A slice of a
pallasite meteorite fragment
of what was once a meteoroid before it collided with Earth, discovered
in Argentina; on display at the Museum of Nature, Ottawa, Canada.
A
meteoroid is a small particle from a
comet or
asteroid.
[1][2] A meteoroid is significantly smaller than an asteroid, ranging from small grains to 1-meter wide.
[3][4][5][6]
The visible streak of light from space debris is the result of heat as it enters a planet's
atmosphere, and the glowing particles that it sheds in its wake is called a
meteor,
or colloquially a "shooting star" or "falling star". Many meteors
appearing seconds or minutes apart, and appearing to originate from the
same fixed point in the sky, are called a
meteor shower. The root word
meteor comes from the
Greek meteōros, meaning "suspended in the air". Objects larger than several meters can
explode in the air and create damage. If a meteoroid,
comet or asteroid withstands
ablation from its
atmospheric entry and impacts with the ground, then it is called a
meteorite.
Around 15,000 tonnes of meteoroids,
micrometeoroids and different forms of
space dust enter Earth's atmosphere each year.
[7]
Meteoroids
Animated illustration of different phases as a meteoroid enters the
Earth's atmosphere to become visible as a meteor and land as a meteorite
In 1961, the
International Astronomical Union defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an
asteroid and considerably larger than an atom".
[8][9] In 1995, Beech and Steel, writing in
Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid would be between 100
µm and 10 meters across.
[10]
Following the discovery of asteroids below 10 m in size, Rubin and
Grossman refined the Beech and Steel definition of meteoroid to objects
between 10 µm and 1 m in diameter.
[4] The smallest asteroid (based on
absolute magnitude) is
2008 TS26 with an absolute magnitude of 33.2,
[11] and an estimated size of 1-meter.
[12] Objects smaller than meteoroids are classified as
micrometeoroids and
cosmic dust. The
Minor Planet Center does not use the term "meteoroid".
Meteoroid composition
The composition of meteoroids can be inferred as they pass through
Earth's atmosphere from their trajectories and the light spectra of the
resulting meteor. Their effects on radio signals also give 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 (see
Meteor showers) 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,
[13] to nickel-iron rich dense rocks. The study of
meteorites also gives insights into the composition of non-ephemeral meteoroids.
Meteoroids in the Solar System
Meteoroids 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.
[citation needed]
The Earth travels at about 29.6 kilometers per second. Thus, when
meteoroids meet Earth's atmosphere head-on (which only occurs when
meteors are in a
retrograde orbit
such as the Eta Aquarids, which are associated with the retrograde
Halley's Comet), the combined speed may reach about 71 kilometers per
second. Meteoroids moving through Earth's orbital space average about
20 km/s.
[14]
On 2013 January 17 at 5:21 PST a 1 meter-sized meteoroid/comet from the Oort cloud impacted Earth.
[15] The meteoroid had a retrograde orbit with perihelion at 0.98 ± 0.03
AU. It approached from the direction of the constellation Virgo, and collided head-on with Earth going 72 ± 6 km/s.
[15]
Meteoroid collisions with Earth and its atmosphere
When meteoroids intersect with the Earth's atmosphere at night, they are likely to become visible as
meteors. If meteoroids survive the entry through the atmosphere and reach the Earth's surface, they are called
meteorites. Meteorites are transformed in structure and chemistry by the heat of entry and force of impact. A noted meteoroid,
2008 TC3,
was observed in space on a collision course with Earth on 6 October
2008 and entered the Earth's atmosphere the next day, striking a remote
area of northern Sudan. It was the first time that a meteoroid had been
observed in space and tracked prior to impacting Earth.
Meteor
A
Leonid meteor, seen in the 2009 Leonid Meteor Shower.
- "Meteor" and "Meteors" redirect here. For other uses, see Meteor (disambiguation).
Photo of a part of the sky during a
meteor shower over an extended
exposure time. The meteors have actually occurred several seconds to several minutes apart.
A
meteor or "shooting star" is the visible streak of light from a meteoroid or
micrometeoroid,
heated and glowing from entering the Earth's atmosphere, as it sheds
glowing material in its wake. Meteors typically occur in the
mesosphere at altitudes between 76 km to 100 km (46–62 miles).
[16] Millions of meteors occur in the Earth's atmosphere daily. Most meteoroids that cause meteors are about the size of a
pebble. Meteors may occur in
showers,
which arise when the Earth passes through a stream of debris left by a
comet, or as "random" or "sporadic" meteors, not associated with a
specific stream of space debris. A number of specific meteors have been
observed, largely by members of the public and largely by accident, but
with enough detail that orbits of the meteoroids producing the meteors
have been calculated. All of the orbits passed through the
asteroid belt.
[17] The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (18 miles/second),
[18] the orbital speeds of meteoroids, and the
gravity well of Earth.
