Reprinted from Meridian Magazine (7 Nov 2007).
©2007 by John P. Pratt. All rights Reserved.
This week you can see an amazing, bright comet which baffles astronomers.
Comet Holmes, which is totally puzzling to astronomers, is currently visible to the unaided eye. Sky and Telescope opens their article saying, "Amateur astronomers the world over have been stunned and amazed by the weirdest new object to appear in the sky in memory."[1] It has the potential to cause comet theory to be rewritten. And you can see it even from most cities. A short drive from the city to darker skies might be worth the trip. Fortunately the moon will not be in the sky for the next week during which the comet should remain visible in the northeast sky after sunset. There are now plenty of pictures of it on the internet.
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Standard comet theory is the "dirty iceberg" model, which claims that comets are conglomerations of rock and ice that orbit the sun. They are supposed to be extremely small, only a few miles across. The theory is that when they get near the sun, some of the ice evaporates, forming both gas and dust tails which both point away from the sun, being blown that direction by particles emanating from the sun. This tail can be millions of miles long and can stream across the sky. Thus, the tail is extremely tenuous. This is all supposed to happen in a very orderly manner, with the comet gradually getting brighter as it approaches the sun in its orbit, and then gradually dimming.
But Comet Holmes breaks most of those rules. It is not a new comet, but was discovered in 1892. It has a period of 6.9 years and is at a distance of about twice as far from the sun as is the earth. That puts it in the asteroid belt between Mars and Jupiter. Most comets are only bright when they are much nearer to the sun, and most come from beyond the distant planet Pluto. This comet is normally so dim it couldn't even be found at all from 1906 to 1964.[4] It would probably be considered to be an asteroid (which are huge chunks of rock) except for the fact that the year it was discovered it looked like a comet when it flared up in a somewhat less dramatic but similar event as is now occurring.
On Oct 24, 2007, it rocketed up in brightness by almost a factor of 1,000,000 in one day! Astronomers use "magnitude" to describe the brightness of stars. The brightest are called first magnitude, and those in the Big Dipper are called second magnitude, being a little dimmer. Five magnitudes is a factor of 100 in brightness. Comet Holmes went from 17th magnitude to nearly 2nd magnitude in 24 hours. Fifteen magnitudes is an increase of 100 x 100 x 100 = 1,000,000 times brighter. And it is now a huge spherical glowing ball of what appears to be mostly dust. Almost certainly something exploded. One astronomer said, "It is as if the earth blew off its crust" which he estimated to be about 1% of the total comet mass. He also referred to viewing this comet as a "once in a lifetime" experience.[5]
The size of the visible "coma," the bright luminescent ball around the head of a comet, is huge. Last night, Sun 4 Nov 2007, it appeared in my 10-inch mirror telescope to be about 10 minutes of arc in diameter, which is a third of the diameter of the full moon. One article reported that it is now more than twice as far from the earth as is the sun.[6] Appearing to be that size at that distance means that the visible sphere is now more than half a million miles in diameter![7] That is quite an increase over the estimate of two miles for the rock/ice head. Starting with a 2-mile diameter rock which ejected 1% of its mass, that would mean that the dust ball now reflecting all that sunlight to us has a density of less than one 2,000th of an ounce per cubic mile. That would be amazing indeed.
To me it appears that this comet, and perhaps most comets, are much larger than previously thought. Comet Holmes is in the asteroid belt area, and would almost certainly be called an asteroid it if hadn't also flared up in a similar way in 1892, allowing its discovery as a comet. Asteroids are known to be much larger than comets. Ceres, the largest, rivals our Moon in size. The real question for comets is to explain exactly where their brightness comes from as they approach the sun, and in this case, to explain how comet-like explosions of such magnitude could occur when the comet is so far from the sun. There is still much to be understood in classical astronomy. Something inside this supposedly dead dirty iceberg exploded in an amazing fashion. This discovery seems to blur the distinction between asteroids and comets. It appears that asteroids may not all be as dead as we had thought.
You can most likely see the comet for at least another week. There is a good map of where to find it in a Sky and Telescope article.[8] Don't miss this opportunity to see a truly marvelous wonder of God.