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Tuesday, February 9, 1999 Published at 14:37 GMT


Eros brought down to size

Near will go into orbit around Eros

Nasa's Near spacecraft has revealed unprecedented new detail about the Eros asteroid. New measurements show the lump of rock to be smaller than previously thought. It has as least two medium-sized craters, a long surface ridge and a density comparable to the Earth's crust.

The Near Earth Asteroid Rendezvous (Near) probe was launched in February 1996 and flew by the asteroid in December last year. It was supposed to go into orbit around the asteroid but an unsuccessful firing of its main engine has put this back to the middle of February 2000.

Nevertheless, Near scientists are pleased with the data gathered in the flyby. They now have 222 photos and supporting spectral observations of Eros, taken from as close as 3,830 kilometres (2,375 miles) from the asteroid.

Smaller size

"The flyby of Eros has given us fundamental information that will help us plan a better orbital mission at Eros," said Dr Andrew Cheng, Near project scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland. "It has taken some of the risk out of our orbit insertion manoeuvre and early operations."

[ image: Eros is slightly smaller than we thought]
Eros is slightly smaller than we thought
Analysis of the data show the size of Eros to be 33 by 13 by 13 kilometres (21 by 8 by 8 miles). Previous radar observations had suggested Eros was 40.5 by 14.5 by 14 km.

The asteroid rotates once every 5.27 hours and has no discernible moons. Its density is approximately 2.7 grams per cubic centimetre (1.55 ounces per cubic inch), close to the average density of Earth's crust.

Elongated ridge

Flyby imaging of the asteroid's surface revealed a prominent elongated ridge that extends along its length for as much as 20 km (12 miles).

"This ridge-like feature, combined with the measurements of high density, suggests that Eros is a homogeneous body rather than a collection of rubble," said Dr Joseph Veverka, of Cornell University, who heads the mission's imaging team. "It might even be a remnant of a larger body that was shattered by an impact."

First observed from the Earth more than 100 years ago, Eros was known to be an S-type asteroid with high concentrations of silicate minerals and metal. However, few details about its structure or composition are observable from the ground.

The Near flyby produced evidence of variations in surface colour and reflected light (or albedo) that suggest the asteroid has a diverse surface make-up. Closer observations during the comprehensive year-long orbital study of Eros will be needed to determine its precise composition.

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