Can’t Seem To Shake Off This Comet Dust

A fingernail moon pokes out from behind bare branches during twilight Thursday morning. Photo: Bob King

With our attention focused on Comet Hartley 2 yesterday, I forgot all about Thursday morning’s crescent moon. OK, I should have arisen earlier when the moon was even more striking in a darker sky, but the sight was refreshing nonetheless. I hope you saw it. Tonight the moon is new and will return as a waxing crescent in the evening sky next week.

Getting up for morning sky watching has been somewhat less painful than usual this month. With Daylight Saving Time now extending into the first week of November, our region will experience its latest sunrise in a long, long time tomorrow – 7:59 a.m. That’s about 10 minutes later than the usual latest sunrise, which occurs in late December. On December 29, the sun comes up about 7:51 a.m. Central Standard Time. If our legislators were to extend daylight time to the end of the year, that sunrise would instead be 8:51 a.m.! Heck, I’d get up every clear morning and poke around the sky before breakfast. One last note about time: don’t forget, this is the weekend we return to standard time. Set your clocks back an hour Saturday night and enjoy the extra snooze time Sunday.

This image montage shows comet Hartley 2 as NASA's EPOXI mission approached and flew under the comet. The images progress in time clockwise, starting at the top left. All comet photos: NASA/JPL-Caltech/UMD

When it’s all said and done, some 120,000 photos will be taken of Comet Hartley 2 by the Epoxi mission, but those first five are simply amazing. When I observed the comet this morning and saw its big, glowing coma, all I could think about were those spectacular jets spewing dust and vapor. As the nucleus rotates end over end in the span of about 18 hours, the jets charge the space around the comet’s nucleus with a temporary atmosphere. I also felt appreciation for the efforts of the people who engineered the Deep Impact probe and got it to its second comet after years of planning and orbital maneuvering. Thanks to the ingenuity of our fellow humans, we can understand and participate in one of nature’s grandest if ephemeral creations – a comet.

This enhanced image, one of the closest taken of comet Hartley 2 by NASA's EPOXI mission, shows jets and where they originate from the surface. There are jets outgassing from the sunward side, the night side, and along the terminator -- the line between the two sides.

Check those geysers out in the photo above. You might wonder if all those gassy jets can propel the comet off its path. After all, according to Isaac Newton, every action has an equal and opposite reaction. While this is certainly true, the gas escaping from a comet has so little mass in comparison to the solid mass of the comet itself, the nucleus is affected very little. Over time however, certain comets with large, active jets do show shifts in their orbits, in part because of the “push-back” effect or what astronomers call non-gravitational forces. These accumulate to change the period of the comet’s orbit or how long it takes to go around the sun. It was first noticed in Comet Encke, which at 3.3 years has the shortest period known of any comet. After many returns, comets eventually degrade and lose most of their ice. They come to resemble dark-crusted asteroids and look like tail-less faint stars, their glory days now over.

This montage shows the only five comets imaged up close with the EPOXI mission spacecraft. The comets vary in shape and size. Comet Hartley 2 is by far the smallest and the most active of small comets. This jet activity can be seen extending from the comet's surface and into its outer shell of gas and dust, or coma

As twilight swelled this morning, I pointed the telescope at newly-discovered Comet V1 (Ikeya-Murakami) in Virgo. It was much smaller than Hartley 2 but every bit as charming in that misty way that comets are. Now that astronomers have calculated a preliminary orbit, I’ll roll out a chart for tomorrow’s blog. Butterscotch-hued Saturn is now up high enough by 6:30 a.m. to find and observe easily in a small telescope. The rings are tipped open considerably more than last year and will guarantee to elicit oohs and aahs if you’re willing to part with a little sleep.