Fireballs Flashing In Our Sky

+ Von Del Chamberlain +

November 25, 1998

We have been hearing much of late about meteors resulting from comets, those cosmic vagabond litterbugs from the suburbs of our planetary city that come speeding into downtown solar system scattering primordial sand and gravel in their wakes. The Leonid Meteor Shower did not live up to expectations in one way, but exceeded them in another. The density of meteors was not as great as hoped, but the number of splendid fireballs was a pleasant surprise. And you did not need to go to Asia to see them.

A fireball is a spectacular meteor, brighter than any of the stars or planets. They vary in brilliance from something like a flashlight aimed at you out of distant darkness, through the range of automobile headlights on high beam approaching on the highway, to dazzling fireworks bursting in the sky. Some illuminate the night-time landscape. Occasionally they can even be seen as resplendent lights burning through clear sky on sunny days.

The meteors that are typical of most meteor showers, commonly referred to as shooting or falling stars, are caused by particles the sizes of sand grains and pellets with an occasional marble thrown into the batch. They are beautiful to witness, especially if enough of them are seen to hold you out at night with upturned eyes. Fireballs are produced by chunks ranging from marbles, golf balls, baseballs, footballs and basketballs, on up to giant boulders and beyond. All of these come streaking into the blanket of air we are snuggled in at speeds of many miles each second--a typical velocity is 40 miles per second. They impact each molecule of air that gets in their way. The reactions are furious, a rapid exchange of energy of motion into heat and light. It is primarily the glowing air we see. Thus the term meteor, literally meaning "of the air," is appropriate. The objects plunging from space into the atmosphere are called meteoroids. A meteoroid the size of a football might produce a moving glowing plasma of radiant air the size of a football stadium, engulfing the grandstands as well as the playing field.

Even the fireballs we saw as part of the Leonid shower were made by rather small clumps of matter. The fact that there were lots of pieces large enough to produce modest fireballs tells us that we were encountering debris recently broken away from Comet Tempel-Tuttle. Most of the comet material was likely frozen gasses which would continue to vaporize and subdivide into smaller pieces as time went on. This is encouraging for meteor shower watchers, creating renewed hope that we might see a truly great shower next November when Earth carries us once more through the pathway the comet took as it rounded the Sun. So, if you enjoyed this years Leonid shower--or if you missed it--you might begin to make plans now for next November. The experts are predicting that it might best be observed from eastern Europe. If you do plan a trip into that part of the world for next November, be sure to include other reasons for your destinations, remembering that we cannot accurately predict exactly when the shower will be most intense, how intense it will be, and, of course, the weather that you might find yourself under at the appointed time. The 1999 Leonid shower can be a good excuse to travel, if you are looking for one.

Fireballs are produced from two different sources. Comets are one source. Asteroids are the other. Comets are made mostly of frozen water and gases with bits of rocky material imbedded in the ices. Asteroids, on the other hand, are boulder and mountain sized objects made of rock and metal. Most comets move in great elongated orbits carrying them from the frigid fringes of the solar system into the fierce heat of the Sun. Asteroids move in orbits well within the zone of planets. Most, if not all, meteoroids from comets are vaporized in their passage through the atmosphere, some of the material eventually filtering down as fine dust. Good-sized asteroidal meteoroids are more likely to deposit recognizable pieces on the ground.

A truly spectacular fireball is one of Nature's grandest spectacles. You never know when you might see one when out at night looking around. When a large meteoroid crashes into the atmosphere it begins to glow about 100 miles up. Penetrating deeper, it rapidly grows in brilliance. A mass of air is pushed in front of it, building pressure that slows the meteoroid and often shatters it into many pieces. Sonic booms are produced by every piece moving beyond the speed of sound. Ceasing to glow while many miles above ground, the fragments drop as dark objects to scatter over the landscape.

An observer on the ground sees the fireball blazing across the sky, flashing brighter at times, perhaps bursting like a roman candle, then disappearing. Beneath the trajectory, explosive sounds are heard a few minutes later, and in the area where pieces rain down they might be seen and heard buzzing through the air and thumping onto the ground. When pieces of these cosmic projectiles are found they are called meteorites, and that is the topic I want to explore with readers in my next column.