Rosetta begins its celestial rendezvous with a comet

A SPACE probe the size of a small fridge has begun its journey towards a comet 200 million km away from earth.

THE European Space Agency’s unmanned Rosetta probe successfully released a lander toward the surface of comet 67P/Churyumov-Gerasimenko tonight, putting on its final seven-hour journey to a historic rendezvous with the fast-moving lump of dust and ice.

The audacious landing attempt is the climax of a decade-long mission to study the 4km wide comet. It is also the end of a 6.4 billion km journey on which Rosetta carried its sidekick lander Philae piggyback.

If successful, it will be the first time that a spacecraft has landed on a comet.

INTERACTIVE: Rosetta’s comet rendezvous

ESA announced that the 100kg lander’s active descent system, which uses thrust to prevent the craft from bouncing off the comet’s surface, could not be activated. Instead, the agency is relying on ice screws and a harpoon system to secure the lander.

Hours later, mission controllers clapped and embraced as the lander’s separation was confirmed.

“Philae has gone — it’s on its path down to the comet,” Rosetta flight director Andrea Accomazzo said. “We are all glad that it worked flawlessly in the past minutes.”

Scientists at the European Space Agency will watch for about seven hours as Philae travels, potentially offering them a view of a dramatic celestial display: as the comet flies closer to the sun, icy material will blast from its surface and form the comet’s halo and tail.

If nothing goes wrong, Philae, which carries 10 instruments for ground analysis, will not only make history by becoming the first craft to survive a landing on a comet, but it could unlock astounding knowledge about the origins of the solar system and life on earth. Comets are believed to be primordial ice and carbon dust left over from the building of the solar system more than 4.6 billion years ago. They have survived virtually intact through millennia and are of enormous scientific interest. It is believed that they

pounded the fledgling Earth at the birth of the solar system, providing it with carbon molecules and precious water and scientists hope the Rosetta mission will offer insight into this theory.

The final checklist and descent was preceded late on Monday by a brief moment of worry when Philae “took a bit longer than expected” to be activated, said Paolo Ferri, mission leader at Darmstadt.

“We were a bit worried at first that the temperature would be wrong (for the descent) but it all worked out. We didn’t lost any time,” Ferri said.

“The robot’s batteries should be charged up by tonight.” Philae has travelled 6.5 billion kilometres on its mothership since the pair were launched more than a decade ago.

But the last 20 or so kilometres of its trek will be the most perilous of all. That is the distance to Comet 67P/Churyumov-Gerasimenko that Philae must cover after separation, which is scheduled at 8.35am GMT (6.30pm AEDT) on Wednesday.

Touchdown will take place about seven hours later, with a confirmation signal expected on Earth around 4.00pm GMT (2.00am AEDT).

But for all this to happen, Rosetta first has to perform a high-precision ballet, more than 500 million km from home.

Philae has no thrusters, which means Rosetta can only eject it when the velocity and trajectory are right.

A tiny inaccuracy means the error in its path will widen during the descent — the probe could miss its landing site and smash into rocks or cliffs nearby.

“Failure is not an option,” Philae scientist Jean-Pierre Bibring of the Paris-Sud University in France said Monday.

In a couple of hours, Rosetta will swerve inward towards the comet and burn its thrusters, the final pre-ejection manoeuvre.

Scant time will be available to verify that these calculations are right before the “go/no-go” for separation is given.

Adding to the tension is the unknown surface of Comet “67P”. It could be tough or brittle, soft or crumbly.

Philae will land at 3.5km per hour, firing two harpoons into a surface the engineers hope will provide grip.

Ice screws at the end of its three gangly feet will then be driven into the comet, while a gas thruster on the top of the probe goes into action to prevent the lab bouncing back into space.

The size and weight of a refrigerator back on Earth, Philae will weigh just one gram — or less than a feather — on the comet.

One of the most complex and ambitious unmanned programs in space history, the Rosetta mission was approved back in 1993.

It took a decade to design and build, and after its launch, another decade to catch up with the comet, using the gravity of Earth and Mars as slingshots with which to build up speed.

Philae accounts for about a fifth of the haul of scientific data expected from the mission.

Its instruments include a mass spectrometer, a hi-tech tool to analyse a sample’s chemical signature.

AFP