Excerpt from  theglobeandmail.com
By Ivan Semeniuk

Half a billion kilometres from Earth and 10 years into its remarkable journey, a small robot is about to plunge into space history.

Pending a final green light from mission controllers on Tuesday night, the robot – nicknamed Philae (fee-lay) – will detach from its mother ship and try to hook itself onto one of the most challenging and mysterious objects in the solar system.

It’s a high-risk manoeuvre with plenty of unknowns. But if it works, then the probe will be able to show us what no one has ever experienced: what it’s like to stand on the surface of a comet.

“Comets are new territory,” said Ralf Gellert, a at the University of Guelph. “There could be some big surprises.”

Prof. Gellert should know. Fifteen years ago, he helped build one of the instruments on the dishwasher-size lander that will reveal the comet’s composition. No such measurement has been made before. Even designing how the instrument should was fraught with challenges since there was so little known about what kind of surface the lander might find itself on.

“Is it an ice ball with rock and trace metals, or a rock ball with ice on it … or ice below the surface? We didn’t know,” he said.
And scientists still don’t.

When the European Space Agency launched the Rosetta mission in 2004, the mission’s target – Comet Churyumov-Gerasimenko – was little more than a fuzzy blip in astronomers’ telescopes. But Rosetta just arrived in August and it’s been in orbit around the comet since then.

What was assumed to be a single, homogeneous lump of ice and rock has out to be a bizarre-looking object in two parts, arranged a bit like the head and body of a rubber duck. By October, scientists had zeroed in on the head portion, which is four kilometres across at its widest point, and settled on a landing site.

Remote sensing from Rosetta suggest that the comet is quite porous, with a surface that is as black as coal and somewhat warmer than expected. In other words, Philae will probably not be landing on skating-rink-hard ice. Yet, whether the surface will be crusty like a roadside snowbank, fluffy like cigarette ash, or something else entirely is anyone’s guess.

And while scientists and engineers say they’ve done everything they can think of to maximize the lander’s chance of success, they acknowledge it’s entirely possible that Philae will encounter something it can’t handle and smash to bits or sink into oblivion.

Yet the landing is more than a daring jaunt to see what has never been seen before. Comets are also among the most primitive bodies in the solar system. Each one is an amalgam of ice and rock that has been around since Earth and its sister planets formed billions of years ago. In a sense, comets are the leftovers of that process – primordial fossils from the birth of the solar system.

The instrument Prof. Gellert worked on, known as the alpha particle X-ray spectrometer (APXS), will help illuminate this early period by making precise measurements of the comet’s elemental ingredients.

It is carried on a robot arm that will a radioactive source near the comet’s surface. The particles and X-rays the comet material gives off as a of this exposure will provide detailed information about what chemical the comet contains. This will be augmented by another experiment designed to drill and extract a comet sample for analysis inside the lander.

Prof. Gellert, who has also been closely involved in NASA’s Mars rover missions, said Rosetta’s long timeline and the many unknowns related to the comet makes this ’s landing a trickier proposition than landing on Mars – but also a tremendously one.

“I think it’s a matter of hope for the best and see what happens.”