View Full Version : Crashing comets in perspective

2003-May-25, 12:22 AM
How dangerous are comets that crash into the Sun? Here's a look at the relative sizes of comets and some other Solar System objects.

Comets seem to come from two sources: the Oort Cloud and the Kuiper Belt.

"In 1950 Jan Oort noticed that no comet has been observed with an orbit that indicates that it came from interstellar space, there is a strong tendency for aphelia of long period comet orbits to lie at a distance of about 50,000 AU, and there is no preferential direction from which comets come. From this he proposed that comets reside in a vast cloud at the outer reaches of the solar system. This has come to be known as the Oort Cloud. The statistics imply that it may contain as many as a trillion (1 X 10^12) comets. Unfortunately, since the individual comets are so small and at such large distances, we have no direct evidence about the Oort Cloud."

"The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets." More here (http://www.nineplanets.org/kboc.html)

The largest object we know of from either the Oort Cloud or the Kuiper Belt is either Pluto (which some consider a Kuiper Belt object) or the newly discovered Kuiper Belt object Quaoar. For reference, Pluto is a bit less than 1/5th the mass of Earth's Moon, and Quaoar is probably about the same size as Pluto's moon Charon--ie around 1/38th the mass of Earth's Moon. Almost certainly the largest Kuiper objects which could be kicked out of orbit to become a short-period comet would be smaller than Quaoar. But Quaoar would be a REALLY HUGE cometary nucleus compared to any we've seen.

So what comets have we seen, size-wise?

"The sizes of cometary nuclei are mostly unknown because the measurement is a difficult one. We have reliable measurements of the sizes of about 10 nuclei. Most of them have diameters from a few km to 10 or 20 km. The nucleus of comet Schwassmann-Wachmann 1 is probably one of the largest (perhaps 20 km), as is the nucleus of comet Hale-Bopp (perhaps 40 km)." More here (http://www.ifa.hawaii.edu/faculty/jewitt/nucleus.html)

Well, from this information I figure that a comet with a 20-km diameter would be roughly 1/74,000th the mass of Quaoar, and a comet with a 40-km diameter would be 1/9300th Quaoar's mass. A comet with a larger diameter than any known, 60 kilometers, would still be only 1/2750th as massive as Quaoar...so if a comet with a nucleus the size of the largest Kuiper Belt object, Quaoar, came zipping into the inner Solar System it would be an astronomical event of way cool proportions! And yet, such an immense comet would still be only 1/38th the mass of Earth's Moon.

Anytime you read that a comet the size of the Moon is heading for the Sun, you had better check the facts: if such a thing was true it would make headlines in every scientific magazine in the world! No comet that size has ever been recorded, glimpsed, or suspected.


For comparison, a customized Ford Mustang might weigh 3450 lbs. Let's say that the Mustang represents the Sun. On that scale, Jupiter is a 3.2-lb kitten, the Earth would be a chunk of gravel weighing .17 ounces (as much as a couple of dimes), the Moon would be a bug weighing .002 ounces, and the amazingly large (for a comet) Quaoar would a speck of dust weighing .00005 ounces. And on the same scale the largest comet nucleus we know of, Hale-Bopp, would be 9300 times smaller than the speck of dust!

So the image of a comet crashing into the Sun as being as insignificant as a mosquito hitting a car windshield is not quite accurate--actually, the largest comet nucleus we know of hitting the Sun would be much, much less significant than a mosquito hitting a car windshield! ;)


Here's a recipe for making a kitchen comet, from this site (http://www.solarviews.com/eng/edu/comets.htm):

Dry Ice (5 lbs) available from ice companies or ice cream parlors; CAUTION: Dry ice is -79 degrees C (-110 degrees F). Any more than brief exposure will cause "burns." Be careful when handling it.

Water (around half a gallon) in pitcher;

Ammonia (a few drops or sprays of window cleaner);

Dirt (fine grained, one handful);

Corn Starch, or Worcester Sauce (a couple of pinches or drops);

These ingredients are either actual components or handy analogous ones. The dry ice is frozen carbon dioxide. Water, ammonia, organic (carbon based) molecules, and silicates are all present on comet nuclei. They have been identified through spectral measurements of comet tails and the collection of tiny ice particles by very high flying research aircraft.


2003-May-25, 12:34 AM
Thanks Dstahl. That's good stuff. I like the comparison with the Ford Mustang - a nice way to help visualize it.

Its good info to keep in mind in preparation for the next conspiracy theory. It also puts in perspective the claim on another thread that solar flare activity is being caused by comets! :D

2003-May-25, 04:55 AM
Lose the dirt & ammonia and add some vodka and a twist and I think the "Comet DStahl" becomes the new favorite beverage at star parties everywhere.