Countdown to comet encounter

Close encounter is set for 2230 GMT, 22 September

By BBC News Online science editor Dr David Whitehouse

Over the next few days, Dr Marc Rayman is unlikely to get much sleep.

The American space agency (Nasa) scientist is mission director on the Deep Space 1 (DS1) probe - and the craft has been aimed straight at a comet.

Preparations are going extremely well, which is remarkable considering the age and debilitated nature of the spacecraft

Rayman and his team, at the Jet Propulsion Laboratory, hope to shepherd the ailing probe through the gas and dust that shrouds Comet Borrelley. If they are lucky, they will get a picture of the very heart of a comet - its mountain-sized nucleus.

Very few probes have managed it. The European craft Giotto's classic image of the peanut-shaped nucleus of Comet Halley is probably the most famous picture.

"People are very excited right now," he told BBC News Online from DS1's control room. "We're mostly focused on keeping the spacecraft healthy, so that the plans we've been preparing over the last few months can be executed.

"Preparations are going extremely well, which is remarkable considering the age and debilitated nature of the spacecraft," he added.

Getting closer

Although DS1 was designed to test a range of novel technologies on one spacecraft, Marc Rayman and his team always had the idea of sending it through a comet when its main work was done.

"DS1 completed its main mission two years ago, so anything else we get is a big bonus. Even before launch, we asked Nasa if we could send it to a comet - but they did not give us the go-ahead until about 10 months after launch."

Giotto's classic image of Comet Halley

With less than a million kilometres to go, they are keeping an eye on the spacecraft and making last minute adjustments.

"We will operate the ion propulsion system to fine-tune the trajectory a bit, and we will collect distant observations of the comet in order to get a better fix as to where it is. We will send this info back to the spacecraft so that it can get even closer to the comet."

Dr Rayman says that the last chance they will have to alter DS1's trajectory will be about 24 hours before its close encounter, which takes place at about 2230 GMT Saturday.

But there is a very real risk that DS1 may not make it at all.

Risky mission

"If we receive data at all - and remember there is a very real risk that the probe will be disabled by dust collisions - the data will include information about the composition of the coma, the gas cloud surrounding the nucleus, as well as what the magnetic and electric fields are like inside the coma."

We will also try to get infrared measurements of the nucleus that will reveal what the surface is made of, and finally we hope to get a black and white image of the nucleus itself."

If DS1 succeeds, we will not have to wait long for the dramatic picture of Borrelley's nucleus.

"We hope to release the image sometime on Sunday or Monday, if we get it. DS1 certainly has less that a 50% chance of succeeding."

But, added Dr Rayman, with hope in his voice: "DS1 has performed spectacularly so far."