[meteorite-list] Secret of Twin Asteroid Birth Revealed

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 25 Aug 2010 19:37:06 -0500
Message-ID: <E3061714F3A7482CA5C21057B3022513_at_ATARIENGINE2>

The Yarkovsky Effect Strikes Again!


Sterling K. Webb
------------------------------------------------
http://www.space.com/scienceastronomy/asteroid-formation-divorced-asteroid-pair-study-100825.html

Secret of Twin Asteroid Birth Revealed
By Mike Wall, SPACE.com Senior Writer
posted: 25 August 2010, 01:09 pm ET

Asteroids are changeable worlds that can split
into pieces, creating two smaller space rocks
with separate paths around the sun, a new study
finds. The process can happen non-destructively -
just add sunlight and lots of time.

The space rock discovery comes from an analysis
of 35 so-called "divorced asteroid pairs" by an
international team of astronomers. First discovered
only two years ago, divorced asteroid pairs are
space rocks that take similar - but separate -
paths around the sun, and have come very close
together at some point in the last million years.
Their origins remained a mystery, until now.

"Asteroids aren't just static boulders floating
around in space," said study co-author Daniel
Scheeres of the University of Colorado, Boulder.
"They're constantly evolving over time."

The research team, led by astronomer led by Petr
Pravec of the Astronomical Institute in the Czech
Republic, used several telescopes around the world
to make the asteroid find. They determined the
sizes of these asteroids by measuring their relative
brightness, and studied the spin rate of each pair
with a technique known as photometry.

"It was clear to us then that just computing orbits
of the paired asteroids was not sufficient to
understand their origin," Pravec said in statement.
"We had to study the properties of the bodies."

The research is detailed in the Aug. 26 issue of
the journal Nature.


Twin asteroid birth predicted:

The asteroids scrutinized in the study were all
on the small side, averaging less than 6 miles
(10 km) wide. Researchers found that all of the
asteroid pairs analyzed shared a specific size
relationship: The smaller one was always less
than 60 percent as big as its companion. These
measurements fit precisely with a theory Scheeres
developed in 2007, which postulated a way that
divorced asteroid pairs could form.

Scheeres' theory addressed the nature and destiny
of "binary asteroid pairs" - asteroids that orbit
each other as they zoom around the sun. Unlike
"divorced pairs," the two asteroids share an
overall path, orbiting together.

Binary asteroid pairs are fairly common in the
solar system. One way they can form, astronomers
think, is via some long-term solar heating. If an
asteroid is small - less than 6 miles or so in
diameter - the sun can help break it apart. Solar
radiation blasting one side can cause the space
rock to spin faster and faster over millions of
years.

"When sunlight shines on asteroids, it can spin
them up like a propeller," Scheeres told SPACE.com.

Most known asteroids in the solar system are
concentrated in a region between the orbits of
 Mars and Jupiter called the asteroid belt, which
is about 200 million miles from the sun. But some
also extend into the inner solar system as near-
Earth asteroids. Astronomers estimate there are
1 million asteroids larger than 0.6 miles (1 km)
wide, in the solar system. NASA's infrared WISE
space telescope has discovered more than 25,000
previously unknown asteroids in the last six months.


More observations needed:
Many asteroids are thought to be "rubble piles,"
rocky bits and pieces held together by each other's
tenuous gravity. If the solar-induced spin gets fast
enough, a chunk on an asteroid's end can split off.

In binary asteroid pairs, the theory goes, this chunk
sticks with the bigger, "parent" asteroid, and the two
rotate around each other. But Scheeres' calculations
predicted that the "baby" can break free if it's less
than 60 percent as big as the parent. The result would
be two space rocks that take slightly different paths
around the sun - a divorced asteroid pair.

The new study's findings confirmed that theory and
should help astronomers understand how asteroids
form and develop - and give them further confidence
that many of their theories and models represent reality.

"It's one thing to do the math and predict these things,"
Scheeres said. "It's another to actually go out and
observe them."
Received on Wed 25 Aug 2010 08:37:06 PM PDT