|A stone from the 1960 Gao-Guenie fall||An iron meteorite from the 1947 Sikhote-Alin fall|
Meteors (often called falling stars or shooting stars) have been observed since prehistoric times, but it wasn't until about 1800 that they were understood to be caused by rocks from Space.
Meteoroids hit Earth's atmosphere at speeds between 11 kilometres per second and 73 kilometres per second.  The slowest are in orbits close to Earth's orbit around the Sun, and hit the atmosphere at a speed equal to escape speed from the Earth.  The fastest are debris from comets in highly elliptical orbits, moving around the Sun in a direction opposite to Earth.  They collide with Earth head-on.
Meteors become visible between 100 and 120 kilometres altitude.  The high-speed collision of a meteoroid with air molecules instantly heats both the surface of the meteoroid and the air to incandescence.  The visible streak is 95% heated air and only 5% vaporized rock.
On a clear, dark night, away from city lights, you can see about five meteors per hour.  The rate can be much higher during a meteor shower, when Earth passes through debris from a comet.
The vast majority of meteors are faint, produced by meteoroids the size of a grain of sand or small gravel, a millimetre to a centimetre across.  Smaller meteoroids don't get bright enough to be seen from the ground.
The smallest micrometeoroids, a few tenths of a millimeter in diameter and less, are stopped by the atmosphere so quickly that they don't heat up enough to vaporize or even melt.  They slowly drift downward, often seeding cloud and raindrop formation.  This one, 0.4 mm long, was found in Antarctic snow.
Thousands of tiny micrometeoroid impact craters have been found on the surfaces of spacecraft.  The largest may be a hole several centimetres across, punched through a solar panel of the Russian space station Mir.  The object which made it was probably the size of a large grain of sand.
Meteoroids large enough to make meteors but too small to reach the ground burn up completely before reaching 50 kilometres.  A typical meteoroid a few millimetres in diameter makes a visible streak tens of kilometres long in less than half a second.  The smoke-like particles and tiny, melted and re-solidified blobs from these meteors also drift downward and often seed cloud or raindrop formation.
Larger meteoroids are infrequent, but if you watch the sky a while, you will see some.  They make brilliant meteors that can last seconds, called fireballs or bolides, and sometimes leave behind faint smoke trails, visible with binoculars by light reflected from cities below or the Sun above, which last for minutes.
The largest meteoroids are very rare.  Few people ever see a meteor made by a meteoroid large enough for pieces larger than microscopic size to reach the ground.  Only about 500 such meteoroids reach Earth's surface in a year.  Of those, only five or six might be located and identified as meteorites.
A meteoroid typically loses 95% to 99% of its mass as it enters the atmosphere.  So a meteoroid which ends up as a 1 kilogram meteorite on the ground must start out with a mass of close to 100 kilograms.  The meteor can look as bright as the Sun and last half a minute or more, depending on the initial size, speed, angle of entry, and composition of the meteoroid.  It usually breaks apart or even explodes while still high in the sky, in which case thousands of small pieces may fall to the ground.  The largest single meteorite body known is the iron-nickel Hoba meteorite in Namibia, Africa, estimated to have a mass of 60 metric tons.
Meteoroids large enough to survive entry generally either slow
to terminal speed or fragment and then slow to terminal
speed before reaching 10 kilometres.  Terminal speed
of a large meteoroid in the lower atmosphere is about 100 to
250 metres per second
The very largest meteoroids reach the ground without slowing to terminal speed.  Such an event has never been seen, but craters such as the 50,000 year-old Barringer Meteor Crater in Arizona show that it has happened in the past.  The passage of a house-sized meteoroid through the upper atmosphere, seen and photographed in the daytime over Utah, Idaho, Montana, and Alberta on August 10, 1972, shows that it can happen again.  It came within 60 kilometres of striking the ground.
The first object to be identified as an asteroid was discovered by Giuseppe Piazzi at Palermo, Sicily, on the evening of January 1st, 1801, using a telescope built for him in England.  Piazzi watched the object each night for four nights as it moved against the background stars before he was convinced that it looked more like a planet than like a comet.  Still, when he announced his discovery to other astronomers, he claimed only that he had found a comet, but one with no tail or nebulosity.  He didn't know what kind of object it really was.
Using Piazzi's observations from 24 nights in January and early February, the German mathematician Karl Friederich Gauss calculated its orbit later that year, and determined that it circled the Sun in the wide gap between Mars and Jupiter, exactly where astronomers throughout Europe had just begun to search for a hoped-for undiscovered planet.
Piazzi named the object Ceres.  It is now known to be the largest asteroid in the inner Solar System, with a diameter of about 933 kilometres -- the distance between Washington DC and Chicago.
This image of asteroid Ceres was made by the Hubble Space Telescope in January 2004.
It doesn't show much detail, but is fantastic considering how small and distant Ceres is.
A NASA Space probe named 'Dawn' should be the first to reach Ceres, in 2015.
