Update 4, Nov. 12, 2014 at 16:05 UTC: T O U C H D O W N!!! At 16:02 UTC (11:02 Eastern) the European Space Agency's Philae lander made contact with the comet 67P/Churyumov-Gerasimenko, launching its harpoons into the icy surface and attaching itself. For the first time in human history, we have reached out and grasped a comet. We expect close-up images of the surface from Philae soon, so stay tuned.
Update 5, Nov. 12, 2014 at 17:00 UTC: Uh-oh. It appears that the harpoons did NOT fire, and Philae is not currently moored to the comet. It's not clear what's going on, and the data are confusing. We should know more soon.
Update 6, Nov. 12, 2014 at 17:50 UTC: Some good news: The first picture of the comet from the Philae lander has been returned to Earth! Taken by the ROLIS camera, the landing site can be seen directly below, on the "top" of the smaller lobe of the two-lobed comet (you can see part of the bigger lobe above and to the right, as well as part of the lander hardware). Although we're still not sure what's going on with Philae as far as attaching itself to the comet, it's great that we're getting telemetry and the cameras at least are working.
Update 7, Nov. 12, 2014 at 18:20 UTC: Another amazing picture from Philae, this time taken moments before touchdown. The scale is unknown (I'm awaiting official word), but you can see boulders and rocks dotting the more-or-less smooth surface. Images like this may prove crucial in understanding what the lander is doing now. There's still no word on the harpoons and the state of the lander.
At approximately 16:03 UTC (11:03 Eastern) Wednesday, Nov. 12, for the first time in human history, a spacecraft is expected to physically land on a comet.
As I write these words, less than a day before the event, the lander Philae has not yet separated from the Rosetta probe. As you read these words—and assuming no roadblocks messed things up—that should have occurred on or about 09:03 UTC (04:03 Eastern time). Philae will then slowly approach the comet 67P/Churyumov-Gerasimenko, taking data and images along the way, and at the right moment will fire harpoons into the comet's surface and then reel itself down.
Update, Nov. 12, 2014 at 14:15 UTC: Philae separated on time! It is now approaching the comet. Everything looks good, except for one small problem: A gas thruster on the top of the lander, designed to absorb some of the rebound after the lander (softly!) impacts the surface, isn't working. Philae will have to rely on its harpoons to make sure it sticks its landing.
Update 2, Nov. 12, 2014 at 14:30 UTC: Philae has released its first image! It shows the Rosetta spacecraft, to which its been attached for 10 years, with a bright sunstrike—a reflection of sunlight on the camera—in the middle. This shows the camera is working, and ready for its historic rendezvous which is on schedule for just after 16:00 UTC (11:00 Eastern time) today!
Update 3, Nov. 12, 2014 at 15:15 UTC: The incredible image below is of the Philae lander shortly after separation, taken by the high-res OSIRIS camera on Rosetta. It shows that the landing legs and instruments have deployed, which is great news. Remember, Philae has been essentially dormant for nearly all of the decade it's been on the way to the comet, so it's very nice to see these mechanical parts deployed. My thanks to Emily Lakdawalla for posting it.
These events happen over night for most of the United States, making it difficult for me to cover everything live. So first, I suggest following my friends Emily Lakdawalla and Karl Battams on Twitter; they are at Rosetta HQ and tweeting everything as they hear it. Emily has posted a very useful timeline of events as well.
Second, I'm embedding a live feed from the European Space Agency. It's not clear when new images will be released, but if you're up you might get a chance to see them. I plan on getting up early so I can find out what's what, and when I do I'll update this post. Think of this as a placeholder until then. But stay tuned! I'll have images as soon as I can get them.
And remember, the times listed here are approximate, so you might want to tune in early so you don't miss anything!