Asteroids
Asteroids, also called minor planets, are chunks of rock and metal that orbit the sun. They are too small to be called planets. Most asteroids circle the Sun in the asteroid belt between the orbits of Mars and Jupiter. Two smaller groups of asteroids, called the Trojan Asteroids, follow Jupiter around the Sun. Astronomers think asteroids are left over rocky chunks that never formed into planets. If all the asteroids were put together they would make a planet smaller than Earth's Moon.
Diagram showing the location of the Trojan asteroids and the asteroid belt.
The first asteroid was discovered in 1801 by the Italian astronomer Giuseppe Piazzi. Named Ceres, at 900 kilometres wide, it is the largest asteroid, and was initially classified as a planet. By the beginning of the 20th century several hundred asteroids had been discovered. Over the last ten years astronomers have searched the night sky for asteroids using automated telescopes, resulting in the discovery of about 200,000 new asteroids. The total number of asteroids with known orbits is about 230,000, of which 73,500 have been officially recognised by the International Astronomical Union. The IAU is responsible for giving asteroids a name and number. For example the asteroid named 4179 Toutatis is asteroid number 4179, meaning it was the 4179th asteroid to be discovered.
Diagram showing the orbit of the Earth crossing asteroid
4179 Toutatis
Asteroid Toutatis
Asteroid 4179 Toutatis was discovered in 1989 by French astronomers and is named after a Celtic god. Toutatis is 2.5 kilometres wide and 5 kilometres long. It moves along an oval shaped orbit around the Sun that extends from just inside the Earth's orbit out to the main asteroid belt between Mars and Jupiter. On 29 September 2004, Toutatis moved past Earth at a distance of about one and a half million kilometres, about four times the distance between the Earth and the Moon – making it the closest approach to Earth for any known asteroid or comet this century.
When the shape of an asteroid's orbit is known its position can be predicted up to a few thousand years into the future. This means astronomers can look into the future and accurately predict if an asteroid will collide with Earth. Asteroids like Toutatis are called Earth crossing asteroids. A consequence of the asteroid's frequent close approaches to Earth is that its trajectory more than several centuries from now cannot be predicted accurately. In fact, of all the Earth-crossing asteroids, the orbit of Toutatis is thought to be one of the most chaotic.
Space probes
Astronomers study asteroids to better understand how our solar system and the planets were made. Before 1991 asteroids could only be studied from Earth. Then the space probe Galileo flew past the asteroid 951 Gaspra during October 1991, taking the first sharp images of an asteroid. In August 1993 Galileo flew past the asteroid 243 Ida.
Near Earth Asteroid Rendezvous or NEAR mission was launched on 17 February 1996 to complete the first close-up study of an asteroid. The asteroid selected was 433 Eros, a near Earth asteroid measuring 33 kilometres long, 13 kilometres wide and 13 kilometres thick. On 27 June 1997 the NEAR spacecraft flew past the asteroid 253 Mathilde. NEAR was to reach Eros two years and 327 days after launch, but was unable to achieve orbit on 20 December 1998, forcing mission designers to add another year and 23 days to the trip. On 14 February 2000 NEAR was placed into orbit around Eros. NASA renamed the NEAR space probe to NEAR-Shoemaker on 14 March 2000 as a tribute to the late Gene Shoemaker – a geologist who realised the important role asteroids play in shaping the planets. NEAR-Shoemaker collected information on Eros' mass, structure, geology, composition, and gravity. On Monday 12 February 2001, the NEAR-Shoemaker spacecraft was deliberately crash-landed on the surface of Eros, ending its mission.
