Meteors and Meteorites
Meteoroids are fragments of asteroids or comets ranging in size from microscopic particles to small boulders.
When they enter the Earth’s atmosphere they burn up creating streaks of light (shooting stars) and are known as meteors.
Meteors appear 50-70 miles (80 to 120 km) above the Earth’s surface and move at speeds of between 25,000 and 160,000 miles per hour (11 and 72 km/s).
If the meteor hits the ground it becomes known as a meteorite. If the meteorite hits a sandstone region, the heat melts the chemicals in the soil forming small, round, glassy objects called Tektites.
There are three types of meteorites:
- Stony which consist of silicates (rocky materials), Chondrites are primitive and have not been melted while Achondrites have been melted;
- Iron which consists of 90% iron and 10% nickel with traces of silicates and has a melted surface;
- Stony-Iron is a mixture, and is one of the rarest type of meteorite. It also has a melted surface.
As the meteorite passes through the atmosphere its outer surface begins to melt, then as it falls to the Earth’s surface it begins to cool forming a glassy coating called a fusion crust. By the time it hits land the meteorite is cold. The leading edge of the meteorite may be curved or are conical in shape. If the meteorite tumbles then its shape could be irregular.
When a comet passes through space it leaves a dust trail. When the Earth passes through this dust trail the microscope particles burns up in the Earth’s atmosphere forming meteor showers. In November 1833, in North American’s east coast, people saw up to 200,000 meteors in an hour.
There are several major meteor showers that occur during the year and are usually named after the constellation from which they radiate.
The main meteor showers are:
- The Geminids originating from Gemini which occurs on the 14 December;
- The Perseids originating from Perseus which occurs in August;
- The Leonids which radiates from Leo occurs in November;
- The Quadrantids occurs in January and its name comes from an abandoned constellation Quadrans Muralis. The meteor shower originates from where the constellations Hercules, Boötes, and Draco meet in the sky.
- The Lyrids radiates from Lyra and occurs in April.
Where a large fragment (many centimetres or metres in size) enters the Earth’s atmosphere, they produce fireballs and may cause a sonic boom and shock waves like the Chelyabinsk meteor which was witnessed in Russia on 15 February 2013. This meteor was travelling at 12 miles per second.
Any small object orbiting the Sun that shows no cometary activity is called an asteroid.
Asteroids, also known as minor planets, are metallic, rocky objects which do not have any atmosphere, although some may contain water. They consist of material that failed to become a planet-size body, and this material forms the main asteroid belt that lies between Mars and Jupiter.
The main asteroid belt is 2.2-3.2 AU from the Sun and is 1 AU thick. There are over 100,000 known objects and 8,500 have precisely determined orbits, but it is not known exactly how many exist.
They range in size from 950 km to a few centimetres (meteoroids).
Most asteroids are piles of rubble held together by gravity. Some are spherical in shape but others are irregular. The asteroid 216 Kleopatra is shaped like a dog bone, others resemble a lumpy potato.
There are 16 asteroids within the main asteroid belt that have diameters of 150 miles (240 km) or more, and most of the asteroids take between 3 and 6 years to orbit the Sun.
The largest asteroids, Vesta, Pallas and Hygiea, are 250 miles (400 km) long and larger.
Ceres (seer-ees), at 590 miles (950 km) in diameter, is a dwarf planet and was discovered by Guiseppe Piazze on 1 January 1801.
By 1868 about 100 new ‘minor’ planets had been discovered.
This had increased to 200 by 1879, and 300 by 1890.
There are four other dwarf planets:
- Pluto – 2,374km in diameter, which used to be called a Planet
- Haumea (hah-oo-may-ah) – 996 km pole diameter, 1,960km equatorial diameter
- Eris – 2,326km diameter
- Makemake (mah-kee-mah-kee) – 1,434km at the equator, 1,422km at the pole
There are two clutches of asteroids orbiting with Jupiter, and these are known as the Trojan asteroids.
Asteroids that pass within 121 million miles (195 million km) of the Sun are known as near-Earth asteroids (NEAs). There are about 500 known NEA’s but there may be thousands that are large enough to cause devastation if they hit us. About 6% of NEA’s belong to the Atens group which lies inside the Earth’s orbit.
Asteroids generally have a low albedo which means it does not reflect much light and this makes them difficult to see.
Missions to asteroids include:
Galileo which approached two asteroids: (951) Gaspra (October 1992) and (243) Ida (August 1993). This mission offered the first close look at the surface of asteroids and it was discovered that Gaspra and Ida have an elongated, irregular shape whilst Ida’s satellite Dactyl has a smooth, regular shape.
The NEAR mission went to Mathilde (flyby June 1997) and Eros (flyby December 1998, survey from orbit February 2000 to 2001, impacted the surface of Eros on 14 February 2001). Mathilde may be a loosely assembled froth of chondrite material, whilst Eros is a homogenous primitive asteroid, its surface is strewn with boulders and it also has flat areas of ‘ponded’ sediments, possibly formed by space weathering.