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.

Meteor's come in a range of sizes, from dust-sized which we see as reflected sunlight in the orbital plane of the Solar System (called zodiacal light) to house-sized.

When a meteor enters the atmosphere friction causes ablation of its surface (i.e. it burns up). If the meteor is small (fist-sized) it vaporizes before hitting the ground. If larger it survives to impact on the ground, although it will be reduced in size during entry into the atmosphere.

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.

Meteors often occur in showers and swarms. A meteor shower or swarm is visible as an increase in the number of ``shooting stars'' (meteors burning up as they hit the Earth's atmosphere) during certain times of the year. The showers often last for a couple weeks, with peaks of a couple days.

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:

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
Note the decreasing size indicating that the largest, therefore the brightest, asteroids were the easiest to locate. By 1890, over 300 asteroids had been discovered. By 1990, over 100,000 known asteroids had been found, most with sizes ranging from 10 meters to 100 km and most found in orbits between the Earth's and Saturn's.

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.

Asteroids are typically rocky, metallic objects that orbit the Sun but are too small to be considered planets. They are made from material left over from the formation of the Solar System that never coalesced into a planet, probably due to the gravitational effects of Jupiter. In fact, if the estimated total mass of all asteroids was gathered into a single object, this object would be less than 1,500 kilometers across, less than half the diameter of our Moon.


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.

This was first noticed by Daniel Kirkwood in 1860, hence are named Kirkwood gaps. This is another example of resonance. This resonance phenomenon has Jupiter passing by any asteroid in the Kirkwood gaps every two or three asteroid years, depending on which gap. The repeated tugging induces an asteroid into larger, longer orbits closer to Jupiter. Eventually, however, an asteroid's resonance with Jupiter disappears as its orbit increases.


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 10km 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:

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.

  • M-type asteroids are rich in metals, mostly nickel and iron, probably remnants of what was going to become the planetary core.


    Well-Studied Asteroids:

    Gaspra is an irregular body with dimensions of about 20 x 12 x 11 km. Its surface reflects approximately 20 percent of the sunlight striking it and it is classified as an S-type asteroid and is likely composed of metal-rich silicates and perhaps blocks of pure metal.

    Ida/Dactyl is a heavily cratered, irregularly shaped asteroid in the main asteroid belt. Ida is classed as a S-type. The greatest discovery from the Galileo fly-by was that Ida has a natural satellite, Dactyl. Dactyl is the first natural satellite of an asteroid ever discovered.

    NASA's Hubble Space Telescope took the above images of the asteroid Ceres over a 2-hour and 20-minute span, the time it takes the Texas-sized object to complete one quarter of a rotation. One day on Ceres lasts 9 hours. The bright spot that appears in each image is a mystery. It is brighter than its surroundings. Yet it is still very dark, reflecting only a small portion of the sunlight that shines on it.


    Comets:

    Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

    Historically, comets were thought to be atmospheric phenomena to early man, rare and objects of great curiosity. Often comets were used by astrologers to foretell future events.

    Comets at various times where thought to be ghosts, filled with poisonous gas, demons or cosmic signs; but are actually just asteroids covered in ice. Tycho Brahe showed there was no parallax and that they must be located at distances greater than the Earth's orbit from the Sun.

    Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size (up to hundreds of km in diameter) and brightness as the comet approaches the Sun.

    Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.

    As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.

    The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun.

    Note that the ion tail points away from the Sun regardless of the orbital motion, thus a comet tail may move in front of the comet's orbit when a comet is leaving the inner Solar System. Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it dissolves into a meteor swarm. For this reason, comets are said to be short-lived, on a cosmological time scale.

    Comets have their origin in the giant Oort cloud that exists outside our Solar System. Perturbations from nearby stars cause the comets in the Oort cloud to fall into the Solar system in hyperbolic orbits (i.e. orbits with only a one time encounter with the inner Solar System).

    If a comet passes to close to Jupiter, its orbit is converted into a highly eccentric ellipse and the comet becomes periodic (i.e. it comes back to the inner Solar System on a regular basis, like Halley's Comet with a period of 76 years).

    One of the most spectacular events of the century was the impact of Comet Shoemaker-Levy 9 on Jupiter in July 11, 1994.