A pair of astronomers monitoring an all-sky camera got a surprise (PDF) when they checked data from last February: a half dozen meteors all seemed to come from the same spot in the sky, indicating they all had a common origin. After doing some calculations, they found that they probably come from a parent comet with an orbit that’s at least 53 years long. Moreover, the orbit of this comet crosses that of the Earth, meaning we may have a close encounter with this object sometime in the future.

And because I can sense the oncoming panic on the web over this news, let me break it down for you. I’ll give you the science (which is cool), how we know this unseen comet may be potentially, um, interesting, then the reason you don’t need to run around in circles screaming (spoiler: it’s rude to others nearby, but also unnecessary).

But just to be up front: should you panic? Nope. We know there are objects out there that could hit us in the future sometime. This comet is in many ways just another one. As I’ll point out below, we pass through lots of meteor streams, so there are plenty of other comets that could hit us. I know, I know, that doesn’t sound reassuring, but think about it: how often is the Earth hit by a comet? Not very often, despite having a few on the list of Potentially Hazardous Objects. So having one more we know about out there isn’t great, but in reality doesn’t really make things any worse for us.

Meatier showers

That picture above is one of the meteors in question. You can see the streak as the tiny bit of rock (probably the size of a grain of sand) glowing as it rammed through the Earth’s atmosphere at about 35 km/sec (22 miles/sec, or nearly 80,000 mph). If you go out on any dark night, you’re bound to see the random meteor or five. But meteor showers are when we see lots of them in a short time, and they occur when the Earth passes through the dust debris left behind by a comet.

Most comets are dirty snowballs: dust, pebbles, and boulders held together by ice (water ice, but also frozen carbon dioxide and other things we normally think of as gases). This makes comets the litter bugs of the solar system, shedding material when the Sun warms them up and turns the ice into gas. The vapor blows off, and the looser material forms a ribbon or stream that stays more or less along the same orbit as the comet.

If the path of the comet intersects the orbit of the Earth, we plow through that material at the same time every year. (more…)

It’s not known how many comets orbit the Sun in our solar system, but the number may be in the trillions. They spend a long, long time in the deep reaches of the outer solar system, only occasionally plunging toward us. If they pass near a planet their orbit can be changed, and some wind up on paths that take them so close to the Sun they burn up. These are called sungrazers.

That is what NASA’s Solar Dynamic Observatory saw on the evening of July 5/6, 2011. This has been seen many times before, but this is the first time one has been seen streaking directly across the Sun’s face!

Here’s the video (I recommend watching it in HD — at least 720p –to make the comet easier to spot):

Did you see it? The whole event took about 20 minutes to unfold, and is seen here highly compressed in time. This is no perspective effect; that comet really was just above the Sun’s surface, and most likely impacted the Sun or disintegrated from the heat. Astronomers are even now going over the data from the event to see if they can determine the comet’s fate.

On the NASA Sun-Earth news site is more information, and a very cool video from SOHO showing the comet’s approach to the Sun.

Credit: NASA/SDO

Rosetta is an amazing probe launched by the European Space Agency. In 2014 it will go into orbit around the comet 67P/Churyumov-Gerasimenko, and actually deploy a lander to sample the surface!

That rendezvous is still years away, but the target is now in sight: Rosetta has returned its first image of the comet.

Oh, very cool! The top image is the wide angle shot, showing a densely-populated star field toward the center of our galaxy; from Rosetta that’s the direction to the comet. The second image zooms in a bit, and you can see some distant stars and nebulosity. The bottom one has been processed to remove the stars, and the nucleus of Churyumov-Gerasimenko stands out.

Note that this image was taken when Rosetta was still 163 million kilometers (100 million miles) from the comet — that’s more than the distance from the Earth to the Sun! That’s why it took a total of 13 hours of exposure time to see the comet in these images; it’s still extremely faint from that great distance.

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A philosopher once asked, "Are we human because we gaze at the stars, or do we gaze at them because we are human?" Pointless, really… "Do the stars gaze back?" Now that’s a question.
-Narrator, "Stardust"

Late last night, the NASA mission Stardust flew within 178 km (110 miles) of the nucleus of the comet Tempel 1, seeing it up close for the first time since July 2005! Here’s one of the better images from closest approach:

[Click to embiggen.]

Wow! The whole flyby sequence has been posted on NASA’s Stardust site (and Emily Lakdawalla at The Planetary Society blog has created a nifty animation of it too).

Ian O’Neill, from Discovery News, posted a nice animation of it as well on YouTube:

To give you an idea of what you’re seeing here, the comet is roughly 7.6 x 4.9 kilometers (4.7 x 3.0 miles) in size.

So, why did NASA fly Stardust past this comet? Ah, set the way-back machine for 5.5 years ago…
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Use the thumbnails and arrows to browse the images, and click on the images themselves to go through to blog posts with more details and descriptions.

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NASA has just released new results and images from the EPOXI spacecraft’s visit to the comet Hartley 2 from November 4… and like the previous ones, these are absolutely stunning jaw-droppers. What scientists have found is that the comet’s solid nucleus is sitting in the middle of a veritable snowstorm!

Wow! Most of those dots are not stars: they are actual snowballs, frozen matter that has been ejected by the comet itself! They range in size from a few centimeters to a few dozen across, so they really are about the size of snowballs you’d use in a snowball fight… or to make a snowman. But I wouldn’t recommend it: a lot of that material is not frozen water, it’s actually frozen carbon dioxide, or dry ice.

As I wrote a few days ago, the comet nucleus — the solid part of the comet at its heart — is only a couple of kilometers across, and composed of rock, water ice, and dry ice. As the nucleus gets close to the Sun, the dry ice turns directly into a gas and spews out of vents in the surface. These plumes of material shoot out, enveloping the nucleus to become the fuzzy head of the comet, and stream away to form the tail.

But we’ve never been close enough and had good enough images to see just what these plumes look like in detail… until now. It’s not just gas, but actual chunks of ice flying out from the nucleus! The comet, for all intents and purposes, is having its own cosmic snowball fight. As you can see in the cleaned picture on the right (click to ensublimate) the comet is sitting in a cloud of thousands, millions of these snowflakes.

How awesome is this?

In fact, so much material is blowing out from the comet that the nucleus is able to cast a shadow in space:

Oh my. How lovely.

But there’s far more than just gorgeous pictures. (more…)

The comet Hartley 2 has come and gone, and the NASA mission EPOXI is also moving on after an exceedingly close flyby of the comet’s solid nucleus. The pictures we got were fantastic and beautiful… but their real power comes from coupling them with spectra.

In the picture above — an enhanced version of one of the images taken during the space probe’s flyby — you can see fan-like emission coming from the comet’s nucleus. These are jets; sprays of material coming out of the nucleus. Comets are made of rock and ice, and when the comet nears the Sun, the heat can turn that ice directly from a solid to a gas. This gas then shoots out from pockets on the nucleus, creating these jets. The EPOXI team (including my old boss, Don Lindler!) made a fantastic animation from a series of observations showing these jets in action.

But what are these ices made of? Lots of things we normally think of as liquids or gas (water, ammonia, carbon dioxide, and so on) exist in comets. In many comets, we see lots of water, and in fact the Swedish satellite Odin detected about 200 kg/sec (440 pounds per sec) of water coming off Hartley 2! So is water powering these jets?

We can find out… using spectra. By breaking up the light from an object into its component colors, we can learn all manners of things including what it’s made of. EPOXI did just that with the jets streaming from Hartley 2, and while it did find water, amazingly, it found a lot more carbon dioxide!
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