Clifford J. Cunningham - Asteroids

A steroids are quirky. Just consider the title of this chapter. It comes from a chapter title of an 1877 book by William Oxley of Manchester in the north of England. His book was as far removed from science as it could possibly be. Are you a stranger? asks someone known as the Recorder. I have lived on the planet Ceres, is the response. I am not a departed Spirit, but am an inhabitant of that planet you call Ceres. The answer is so preposterous that one can easily miss the two dramatic assumptions it contains: first, the belief, widespread in the nineteenth century, that other celestial bodies (including asteroids) can be inhabited; and second, that Ceres is a planet.

The simple question what are asteroids? has no simple answer. Are they at least partly metallic objects orbiting the Sun between Mars and Jupiter? Are they dead comets? Are they objects in the outer solar system that were subject to weathering by liquid water? Are they interstellar visitors? Are they companions of the planets, following or leading the planets in the same orbits? Are they pristine objects from the origin of the solar system? Asteroids are all these things and more, making them the most interesting and diverse diminutive objects in the solar system.

In essence asteroids are small objects, composed of silicates or metal, that can be found throughout the solar system. Comets are composed of silicates and ice. The major distinguishing factors between asteroids and comets are their source regions and the presence, or lack, of ice and frozen gases. Meteorites typically originate from either asteroids or comets, although a few come from the Moon or Mars owing to impacts on their surface by asteroids, meteoroids or comets. When they survive the descent through Earths atmosphere, they are termed meteors. Asteroids, comets and meteors are now collectively known as small bodies.

The key word here is small. They are also called minor planets or, in French, petites planetes and, in German, kleine Planeten. The International Astronomical Union (IAU), which prefers minor planet to asteroid, developed the unattractive designation dwarf planet to encompass the largest of these objects. Thus Ceres, the first asteroid discovered, has in the twenty-first century assumed the guise of a dwarf, cut from the same cloth as Pluto. The very suitability of the (IAU) (whose membership comprises some 14,000 professional astronomers worldwide) to vote on and adjudicate such matters as designating Ceres and Pluto as dwarf planets is still hotly contested by many of those astronomers.

It was Giuseppe Piazzi and his assistant Niccol Cacciatore who discovered Ceres on New Years Day 1801. Never before has a century been inaugurated with such an epoch-changing astronomical event. But on that evening, neither man knew they had just made a major discovery. Piazzi saw through his astronomical instrument atop the Royal Palace in Palermo a star which his assistant duly recorded. It was part of their grand effort to create an improved star catalogue and for this a special kind of telescope a vertical circle was essential. Accuracy was also necessary, and for this purpose Piazzi had commissioned Jesse Ramsden in England to make a circle with a diameter of 1.5 m (5 ft). It was the best such instrument in the world.