I’ve been meaning to let y’all know about this: a while back I wrote how Emily Lakdawalla of the Planetary Society blog put together a scaled asteroid/comet size comparison chart. Well, she’s updated it to include the nucleus of Hartley 2, visited by the EPOXI spacecraft last month.

The arrow points to the new addition of Hartley 2. It’s tiny! Well, compared to Lutetia, an asteroid well over 100 km across. Mind you, this chart shows every asteroid and comet we’ve ever visited and photographed with spacecraft! That’s 14 in total; I don’t know whether to be amazed that it’s that many, or sad that it isn’t more. Maybe a little of both. But Dawn will visit the asteroids Ceres and Vesta starting next year. More comets are coming up too, which means more incredible pictures and important science is coming up in the near future, too. And that’s definitely good news.

Here are some other images of the comet Hartley 2. Use the thumbnails and arrows to browse, and click on the images to go through to blog posts with more details and descriptions.

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Using a giant radio telescope like a cop’s radar gun, astronomers have made some pretty cool images of the nucleus of the comet Hartley 2:

Hartley 2 is a comet that is currently very close to the Earth as these things go: last week it passed us at a distance of about 18 million km (11 million miles). Astronomers took advantage of the close pass to ping the comet with radar pulses. By timing exactly how long it took the pulses to go from the telescope to the comet and back to Earth, they can create a map of the comet’s shape and other characteristics — something like how dolphins and bats use echolocation to map their surroundings… though, as Emily Lakdawalla at The Planetary Society Blog explains, it’s a bit more complicated.

From the images, it looks like the nucleus — the solid, central part of a comet — is highly elongated, about 2.2 km (1.4 miles) in length, and rotates once every 18 hours. We’ve only seen a handful of comets up close, and in general the nuclei are potato-shaped, so this one fits that description. The image has a scale of about 75 meters per pixel.

These observations were made to help out the EPOXI space mission, which will pass just 700 km (420 miles) from the nucleus of Hartley 2 on November 4. That means we’ll be getting some really cool close-up images and data from the comet very soon! Stay Tuned.

Emily Lakdawalla — scientist, blogger, and all around cool chick — has just posted a totally awesome scale diagram comparing every asteroid and comet visited by spacecraft. It features pictures of all the rocks, each of which she has carefully resized so you can see just how big they are relative to each other:

Whoa. Look how big Lutetia, just visited by Rosetta, is compared to everything else! And yet, at 130 km across, it’s a dot compared to our Moon. In fact, you could smash together all the known asteroids in the main belt between Mars and Jupiter and they’d be far smaller than our rocky satellite.

Still, small doesn’t mean "uninteresting". These rocks in Emily’s diagram are all fascinating beasts, and the more we learn about them the more compelling they become. And there’s more to come, with the Dawn mission about to see the big asteroids Vesta and Ceres up close… and go read Emily’s blog about this to see how they’d fit on the diagram (hint, they don’t, and by a long shot). You’ll also find a much larger version of the diagram there, and you really, really should look at it. Wow.

I love me some comets.

I’ve seen quite a few in my time. Some were faint smudges in a big telescope’s eyepiece, some seen only in distant spacecraft images, and some so bright they were obvious and awesome to my naked eye.

They used to be considered harbingers, omens up for interpretation by mystics and people looking for reasons things happened the way they do. In reality, comets are just a class of objects in our solar system along with planets, asteroids, dust, and one biggish star.

Hmm. Did I say "just"? That’s unfair. They are gorgeous, interesting objects, worthy of study. And 100 years ago today — April 20, 1910 — we got a pretty good look at the most famous of them all, Comet Halley, as it passed the Earth at a distance of just 23 million km (14 million miles). It got so bright that it was obvious even when seen from cities. As geometry would have it, the Earth even passed through the comet’s tail, sparking fears of widespread death (cyanogen was detected in the comet, making people think it would poison them). It was the talk of the planet, featured in magazines and papers across the globe. For your history enjoyment, here is one of those articles from the 1910, transcribed by James Brooks. It gives a great flavor of the times.

To celebrate this remarkable centennial anniversary, I have put together Ten Things You Don’t Know About Comets. I imagine some readers will know some of these, and some will know all ten, but if you do you can still enjoy the pretty pictures — and make sure you click on them to embiggen ‘em. And if you like this, I have several others, too (Ten Things You Don’t Know About… the Earth, Black Holes, Hubble, the Sun, Pluto, and the Milky Way), so check ‘em all out and see how many things you don’t know.

The European Space Agency probe Rosetta is on its way to comet 67/P Churyumov-Gerasimenko (by way of asteroid 21 Lutetia next July), where it will arrive in May of 2014. It will be dropping a lander — the first ever attempted on a comet — and our knowledge of these fuzzy visitors will increase enormously.

But getting there is tough, and involves swinging by the Earth three times and Mars once. The final gravity assist will occur on November 13, with closest approach at 08:45 CET (over, roughly, the island of Java) when it’ll be moving past us at 13.3 km/sec (almost 30,000 mph). While it’s passing us by it will observe both the Earth and Moon, doing as much science as it can before heading out into deep space. Specifically, it will add its sensors to those already studying water on the Moon, as well as aurorae on Earth.

You can follow the action on the Rosetta blog. In fact, just the other day they posted this awesome shot of the Moon from Rosetta:

That was taken form a distance of 4.3 million kilometers (2.5 million miles), ten times the distance of the Moon from the Earth. The images as it gets closer will be even cooler.

So stay tuned! This is a very exciting mission, especially next year when it passes Lutetia! I can never see enough closeup pictures of asteroids.

Spacecraft image credit: ESA, image by AOES Medialab