CU: Some Asteroids Live in Own 'Little Worlds'

[bystander]

Some Asteroids Live in Own 'Little Worlds'
Colorado University, Boulder | 25 Aug 2010

Illustration of a binary asteroid. Courtesy ESO/L. Calcada

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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.

Sunlight spawns many binary and 'divorced' binary asteroids
University of California, Berkeley | 25 Aug 2010

[img3="Over tens of millions of years, a rubble-pile asteroid less than 10 kilometers in diameter can spin up to a speed that allows the asteroid to split in two, forming a proto-binary system. If the smaller companion is less than 60 percent the size of the larger asteroid, they will gently separate from one other at a relatively low velocity. The system will eventually become an asteroid pair, or "divorced" asteroid.
(Image: Pravec, et al.)"]http://www.berkeley.edu/news/media/rele ... oid650.jpg[/img3]

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.

Formation of asteroid pairs by rotational fission - P Pravec et al

• Nature 466 (26 Aug 2010) DOI: 10.1038/nature09315

Asteroids two-by-two

• Nature 466 (26 Aug 2010) Editor's Summary

ScienceShot: Giving Birth, Asteroid Style
Science NOW | 25 Aug 2010

[i]Credit: NASA/JPL[/i]

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Calling asteroids rocky is a misnomer. Recent space missions have shown them to be surprisingly loose agglomerations of pebbles that can barely hold themselves together gravitationally. And that may explain the phenomenon of asteroid pairs. Reporting online today in Nature, researchers say that asteroids can literally spin themselves apart, as in the simulations above, essentially giving birth. The split happens when sunlight heats the irregular surface of an asteroid unevenly, causing it to begin spinning, like a pinwheel in the wind. Eventually the asteroid rotates so fast that a big chunk breaks off. The researchers have observed 35 asteroid pairs that they think formed in this way, with the offspring less than 60% of the mass of the parent. The resulting pairs don't form binary systems, however: Lacking sufficient gravity from the parent, the newborn asteroid goes its own way, but follows its parent's original orbit.

[Beyond]

By seeing all the poch marks on them, i figured that the astroids just got hit by speedier stuff and had pieces blasted off of them. I never would have thought that sunlight hitting them would cause them to spin apart. If anything i would have figured that perhaps the sunlight hitting them on one side would cause stress and a piece would pop off much like putting an icecube into a liquid and hearing a little pop and seeing two pieces of ice in your glass.

[bystander]

Avoiding an Asteroid Collision
American Friends of Tel Aviv University | 13 Sept 2010

TAU researchers reveal a new dimension in the study of asteroid pairs

Though it was once believed that all asteroids are giant pieces of solid rock, later hypotheses have it that some are actually a collection of small gravel-sized rocks, held together by gravity. If one of these "rubble piles" spins fast enough, it's speculated that pieces could separate from it through centrifugal force and form a second collection — in effect, a second asteroid.

Now researchers at Tel Aviv University, in collaboration with an international group of scientists, have proved the existence of these theoretical "separated asteroid" pairs.

Ph.D. student David Polishook of Tel Aviv University's Department of Geophysics and Planetary Sciences and his supervisor Dr. Noah Brosch of the university's School of Physics and Astronomy say the research has not only verified a theory, but could have greater implications if an asteroid passes close to earth. Instead of a solid mountain colliding with earth's surface, says Dr. Brosch, the planet would be pelted with the innumerable pebbles and rocks that comprise it, like a shotgun blast instead of a single cannonball. This knowledge could guide the defensive tactics to be taken if an asteroid were on track to collide with the Earth.

A large part of the research for the study, recently published in the journal Nature, was done at Tel Aviv University's Wise Observatory, located deep in the Negev Desert — the first and only modern astronomical observatory in the Middle East.

Tel Aviv University astronomers reveal the nature of "separated asteroids"