Tim Furniss/LONDON
The Near Earth Asteroid Rendezvous (NEAR) spacecraft has been thwarted in its attempt to make interplanetary history on 10 January by becoming the first craft to enter orbit around an asteroid.
The NEAR, the first craft in the NASA Discovery programme to be launched - on 17 February, 1996, - was to have travelled 2.4billion kilometres to reach asteroid 433 Eros to embark on the first close-up, comprehensive study of such a body.
An engine burn on 20 December was aborted and contact with NEAR was lost for 24 hours, resulting in the craft instead making a mere fly-by of Eros, placing it on course for another orbital attempt in May 2000.
The NEAR's rendezvous with Eros called for a series of precise manoeuvres that began with the fated burn on 20 December. The spacecraft's velocity had to be increased in increments with a series of engine burns so it could catch up with the faster-moving asteroid.
This was to be followed on 28 December with a second burn to increase the speed by 294m/s, at a distance of 21,000km from Eros, reducing the spacecraft's speed relative to the asteroid to less than 30m/s.
On 3 January, 1999, a third burn was planned to reduce the relative speed by a further 22m/s at a distance of 5,000km so that, on 10 January, the NEAR was scheduled to lock into orbit around Eros with a final burn reducing relative speed by 8m/s at a distance of 1,000km.
The $150 million NEAR Discovery mission, with its "faster, better, cheaper" label, is managed by the John Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, which also built the 805kg spacecraft.
Eros will eventually become only the fourth object in the solar system - apart from the earth and the moon - to be orbited by a spacecraft, following Venus, Mars and Jupiter. The NEAR will also make the closest orbit of any spacecraft - just 15km from the asteroid's surface.
Eros is not the first asteroid to be visited by a spacecraft, however. The NEAR itself made a fly-by of asteroid 253 Maltide in June 1997 en route to Eros - which also involved an earth swing-by in January 1998 - and the Galileo spacecraft, which is now in orbit around Jupiter, briefly explored the asteroids Gaspra and Ida in 1991 and 1993, respectively.
Despite those fly-bys, NEAR mission manager Dr Robert Farquhar of APL says: "What we know about asteroids is very limited. Now, for the first time, we're going to go into orbit around an asteroid and study it intensely for a year. We expect to get astounding information."
The NEAR's year-long mission to unlock the secrets of asteroid Eros may be more technically, than scientifically, challenging because it will attempt to orbit a tumbling, irregularly shaped body at very low altitude. Eros is a cigar-shaped body 35km long, 16km wide and 6km deep, rotating on its axis every five and half hours.
"Never before has any small body been orbited by a spacecraft," says APL, "but the additional task of manoeuvring a spacecraft to within 15km of the asteroid's surface during its orbit makes the challenge even more complex."
Dr Joseph Veverka of Cornell University, the mission's science team leader, says the challenges facing the NEAR are significant. "This will be the first characterisation in detail, not only of the surface of an asteroid, but of the interior of the asteroid, and the history that this asteroid has gone through based on its surface characteristics and materials composition."
A suite of six instruments, including a 1m-resolution charge-coupled-device imager, will take millions of measurements and images over the entire surface of Eros from various altitudes. From these data, scientists will determine the asteroid's physical and geological properties and the elements and minerals it contains.
For a year, NASA's Deep Space Network will transmit data from the spacecraft to NEAR's Science Data Center, at APL.
Regular tweaking of the spacecraft's orbit will be needed to ensure that its instruments are used to their full advantage. This will include taking it down to only 15 km from the asteroid.
By the end of the NEAR's mission in 2001, scientists expect to know much more about Eros and near-earth asteroids. and gain insight into the earth's origin and the formation of the solar system.
ASTEROIDS
Eros was the 433rd asteroid to be discovered, in 1898. Asteroids are regarded as primordial material left over when the planets were formed and are often called minor planets. They are relatively small rocky bodies in elliptical orbits around the sun, mainly in a belt between the orbits of Mars and Jupiter.
Of the 2,500 known asteroids, 95% are in this region, including Ceres, Pallas and Juno - the first three to be discovered in 1801-7. Ceres has a diameter of 955km. Some asteroids are no bigger than boulders, however.
Some of the main belt asteroids have more elliptical orbits, with a closest point nearer to the sun, in the region of the inner planets, Mercury, Venus and earth. Asteroids sometimes fly quite close to the earth. In January 1975, Eros passed within 22.5 million kilometres of the planet, appearing as a swiftly moving, faint star-like object at seventh magnitude. Its shape suggests it might have broken off a larger asteroid.
Source: Flight International
