Meteors
and meteorites
Meteors are pieces of stray extraterrestrial rock, sometimes with
a high proportion of metals like iron, that enter the Earth's atmosphere.
Meteorites are the remnants of meteors that actually hit the Earth.
Meteors or meteorites come from a variety of sources and are best described
as bits of planetary debris as well as stray asteroids.
Mars is source of meteor material; in fact, a dog was killed by a meteor
in Alexandria, Egypt, in 1911, by what is now believed to be a piece
of Mars rock. It has been calculated that thousands of tonnes of Martian
rock and dust fall on Earth each year, this massive quantity of debris
floating in space is due to rock having been blasted from the surface
of Mars by asteroid impacts occurring over the past few million years.
A similar amount of material from Earth may also be colliding with Mars
each year, and some scientists have theorised that this may be a way
that life could "jump" from one planet to another in the form
of rock dwelling bacteria.
Shooting stars and meteor showers
Shooting stars are small dust particles entering
the Earth's atmosphere and burning up as they shoot across the sky.
The material that forms shooting stars is generally dust from passing
comets which has spread out into space, and gone into orbit around the
Sun. When the Earth passes through one of these comet dust orbits, a
meteor shower occurs. For a short period of time tens to hundreds of
shooting stars may be seen in the night sky each hour as the dust grains
strike the upper atmosphere of the Earth. Although random meteors can
be observed nightly, they occur at a much higher rate during a meteor
shower. It has been calculated that the Earth is showered with 400 tons
of asteroid and cometary material daily!
Interplanetary space and the solar
wind
Besides the leftovers from comets, meteors, and asteroids, the space
within the Solar System contains protons, electrons, and ions all streaming
outward from the Sun in the form of the solar wind. Giant flares or
sunspots on the Sun's surface expel matter, along with light and other
high-energy radiation, that contribute to this interplanetary medium.
Exactly where the boundary between the interplanetary medium and the
interstellar medium lies has not yet been determined, but four spacecraft
have recently passed the orbit of Pluto with velocities that will allow
them to escape from the Solar System. Thus, this boundary may well be
crossed in the near future.
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