UPDATE (12:59 p.m. EST): Philae has returned a photo from the descent stage, about 3 kilometers (1.86 miles) from the surface! See the photo here: http://www.esa.int/spaceinimages/Images/2014/11/ROLIS_descent_image
UPDATE (12:52 p.m. EST): No pictures from the surface yet. Also, according to ESA Operations, it looks like mission control was mistaken in announcing that Philae's anchor harpoons had fired upon landing. Now they are investigating why the harpoons did not launch. However, the lander is safe and still resting on the surface of the comet.
UPDATE (11:43 a.m. EST): The excellent host of the ESA live stream predicts the first images from the Philae lander will be arriving in about an hour, so watch out for them at about 12:40 p.m. EST!
UPDATE (11:10 a.m. EST): The landing is a success! Philae has fired harpoons to anchor itself to the comet, and it is communicating with ESA mission control!
So far, human vehicles have only landed safely on a handful of surfaces in the solar system: the moon, Venus, Mars, Saturn's moon Titan and two asteroids. Now we're all getting ready to add another object to the list. This morning, the European Space Agency (ESA) is performing a soft landing on a comet for the first time in the history of human space exploration. Watch the ESA live stream of the landing here: http://new.livestream.com/esa/cometlanding
As of about 10:20 a.m. EST (when I'm writing this), the unmanned Rosetta mission has already performed a successful separation maneuver between the main spacecraft and its landing vehicle Philae. Right now, Philae is scheduled to move in for touchdown on the dusty comet 67P/Churyumov-Gerasimenko at about 16:00 GMT, which means about 11 a.m. EST. If you watch the live stream, you should be able to see the first images coming from the landing and share in the dusty cosmic glory of Philae, Rosetta and the ESA team.
Supplements, as I add them:
But we've already landed on two asteroids! How is a comet any different from an asteroid? The short answer is that an asteroid is a generally solid, rocky, metallic body, while a comet is a "dirty snowball," or a mixture of dust, ice, frozen gases and rocky pieces. But the composition of individual comets may vary. For example, it might make more sense to call this particular comet (67P) something like a "frozen dustball." According to an ESA statement, "Comet 67P is classed as a dusty comet, with a dust to gas emission ratio of approximately 2:1."
I don't remember this launch. How long has the Rosetta mission been in space? This one has been a long time coming. The Rosetta vehicle launched from Earth on March 2, 2004. It's been in space for more than a decade. The spacecraft finally had its rendezvous with 67P this past August, and it has been flying along since then as the comet orbits the sun.
How big is the comet 67P/Churyumov-Gerasimenko? The most recent mass estimation of 67P that I can find is this ESA blog post, which puts the comet at 10 trillion kilograms, which is about 11 billion tons. As for the volume, I can't do much better than this ESA illustration, putting 67P's estimated 4000 meters next to earthly landmarks for comparison:
However, I think it's important to note that these estimates both came from before Rosetta approached 67P up close, and both could be revised. We already know, for example, that 67P's shape is not exactly oblong-potato-ish, like it appears in the picture above. Instead, it is a sort of double-lobed L-shape, like you can see in the image at the top of this blog post.