[meteorite-list] Asteroid Siblings Oddly Grouped by Orientation

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:21 2004
Message-ID: <200209050131.SAA21822_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/asteroid_siblings_020904.html

Asteroid Siblings Oddly Grouped by Orientation
By Robert Roy Britt
space.com
04 September 2002

Within a family of more than 200 asteroids cleaved from a larger space rock
in some long-ago catastrophe, siblings are mysteriously grouped into two
categories defined by sharply different rotation orientations, a new study
shows.

The finding reveals a puzzling void of knowledge about how asteroids form
and evolve.

Researchers had expected that each rock in the so-called Koronis family of
asteroids would spin at some randomly determined rate. This action would be
a result of the chaos of their birth, and the axis of each asteroid's
rotation would point in some random direction. The family was originally
created in a collision between two large asteroids, experts generally agree.

MIT researcher Stephen Slivan examined changes in brightness of ten Koronis
asteroids, caused by the objects' non-spherical shape and changes in
sunlight reflection as they rotate. The technique is akin to spinning a
potato under a spotlight: Its sides reflect more light than its ends.
Measuring the time between peaks and dips in brightness provides a rotation
rate and other information.

Slivan found seven of the 10 asteroids clumped into two groups based on
their rotation rates. Three larger asteroids revolved about their axes in
about 14 hours, while four smaller rocks took about 8.5 hours, on average,
to complete a rotation.

More significant and perplexing was that all ten asteroids were fairly
neatly divided into two groups based on the orientation of their spin axes.
Most objects in the solar system rotate about an axis that is not perfectly
aligned with that of the Sun. Earth, for example, is tilted about 23.5
degrees.

In the sample of Koronis asteroids, Slivan found six whose axes generally
point one way while four point in a distinctly different direction.

"I was looking for a subtle effect," he said of these so-called spin
clusters. "This hits you in the face."

The results, which will be detailed in the Sept. 5 issue of the journal
Nature, generate questions rather than supplying answers.

"The fact that the spin clusters exist at all tells us that even what we
thought we understood about family formation and/or what happens afterwards
is at best very incomplete, because we have no consistent explanation at
present for the observed clustered spin distribution," Slivan told
SPACE.com.

Of the likely collision that created the family, Slivan said the amount of
energy released was "much, much greater than anything in human experience,
so much so that we don't really understand how the process unfolds."

Like most asteroids, the Koronis family orbits the Sun between Mars and
Jupiter. Astronomers are eager to learn more about the characteristics of
such asteroids so they can consider how best to divert or destroy one that
might someday be found on a collision course with Earth. Further, grasping
the evolutionary history of asteroids would improve understanding of the
solar system's development.

The Koronis family has a history of presenting puzzles, including one
regarding the timing of its creation.

One of its largest family members, Ida, is peppered with craters, a scene
some researchers think indicates the initial birth must have taken place
three billion years ago or more. But Ida has a moon, Dactyl, that other
scientists say must have been created in that initial collision. Yet Dactyl,
they say, can't be more than 100,000 years old.

Slivan speculates that the spin clusters might be the result of subsequent
collisions, which could have generated asteroidal grandchildren with similar
spin rates. Or, he suggests, some effect over time might gradually nudge
asteroids with random spin characteristics toward those observed in the
study.

Only a broader study of asteroids outside the Koronis family will fully
answer these questions.

"For now it's a big puzzle," he said, adding that any solution is "quite a
ways off in the future."
Received on Wed 04 Sep 2002 09:31:37 PM PDT