Comet Hartley 2 and the Amazing Tail!

"I have just gone over my comet computations again, and it is humiliating to perceive how very little more I know than I did seven years ago when I first did this kind of work." -Maria Mitchell, Astronomer

If you were around back in 1997, perhaps you remembered seeing Comet Hartley 2 back then, as it brightened and came close to Earth, as captured by NASA.

Unlike the other comets you know of, like Halley's Comet, Hartley 2 returns close to Earth every 6.5 years, while most comets take nearly a century!

Comet Hartley 2 doesn't look like it originates from the Kuiper Belt. Rather, it looks like it's some sort of asteroid, flung close to the inner solar system by, perhaps, a chance encounter with Jupiter!

In fact, if you look up in the night sky now (or rather, in the early morning sky for my Northern Hemisphere friends), you might see something like this:

And where would you look to find it? Well, over the last few days of October, it passed through the constellation of Gemini, and you can pretty easily extrapolate where to look next from this picture.

(As we pass into late November, those of you in the Southern Hemisphere will get a better view.)

Well, NASA's EPOXI mission just yesterday passed within 700 km of this comet, and what it found was unprecedented! You see, we've gotten pretty close to other comets before, with some exciting results.

After all, as just a fuzzy streak in space, it's pretty difficult to get a look at what's going on with the nucleus of these comets!

Unfortunately, none of these comets were doing, at the time, what comets are best known for: emitting a giant tail of gas and dust!

But here they are, the stunning images of Comet Hartley 2!

Thanks to APOD for putting these together.

This potato-shaped comet is tumbling through space, and at only 1.25 miles (2 km) in diameter, it's the smallest comet we've ever imaged up close!

But it's also emitting tiny amounts of debris, making its tail, and it's doing so from two different spots!

But what's most remarkable about this is what you can't see with your naked eye. Unlike other comets, with their tails made mostly of ice, melting into water, the tail of Comet Hartley is mostly rocky dust and carbon dioxide, known commonly as dry ice!

Why is this comet so different than practically all of the others?

Because it's not a traditional comet; it's an asteroid that simply got too close to the Sun! On Earth, if you're a small rock, and you want to escape from the Earth's gravity, you need to move at about 25,000 miles per hour (about 11,000 meters per second).

But on Comet Hartley 2? It's so small and light, that if you move at a mere 1.4 miles per hour (or just 0.6 meters per second), you'll escape from the surface of the comet, and leap off into interplanetary space! That's slow enough that hitting a dry ice molecule with a beam of sunlight will totally give it enough energy to do just that!

And that's why this comet has a tail, even being as unusual as it is! So enjoy it while we can see it, because it's shrinking and losing mass all the time, but it's bright and brilliant right now! Like all things, Comet Hartley 2, won't be around forever. So enjoy the light from this tiny rock, and thanks to Jupiter for sending it our way!