[meteorite-list] New International Study Shows Some Asteroids Live in Own 'Little Worlds'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 26 Aug 2010 16:28:37 -0700 (PDT)
Message-ID: <201008262328.o7QNSbsp015333_at_zagami.jpl.nasa.gov>

http://www.colorado.edu/news/r/11335e94d14fe23de65b3cbbabd145dc.html

New International Study Shows Some Asteroids Live in Own 'Little Worlds'
University of Colorado at Boulder
August 25, 2010

While the common perception of asteroids is that they are giant rocks
lumbering about in orbit, a new study shows they actually are constantly
changing "little worlds" that can give birth to smaller asteroids that
split off to start their own lives as they circle around the sun.

Astronomers have known that small asteroids get "spun up" to fast
rotation rates by sunlight falling on them, much like propellers in the
wind. The new results show when asteroids spin fast enough, they can
undergo "rotational fission," splitting into two pieces which then begin
orbiting each other. Such "binary asteroids" are fairly common in the
solar system.

The new study, led by Petr Pravec of the Astronomical Institute in the
Czech Republic and involving the University of Colorado at Boulder and
15 other institutions around the world, shows that many of these binary
asteroids do not remain bound to each other but escape, forming two
asteroids in orbit around the sun when there previously was just one.
The study appears in the Aug. 26 issue of Nature.

The researchers studied 35 so-called "asteroid pairs," separate
asteroids in orbit around the sun that have come close to each other at
some point in the past million years -- usually within a few miles, or
kilometers -- at very low relative speeds. They measured the relative
brightness of each asteroid pair, which correlates to its size, and
determined the spin rates of the asteroid pairs using 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," said Pravec.
"We had to study the properties of the bodies. We used photometric
techniques that allowed us to determine their rotation rates and study
their relative sizes."

The research team showed that all of the asteroid pairs in the study had
a specific relationship between the larger and smaller members, with the
smallest one always less than 60 percent of the size of its companion
asteroid. The measurement fits precisely with a theory developed in 2007
by study co-author and CU-Boulder aerospace engineering sciences
Professor Daniel Scheeres.

Scheeres' theory predicts that if a binary asteroid forms by rotational
fission, the two can only escape from each other if the smaller one is
less than 60 percent of the size of the larger asteroid. When one of the
asteroids in the pair is small enough, it can "make a break for it" and
escape the orbital dance, essentially moving away to start its own
"asteroid family," he said. During rotational fission, the asteroids
separate gently from each other at relatively low velocities.

"This is perhaps the clearest observational evidence that asteroids
aren't just large rocks in orbit about the sun that keep the same shape
over time," said Scheeres. "Instead, they are little worlds that may be
constantly changing as they grow older, sometimes giving birth to
smaller asteroids that then start their own life in orbit around the sun."

While asteroid pairs were first discovered in 2008 by paper co-author
David Vokrouhlicky of Charles University in Prague, their formation
process remained a mystery prior to the new Nature study.

When the binary asteroid forms, the orbit of the two asteroids around
each other is initially chaotic, Scheeres said. "The smaller guy steals
rotational energy from the bigger guy, causing the bigger guy to rotate
more slowly and the size of the orbit of the two bodies to expand. If
the second asteroid is small enough, there is enough excess energy for
the pair to escape from each other and go into their own orbits around
the sun."

Several telescopes around the world were used for the study, with the
most thorough observations made with the 1-meter telescope at Wise
Observatory in the Negev Desert in Israel and the Danish 1.54-meter
telescope at La Silla, Chile. "This study makes the clear connection
between asteroids spinning up and breaking into pieces, showing that
asteroids are not static, monolithic bodies," said Vokrouhlicky.

The asteroids that populate the solar system are primarily concentrated
in the main asteroid belt between Mars and Jupiter some 200 million
miles from the sun, but extend all the way down into the inner solar
system, which are known as the near-Earth asteroids. There are likely
about a million asteroids larger than 0.6 miles, or 1 kilometer, in
diameter orbiting the sun. Last month, NASA's WISE spacecraft spotted
25,000 never-before-seen asteroids in just six months.

Astronomers believe most asteroids are not solid chunks of rock, but
rather piles of debris that come in shapes ranging from snowmen and dog
bones to potatoes and bananas, with each asteroid essentially glued
together by gravitational forces.

"Sunlight striking an asteroid less than 10 kilometers across can change
its rotation over millions of years, a slow motion version of how a
windmill reacts to the wind," said Scheeres, who has studied asteroids
for the past decade. "This causes the smaller asteroid to rotate more
rapidly until it can undergo rotational fission. It's not hard for these
asteroid pairs to be pushed over the edge."

CU-Boulder doctoral student Seth Jacobson of CU-Boulder's astrophysical
and planetary sciences department, a co-author on the Nature paper, said
the most surprising part of the study was showing that sunlight played
the key role in "birthing" asteroids. "There was a time when most
astronomers referred to asteroids as vermin," said Jacobson. "But the
more we learn about them, the more exciting they are. They are not just
big chunks of rock, but have the dynamic ability to evolve."

The asteroids in the study ranged from about 1 kilometer to about 10
kilometers or about 0.6 miles to 6 miles in diameter, said Jacobson. He
said one of the biggest questions is what lies beneath the surfaces of
asteroids. "This is something we just don't know yet," he said.

Asteroids have become a hot topic, said Scheeres. The Japanese
spacecraft Hayabusa made two landings on the asteroid Itokawa in 2005
before its recent return to Earth -- the first spacecraft ever to visit
an asteroid and return to the planet. Scientists are hopeful the
spacecraft recovered at least some particles from the asteroid, which
may give them more information about the origin and evolution of the
solar system roughly 4.6 billion years ago.

President Barack Obama this year announced his vision for planetary
exploration that involves skipping future manned moon landings in favor
of sending astronauts to a near-Earth asteroid in the next two decades.
Obama and others see a successful manned asteroid landing as a stepping
stone to eventually landing humans on Mars.

"Asteroids are important to understanding life on Earth," said Pravec.
He pointed to the Chicxulub asteroid believed to have plowed into the
Yucatan Peninsula 65 million years ago and caused dinosaurs to go
extinct, essentially resetting the evolutionary clock on Earth. Some
asteroids have even been found to contain amino acids -- the building
blocks of life -- causing some scientists to speculate that life on
Earth could have come from asteroids pelting the planet.

Other co-authors of the study are from institutions in North Carolina,
California, Massachusetts, Chile, Israel, Slovakia, the Ukraine, Spain
and France.

Contact

Daniel Scheeres, CU-Boulder, 303-492-7420
Daniel.Scheeres at colorado.edu

Seth Jacobson, CU-Boulder
Seth.Jacobson at colorado.edu

David Vokrouhlicky, Charles University, Prague, +420-2-21912574
vokrouhl at cesnet.cz

Jim Scott, 303-492-3114
Jim.Scott at colorado.edu
Received on Thu 26 Aug 2010 07:28:37 PM PDT