An asteroid is a small Solar System body that is in orbit around the sun. Unlike comets, which are made of ice and generate a “tail” from evaporating material, the asteroids are made of rock, metal or organic materials with a smattering of water thrown in for good measure. The asteroids are thought to be the remnants of proto-planets that never quite became planets in the early Solar System. There are literally millions of them in the Asteroid Belt between the orbits of Mars and Jupiter. Other asteroids are in near Earth orbits.
With so many known asteroids why would any stand out as remarkable?
First consider size. Asteroids are widely distributed by size. The largest three in the Asteroid Belt are 1 Ceres, 2 Pallas and 4 Vesta. The number indicates the order in which they were found. The smallest are just centimetres in diameter.
Ceres was the first asteroid to be observed. It was discovered in 1801 by Giuseppe Piazzi. Ceres is considered to be an almost intact relic from the time of planetary formation. Ceres is thought to have a rocky core and an icy surface made up of water ice, carbonates and clays. Its water content is unusual among asteroids.
Ceres has the distinction of being the first asteroid to be reclassified as a dwarf planet. In 2006 the International Astronomical Union defined a dwarf planet to define a body is in orbit about the sun, has sufficient mass to be spherical as a result of its own gravity but fails to clear its own neighbourhood of other orbiting bodies. Pallas and Vesta are not classified as dwarf planets because it is not clear whether their surfaces are spherical.
Pallas was the second asteroid to be discovered, in 1802. Pallas is though to be octahedral in shape with a mean diameter of 544 kilometres. The surface of Pallas has a low water content. Pallas is thought to be made of silicates. Pallas has a highly unusual orbit. Most asteroids orbit in the same plane and follow relatively circular orbits. The orbit is Pallas is elliptic and inclined at 38 degrees to the elliptic. Pallas also spins on its side: the spin axis is at 78 degrees. Consequently one side of Pallas is in near permanent day and the other in near permanent night.
Vesta was discovered in 1807. Vesta has a mean diameter of 529 kilometres and is the brightest asteroid. Although similar in volume to Pallas Vesta is the second largest asteroid by mass. Vesta has a nickel iron core with a rocky crust. It 25% more massive than Pallas. Vesta is the only known asteroid that has reshaped its surface and has a composition similar to some meteorites found on Earth. A huge impact crater covers most of the southern hemisphere. The crater was formed in an impact event less than 1 billion years ago which generated many smaller asteroids and meteors. Scientists believe that the HED class of meteorites which are found on Earth contain some of the impact material from Vesta.
More discoveries await: the Dawn Mission, launched in 2007, will reach Vesta in 2011 and at Ceres in 2015.
Next consider asteroids that are scientifically distinguished, mostly for their role is spaceflight exploration.
The Galileo space mission was launched on a fourteen year mission on October, 18 1989. During that mission Galileo has taken close up photographs of two asteroids, witnessed the impact of comet Shoemaker-Levy 9 into the Jovian atmosphere and discovered the Jovian rings. 951 Gaspra was the first true asteroid to be photographed in close-up during the 1991 flyby. In 1993 Galileo photographed 243 Ida. Ida is a pock marked tiny place just 31 kiomoteres in diameter. It considered to be the most heavily cratered body in the Solar System. It is famous for having a companion. Dactyl is 1.4 kilometres in diameter and rotates in a circular orbit about 70 kilometres distant from Ida.
433 Eros has several claims to fame. It is a near Earth object which was discovered in 1898. Eros is first asteroid to be reached by a space prove. The Near Earth Asteroid Rendezvous (NEAR) Shoemaker probe was the first space program dedicated to asteroid research. When NEAR Shoemaker touched down on Eros on February 12, 2001 it failed to associate the asteroid with meteorite materials that have been found on Earth. The association of Eros with meteorite material remains inconclusive.
25143 Itokawa has achieved recent fame as the first asteroid from which samples have been returned to Earth. The Japanese Hayabusa probe brought these samples back on June 13, 2010.
21 Lutetia has the distinction of being the largest asteroid visited to date. The European Rosetta probe had a close encounter in July 2010.
From the point of views of mankind the most notable asteroids are those threaten the planet Earth. An asteroid on collision course with the Earth would bring with it a huge amount of kinetic energy by virtue of its mass and velocity. An asteroid with 1 kilometre or more strikes the Earth every half million years on average. Very large collisions with an asteroid of more than 5 kilometres takes place, typically once on every ten million years. Very large impacts have the potential to throw so much dust into the atmosphere that the climate is severely disrupted. The last notable impact is believed to have taken place 65 million years ago and caused the mass extinction of plants and animals including the dinosaurs. In March 2010 scientists writing in Science Magazine confirmed that the leading contender for the extension was an asteroid with 15 kilometres in diameter. Scientists believe that the asteroid causing the extinction broker away from an even larger asteroid. Their leading contenders are 298 Baptisma and the newly discovered P2010 A2. Both are members of the Flora group of asteroids which share a similar orbit. P2010 A2 has a second point of interest. When discovered in January 2010 it was thought to be a comet because it had “a tail”. Subsequent analysis has confirmed that it is an asteroid with a gravel tail caused by a very recent impact in the last few years. P2010 A2 is the first known asteroid with a gravel tail.
Icarus is probably the most famous asteroid of all time. This asteroid was discovered by Walter Baade in 1949. Icarus It has a diameter of 1.4 kilometres. It makes a close approach to Earth in gaps of 9, 19 and 38 years. In 1968 it made a particularly close approach, passing within 4 million miles of the Earth. In 1967 Professor famously asked his student class at MIT what could be done about an approaching asteroid. The students proposed pioneering work, known as Project Icarus. in which the asteroid could either be blown apart by nuclear weapons or simply distracted onto a different course. In 1979 Project Icarus was the basis of the film Meteor. The asteroid, Orpheus, which stars in the film is fictitious. As of May 2010, 7,075 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 500 – 1,000.
Although there are many asteroids within the Asteroid Belt and in Near Earth Orbit a few of them have memorable traits or stories of discovery to tell.
A guide to notable asteroids The Planetary Society