Astronomy and Astrophysics

How Kleopatra Got Its Moons

Berkeley — The asteroid Kleopatra, like its namesake, the last pharaoh and queen of Egypt, gave birth to twins – two moons probably spawnedby the asteroid sometime in the past 100 million years.

In the February issue of the journal Icarus, a team of French and American astronomers, including Franck Marchis, a research astronomer at the University of California, Berkeley, and Pascal Descamps, an astronomer at the Institut de Mécanique Céleste et de Calculs des Éphémérides (IMCCE) of the Observatoire de Paris, report the discovery and also confirm earlier reports that the asteroid is shaped like a dog bone.

In addition, the team’s detailed study of the asteroid using small telescopes as well as the large Keck II telescope is Hawaii allowed it to determine the precise orbits of the twin moons and calculate the density of Kleopatra, showing that the asteroid is probably a big pile of rock and metal rubble.

“Our observations of the orbits of the two satellites of 216 Kleopatra imply that this large metallic asteroid is a rubble pile, which is a surprise,” said Marchis, who is also a planetary scientist at the SETI Institute in Mountain View, Calif. “Asteroids this big are supposed to be solid, not rubble piles.”

The Committee on Small Body Nomenclature of the International Astronomical Union accepted the proposal of Marchis and his collaborators to name the moons after Cleopatra’s twins by Mark Antony: Cleopatra Selene II and Alexander Helios. The outermost moon is named Alexhelios and the innermost moon is Cleoselene. In Greek mythology, Helios and Selene represented the sun and moon, respectively.

Kleopatra, about 217 kilometers long, is one of several large asteroids recently found to be composed of rocky rubble held together by mutual gravitational attraction. Others are 87 Sylvia, which is 280 kilometers in diameter; 90 Antiope, 86 kilometers in diameter; 121 Hermione, 190 kilometers in diameter; and 22 Kalliope, 166 kilometers in diameter. During the past five years, Marchis and Descamps were involved in the determination of the density of all of these multiple asteroids, that is, asteroids with moons.

The proportion of large asteroids in the solar system that are rubble piles is unknown, but the fact that, so far, all multiple asteroids are porous collections of gravitationally-bound chunks could have implications  for how planets form, Marchis said. The collisions between two asteroids are as likely to break up both bodies as to coalesce into one large asteroid, making planet formation a slow process. Rubble piles asteroids, however, would merge more easily during a collision.

“If a large proportion of asteroids in the early solar system were rubble-pile, then the formation of the core of planets would be much faster,” Marchis said.

The rubble pile structure explains Kleopatra’s shape and its two moons, the authors argue. The asteroid probably coalesced from the remains of a rocky, metallic asteroid smashed to smithereens after a collision with another asteroid, which could have occurred anytime since the origin of the solar system 4.5 billion years ago. Based on Descamps’ theory of binary asteroid formation, the rubble pile was set spinning faster by an oblique impact 100 million years ago.  The spinning asteroid would have elongated and slowly thrown out the most distant moon. The inner moon was likely shed more recently, perhaps 10 million years ago.

The asteroid 216 Kleopatra was discovered in 1880 and was shown in 2000 to have an unusual, elongated shape reminiscent of a dog bone. Subsequent radar observations confirmed the shape, but Marchis and his colleagues wanted higher resolution images to determine whether the two lobes of the dog bone are touching or are two separate bodies, and also to calculate the density. Using adaptive optics on the Keck II telescope, the astronomers obtained in 2008 the best images yet and confirmed that the asteroid is one double-lobed body. They also discovered the two moons.

By reanalyzing occultations of other stars by the asteroid, which were observed by smaller telescopes around the world, the researchers were able to refine the size and shape of the asteroid. Some of the observations they used took place 30 years ago.

