Asteroid Triple Threat

2001 SN263 has now been revealed as the first near-Earth triple asteroid ever found.
Credit: Cornell University / Arecibo Observatory

Once considered just your average single asteroid, 2001 SN263 has now been revealed as the first near-Earth triple asteroid ever found. The asteroid – with three bodies orbiting each other – was discovered this week by astronomers at the sensitive radar telescope at Cornell University’s Arecibo Observatory in Puerto Rico.

Cornell University and Arecibo astronomer Michael C. Nolan said he and his colleagues made the discovery when they obtained radar images Feb. 11. The group subsequently took more images to learn that the three objects – about 7 million miles from Earth – are rotating around each other.

The main, central body is spherical with a diameter of roughly 1.5 miles (2 kilometers), while the larger of the two moons is about half that size. The smallest object is about 1,000 feet across, or about the size of the Arecibo telescope.

Other triple asteroids exist in the asteroid belt (between Mars and Jupiter) and beyond, but this is the first near-Earth system where the actual shapes of objects can be clearly seen.

This painting depicts an asteroid slamming into tropical, shallow seas of the Yucatan Peninsula. This famous impact, which occurred approximately 65 million years ago, is believed to have caused the extinction of the dinosaurs and many other species on Earth.
Credit: Donald E. Davis

The Arecibo telescope is operated for the NSF by Cornell’s National Astronomy and Ionosphere Center.

"This discovery has extremely important implications for ideas about the origins of near-Earth asteroids and the processes responsible for their physical properties," said Nolan. "Double, or binary, asteroid systems are known to be fairly common – about one in six near-Earth asteroids is a binary – but this is the first near-Earth triple system to be discovered."

The object was first discovered visually Sept. 19, 2001, by the Lincoln Near Earth Asteroid Research (LINEAR) project, part of the Massachusetts Institute of Technology’s Lincoln Laboratory. The orbits of binary – and now triple – asteroid systems unveil the mass and allow astronomers to assess whether they are stable over millennia or have formed very recently. Previous radar investigations of binary near-Earth asteroids have disclosed extraordinary physical and dynamical characteristics.

Nolan said this discovery prompts several important questions: Are the objects orbiting in the same plane? How rapidly are the orbits changing with time? Did the moons form when this asteroid formed in the main asteroid belt, or after it arrived in near-Earth space?

Because of the small sizes and irregularly shaped components, 2001 SN263 should offer unique insights relative to the much larger triple systems in the main asteroid belt, said Nolan. "Examining the orbits of the moons as we continue to observe 2001 SN263 over the next few weeks may allow us to determine the density of the asteroid and type of material from which it is made," he said. "We will also be studying its shape, surface features and regolith [blanketing material] properties."

The Arecibo Radio Observatory in Puerto Rico.
Credit: NAIC – Arecibo Observatory, a facility of the NSF

Radar observations by the Arecibo Observatory can image a much larger fraction of the population of near-Earth asteroids than spacecraft. For example, Arecibo has discovered more than half of the near-Earth binary asteroid systems discovered since 1999. Continued observations will undoubtedly lead to the discovery of new classes of objects, such as this triple system. While the Arecibo telescope is capable of these investigations, the future of the radar program and the entire telescope are in considerable doubt due to NSF budget cuts.

Cataloging and monitoring near-Earth asteroids helps scientists understand these important celestial bodies. Asteroid impacts have played a major role in the history of life on Earth, causing mass extinctions and potentially delivering organic molecules important for the origin of life on our planet. Keeping an eye on nearby asteroids also helps scientists understand the potential for future impacts and the threat they may pose for the future of life on our planet.

Nolan’s collaborators on the project are Ellen S. Howell, Arecibo Observatory/Cornell University; Lance A.M. Benner, Steven J. Ostro and Jon D. Giorgini, Jet Propulsion Laboratory/California Institute of Technology; Michael W. Busch, Caltech; Lynn M. Carter and Ross F. Anderson, Smithsonian Institution; Chris Magri, University of Maine at Farmington; Donald B. Campbell and Jean-Luc Margot, Cornell; Ronald J. Vervack Jr., Johns Hopkins University Applied Physics Laboratory; and Michael K. Shepard, Bloomsburg University.


Related Web Sites

Astrobiology Roadmap Goal 6: Life’s Future on Earth and Beyond
Zipping Through the Neighborhood
Small Rocks Make Big Impacts
The Art of Asteroid Avoidance
Deflecting Don Quijote
Impact Hazards Website