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.
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.
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.
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?
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?
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.
