November 27, 2006

Arecibo Observes Asteroid

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If you’ve ever worried about whether movies like “Armageddon” could really happen, rest easy. Astronomers recently used new data from Cornell’s Arecibo Observatory in Puerto Rico to declare the planet safe from near-Earth asteroid KW4 for at least another thousand years.

A team of astronomers, including Steve Ostro, senior research scientist at the NASA/Caltech Jet Propulsion Laboratory in Pasadena, and Prof. Jean-Luc Margot, astronomy, made the most detailed observations to date of near-Earth asteroid (66391) 1999 KW4 (KW4), which is classified as a Potentially Hazardous Asteroid. KW4 is actually a binary system composed of a small asteroid orbiting a larger asteroid. Although the system was discovered in 1999, its twin nature was not known until 2001, when Margot and his colleagues observed it with the radar systems of Arecibo and NASA’s Goldstone Deep Space Network.

“Without radar, [an object] is unresolved — you don’t know if it’s a binary or any of its geological features or anything,” Margot said.

Alpha, the larger member of the duo, is about one-and-a-half kilometers in diameter and, according to Margot and his colleagues’ report, is “in a highly unusual physical state.”

Most of it is nondescript, except for a sudden, distinct ridge that extends all the way around its equator, standing several hundred meters high. This ridge was formed because Alpha is spinning so fast that free particles on its surface were pulled towards the equator and held there loosely in a “nearly weightless environment,” according to the report.

If it spun any faster, pieces of the upper soil would break free of Alpha’s gravity and spin off into orbit. Alpha is literally spinning as fast as it can without falling apart — the universe’s greatest rollercoaster ride.

Beta, Alpha’s smaller sibling, is about one-third its size and orbits around its equator once every 17 hours. The fact that KW4 is a binary rather than a single asteroid helped scientists pin down specific details about the system’s composition.

“Having the little guy there lets us find out the mass of the system by measuring the orbital rotation period,” Margot said.

From that, they could determine parameters like Alpha’s density that would otherwise be unknowable from Earth-based observations.

By using the powerful radar systems at Arecibo and Goldstone, scientists also attained “extraordinary precision on position, range and velocity,” according to Margot. “[Radar] very accurately predicts the position of objects over centuries or even millennia,” he said.

He and his colleagues determined that KW4 has not and will not come closer to the Earth than the moon (about 200,000 miles) between 1179 and 2946.

“There are hundreds of approaches, but they’re all fairly safe,” said Margot.

Margot and his colleagues’ study of KW4 provides the astronomical community with new and unique insights into the formation and evolution of near-Earth asteroids. It also underlines the importance of having a variety of powerful radar facilities: NASA’s Goldstone system is more fully steerable, so it provided the longest continuous sequence of images, but Arecibo is an order of magnitude stronger, and provided useful data even when the asteroid was comparatively far away. Used in parallel, these telescopes provided astronomers with the most comprehensive and precise information available to modern science, which, if KW4 had turned out to be a threat, would be of vital importance. “If you want to mitigate, you want as much information as possible,” said Margot.

Margot and his colleagues’ findings were published in the November 24 issue of Science.