Comet-chaser nears duck-shaped prey

AFTER a decade-long quest spanning six billion kilometres, a probe is about to come face to face with a comet. So why has it been so elusive, and why is it so important?

Rosetta Orbiting Comet 67p Churyumov Gerasimenko

AFTER a decade-long quest spanning six billion kilometres, a European probe is about to come face to face with a comet.

The moment on Wednesday will mark a key phase of the most ambitious project ever undertaken by the European Space Agency (ESA) — a 1.3 billion euro ($1.95 billion) bid to get to know one of the Solar System’s enigmatic wanderers.

More than 400 million km from its March 2004 launch site, the spacecraft Rosetta will meet its prey, Comet 67P/Churyumov-Gerasimenko (C-G).

To get there, Rosetta has had to make four fly-bys of Mars and Earth, using their gravitational force as a slingshot to build speed, and then enter a 31-month hibernation as light from the distant Sun became too weak to power its solar panels.

It was awakened in January.

The three-tonne craft should on Wednesday be about 100km from the comet.

“It’s taken more than 10 years to get here,” spacecraft operations manager Sylvain Lodiot said.

“Now we have to learn how to dock with the comet, and stay with it for the months ahead.” Astrophysicists believe comets are clusters of the oldest dust and ice in the Solar System, the rubble left from the formation of the planets 4.6 billion years ago.

These so-called dirty snowballs could be the key to understanding how the planets coalesced after the Sun flared into life.

One theory, the pan-spermia hypothesis, is that by bombarding the fledgling Earth with water and organic molecules comets helped kickstart life.

The destination for a historic mission ... Picture: European Space Agency

Until now, exploration of comets mainly entailed photographs taken by probes thousands of kilometres away on unrelated missions. Exceptions were the US probe Stardust, which brought home dust snatched from a comet’s wake, while Europe’s Giotto ventured within 200km of a comet’s surface.

On November 11, the plan is for Rosetta to approach within a few kilometres of the comet to send down a 100kg refrigerator-sized robot laboratory, Philae.

The laboratory will carry out experiments in cometary chemistry and texture for up to six months.

After Philae expires, Rosetta will accompany C-G as it passes around the Sun and heads out towards the orbit of Jupiter.

Before November’s landing, Rosetta’s operators still have plenty to do.

The first few weeks will be a get-to-know exercise, as the spacecraft traverses elongated circuits of the comet, scanning its surface.

The probe, which will have to avoid ice crystals and dust particles, will be seeking a suitable landing site for Philae. Last month as Rosetta closed in, its cameras revealed that C-G resembled a duck — two lobes, one big and the other small, connected by a “neck”.

“That was a bit of a surprise,” said Philippe Lamy of the Astrophysics Laboratory in Marseille.

“Several theories have already been aired to explain this shape, but the likeliest in my book is that it came from two bodies which fused while the Solar System was being formed.” The unexpected shape will limit the choice of landing site, Lamy said.

“You can reasonably argue that it will impose additional constraints.”

European Space Agency's comet-chasing probe Rosetta. Picture: ESA