The team also charted the orbits of the two small moons, each about 8 kilometers across, providing enough information to derive the mass of the asteroid. Given the size, shape and mass, the astronomers then calculated the asteroid’s density: 3.6 grams per cubic centimeter. If the bulk of the asteroid is iron, a common component with a density of about 5 grams/cubic centimeter, then the asteroid is between 30 and 50 percent empty space, the team concluded.

The team continues to observe large asteroids in search of ones with moons, which allows them to calculate the density and determine the prevalence of porous, rubble pile asteroids.

The coauthors of the paper include Imke de Pater, UC Berkeley professor of astronomy; UC Berkeley assistant researchers Michael H. Wong and Gaspard Duchêne; Jérôme Berthier and Frédéric Vachier of the Institut de Mécanique Céleste et de Calcul des Éphémérides at the Observatoire de Paris; and former UC Berkeley student Brent Macomber, now  at Texas A&M University.

Long-lost, Dangerous Asteroid is Found Again

Echoing the re-discovery of America by the Spanish long after an earlier Viking reconnaissance, astronomers have learned that a recently observed asteroid - one that could potentially hit the Earth - was actually first observed nearly a half-century ago. Researchers at the Minor Planet Center of the Smithsonian Astrophysical Observatory in Cambridge, MA have confirmed work by SETI Institute astronomer Peter Jenniskens that the recently discovered asteroid 2007 RR9 is in fact the long-lost object 6344 P-L.

6344 P-L was last seen in 1960, and ever since has had the peculiar distinction of being the only Potentially Hazardous Asteroid without a formal designation. "The object was long recognized to be dangerous, but we didn't know where it was," says Jenniskens. "Now it is no longer just out there."

A designation as Potentially Hazardous means that 2007 RR9 is one of 886 (not 887) known asteroids bigger than 150 m (500 ft) in diameter that come to within 0.05 astronomical units of Earth's orbit (roughly 7,480,000 km or 4,650,000 miles). The size is estimated on the basis of the object's observed brightness and an assumed reflectance of 13 percent.

Jenniskens believes that this object may not, in fact, be an asteroid. "This is a now-dormant comet nucleus, a fragment of a bigger object that, after breaking up in the not-so-distant past, may have caused the gamma Piscid shower of slow meteors (IAU #236) that is active in mid-October and early November," he says. 2007 RR9 moves in a 4.70-year orbit, nearly all the way out to the distance of Jupiter. Because of this elongated orbit, it has a Tisserand parameter of T = 2.94, which defines it dynamically as a Jupiter Family Comet (T = 2.0 - 3.0), not an asteroid (T > 3.0).

So far, this object has not yet been seen to be even weakly active, but the now dormant comet is still moving closer to the Sun. It is sliding rapidly toward visibility in the southern hemisphere, and is expected to brighten to magnitude +18.5 in mid-October. According to Gareth V. Williams of the Minor Planet Center, it will pass Earth around November 6 at 0.07 AU, when the minor planet is at high latitudes in southern skies.

The original designation of P-L stands for "Palomar-Leiden," the juxtaposition of two observatory names that reflect what was a very fruitful collaboration by the trio of pioneer asteroid searchers Tom Gehrels of the University of Arizona, and Ingrid van Houten-Groeneveld and her husband Cornelis Johannes van Houten. Gehrels made a sky survey using the 48-inch Schmidt Telescope at the famed Palomar Observatory, long before modern asteroid reconnaisances, and shipped the photographic plates to the van Houtens at Leiden Observatory in the Netherlands. There, Ingrid discovered 6344 P-L on four plates taken on September 24-28, 1960. The trio are jointly credited with several thousand asteroid discoveries, but only 6344 P-L is a potential danger to Earth.

Peter Jenniskens is a meteor astronomer with the SETI Institute and author of "Meteor Showers and their Parent Comets" published by Cambridge University Press (2006). He is also credited with the identification of the parent body of the Quadrantid meteor shower. As it happens, he graduated from Leiden Observatory in 1992, before joining the SETI Institute.

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 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.

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