Bolide

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The word bolide comes from the Greek for bolis which can mean a missile or to flash. The International Astronomical Union has no official definition of bolide and generally considers the term synonymous with fireball. The term is more often used among geologists than astronomers where it means a very large impactor. For example, the United States Geological Survey uses the term to mean a generic large crater-forming projectile to imply that we do not know the precise nature of the impacting body … whether it is a rocky or metallic asteroid, or an icy comet, for example. Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes sometimes called a detonating fireball.

Since bolide and fireball are often interchanged, here is the definition of a fire ball: a meteor brighter than any of the planets, magnitude -4 or greater. There are other definition that are all just variations of this one.

The composition of a bolide can be determined as it passes through Earth’s atmosphere from its trajectory and the light spectra of the resulting meteor. The effects on radio signals also yield information, especially useful for daytime bolides which are otherwise very difficult to observe. From these trajectory measurements, bolides have been found to have many different orbits, some clustering in streams often associated with a parent comet, others apparently sporadic. Debris from bolide streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice, to nickel-iron rich dense rocks.

Bolides travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 42 kilometers per second through space in the vicinity of Earth’s orbit. The earth travels at about 29 kilometers per second. So, when bolides meet the Earth’s atmosphere head-on, the combined speed may reach about 71 kilometers per second.

Bolides are an interesting aspect of astronomy that do not get enough study time. There is great work left for amateurs to do.

There is some good information on a bolide on Wikipedia. Here on Universe today we have many good articles on them. Two very interesting ones are here and here. Astronomy Cast has a great episode about meteor showers, also.

References:
USGS
Wikipedia