International study shows some asteroids
live in own little worlds
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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.
