Satellite to encounter Eros on St Valentine's Day

US space scientists are hoping for a remarkable St Valentine's Day greeting this afternoon: word from 161 million miles away that the first spacecraft to orbit an asteroid has arrived safely.

The Near Earth Asteroid Rendezvous (NEAR) satellite was launched on February 17th, 1996, its eventual goal being to meet up with asteroid Eros, a 21-mile-long peanut-shaped lump of rock located in the asteroid belt beyond Mars.

The satellite's roundabout journey brought it close to the asteroid Mathilde in June 1997 before a return visit to Earth and a gravity powered swing-by to carry it on to its target, which at the moment is about 161 million miles away. All going well a final short burst of NEAR's rocket engines will insert it into orbit by about 4.30 p.m. Irish time.

If it makes it without mishap - and its controllers at NASA's Jet Propulsion Laboratory and Johns Hopkins Applied Physics Laboratory are confident it will - the event will mark a stunning turnaround in the satellite's fortunes. It was almost given up for dead in December 1998 when the $224.1 million mission was put in jeopardy by rockets that didn't fire as planned.

NEAR went silent for 27 hours and was feared lost before being brought back on line. In January 1999 controllers again began the delicate task of manoeuvring the spacecraft into position and by this afternoon NEAR should be in a "loose adjustable orbit", according to NASA.

Initially it will be about 300 miles above the surface but between now and the end of May it will descend to about 30 miles, where it will remain for the rest of the year.

"What most of us hope is that after most of the mission's goals have been accomplished the spacecraft will go even closer, perhaps five kilometres (three miles) above the surface, to obtain much higher resolution pictures," said senior researcher Dr Peter Thomas. Controllers might even try to bring NEAR right on to the surface.

Even without this dramatic attempt, the mission will deliver a wealth of knowledge about a little-known object. NEAR should be able to determine whether Eros is just a pile of rubble or a single lump of something solid. Yesterday the satellite was to have carried out a "flyby", passing directly between the asteroid and the sun at an angle designed to cast a shadow on to the surface. This will permit NEAR to use its infrared spectrometer to read the asteroid, telling researchers what Eros is made of and how its surface might have changed after long exposure in space.

As ever, NASA missions like to adopt a theme; for example, moon walks or landings on Mars on US Independence Day. This time St Valentine's Day was the target date for a visit to Eros, named after the Greek god of love. How appropriate, given the day that is in it.