For the First Time, Humanity Has Landed Something on a Comet

Questions, and answers, about a union more than a decade in the making

European Space Agency
Update: We have, officially, now landed on a comet.
Wait, we're landing on a comet?
Which comet?
It's called 67P/Churyumov-Gerasimenko; it was discovered in 1969 (and named, as you may have guessed, for its discoverers). And it's small. Its nucleus—the solid part of a comet, sometimes called an "icy dirtball"—is only about 2.5 miles, or 4 kilometers, wide.
The comet is 311 million miles from Earth, and traveling through space at more than 34,000 miles an hour.
And what's actually doing the landing?
A robot named Philae—a lander about the size of a washing machine, and weighing 220 pounds. It's named after Philae Island in the Nile—the site of the discovery of the obelisk that was used, along with the Rosetta Stone, to decipher ancient Egyptian hieroglyphics.
So how do you land a robot on a comet that's hurtling through space?
It involves, in this case, a pair of spacecraft. The Rosetta craft, which the European Space Agency launched in 2004, has spent the past 10 years traveling to 67P. It has been orbiting the comet since this August. And, today, it's essentially dropping Philae onto the comet.
For more, here's a great (un-embeddable) animated explainer, from the European Space Agency.
Have we done something like this before?
No! This is a first. Other spacecraft have made contact with comets before, but only as crash-landings. (Take, for example, the aptly named Deep Impact probe, which made impact with a comet in 2005.) This is humanity's first attempt to soft-land on a comet—and actually landing means that Philae, if all goes according to plan, will be able to analyze the comet with the help of its scientific instruments.
Why land on a comet?
The short answer: for science. The comet is thought to hold material dating from the origin of the solar system, some 4.5 billion years ago.
How many other places have we landed on previously?
Only seven others. Soft-landing is really, really difficult—each landing (take the landing of the Curiosity rover on Mars) has been an individualized feat of engineering and ingenuity.
But doesn't a tiny comet have almost no gravity? How will Philae stay in place once it's landed?
Immediately at touchdown, Philae will fire two harpoons from its legs. Each of these has screws; they'll screw down three feet into the comet.
Has anything gone wrong in the mission so far?
A little bit, actually. Originally, a thruster on top of Philae was supposed to help push the lander into the surface of the comet; that system, ESA has said, isn't working.
How long will Philae be doing its thing on the comet?
The lander has solar panels that will recharge its battery until March—at which point the surface of the comet will become too hot for Philae to survive. "There is a fairly natural end to the mission," Rosetta's project manager has said.
Where are we landing on the comet?
A site named Agilkia (named, through an ESA contest, in honor of Agilkia Island). The site is on the "head" of the comet.
How was that site chosen?
The Rosetta has been, since August, scouting the comet for a good landing site. (In the process, it's been taking some amazing photos.) 67P has canyons and cliffs—which make it interesting to look at, but perilous to land on. Scanning the photos, mission scientists chose a site that balances minimally hazardous terrain with scientific potential.
I've heard something about the "seven hours of terror."
That's a reference to NASA's mission to Mars with its landing of the Curiosity rover. It's also a reference to the seven hours it will take Philae to get to the surface of the comet. It's also a reference to the fact that Philae, which has spent 10 years fixed to the side of Rosetta, can't be steered. Which means that, now that is has been released, it is on its own.

Can I watch the landing as it happens?

Definitely. In fact, you have a couple options. Here's the live feed from ESA. And if you prefer your comet news in comic form, XKCD is, awesomely, live-drawing the whole thing. Because the Internet.