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Front: Scientists hope there's no twist in the tail as Rosetta heads for comet rendezvous: Craft approaches 'Chury' today after 10-year chase It will study surface before sending in landing probe

August 6, 2014

Ian Sample Science editor



With a gentle kick from its onboard thrusters, a European spacecraft will arrive at a speeding comet this morning and prepare for a spectacular first in space history.

Planned for 10amUK time, the seven-minute burn should bring the euros 1bn (pounds 790m)Rosetta probe to within 100km of the strangely-shaped comet which is tearing through space at up to 135,000km/h.

The comet is so far from Earth that mission controllers at the European Space Agency (ESA) will have to wait half an hour before they receive any signal back that the manoeuvre has worked. For those who have devoted much of their careers to the mission, the pause will not be pleasant.

"It's the first time we have ever done this," said Matt Taylor, a project scientist on Rosetta at ESA in the Netherlands. "Even the smallest manoeuvre can go wrong. Nothing is straightforward in spaceflight."

The Rosetta probe launched from French Guiana more than 10 years ago on a mission to chase down comet 67P/Churyumov-Gerasimenko, known to some space researchers as "Chury".

But catching up with the comet is merely the end of the beginning of the mission. In the coming weeks, Rosetta will become the first spacecraft in history to go into orbit around a comet and, on 11 November, drop a lander on to the surface.

The gravitational pull of the Mont Blanc-sized comet is so weak that the lander, Philae, must attach itself with an explosive harpoon.

With Philae latched on and Rosetta circling above, the mission will give scientists their first close observations of a comet as it streaks towards the sun and becomes active. As the solar radiation rises, the comet will start billowing hundreds of kilos of dust and gas that could grow into a spectacular tail more than one million kilometres long. But images taken by Rosetta's cameras have already shown that the landing might be trickier than mission controllers had hoped. The comet appears to be two space rocks stuck together. With one lump smaller than the other, the comet looks like an enormous rubber duck.

Rosetta will spend August flying around the comet in a triangular orbit to give scientists time to map the terrain, and the shape and strength of its gravitational field. Armed with those details, Rosetta will then move closer, to within 30km of the surface, and settle into a more conventional circular orbit.

"The time pressure at the moment is phenomenal. It's a race against the clock to learn about the comet and select a landing site. We have to land before the comet becomes too active," said Taylor. "We'll get an inference of what's possible in September, but we won't want to land near the neck of these two parts of the comet. We need the best communications with the orbiter and also to maximise the sunlight the lander receives to give it the best chance to survive as long as possible."

Once in orbit, instruments on board the spacecraft will start collecting and analysing dust and gas coming off the comet. If Philae lands safely in November, it will relay measurements from its own suite of instruments back to the mother ship. While the electronics on Philae will probably die as the comet swings around the sun, the little lander may cling on for several laps around the sun before it loses its grip and falls off.

Comets are formed from debris left over from the early solar system around 4.6bn years ago. By studying the composition of Churyumov-Gerasimenko, scientists hope to learn more about how Earth and the other planets in the solar system came to be.

The comet swings past the sun at a distance of 185m kilometres before heading out to 800m kilometres in the deeper reaches of the solar system.

"The reason we look at comets is that they were there right at the beginning. Leaning about them gives us an idea of where Earth came from and where the whole solar system came from, what that primordial material at the beginning of the solar system was made up of," said Taylor.

One of the main instruments on the lander was built by researchers at the Open University in Milton Keynes and the Rutherford Appleton laboratory near Didcot. Called Ptolemy, it will take samples drilled from the comet and bake them at different temperatures. By analysing the gases given off, researchers can build up a picture of the comet's composition.

Measurements from Ptolemy might shed light on whether Earth originally formed as a dry planet that was seeded with water and organic molecules carried by comets that slammed into the planet.

Simeon Barber, project manager for the Ptolemy instrument at the Open University, said: "It's fair to say everyone was a bit gobsmacked when they first saw these images coming back of the comet's nucleus. The scientists are saying, which bit do we want to land on, and the engineers are saying, blimey, how will its shape affect the gravitational field?

"At the end of the day we just want a safe landing. We've been waiting a long time for this."

Hermann Bohnhardt, an astronomer at the Max Planck Institute for Solar System Research in Lindau and lead scientist on the Philae lander, said information from Rosetta's sensors will help the team to select the best landing site in the coming weeks. So little is known about the comet's makeup that Philae's harpoon was designed to penetrate a surface as hard as a table or as soft as snow.

"The nucleus shape came as a surprise and, of course, it has implications for the landing site selection," Bohnhardt said. "Apart from the shape itself, also the gravity field, the nucleus rotation, the activity and illumination of the surface play important roles and, on top of all, scientific criteria related with physical properties of the nucleus and its surface layers."

The European Space Agency is live-streaming the final burn that will rendezvous Rosetta with the comet today.

Captions:

11/11

Date this year on which the Rosetta is scheduled to launch a probe to study the chemical makeup of the speeding comet


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Source: Guardian (UK)


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