Fact Impact - Asteroids

Fact Impact: A high-speed run-down of facts about Asteroids

25 September 2011

Interview with

Anna Dodge and Carolin Crawford

Part of the show Dark Planets and Dark Matter

951_Gaspra.jpg

Credit:

CC0 - NASA

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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