What are fireballs and bolides, and how often can you see them?

Fireballs and bolides are more massive than the objects that produce ordinary meteors which are the size of a grain of sand and weigh less than a gram. Fireballs can be as big as a pea and weigh several ounces; or can be a foot across or more and weigh many pounds. A good book on this is Rocks from Space by O. Richard Norton, published in 1994 by the Mountain Press Publishing Company in Missoula Mountana.

Fireball light is generated by the heating of the body as it enters the atmosphere at 10 - 20 kilometers per second. This heats the body surface to 3000 K and liquefies its surface. A region of the atmosphere up to 100 feet in diameter is also ionized and the gas glows. It is this atmospheric effect that you see from the ground. The colors are usually white near the start of the trail, and fade to red as the fireball has been slowed to low enough speeds where the efficiency of ionizing the atmosphere is small. The colors can tell something about the composition of the fireball: nickel produces a green color, sodium a yellow color and magnesium a blue-white color. The atmosphere ionization gives off the colors of greens and reds from oxygen, nitrogen and other gases.

Fireballs that produce actual meteorites that impact the ground, produce noises resembling sonic booms, or hissing. Even a 'whomping' sound has been reported just before impact. These sounds occur seconds of minutes AFTER the fireball has vanished because the plume burns out 10 - 40 miles above the ground; too far for sound waves to propagate.

Fireballs loose mass at a prodigious rate depending on their path through the atmosphere and their speed. At 12 miles per second, at a 45 degree angle through the atmosphere for a 1-ton object, more than 80 percent of the original mass can survive. At 24 miles per second, only 50 percent survives to the ground. Objects less than 10 tons actually get aerobraked so that by an altitude of 10 miles, they are now free-falling to the ground, having lost all of their original 'cosmic' velocity. The object begins to speed up as it begins to fall at 32 feet per second per second. Objects up to several tons impact the Earth and make 'holes' only slightly larger than they are physically. Many fireballs simply break up into a shower of smaller particles if they are made of stony material which isn't as strong as pure iron/nickel falls. A 10 ton fireball remnant impacts the Earth at 24 miles per second or 5200 miles per hour.

How often might you expect to see fireballs? According to a book Fireballs, Meteors and Meteorites by Harold Povenmire published in 1980, if an observer, or group of observers, see 100,000 meteors, about 900 of these would be brighter than an apparent magnitude of -3.0; brighter than the planets Venus or Jupiter. About 50 would be brighter than magnitude -8.0 making them of the 'fireball' class. As you see, they are rare, but statistics being what it is, SOMEONE casually looking at the sky on a clear night will see one of these every day since there are thousands of people 'casually' looking up every night!


Copyright 1997 Dr. Sten Odenwald
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