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."
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