Scientists get near the real Eros

Eros: Not just a pile of rubble

By BBC News Online science editor Dr David Whitehouse

Astronomers have just released the results of the most detailed orbital survey of an asteroid ever undertaken.

The data covers the space rock known as Eros, which has been under close study by the Near-Shoemaker spacecraft.

Images show lots of small boulders

Eros follows an orbit that takes it from relatively close to Earth out to beyond the orbit of Mars - a journey lasting 1.76 years.

The new research, published in the journal Science, has revealed in great clarity many of the asteroid's major properties, such as its mass, topography and composition.

Eros, discovered in 1898, measures 34km by 11km by 11km. It spins on its axis every 5.3 hours. Its surface is covered in craters and a surprisingly large number of house-sized boulders.

Violent collisions

And this is the big puzzle for astronomers: how can something so small have so much debris lying around on its surface?

Researchers may be getting closer to some answers thanks to information sent back to Earth by Nasa's Near Earth Asteroid Rendezvous spacecraft (recently re-named in honour of the famous geologist Eugene Shoemaker).

Eros facts

Classification
S-type asteroid

Size
34x11x11 km

Mass
6.7x10e15 kg

Volume
2503 cubic km

Rotation period
5.3 hours

Distance from Sun
168m - 258m km

Orbital period
1.76 years

Professor Joseph Veverka, of Cornell University, said in his Science paper that Eros had a surface "saturated" with tiny craters smaller than one kilometre (0.6 of a mile) in diameter and was "abundant" with rocks that measured 30 to 100 metres (33 to 109 yards) across.

The craters and the boulders, said Professor Veverka, indicated many violent collisions with other space bodies. But why they were still there on an asteroid which had a weak gravitational field was a mystery, he said.

"Intuition and calculation tell you that most of the debris produced in a collision would have escaped - but the surface is full of it."

Eros has a banana-shape and the speed required for any debris to break free from the pull of the asteroid can vary widely from place to place. In some regions, the so-called escape velocity is just 3.1 metres per second. In others, it is five times higher.

Near's Laser Rangefinder has probed every corner

It is possible that debris could be thrown out at very low speeds only to fall back on to Eros. The ejected material could possibly end up in the same orbit as Eros where it is retrieved at a later time.

Following Near's scrutiny, Eros has been confirmed as an S-type asteroid, the most common type in the inner asteroid belt. It is a primitive relic from the birth of the Solar System.

Near's observations show no evidence that Eros has ever gone through a major heating event in which metals are segregated from silicates to form an iron-rich core and rocky mantle - such as happened on Earth.

Eros has a surface "saturated" with tiny craters

As the spacecraft orbits Eros, its Laser Rangefinder (NLR) fires a laser pulse every second and records how long it takes the beam to be reflected back.

The NRL's data gave Professor Maria Zuber and colleagues, from Nasa's Goddard Spaceflight Center, the information they needed to construct a detailed topographic map of the asteroid's surface.

The researchers have concluded that Eros is far from a loose pile of rubble held together by gravitational attraction, as some researchers have suggested.

And by looking at Near's orbit around Eros, and the asteroid's detailed shape, Dr Donald Yeomans, of Nasa's Jet propulsion Laboratory, has estimated that Eros weighs approximately 6.7 million billion kilograms.