A second asteroid was discovered the next year.  Approximately 580 by 470 km, Pallas isn't much more than half the size of Ceres, and is decidedly non-spherical.
Piazzi's initial estimate of the size of Ceres was far too large.  He thought it was larger than Earth.  After the discovery of Pallas, William Herschel determined that the new bodies are very small compared to the other planets, but severely underestimated the diameter of Ceres as only a quarter of what we now measure.  He proposed that these bodies should be called "asteroids" rather than planets, and the term has become standard in English.  Other terms used synonymously include planetoid and minor planet.
Two more asteroids were discovered in the next five years.  Juno was the third discovered.  It is less than half the size of Pallas.  Vesta, discovered in 1807, is about the same size as Pallas.  Although much smaller than Ceres, it gets a bit closer to Earth, and is lighter in color than most asteroids, so when Vesta and Earth are close together, Vesta can sometimes be seen with the naked eye, unlike any other asteroid.
Curiously, there were no further discoveries between 1807 and 1845, but from 1845 to 1890 more than 300 asteroids were found.  In 1891, photographic techniques for asteroid hunting were introduced, resulting in a much higher discovery rate.  As of September 2006, over 136,000 asteroids have been catalogued, and an even larger number are known but have not yet had their orbits determined precisely.
Most asteroids discovered over the last 200 years orbit between Mars and Jupiter, in the asteroid belt, often referred to as the main belt within the context of discussion of asteroids.
A survey completed in 2002 with the European Space Agency's Infrared Space Observatory indicates that there are between 1 million and 2 million asteroids larger than a kilometre diameter in the main belt.
The orbits of asteroids are greatly influenced by the gravitational pulls of other planets.  Jupiter's gravity keeps asteroids within the main belt and in its own orbit.  Many near-Earth asteroids are clearly affected by Earth's gravity, as their perihelia or aphelia are very close to Earth's orbit.  All the planets except Mercury, Venus, and Earth have small moons which are undoubtedly captured asteroids.  Some larger moons of the outermost planets may also be captured asteroids.
As techniques improve and dedicated searches are started, more and more asteroids are found outside the main belt -- some near Earth, some in the outer reaches of the Solar System.
Near-Earth asteroids (NEAs) have orbits which closely approach Earth's orbit.  It is estimated that as many as 2000 NEAs are larger than a kilometre in diameter.  Another term used is Near-Earth Objects (NEOs), which includes comets that approach Earth's orbit.
Atens cross Earth's orbit and have a period shorter than one year.  More than 300 known.
Apollos cross Earth's orbit and have a period longer than one year.  About 2000 known.
Amors reach perihelion just outside Earth's orbit.  Most cross the orbit of Mars.  Over 1700 known.
Main belt asteroids orbit between Mars and Jupiter, about 2-4 AU from the Sun.  Over 330,000 known.
Trojans orbit at Jupiter's Lagrange points, 60 degrees ahead of and behind Jupiter.  Over 2000 known.  Asteroids recently discovered in the orbits of Mars and Neptune are also being called "Trojans".
Centaurs orbit in the outer Solar System, beyond Jupiter.  Aphelion can be well beyond the orbit of Neptune.  About 100 known.  Centaurs are thought to be asteroids scattered sunward from the Kuiper belt.
Transneptunians or trans-Neptunian objects (TNOs) orbit mostly beyond Neptune.  Over 1000 known.
Kuiper belt objects (KBOs) Are TNOs in a broad disk beyond Neptune, hypothesized to be the source of most short-period comets.  The largest cluster of KBOs orbit between 41 and 47 AU from the Sun, and are herded into that band by gravitational tugs from Neptune.
Plutinos are KBOs in orbits similar to that of Pluto, having a 2:3 resonance with Neptune's orbital period.
Scattered disk objects (SDOs) are TNOs which orbit beyond the hypothetical limits of the Kuiper belt, and are thought to have been scattered outward from the Kuiper belt.
Oort cloud objects (OCOs) are asteroids hypothesized to orbit in a vast spherical shell at the outer limits of the Solar System.  When one happens to be deflected toward the inner Solar System -- perhaps by a passing star or planet -- it can become visible as a long-period comet.
Long-period comets are seen only once as they pass through the inner Solar System and then leave, not to be seen again for thousands of years.  They are thought to originate in the Oort Cloud.
Short-period comets or periodic comets have orbital periods of about two hundred years or less.  Many of them are thought to originate in the Kuiper belt.
Bodies with similar orbits often have similar physical characteristics because they usually originated in the same part of the protosolar disk and have similar histories.
Dust and ice particles in the protosolar disk frequently collided and stuck together, forming ever-larger bodies called planetesimals.  The planetesimals, in turn, collided with each other.  In low-speed collisions they would stick together, while high-speed collisions often broke them apart again.
When a planetesimal grew massive enough to attract other planetesimals nearby and hold on to them by gravity, it became able to withstand collisions at higher speeds without breaking apart.  It also became able to hold on to gas molecules.  This cleared a broad lane in the protosolar disk, and formed a sub-disk of gas and particles which could become a planet and moons.
