Just an hour ago as I write this, the NASA spacecraft EPOXI passed just 700 km from the nucleus of comet Hartley 2! The flyby was successful, and it took incredible images of the comet's solid heart:
Wow! These images are in order of approach (left to right, top to bottom), as EPOXI flew by. We knew from ground-based radar that the nucleus wasn't round, but these pictures clearly show it to be shaped like a peanut. That's not too surprising; this shape is common in asteroids and comet nuclei. But what detail! You can see the surface is irregular and contoured. There's a groove of some sort on the left side, and what might be an impact crater or pit on the very left tip.
And those bright streamers of light? Those are jets of gas shooting away from the comet, formed when frozen material on the comet surface gets heated by the Sun, expands, and shoots away!
Amazing. And very lovely.
Here's the fourth image a bit bigger:
Looking more carefully, weird things pop up. The round ends of the nucleus are bumpy and rough, indicating material is loosely aggregated there. But the waist looks smooth! There also appear to be bands of material circling the round ends separating the waist from the tips. You can also see what looks like a large boulder or spire of ice on the right side of the smooth section, poking up into the Sun. That's a testament to the weak gravity of the comet; the nucleus is only about 2.2 km (1.4 miles) across, so there's not much mass there. That piece jutting up is probably about a hundred or so meters high.
[UPDATE to note that there will be a live press conference at 20:00 UT (16:00 Eastern US time) where scientists will discuss these images. You can watch it live on NASA TV and on JPL's Ustream channel. Also, Emily Lakdawalla has written her thoughts on the comet on her blog, too.]
And mind you, these are not the highest-resolution images! These are medium-res; even better ones will be coming soon. Trust me, as soon as they're available I'll have them here. I want to add that this part of the mission is a bonus; the mission was extended after the Deep Impact part back in 2005. Renamed EPOXI, it has gone on to do some amazing work in characterizing how we look for extrasolar planets. It was a very smart decision to continue this mission, and now we have these images and data, a whole new comet to investigate and understand.
Congratulations to the EPOXI team for a successful and amazing cometary encounter!