Posted on 10-2-2004

Off to see the Swiss cheese

Europe's next space shot is on a 10-year mission to explain.

A three tonne spacecraft is about to begin a 10 year journey to answer one of the simplest questions ever put by science: what does a comet feel like? The answer is one of the most complicated space explorations ever.
Rosetta is a European mission destined for one comet last year and then redirected to another after its proposed launcher failed. A box of instruments the size of a garden shed will be fired into orbit around the sun by an Ariane 5 rocket from Kourou on February 26, for a date with Comet 67P/Churyumov-Gerasimenko on the far side of Jupiter in 2014.

It will whizz past Mars once and the Earth three times, each time picking up speed from a "gravity assist" manoeuvre to catch up with its mysterious fellow-voyager. Then it will go into orbit around the comet, and daintily drop a package of instruments on to its surface. By that time, the 3m spacecraft and its 4km companion will be 675 million km from the sun and cruising at 135,000 kph.

Four weeks ago a spacecraft called Stardust sailed through the dust of comet Wild-2 on the far side of the sun to take a sample of comet fabric and bring it back to earth. In December, the Americans launch Deep Impact, a hit- and-run probe designed to fly past and fire a probe deep into the fabric of comet Tempel 1 in 2005, excavating a hole the size of a building site. Rosetta is the only one to take a long, slow look at one of the ghostly messengers.

Taking its name from the slab of black stone now in the British Museum, used by archaeologists to decipher Egyptian hieroglyphics, Rosetta will provide the key to unlock a much bigger, older mystery. Earth, Mars and the moon are pulverised, cooked, eroded and recycled bits of brickdust from which the solar system was fashioned. Comets, parked in a kind of builder's yard far beyond the orbit of Pluto, occasionally straying near the sun to flare up into a spectacular trail of ice and gleaming dust, are the unaltered stuff of which the planets were made. So far, says David Hughes, astronomy professor at the University of Sheffield and comet expert, planetary probes have snatched close-up pictures of just three. Astronomers now know what a cometary nucleus looks like.

"So we have some information. There are to me, two huge snags. One is that we haven't the foggiest idea what the density is. The density could be the density of dirty ice, or extremely fluffy snow. We haven't the foggiest idea. This is one of these mega-mysteries. When you take a photograph you look in essence at the top fraction of a millimetre of the object and what I want to know is what the thing is really like inside."

Hughes has proposed that the cometary nucleus could contain huge voids or cavities - the Swiss cheese theory of comet fabrication. But Swiss cheese has tensile strength as well as holes. Comet nuclei, he guesses, will have little strength. Shoemaker-Levy 9, the comet that slammed into Jupiter in fragments in 1994, had been pulled apart by the tidal forces of the planet.

"This meant that the bits that it contained were literally just resting against each other. To me, when it comes to comets, I just want to know how massive they are and what they are made of inside. I just have no idea."

Comets are so much ice and dust: most of the ice is water, but a tiny fraction is frozen carbon monoxide, methane, carbon dioxide and other organic chemicals. They are believed to be the source of the Earth's oceans; and may also have delivered the biochemical broth from which life was brewed. Rosetta will drill beneath the crust to sample the material underneath.

Hughes remembers a US colleague at a conference who proposed excavating a comet nucleus with a rock drill. "Then it was my turn and I stood up and I had a spoon in my pocket for some unknown reason - I had probably just been at a coffee break - and I said 'As far as I am concerned if you want to collect some cometary surface material all you want is a spoon!'

"I said the material was more akin to a McDonald's milkshake than it is to a great lump of solid ice. But we are that ignorant: there are people still talking about the icy nucleus and other people talking about the snowy nucleus. And if I chucked a ball of ice at you and then a ball of snow at you, you'd realise the difference. So what I really want to do is to get inside a bloody comet.

"This is where Deep Impact is going to be huge fun. It is a typical brute force and ignorance experiment. You are going there with a great big ball of pure copper and you are just bloody well hurling it at the comet. What's going to happen? We don't know. There is one possibility that this ball is going to go straight through, come out the other side and the comet is going to look like a glorified Polo mint. The other possibility is that it is going to bounce off the surface. And we are that ignorant."