Less-massive planetesimals which were not swept up into a planet became asteroids.
The term planetesimal is sometimes used as another synonym for asteroid.
While planets and asteroids were forming, the Sun was heating up at the center of the protosolar disk.  Powerful solar wind from the young Sun blew away Mercury's atmosphere, and eventually cleared the entire Solar System of gas and dust.  Some of it was captured by the more massive outer planets, but most was blown into interstellar Space.  Sunlight heated asteroids in the inner Solar System, evaporating most or all of the ices they contained, at least near their surfaces.  They would have looked like comets as the vapor was blown away by solar wind.
The asteroids were also heated by the collisions that formed them, plus radioactive decay of unstable atomic nuclei throughout their volumes.  The largest -- including Ceres and perhaps Vesta and others which have since been broken up in collisions -- were heated enough to melt through, so that denser materials such as iron and nickel sank to the center, while lighter material such as silicates floated to the surface.
Main belt asteroids are mostly silicate rock, with significant amounts of other elements that form dust particles, including carbon, iron, and nickel.
Ceres comprises 25 percent of the Main Belt's total mass. However, Pluto, one of the largest Kuiper Belt objects, is 14 times as massive as Ceres.
The giant planets Jupiter, Saturn, Uranus, and Neptune appear to have thrown vast numbers of asteroids into the outermost reaches of the Solar System early in the Solar System's development.  These asteroids formed very far from the Sun, where water, carbon dioxide, ammonia, methane, and other readily-evaporated molecules are solid ices, so the asteroids include large amounts of those materials.
When gravitational interactions with other bodies happen to send an icy asteroid close to the Sun, the ices warm and vaporize again, releasing gas and dust particles which are blown away by the solar wind.  We see the gas and dust as a comet.  The icy asteroid itself is usually referred to as a comet even when it is far away from the Sun and has no visible coma or tail.  The asteroid which produces a visible comet is termed the comet's nucleus.
The gas giant planets threw asteroids out of the inner Solar System in all directions, so comet nuclei form a huge spherical shell around the Sun called the Oort Cloud.  The total mass of all bodies in the Oort cloud is probably greater than that of Earth, and could be greater than the mass of Jupiter.
An asteroid is a solid body in Space, smaller
than a planet but large enough to be seen at a distance.
A meteoroid is a solid body in Space, too small
to be seen at a distance, which is discovered when it strikes
something (such as the atmosphere of a planet or the surface of
another body), causing a momentary flash of light (a meteor) or
other disturbance in whatever it strikes, or after it makes a
crater or deposits meteoritic material.
These descriptions are consistent with virtually all existing
uses of the terms, so applying them as definitions does not alter
the meaning of either term.  They are more practical than
definitions based on size or mass: When you find a small body in
Space with a telescope, you know immediately that it is large
enough to be called an asteroid.  Likewise, when you see a
meteor, you know that the body causing it is a meteoroid.
These definitions preserve the original reason for having two
separate terms, as opposed to an arbitrary distinction based
on size, mass, or other features.
A similar distinction can be drawn between asteroids and
comets.  A Comet is the visible cloud of
gas and dust given off by an icy body in Space when it is warmed
by sunlight.  The nucleus of a comet is otherwise generally
too small to see except at close range.
The Gao and Sikhote-Alin meteorites
Dawn  Probe should reach Vesta in August, 2011 and Ceres
in February, 2015.  NASA / JPL website.
New Horizons  Probe is enroute to Pluto.
Johns Hopkins University Applied Physics Laboratory website.
To my Space and Science home page
Distinguishing asteroids from meteoroids
Distinguishing asteroids from comets
April 1, 2009
An asteroid is a solid body in Space, smaller than a planet but large enough to be seen at a distance.
A meteoroid is a solid body in Space, too small to be seen at a distance, which is discovered when it strikes something (such as the atmosphere of a planet or the surface of another body), causing a momentary flash of light (a meteor) or other disturbance in whatever it strikes, or after it makes a crater or deposits meteoritic material.
These descriptions are consistent with virtually all existing uses of the terms, so applying them as definitions does not alter the meaning of either term.  They are more practical than definitions based on size or mass: When you find a small body in Space with a telescope, you know immediately that it is large enough to be called an asteroid.  Likewise, when you see a meteor, you know that the body causing it is a meteoroid.
These definitions preserve the original reason for having two separate terms, as opposed to an arbitrary distinction based on size, mass, or other features.
A similar distinction can be drawn between asteroids and comets.  A Comet is the visible cloud of gas and dust given off by an icy body in Space when it is warmed by sunlight.  The nucleus of a comet is otherwise generally too small to see except at close range.
The Gao and Sikhote-Alin meteorites
Dawn  Probe should reach Vesta in August, 2011 and Ceres in February, 2015.  NASA / JPL website.
New Horizons  Probe is enroute to Pluto.  Johns Hopkins University Applied Physics Laboratory website.
To my Space and Science home page