Meteors

The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the Solar System falls into Earth's atmosphere creating temporary incandescence resulting from atmospheric friction.

A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized.

About 25 million meteors enter the Earth's atmosphere every day (duck!). Most burn up and about 1 million kilograms of dust per day settles to the Earth's surface.

Meteorites have proven difficult to classify, but the three broadest groupings are:

Chondrites Pallasites Irons

The most common meteorites are chondrites, which are stony meteorites. Radiometric dating of chondrites has placed them at the age of 4.55 billion years, which is the approximate age of the Solar System. They are considered pristine samples of early solar system matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration.

Other meteorite types which have been geologically processed are irons and pallasites. Iron meteorites are classified into thirteen major groups and consist primarily of iron-nickel alloys with minor amounts of carbon, sulfur, and phosphorus. These meteorites formed when molten metal segregated from less dense silicate material and cooled, showing another type of melting behavior within meteorite parent bodies. Thus, meteorites contain evidence of changes that occurred on the parent bodies from which they were removed or broken off, presumably by impacts, to be placed in the first of many revolutions. Pallasites are stony iron meteorites composed of olivine enclosed in metal.

The reason for meteor showers and swarms is shown below. Old comets breakdown into individual rocks over time (several thousand years). These rocks are clustered together at first as a swarm, then later spread out along the old comet orbit. When the Earth passes through the old orbit it encounters a fraction of the meteors causing a shower or swarm.

Asteroids

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, they were often distinguished from asteroids of the asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively. The size of asteroids varies greatly, some reaching as much as 1000 km across.

Asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are composed of dust and ice. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice. The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials.

The gap between Mars and Jupiter was of great historical interest due to the Bode-Titus relation. In 1801, first minor planet (later termed asteroid) between Mars and Jupiter was discovered. Then followed an increasing number of further discoveries of minor planets.

date asteroid size 1801 Ceres 500km 1802 Pallas 290km 1804 Vesta 260km 1806 Juno 150km

Most asteroids are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's orbit and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona and Clearwater, Canada.

Kirkwood gaps :

If you plot the radius of the orbits of the asteroids you do not get a smooth `bell-curve' shape. Instead, their are interruptions and the numerical value of these `gaps' corresponds to orbital periods that are integer fractions of Jupiter's orbital period.

Trojan Family :

Asteroids are gathered into ``families'' based on their orbital characteristics.

The Trojan family of asteroids are located at either the leading or trailing Lagrangian points in Jupiter's orbit. These are two stable points in the gravitational attraction between Jupiter and the Sun. Both points are populated with a cluster of asteroids that have be captured in these stable points over the life of the Solar System.

Apollo Family :

The Apollo family of asteroids are those with near-solar orbits. These are objects with highly eccentric orbits that are Earth-crossing. Earth-crossing, of course, means the potential for impact with the Earth (i.e. the objects that caused mass extinctions).

Barringer Clearwater

For reference, a 1 kilometer sized asteroid would impact with the Earth releasing the same energy as a 20,000 megaton blast. It would leave a 13 km sized crater at the impact point and throw enough debris into the atmosphere to cause nuclear winter.

Past near misses:

object name date closest distance Eros 1931 23 million km Icarus 1968 6.4 million km unnamed 200 m object 1972 60 km (!) unnamed 20km object 1988 1.1 million km

Amor Family :

The Amor family are asteroids with Mars-crossing orbits. We believe they evolve by interaction with Mars into Apollo asteroids, so represent a groups of objects in transition.

Hirayama/Koronis Families :

The Hirayama/Koronis families are groups of asteroids that travel in a cluster along the same orbit. They are probably remnants of a single, large body that was broken into a group of smaller asteroids.

Asteroid Composition :

The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust. 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid.

Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume.

Asteroids come in three flavors, C-type or carbonaceous, S-type or silicate and M-type or metal.

C-types tend to have a large fraction of carbon in their make-up and, thus due to radiation darkening, are the least reflective of the two groups. About 75 percent of all asteroids are C-types, and the C-types are more common in the outer Asteroid Belt.

S-type asteroids are rich in rocky or silicate materials and are more reflective. S-types are rarer and occupy the inner Belt.