More About Comets And Comet Elenin

Comet McNaught photographed this morning from Austria. The comet shows a blue-green, gaseous coma and a short tail to the west. Credit: Michael Jaeger

Comets have been in the news a great deal this year. Currently only one is bright enough to see in 6-inch and larger scopes – Comet C/2010 C1 McNaught. It’s sailing eastward through Pegasus the Winged Horse low in the eastern sky just before the start of dawn, and shines weakly at around magnitude 9.5.

This map shows Comet McNaught during its travels through Pegasus this month. It's a fuzzy 9th magnitude object. Created with Chris Marriott's SkyMap software

Interested amateur astronomers can get an idea of where to look using the map at right. For a more detailed map, go to the IAU Minor Planet Center’s Orbital Elements for Software Packages page. There you can download the latest positional information for your particular sky program and create your own locator maps.

Comet Elenin on April 2, 2011 is just a small, fuzzy object in this time exposure photo made through a 12-inch telescope. The stars are trailed because the photographer kept the scope centered on the moving comet. Credit: Bernhard Haeusler

Then there’s much-talked-about C/2010 X1 Elenin. Although there are many photos of it, only two people I know of have seen Elenin visually through a telescope, because it’s still extremely faint.

On the one hand, I’m delighted there’s so much interest in the comet, which may become bright enough to see with the naked eye this October. On the other, I’m amazed that some people suspect there’s some kind of cover-up about the comet’s identity or that its passage through the inner solar system could cause earthquakes and a shift in Earth’s poles.

We’re a superstitious people at heart. The ancient Romans would gladly have loaned an ear to our cometary fears. They shared the same feelings some 21st century humans  still have about these unexpected visitors from the distant realms of the solar system. People of long ago took comets as omens of change, mostly for the worse, because they came unexpectedly and no one really knew what they were. Thanks to hundreds of years of telescopic observation, orbital calculations and a handful spacecraft flybys, including the Stardust sample return mission, we have a darn good idea in 2011 of what comets are.

The orbit of a typical comet. Unlike planets, comet orbits are often very elongated or cigar-shaped. Many arrive from a great distance, swing around the sun and then return to the outer solar system. Credit: NASA

While future studies will undoubtedly teach us more, this we know. Comets are small, friable bodies ranging in size from about 1/4 mile to several tens of miles in diameter and composed of a mixture of ice,  dust and small bits of rock. When one travels through the inner solar system, the sun heats the comet, causing ice to vaporize and form a fuzzy envelope of gas and dust around it called a coma. The pressure of sunlight pushes some of the dust away from the coma to form a tail. Solar ultraviolet light excites some of the other gases to glow in the form of a second ‘ion’ tail.

Most meteor showers occur when the Earth’s orbit crosses that of a comet. Then we encounter the dusty debris left behind, particles of which burn up as flashes of light in our upper atmosphere. That’s a ton more information than was available to our ancestors.

Once a new comet is spotted, by professional sky surveys using automated telescopes, spacecraft or amateur comet hunters (yes, even amateurs still occasionally nab one before the professionals!), its orbit is determined through careful observation of its nightly track across the sky. Once we know a comet’s orbit, astronomers can then predict where the comet will be in the days, months and even years ahead. Orbits tell us things like how close a comet will get to the sun and Earth and how long before it returns for another visit. To date, over 4000 comets have been discovered.

Outline of the 110-mile diameter buried impact crater in the Yucatan Peninsula. Credit: NASA

No comet has been positively confirmed to have hit the Earth. Meteorites from asteroids have, and there’s no doubt that sometime in the past, comets have done the same, but it’s never been recorded in history. The Chicxulub crater in the Yucutan, widely believed to have led to or accelerated the extinction of the dinosaurs 65 million years ago, was created by a six-mile-diameter impactor. The jury’s still out on whether comet or asteroid. By the way, there’s nothing to prevent this from happening again sometime in the future. Earth doesn’t carry pack a magic, impact-deflecting umbrella.

Comet Biela and its breakaway companion comet in February 1846. Credit: E. Weiss from Bilderatlas der Sternenwelt

Being crumbly, comets sometimes break into two or more pieces when heated and gravitationally strained by the sun or large planets like Jupiter. Breakups can release lots of extra dust and gravel bits into the comet’s orbital path. If at some point, Earth intersects a shattered comet’s orbit, we experience a meteor shower.

Biela’s Comet, first spotted in 1772 and recognized as a periodic or returning comet by Wilhelm von Biela in 1826, gave 19th century observers a little surprise. It was seen again in 1832, but when it returned in 1845, Biela had split into two separate comets traveling side by side. Neither was seen during the 1859, 1865 and 1872 returns, however a spectacular meteor shower appeared in November 1872 from the area of the sky where the comet had been predicted to cross in September that year. Because it was known that Comet Biela’s orbit intersected Earth’s, it appears that the breakup of the comet led to the shower.

During the 1832 pass and later predicted returns, there were concerns Biela’s Comet or its coma would impact the Earth.  The only ‘terrible’ thing that happened was a nice series of meteor showers during the remainder of the 19th century.

An illustration from a Chilean newspaper playing upon fears of a collision between Biela's Comet and Earth in 1877. The words say "end of the world".

Depending upon where the Earth happens to be in its orbit when a comet drops by, our distance from it can vary from one return to the next. Only 3.2 million miles separated Earth from Biela in 1805. The closest- ever recorded approach of a comet was Lexell’s Comet in July 1770, when it zipped by Earth at a distance of only 1.37 million miles. No meteors were seen and no cometary tragedies were noted. More recently, Comet IRAS-Aracki-Alcock passed some 3 million miles from us in the spring of 1983. I was around for that one. Its nearness meant the fuzzy visitor moved through the sky with great speed. I watched it travel from one end of the heavens to the other in just three days. Telescopic views were incredible, with a bright central nucleus surrounded by a gigantic coma.

Let’s return again to Comet Elenin. At closest approach on October 17, the comet will be 21 million miles from our planet and 2.4 million miles above our orbit. As comets go, that’s sort of close but not unusual or exceptional. I only hope the Elenin will brighten up as hoped, so we can get a nice view in binoculars.

Sure, anything can happen, but the more fantastic the scenario, the less probable its likelihood. Elenin might split into two or three comets or see its orbit altered slightly due to outgassing of material – even that wouldn’t be unusual. In the meantime, I’ll put my money on orbital mechanics and past experience as guides for predicting its likely future behavior over some of the pseudo-scientific and patently untrue forecasts posted online in recent weeks.

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