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A meteoroid can be an object from the size of a grain of sand to the bulk of a boulder that is traveling in space. There are several official definitions of what is a meteoroid . They all differ based on the largest objects to be included in the group. If a meteoroid enters the atmosphere of Earth or any other body the visible trail that it leaves is called a meteor. Once it impacts the surface it is called a meteorite. These are commonly called shooting stars.
The composition of a meteoroid can be determined as it passes through Earth’s atmosphere from its trajectory and the light spectra of the resulting meteor. Meteoroids effects on radio signals also yield information, especially useful for daytime meteors which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams often associated with a parent comet, others apparently sporadic. Debris from meteoroid 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.
Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 42 km per second through space in the vicinity of Earth’s orbit. The earth travels at about 29 km per second. So, when meteoroids meet the Earth’s atmosphere head-on, the combined speed may reach about 71 km per second.
A meteor is the visible streak of light that occurs when a meteoroid enters the Earth’s atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Millions of meteors occur in the Earth’s atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between 65 and 120 km above the earth. They disintegrate at altitudes of 50 to 95 km. Meteors have roughly a fifty percent chance of a daylight, or near daylight, collision with the Earth as the Earth orbits in the direction of roughly west at noon. Most meteors are, however, observed at night as low light conditions allow fainter meteors to be observed.
Meteoroids are a fascinating glimpse into the content of space objects. Many of the things that we know about asteroid content have come from studying meteorites. There is a good article on the different phases of a meteoroid here. Universe Today has a great article on what a shooting star is and another on the Tunguska meteoroid. Astronomy Cast has an episode on the dust particles in space.