Jimmy Westlake: Here comes Comet ISON
October 14, 2013
Steamboat Springs — Ready or not, here comes Comet ISON.
No one knows how brightly Comet ISON will shine after it swings around the sun Thanksgiving Day. Right now, it is a faint wisp of light in the pre-dawn sky, invisible to the unaided eye, but very close to the bright planet Mars and visible in backyard telescopes.
The behavior of a new comet always is tricky to predict. As comet-hunter David Levy says, "Comets are like cats; they have tails and do precisely what they want." Comet ISON is not brightening very much as it plummets toward the sun, so it seems pretty clear that the most optimistic predictions of a "comet of the century" won't come to pass. But that still leaves plenty of room for a "great comet." Here are three of the possible futures for Comet ISON.
Comet ISON might crumble and fizzle out before it ever reaches the sun in late November. That would be a great disappointment for all of us. Best estimates of the comet's size using the Hubble Space Telescope, however, are about 1 mile in diameter, and most comets that large can survive a close brush with the sun.
Comet ISON might break up as it rounds the sun or soon thereafter. Passing less than one solar diameter from the sun's fiery furnace Thanksgiving Day, the comet will be heated to a temperature of 5,000 degrees, hot enough to melt iron, let alone ice. Tidal forces of the sun pulling and stretching the sizzling snowball might result in a breakup, exposing its icy interior to the solar heat and releasing even more steamy gas into space. This could cause the comet to brighten exponentially before evaporating completely. It also might produce the best possible comet spectacle from here on Earth.
Finally, Comet ISON might hold together, execute its hairpin turn around the sun and swing out toward Earth in December. If it survives its death plunge, we still might be treated to a spectacular view as the comet heads back out into deep space. It passes closest to Earth on Dec. 26, at a distance of about 40 million miles.
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Comet ISON passed less than 7 million miles from the surface of the planet Mars on Oct. 1, much closer than it ever will come to Earth. Several of our spacecraft on or around Mars observed the comet at that time. It continues to hang out close to Mars in our earthly skies, making the faint comet easy to locate this month by using Mars as a guidepost.
Mars and the comet break the horizon here in Northwest Colorado at about 3 a.m., but it will take another hour or two for them to rise over the mountains and become easy to spot. Look due east at about 5 a.m. Mars and Comet ISON will appear about one-quarter of the way up toward the zenith. Comet ISON will be passing only a degree or two to the left of Mars this coming week, both close to the bright blue star Regulus in Leo the Lion. That's about the width of your thumb held out at arm's length.
On Friday, the comet will appear closest to Mars, about 1 degree to the left of the planet, the width of a pinky finger. You will need strong binoculars or a medium-sized telescope to see the wispy comet, but it will be well worth the effort. Then, you will be able to say that you saw Comet ISON when it was just a little guy, long before it became a famous household name around our Thanksgiving dinner tables.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy's astrophotography website at http://www.jwestlake.com.
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