Sunlight spawns many binary and
“divorced” binary asteroids
BERKELEY — The asteroid belt
between Mars and Jupiter is often depicted as a dull zone of dead
rocks with an occasional wayward speedster smashing through on its
way toward the sun.
A new study appearing in the Aug. 26 issue of the journal
Nature paints a different picture, one of slow but steady
change, where sunlight gradually drives asteroids to split in two
and move far apart to become independent asteroids among the
millions orbiting the sun.
“This shows that asteroids are not inert, dead bodies of no
interest,” said study co-author Franck Marchis, a research
astronomer at the University of California, Berkeley, and the SETI
Institute in Mountain View, Calif. “In fact, small asteroids very
slowly evolve into binaries and, eventually, divorced
binaries.”
Marchis, who studies double- and triple-asteroid systems, teamed
up with former UC Berkeley undergraduate Brent Macomber to analyze
two pairs of former or “divorced” binaries, which are asteroid
pairs that have drifted apart and are no longer gravitationally
bound to one another. Macomber, now a graduate student at Texas
A&M University, participated through UC Berkeley’s
Undergraduate Research Apprentice Program (URAP), which matches
students with researchers in need of assistance.
Marchis and Macomber contributed their findings to a group of
astronomers in the Czech Republic, who analyzed the evolution of 35
pairs of divorced binaries. The leader of that group, Petr Pravec
of the Astronomical Institute in the Czech Republic, and 25
colleagues from 15 other institutions published the results this
week, showing that all of the asteroid pairs have similar relative
masses and relative velocities that point to a similar origin by
fission.
The conclusion fits a theory of binary asteroid formation
originated by co-author Daniel Scheeres, a professor of aerospace
engineering sciences at the University of Colorado, Boulder. His
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 the size of the larger asteroid. Of all the
asteroid pairs in the study, the smallest of each pair was always
less than 60 percent of the mass of its companion asteroid.
Of the estimated one million asteroids 1 kilometer or more in
diameter orbiting the sun, many are thought to be rubble piles of
smaller rocks gravitationally bound together. Previous research has
shown that asteroid rubble piles less than 10 kilometers in
diameter can start rotating faster because of the
Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP)
effect: the imbalance between sunlight absorbed on one side of an
out-of-round asteroid and heat radiated on the other makes it
spin.
“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,” Scheeres said.
As an asteroid spins up, the equator bulges and the rocks at the
extreme edge eventually reach escape velocity and detach. The
detached rocks coalesce into a moonlet and, over millions of years,
the primary and secondary asteroids “separate gently from each
other at relatively low velocities,” Scheeres said.
“This slow process, rather than catastrophic demolition,
replenishes the population of binary asteroids, and accounts for
the many binaries and ex-binaries that we see,” Marchis said,
noting that 10-15 percent of all small asteroids could be part of
binary systems.
The researchers focused on so-called “asteroid pairs”:
independent asteroids in the same orbit around the sun that have
come close to each other – usually within a few miles –
at very low relative speeds at some point in the past million
years. Asteroid pairs were first discovered in 2008 by co-author
David Vokrouhlicky of Charles University in Prague, Czech Republic,
but their formation process remained a mystery prior to the new
study.
Suspecting that asteroid pairs were at one point binary asteroid
systems, Pravec asked collaborators to measure two characteristics
of each of the 35 asteroid pairs: the relative brightness of each
asteroid – which correlates to its size – and the spin
rates of the asteroid pairs using a technique known as
photometry.
The 35 asteroids in the study ranged from about 1 to 10
kilometers (0.6 – 6 miles) in diameter. Observations were
contributed by co-authors from institutions in North Carolina,
California, Massachusetts, Chile, Israel, Slovakia, the Ukraine,
Spain and France.
Macomber’s contributions to research are not unusual for a UC
Berkeley undergraduate. More than 1,400 students were involved in
research last year in all fields of science, social science and the
humanities.
“In the three years that I worked with Dr. Marchis, I got more
experience than I could have possibly imagined in all aspects of
observational astronomy, everything from planning a night of
observations, to collecting data with advanced adaptive optics
imagers, to processing the data after the observations are
completed,” said Macomber, who obtained his bachelor’s in physics
and astronomy in December 2008, worked for a semester with Marchis
at the SETI Institute and is now a Bradley Fellow in the Department
of Aerospace Engineering at Texas A&M. “The most important
thing I learned was how real science works, the process of
collaborating with a team around the world to collect observations,
analyze them and publish scientific results.”
“When students work with us, they can be involved in
state-of-the-art research and make a real contribution to science,”
Marchis said.
The contributions of Marchis and Macomber, obtained using the
Lick Observatory’s 1-meter Nickel telescope, were supported by the
National Science Foundation.