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SYFY WIRE Bad Astronomy

A Comet and Two Planets…Including Home

By Phil Plait

For the next week or two, the bright comet called C/2011 L4 (Pan-STARRS) is gracing our skies. For Northern Hemisphere observers, look west shortly after sunset to spot the cosmic visitor; in fact, tonight the thin crescent Moon will be hanging nearby like a landmark (skymark?) to help you spot it.

Itâs been cloudy here in Boulder (it snowed all during sunset last night, totally destroying any chance of seeing the comet for me) but some places are better than others for viewing. One of the best is from the STEREO B spacecraft, one of two twin space probes in orbit around the Sun. The STEREO pair were launched in opposite directions to give us a view of the entire Sun, instead of just the part facing us at any given time, and have been returning fantastic data about the Sun since then.

STEREO B has moved far enough around the Sun that it now can see the comet in the same field of view, and the pictures itâs been sending back have been amazing. But this one literally made my jaw drop:

As you can see from the labels, thatâs the comet at the bottom. The Sun is just off the left hand edge of the frame, and all the odd fuzzy stuff is material blown off by the Sun in the solar wind (and gusts of wind making up solar storms). You can also see Mercury there, bright enough to overflow the cameraâs pixels, causing the light to bloom (creating those long vertical spikes).

But the coolest part of this is the Earth itself, hanging in space off to the right! STEREO B is more than a third of the way around the Sun as seen from Earth, so when it looks at the Sun itâs also looking back on us. From its position, 280 million kilometers away (175 million miles), it can see the Sun, Mercury, the comet, and the Earth all at a glance. Incredible.

That STEREO image may look a little funny to you; see how everything has a white side and a black side? No, this isnât from an old episode of Star Trek. Itâs because this is whatâs called a âdifference imageâ: Itâs literally one image subtracted from another. When you do that, it minimizes things that donât change, like stars, and accentuates things that do. If something doesnât change from one image to the next it gets subtracted away, but things that move leave that white/black feature. In this case, the planets and comet moved a tiny bit due to their own motion as well as that of STEREO B. The wind from the Sun did as well, but itâs a big, fuzzy feature, so you get that odd patchiness to it.

That image was taken on Mar. 10, when Pan-STARRS was closest to the Sun. On that same day STEREO B took this image of the comet:

You can clearly see the different tails of the comet. Multiple tails are common; one is made of dust, ground-up rock freed as the frozen ice on and in the comet warms up and sublimates, turning into a gas. The gas itself gets whacked by the solar wind, gets stripped of electrons, and glows like a neon sign, creating the ion tail. The dust tail tends to curve away from the comet, falling behind in the cometâs orbit, while the ion tail gets blown straight back by the particles in the solar wind which are moving far faster than the comet itself. Different ions (like nitrogen, sodium, and so on) behave differently, so you can have several ion tails as seen here.

STEREO Bâs view is amazing, but still, thereâs nothing like seeing it for yourself. Check Sky and Telescopeâs site for viewing info, or the one for Astronomy magazine. I hope you get a chance to see it!

Come to think of it, I hope I do too. And I'll be posting more amazing pictures of this comet soon, so Stay Tuned.

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