Fact Impact - Asteroids
25 September 2011
Anna Dodge and Carolin Crawford
- There are several million asteroids contained within our Solar System.
- Most travel in orbits at distances between 300 to 600 million km from the Sun
- ... in a region of space located between the orbits of Mars and Jupiter known as the asteroid belt
- ... which contains over forty thousand known objects of over 1km in diameter.
- Although it contains such a high number of objects, the asteroid belt is still only thinly populated, and several robotic spacecraft have travelled safely through it...
- ... the first one being Pioneer 10 in 1972.
- Overall, over 100,000 asteroids have been confirmed as detected, and at least 30,000 have well -determined orbits
- Based on both the spectra of their reflected light and the composition of samples that have fallen to Earth as meteorites, asteroids can be classified into 3 types - according to whether their composition is primarily carbon-rich, stony or metallic
- The majority - over 75% - are the dark carbon-rich objects
- The asteroids are thought to be the debris from shattered planetismals that were prevented from forming planets due to the strong gravitational tides created by nearby giant planet Jupiter
- The total mass of all the objects in the asteroid belt is still only about 4% of the mass of the Earth's Moon
- Asteroids have a wide range of size, from almost 1000km across down to 10s of metres in diameter
- And the number of asteroids increases rapidly with decreasing size.
- Very few asteroids are round, and most are irregular in shape; this is partly due to collisions with other asteroids
- and because they are not massive enough to have enough gravity to pull themselves into a spherical shape
- Some asteroids even have their own satellite moons, such as little (1.5 km-across) Dactyl that orbits the 55km-long asteroid Ida.
- The first asteroid found was Ceres, which was discovered by accident by the Italian priest and astronomer Giuseppe Piazzi on the 1 January 1801.
- The name 'asteroid' was originally proposed by the British astronomer William Herschel, and was taken from from the Latin words aster, meaning star, and oid meaning rock or planet.
- Three more asteroids - Pallas, Juno and Vesta -- were discovered in the next 6 years...
- After that it was another 38 years until any more were found!
- Ceres is the largest object in the asteroid belt, and contains about 1/3 of the belt's mass...
- ... although with a diameter of 925km, more recently it has been reclassified as a dwarf planet.
- Even though it is nearer to Earth than Jupiter, it is too dim to be seen in the night sky with the unaided eye
- With a diameter of 535km, Vesta is the second most massive asteroid in the belt, containing about 10% of the mass in the belt.
- Analysis of the light reflected from the surface of Vesta reveals large basalt-rich regions suggesting that lava must have once flowed on the surface
- NASA's Dawn spacecraft was launched in September 2007 and it arrived at Vesta in July this year.
- This is the largest asteroid that has been visited to date...
- ... the first spaceprobe to visit an asteroid was NEAR Shoemaker, which landed on the surface of the asteroid Eros in 2001...
- And the Japanese spacecraft Hyabusa returned a sample from the surface of asteroid Itokawa in 2010
- After orbiting Vesta for 12 months Dawn will travel to examine Ceres in detail, arriving in 2015
- Vesta has a colossal crater 460km in diameter near its south pole, which lends the asteroid an overall look of a punctured football
- This crater is thought to be due to an enormous impact event less than a billion years ago
- Which spread debris from the asteroid far and wide.
- Indeed, about one in every 20 meteorites that falls to the surface of the Earth is probably a bit of Vesta