A “shooting star” is the term commonly used for a meteoroid when it enters the Earth’s atmosphere and burns up, leaving a streaking trail of light in the night sky. A meteoroid is essentially a very small space object, ranging in size from a grain of sand to a boulder. Technically, a meteoroid becomes a meteor when it enters the atmosphere, and if any of the debris is large enough to actually make it to the surface, that debris is referred to as a meteorite.
Currently, the International Astronomical Union has a vague official definition of a meteoroid as something larger than mere atoms, but smaller than true asteroids. Another traditional boundary between a large meteoroid and a small asteroid is a diameter of ten metres, but asteroid too has never been formally defined by size. In any case, regardless of size, the objects are relatively similar. They formed either very early on in the solar system, and never had a chance to accrete into larger objects (the process which ultimately led to formation of the planets), or were once part of larger objects which were broken apart by impact events. A third group of small orbiting bodies, comets, are distinguished by the fact that they follow very long elliptical orbits taking them from the inner solar system out to the Kuiper Belt, beyond the orbit of Neptune.
Meteoroids are small enough that they pose no threat to life on Earth when they strike the planet’s atmosphere. However, the dividing line between harmless and threatening is surprisingly small, given the size of the solar system. Even a object a few tens of metres across is capable of unleashing energy equivalent to a nuclear bomb, as occurred over Tunguska, Russia, in 1908. Such objects probably come along about once in a century or less, whereas on any given day at least one object a foot or so in diameter across is likely to hit the Earth. Meteors are small enough that they are entirely or nearly entirely consumed by the heat generated by friction in the upper atmosphere. It is this process of burning up which also produces the tell-tale flare high in the sky. Significantly large meteors can form fireballs and possibly sometimes even sounds heard on the ground. There are reports of detectable sounds during meteor showers, although that phenomenon has not received much careful study.
Meteors have obviously been observed for thousands of years of recorded history, although it was not until astronomical advances of the Enlightenment period (the 18th and 19th centuries) that astronomers came to understand their significance. We now understand, for example, that the so-called “meteor showers” (predictable periods of very high-frequency meteor activity) happen when the Earth’s orbit sweeps through a band of rich debris left by disintegrating asteroids or comets. For example, the Perseid Meteor Shower, which occurs in mid-August each year, was initially named after the constellation Perseus because that was the section of the sky from which the meteors seemed to appear. We now understand that Earth is flying through a debris cloud left by the Swift-Tuttle comet, which has a highly elliptical 134-year orbit and will swing past Earth again in 2126.
Meteorite debris recovered on the surface also represents a valuable resource for the study of chemistry and geology in the early solar system. By definition, meteorites are objects which formed very early on in the solar system’s history, when very small objects were still accreting through collisions in the disk of gas and dust surrounding our young Sun. In a recent Arizona study, meteorite Northwest Africa 2364 (found in Morocco several years ago) was dated to two million years older than the previous oldest-known object. This bumped back the age of the solar system by the same amount of time, so that it is now estimated at 4,568,200,000 years old.
