PARIS: European scientists were jubilant on Saturday after the pride of their space fleet, racing towards a rendezvous with a comet in 2014, enjoyed a close encounter with an asteroid likened to a “diamond in the sky.”

The unmanned craft Rosetta skimmed past an asteroid called (2867) Steins late Friday in a choreographed operation 360 million kilometres (225 million miles) from Earth that mission leaders admitted tested the 1.45-billion-dollar probe to the limit.

“Steins looks like a diamond in the sky,” said Uwe Keller, in charge of Rosetta’s main cameras. “We observed a new jewel in the solar system.”

It is the first time that a European spacecraft has performed a flyby of an asteroid, part of the intriguing primordial debris left from the building of the solar system.

Pictures unveiled at a webcast press conference by the European Space Agency (ESA) revealed a four-pointed greyish rock pitted with two large craters as well as a curious line of seven smaller ones.

The flyby had been ordered four-and-a-half years into Rosetta’s decade-long trek, which will take the scout to a rendezvous with Comet 67P/Churyumov-Gerasimenko in 2014.

The orbiter will circle the comet and send down a miniature lab to carry out a range of experiments on its crusty surface.

Fifteen instruments were switched on for the flyby, which saw Rosetta swoop to within 800kms of Steins, a wafer-thin margin in the infinity of space.

Pictures were relayed back to mission control in Darmstadt, Germany, via a NASA ground station, followed by scientific data, although it could take days or weeks to gain a full analysis.

Rosetta team members, giving a preliminary appraisal from Darmstadt, said the operation went stunningly well.

The spacecraft had to perform a 180-degree flip to get the best view as the asteroid raced past at a relative speed of 30,720 kph and used an optical navigation system to keep automatically on course.

“It was a race against time. The last command (to Rosetta) was three hours before the close encounter,” Andrea Accomazzo, in charge of spacecraft operations, said.

“The dynamics of the spacecraft were really pushed to the limit... (but) it was a big success,” he added.

“This was a very important testing of the waters,” said ESA’s director of science, David Southwood. “We are really far out on the frontier. We are very far away, 360 million kilometres is not just down the street.”Early measurements from ground-based telescopes had put Steins at 10 kilometres in diameter, which was later refined by NASA’s Spitzer observatory to 4.6 kilometres.

Data sent back by Rosetta fine-tuned this to five kilometres, which was a welcome confirmation of Spitzer’s accuracy, said Keller.

Images showed a large crater about two kilometres across on the top of the asteroid, and a somewhat smaller one on its shadowed site, which suggests that the entire rock could be “fractured,” he said.

Seven small craters run down one side of the asteroid in a line from top to bottom, phenomena which have so far been seen only on the Moon or other satellites, said Keller.

Known as ejecta, these mini-craters are typically caused by the impact of material thrown out when an object smacks into a surface.

Asteroids are a source of deep scientific interest because of the clues they offer into the physical composition of the planets and the impact of the harsh environment of space.

“If we want to know how our Solar System formed and evolved subsequently.... then we have to look into other Solar System bodies,” said project scientist Rita Schulz.

“Asteroids in the asteroid belt keep a memory in that part of the Solar System of when planets were formed.”

Understanding their structure, density, orbit and spin is also useful for helping Earth to defend itself against any rogue rock that may be deflected on to a collision course with our planet.

Steins falls into a so-called Type E asteroid, comprising mainly silicates and basalts, but about which very little is known in detail.

Friday’s operation was the eighth flyby of an asteroid. Six have been carried out by the United States, which also performed the first, in 1991. The seventh was carried out by the Japanese probe Hayabusa in 2005.

Rosetta will meet a second in July 2010, zipping past a 100-kilometre rock called 21 Lutetia.

—AFP