[meteorite-list] Asteroid Moons Pulled In By Gravity

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:34 2004
Message-ID: <200212121802.KAA25741_at_zagami.jpl.nasa.gov>

http://www.zwire.com/site/news.cfm?newsid=6363492&BRD=1692&PAG=740&dept_id=331520&rfi=6

Asteroid moons pulled in by gravity
By LIDIA WASOWICZ
United Press International
December 11, 2002

PASADENA, Calif., (United Press International via COMTEX) -- In
a theory-smashing discovery, astronomers said Wednesday they have found the
pull of gravity, not a clash of the titans, spun companion moons into
asteroid orbits on the edge of the solar system.

Since observations from the spacecraft Galileo first revealed in 1993 a
binary asteroid system -- the primeval, icy space rock Ida orbited by its
satellite Dactyl -- in the main asteroid belt between the planets Mars and
Jupiter, astronomers have observed more than a dozen pairs of such frozen
relics of the solar system's beginnings.

Scientists long have thought such twin worlds -- exemplified by Earth and
its moon -- resulted from the collision of large heavenly bodies. However,
such crashes rarely occur in the deep freeze of the outermost region of the
solar system, where asteroid pairs were revealed for the first time last
year.

There, in the area known as the Kuiper belt, which stretches from just past
the frigid, cyclone-whipped planet Neptune to beyond the farthest reaches of
the tiny misfit Pluto's highly elliptical orbit, some other forces must have
been at work.

"In the Kuiper belt today, there just aren't that many collisions between
large objects, so it's a little hard to understand how there could be as
many large binary systems formed by this mechanism as we actually observe,"
Daniel Durda of the Southwest Research Institute in Boulder, Colo., who
analyzed the findings, told United Press International.

Intrigued by the mystery, a team of space watchers from the California
Institute of Technology set out to solve the puzzle.

"Previous attempts to explain Kuiper belt binaries relied upon physical
collisions," lead study author Re'em Sari told UPI.

"However, collisions are very rare in the Kuiper belt. Moreover, when a
binary is formed by a collision, it tends to be close, i.e., the separation
between the two component bodies is only a few times larger than the bodies'
diameters," he explained. "By contrast, the separation in Kuiper belt
binaries is hundreds or even thousands of diameters. Thus, it is implausible
that Kuiper belt binaries formed through collisions."

Rather, Sari and his colleagues suggest in the Dec. 12 issue of the British
journal Nature, the double worlds might have sprung from close encounters of
the gravitational kind. Specifically, they propose the gravitational effects
during the period of runaway accretion in the early solar system could have
generated perhaps 5 percent of the binaries among Kuiper-belt objects.

"We propose that gravity alone is responsible for the formation of the
binaries," Caltech researcher Yoram Lithwick told UPI. "With the help of its
gravitational field, a body can reach out to large distances, and so it can
capture a companion that is initially quite distant."

However, the mutual gravitational attraction of two bodies passing by each
other merely will deflect them from their initial trajectories, he said. For
them to slow down sufficiently to become bound as a binary, they must
dispose of some of their energy.

"We propose ... gravity is responsible for this energy loss," Sari said.
"The two bodies can lose some energy if there is a third body nearby --
close enough to feel the two bodies' gravitational fields. In certain
configurations, the two bodies will transfer some of their energy to this
third body (or a swarm of tiny bodies), resulting in a binary."

The new results reflect the wide variety of mechanisms engaged in forming
satellites around minor planets, said Durda, who in his own research is
working out asteroid collision models of satellite formation.

"Many near-Earth asteroid binaries may form through tidal breakup when
passing near the Earth, many main-belt asteroid satellites may form through
impacts and collisions, and now we're coming to understand that many of the
binaries in the outer solar system may have formed in primordial times
through comparatively gentle gravitational encounters," he told UPI.

The solution stands to shed light on an array of topics of high interest to
Earthlings.

Many comets -- primitive snowballs that hold frozen records of solar system
origins -- and other visitors from space that occasionally stop by Earth
hail from the Kuiper belt, home to icy leftovers from the formation of the
large planets 4.5 billion years ago.

"Since the Kuiper belt binaries are relics from the early solar system, they
can teach us about this early history -- in particular, how the objects that
are presently orbiting the sun (e.g., Kuiper belt objects, planets and
moons) were built up from much smaller building blocks to their present
size," Sari explained.

Scientists' evolving understanding of asteroid satellite systems carry more
practical implications as well, Durda added.

For example, astronomers estimate some 17 percent of near-Earth asteroids
come as twosomes, while models for satellite formation indicate many of
these objects are little more than rubble piles, he said.

"That, in turn, has important implications if we ever find ourselves in a
position to need to deflect an asteroid threatening us with an impact,"
Durda noted. "These rubble pile objects may be much more difficult to push
on or disrupt than solid, monolithic mountains of rock."

Planetary scientists value the Kuiper belt -- whose existence was predicted
in 1951 by Dutch-American astronomer Gerard Kuiper and proven only a decade
ago -- both as a secure safe for holding mementoes of the birth of the solar
system and a pristine laboratory for studying planet formation and
evolution.

"When did (the outer) planets form? Why are their compositions different --
more gas and ice-rich -- than the inner planets? Did the formation of these
large (Kuiper belt) objects scatter debris into the inner solar system to
impact the terrestrial planets and the early Earth?" Durda asked.

"Lots of very interesting questions that we may help to answer in part if we
can better understand exactly when and under what circumstances the remains
of planet-forming material in the outermost solar system -- the Kuiper belt
objects -- got their satellites," he said.

Since the discovery of the first Kuiper-belt object in 1992, 617 other such
objects have been identified, helping put the seemingly anomalous Pluto into
perspective. Long regarded as a planetary misfit for its unusual orbit that
crosses Neptune's, as well as its ice-rock composition and location on the
outskirts of the solar system, Pluto seems to fit in a bit better as the
largest member of a disk of icy minor planets left over from the formation
of the outer planets, Durda said.

Since the first KBO moon was discovered in 2001, another six binary systems
have been observed with ground-based telescopes and the Hubble Space
Telescope, bringing the total to eight, including Pluto/Charon. Among the
main-belt asteroids, some nine have been found to have moons.

Astronomers expect to find many more binaries, although some of them might
be separated by such small distances they could appear as one object to an
Earth-bound onlooker.

"The discovery that even small, minor planets can have satellites of their
own makes the solar system a more complex, interesting and wondrous place to
live," Durda said. "It reminds us that we will never run out of new and
interesting worlds to explore and phenomena to understand when they continue
to pop up even here in the backyard of our own solar system."
Received on Thu 12 Dec 2002 01:02:42 PM PST