杏吧原创

THE Rosetta spacecraft made history last week when it became the first to orbit a comet, but it has no time to rest on its laurels. It must now get to work on its main mission: unlocking the secrets of comet 67P/Churyumov-Gerasimenko and its links with life on Earth.

The European Space Agency (ESA) probe has already beamed back incredible pictures of a wildly alien landscape. 鈥淲e鈥檝e never seen a comet that close, that high-res 鈥� it鈥檚 an all-new world,鈥� says Holger Sierks, the lead researcher for Rosetta鈥檚 main camera, based at the Max Planck Institute for Solar System Research in G枚ttingen, Germany. But what we can鈥檛 see will be just as revealing, as Rosetta increases its detailed observations of the comet鈥檚 molecular composition, internal structure and more.

These details could tell us how Earth got its water. About 4.6 billion years ago, a cloud of dust and gas began clumping together to form the sun and planets of our solar system. Planets have churned and reprocessed that original material, but the unused bits became asteroids and comets, which are essentially pristine planetary building blocks.

鈥淚f you want to know what was there to start with, you鈥檝e got to go and study these things which were there at the time,鈥� says Rosetta team member of the Open University in Milton Keynes, UK. 鈥淵ou cannot address that question by studying the Earth, because anything that went together to make the Earth has been mixed up.鈥�

Comets may have brought water and the carbon-based molecules necessary for life as they rained down on the early Earth鈥檚 molten surface. But other theories suggest the oceans formed in situ on Earth as the planet鈥檚 atmosphere evolved. Results from previous comet-fly-by missions 鈥� which have snapped photos and even grabbed particles from a comet鈥檚 tail 鈥� have proved inconclusive, but Rosetta鈥檚 extended stay and carefully chosen toolkit mean it is well placed to provide answers.

As you read this, Rosetta will be training its array of instruments on 67P. One can grab dust ejected from the comet and place it under an on-board microscope to see what it鈥檚 made of. Others will eyeball the gas tail streaming from the comet, while yet more investigate its magnetic and electrical properties.

But our closest look at 67P will come in November, when Rosetta鈥檚 Philae lander will physically dig into the comet鈥檚 history. Getting the washing-machine-sized lander to the surface will be tricky. From the moment Rosetta arrived on 6 August, ESA researchers have been gathering data to select a place to touch down. The comet鈥檚 bulbous shape and resulting strange gravity field rule out some areas, but a number of feasible landing spots have presented themselves (see 鈥�Green marks the spot鈥�, below).

Incoming data on the comet鈥檚 mass, density and surface will help ESA narrow the choice to as many as five possible zones, each 600 metres across. The smallest details will be considered: if one of the lander鈥檚 feet settles on a stone just 30 centimetres high, for example, it could tip over. 鈥淭his is the difficult part, to find a sufficiently large area that is sufficiently flat,鈥� says Jens Biele of the German space agency DLR, which built the lander.

Harpoon anchor

On 11 November, Rosetta will dive from 30 kilometres above 67P鈥檚 surface to 10 kilometres, fast enough to ensure it will pass over the comet rather than crash should anything go wrong. At the right moment, Philae will be ejected backwards, allowing it to gently fall to the comet鈥檚 surface, which could take as long as 12 hours. At touchdown, the lander will deploy a harpoon to anchor itself. Philae will only be able to communicate with Earth at certain points during the descent. 鈥淚t will be nerve-wracking,鈥� says Biele.

The forces Philae records as it touches down will tell researchers whether the surface is like fresh powdered snow, hard ice or something in between. It will then drill up to 23 centimetres into the comet to take samples, untouched since the solar system formed, and identify the molecules inside. 鈥淲e will really touch and taste the comet,鈥� says Biele.

One instrument will measure water isotopes in the comet. If they match the isotopes on Earth, it will suggest that comets brought our planet鈥檚 water. 鈥淚f it came from comets, what you鈥檝e got is a sample of frozen primordial soup,鈥� says Wright.

聯If Earth鈥檚 water came from comets, what we鈥檝e got is a sample of frozen primordial soup聰

Philae will also look at complex organic molecules thought to be precursors to life, particularly whether their form is the left-handed or right-handed mirror image. Inorganic processes produce both kinds of molecule in equal measure, but for some reason life on Earth is only left-handed. It is possible that this handedness is produced by the interaction of ultraviolet radiation with icebound organic molecules in a vacuum. In other words, sunshine on comets in space produced a plethora of left-handed molecules which could then have crashed into early Earth. 鈥淚t would help to solve the enigma of why life took one side and not the other,鈥� says Biele.

Philae and Rosetta will work together to scan the comet鈥檚 interior using radio waves, to reveal whether it is solid or more of a rubble pile inside 鈥� a technique that could prove useful for future asteroid miners (see 鈥�Asteroid education鈥�).

鈥淭he team was so fortunate to pick such a great comet,鈥� says of the University of Washington in Seattle, who led NASA鈥檚 Stardust mission to bring comet dust back to Earth. 鈥淭his really looks like a Rosetta Stone that may tell us how comets work.鈥�

Leader: 鈥�鈥淪pace is the place to solve the riddle of life, maybe鈥�鈥�

Green marks the spot

Comet 67P鈥檚 bulbous shape, which has been likened to a rubber duck, means only a few spots are suitable for landing. Those highlighted in green are聽probably safe places for a聽lander to touch down. The other colours mark where sunlight falls聽鈥� blue for always dark, red for always light, and the rest in between.

This article appeared in print under the headline 鈥淭o touch and taste a comet鈥