Observations of double asteroid stress Arecibo radar's vital role in identifying threats in Earth's vicinity

Researchers using the Arecibo Observatory's powerful radar have made the most detailed observations ever of a binary near-Earth asteroid (NEA) -- two clusters of rubble circling each other -- offering new clues about how such systems formed, the properties they share and the dynamics of their motion.

The observations, made by Steve Ostro, senior research scientist at the NASA/Caltech Jet Propulsion Laboratory in Pasadena (who earned his master's degree in engineering physics at Cornell), Jean-Luc Margot, assistant professor of astronomy at Cornell, and their colleagues, describe asteroid (66391) 1999 KW4 (called KW4). Their report appears in the latest issue (Nov. 24) of the journal Science. The double asteroid also appears on the cover.

KW4, they say, is actually a pair of light, porous clusters of rubble that circle each other as they orbit from a point closer to the sun than Mercury and then outward -- occasionally passing very close to Earth along the way. The bodies were discovered in 1999 but were not known to be binary until they were observed in May 2001, when they came within about 2.98 million miles of Earth -- their closest pass until 2036.

The researchers used antennas at Arecibo and NASA's Goldstone Deep Space Network -- the only telescopes with the radar capability for such observations. Arecibo, in Puerto Rico, is managed by the National Astronomy and Ionosphere Center at Cornell for the National Science Foundation.

KW4 is a valuable source of information for planetary scientists studying the formation and evolution of NEAs -- as well as for researchers studying how to mitigate the potential threat they pose to Earth. KW4 is classified a Potentially Hazardous Asteroid, but data show that its path will not intersect Earth's for at least 1,000 years.

Unlike single asteroids, many of whose physical properties are impossible to determine from Earth-based observations, binaries can reveal information about their mass and density by their interaction with each other. The researchers were able to reconstruct the orbit, mass, shape and density of KW4's two components, Alpha and Beta. They found an oddly shaped pair of dance partners, with Alpha, by far the larger (1.5 kilometers, or a little less than one mile, in diameter) of the two, spinning as fast as possible without breaking apart, and the smaller and denser Beta wobbling noticeably as it orbits its partner.

"It's the first time we have very detailed high-resolution images that allowed us to derive the shape of both components," said Margot. Viewed pole-on, Alpha looks circular; but from the side it looks more like a squashed diamond with rounded edges, showing a distinct ridge at the equator. A particle on Alpha's surface will be pulled toward the equator -- which means, strangely, that the body's highest point is also its lowest.

The study also involved the most precise tracking of an irregularly shaped binary system's motion -- information vital in learning how the two asteroids formed.

"The overwhelming majority of these binaries have primary components whose spins are very near the maximum of what they can sustain. It's a distinctive feature," said Margot. That indicates the systems could have been a single asteroid -- or pieces of a larger asteroid -- that were sent spinning by a close encounter with another body or by the effects of sunlight.

The system's orbit has brought it within about 9.3 million miles of Earth or closer dozens of times in the last several millennia, but not near any other planet.

As a whole, the Arecibo/Goldstone data on KW4 take the understanding of NEAs to a new level of precision, say researchers. The study also highlights the value of both telescopes involved: NASA's Goldstone, which is more steerable, and Arecibo, whose radar is an order of magnitude more powerful.

"They are complementary and both are essential," said Margot. Goldstone can track objects over a longer time period, but "you couldn't do it at this level of precision without the Arecibo data."

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Blaine Friedlander