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 professor of physics 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 direct measurement has been made before. Even designing how the instrument should work 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 turned 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 two-and-a-half kilometres across at its widest point, and settled on a landing site.
Remote sensing data 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.
"We will need to be a bit lucky," said flight director Andrea Accomazzo during a status briefing in Germany on Monday at the European Space Operations Centre.
Until now, comet missions have been remote and fleeting affairs. Among the first was a flyby of Comet Halley in 1986 by an earlier European probe called Giotto. Its success generated the momentum for Rosetta.
In the mid-2000s, NASA managed to scoop up some dust from a comet's tail and even fire a massive projectile into another comet's nucleus to see what kind of material it kicked up. But Rosetta marks the first time a comet has been orbited and the first attempted landing.
The results promise to revolutionize the study of comets – particularly if the orbiter and lander can together witness the comet "turn on" as it approaches its closest point to the sun in mid-2015 and starts to vaporize with increasing vigour.
"There are still massive debates" on the detailed structure and behaviour of comets, said Matt Taylor, project scientist for the mission. "This is what Rosetta is there to do, to see how a comet works."
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 place a radioactive source near the comet's surface. The particles and X-rays the comet material gives off as a result of this exposure will provide detailed information about what chemical elements 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 week's landing a trickier proposition than landing on Mars – but also a tremendously exciting one.
"I think it's a matter of hope for the best and see what happens."