NASA providing additional support for the
planetary radar program at Arecibo.

Related World Wide Web sites: The following sites provide additional
information on this news release. Some might not be part of the Cornell
University community, and Cornell has no control over their content or
availability.

o Science Express: http://www.sciencexpress.org

o Images of binary asteroid 2000 DP107:

http://www.gps.caltech.edu/~margot/2000DP107
Received on Mon 15 Apr 2002 12:06:07 PM PDT