Want to stay on top of all the space news? Follow @universetoday on TwitterWhen were asteroids discovered is a subject that could take years to write. There are a few thousand known asteroids. What I will do is go over the first four discovered and give a few details about each.
The first asteroid to be discovered was Ceres, on 1 January 1801, by Giuseppe Piazzi on accident. At first he thought this was a comet, then later a planet! When it was realized it was too small Sir William Herschel (the astronomer who discovered Uranus) made up the word “asteroid” to describe it, from the Latin word aster meaning star and the -oid meaning rock or planet. In other words a star-like planet because he couldn’t see any details due to the small size of the object. By 1807 another 3 asteroids were discovered, but no more were found until 1830 when a persistent asteroid hunter named Karl Ludwig Hencke found a fifth and sixth asteroids. Ever since then at least one new asteroid has been found each year.
The second asteroid to be discovered was Pallas. It one of the largest asteroids and is located in the main belt. Its diameter is some 550 km. It has a slightly low mass at 2.11 1020kg. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft at this time, but who know what advances will be made in the near future.
The third asteroid to be discovered was Juno. Juno was discovered on September 1, 1804, by German astronomer Karl Harding and named after the mythological figure Juno, the highest Roman goddess. It is the second most massive S-type asteroid after 15 Eunomia. Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent magnitude for a small object not near the inner edge of the asteroid belt. Juno can reach +7.5 at a favourable opposition, which is brighter than Neptune or Titan, and is the reason for it being discovered before the larger asteroids Hygeia, Europa, Davida, and Interamnia.
The fourth asteroid to be discovered was Vesta. It may have been discovered fourth, but it is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt. Vesta lost some 1% of its mass in a collision less than one billion years ago. Many fragments of this event have fallen to Earth as Howardite-Eucrite-Diogenite meteorites which are a rich source of evidence about the asteroid.
As you can see answering when were asteroids discovered is really the stuff that entire books are written about. Here on Universe Today we have a good article on each of these asteroids, Ceres, Pallas, Juno, and Vesta.