Two days ago the IAU's quaintly named Central Bureau for Astronomical Telegrams announced that astronomers have discovered a triple asteroid passing in Earth's vicinity.

This isn't the first rock trio — out in the main asteroid belt, 87 Sylvia has two small moons (Romulus and Remus) and so does 45 Eugenia (Petit-Prince and another designated S/2004 (45) 1). Multiple asteroids are no longer big news. Based on the terrestrial cratering record, about one in six impacts with Earth involves a double object.

Triple asteroid

A visualization of the radar echoes acquired February 13, 2008, from asteroid 2001 SN263. Radar illumination is from the top. Echoes from the two satellites appear thinner because they are rotating slowly and therefore don't produce as much Doppler shifting (horizontal axis) as the main mass.

National Astronomy and Ionosphere Center

But this one is a rather different animal. First, it's in an Amor-type orbit, meaning that it comes to within 96.4 million miles of the Sun but doesn't quite cross the orbit of Earth. Second, its three components are rather similar in size. The main body is roughly 1½ miles (2 km) across, whereas the other two are closer to 1,000 feet (300 meters).

The threesome came to light because a team headed by Cornell astronomer Mike Nolan used the Arecibo radio telescope in Puerto Rico to bounce radar pulses off a small asteroid passing within about 7 million miles of Earth. When three echoes came back on Tuesday, Nolan knew they'd found something special.

First seen in September 2001, this object has two official designations: 2001 SN263 (its ID when discovered) and 153591 (assigned once its orbit was known well). But this alphanumeric jumble gets worse: according to IAU convention, for now the two little satellites will be known as S/2008 (153591) 1 and S/2008 (153591) 2. Can't we just call them Moe, Larry, and Curly and be done with it?

Nolan notes in a press release that not much is known about the trio. But more observations in the coming days might determine whether the moonlets are orbiting in the same plane, the masses of all three objects, and whether the trio formed in the asteroid belt or due to a close brush with Earth.

The 305-meter (1,000-foot) radio telescope near Arecibo, Puerto Rico, has been used for radar probing of solar-system objects since the early 1960s. The facility underwent a $27 million upgrade in the mid-1990s.

Photograph by David Parker (Science Photo Library); courtesy NAIC/Arecibo Observatory

Because it's in a near-Earth orbit, breakup specialist Derek Richardson (University of Maryland) thinks 2001 SN263 could have been ripped apart by Earth's gravity in the not-too-distant past. "You need to get within 4 Earth radii or so [about 15,000 miles or 25,000 km] for anything interesting to happen."

He doesn't think the breakup occurred in the asteroid belt, citing the unlikelihood of the threesome staying intact for the millions of years it would have taken to migrate inward. Nor does he think subtle solar forces caused it to spin so rapidly that it flew apart.

Incidentally, Arecibo's unique radar observations may come to an abrupt end in 2011 (along with its other valuable contributions). That's because a 2005 review by the National Science Foundation decided that funds for the facility could be better spent elsewhere. But the facitity's astronomers aren't going down without a fight.


Image of Tony Beresford

Tony Beresford

February 15, 2008 at 7:58 pm

In its radar role the Arecibo facility
is a civil defence facility and should
be funded by the branch of the US Government
responsible for such jobs, not the

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Image of Mike Howell

Mike Howell

February 19, 2008 at 12:29 pm

Arecibo's "radar role" has nothing to do with civil defense.
In this instance it merely means using pulsed radio waves and their echos to image astronomical objects. This same techniques has also been used to accurately determine distances to the Moon and other objects.

The design of the Arecibo system in fact makes it useless for
detection of objects near the Earth, such as aircraft.
RADAR (Radio Detection And Ranging) is merely a technique.
There are many uses that have no military applications.

Oops, time to get the coffee out of the microwave.

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