Discovery

The first asteroid (Ceres) was discovered on Jan. 1, 1801, by Giuseppe Piazzi at Palermo, Italy. Owing to illness, Piazzi was only able to observe the object until February 11, and, as no one else was aware of its existence, it was not re-observed before it moved into the daytime sky. The short arc of observations did not allow computation of an orbit of sufficient accuracy to predict where the object would reappear when it moved back into the night sky, and so it was “lost.”

The introduction of photography to the search for new asteroids by the German astronomer Max Wolf in 1891 accelerated the discovery rate. By this time 322 asteroids had been identified - largely through the use of  Bode's law of planetary distances. By the end of the 19th century, 464 had been found. The asteroid designated 323 Brucia, detected by Wolf in 1891, was the first to be discovered by means of photography.

Measurement Techniques

The first measurements of the sizes of asteroids were made in 1894 and 1895 by the American astronomer Edward E. Barnard, who used a filar micrometer (an instrument normally employed for visual measurement of the separations of double stars) to estimate the diameters of the first four asteroids. The first four asteroids came to be known as “the big four,” 1 Ceres (930 km-largest), 2 Pallas (535 km), 3 Juno (240 km), and 4 Vesta (520 km). The fourth largest asteroid, 10 Hygiea, has a diameter of about 410 km. The number in front represents the order of discovery.

The most widely used technique for determining the sizes of asteroids is that of thermal radiometry. This technique makes use of the fact that the infrared radiation (heat) emitted by an asteroid must balance the solar radiation it absorbs. The only technique that measures the diameter directly (i.e., without having to “model” the actual observations) is that of stellar occultation. In this method, investigators measure the length of time that a star disappears owing to the passage of an asteroid between the Earth and the star. Then, using the known distance and the rate of motion of the asteroid, they are able to determine the latter's diameter uniquely.

Check out the Asteroid Fact Sheet at:

http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html

Reflectivity

A parameter closely related to size is albedo, or reflectivity. This property also provides compositional information. Albedo is the ratio between the amount of light actually reflected and that which would be reflected by a uniformly scattering disk of the same size. Snow has an albedo of approximately 1 and coal an albedo of about 0.05. An asteroid's apparent brightness depends on both its albedo and diameter as well as on its distance.

Earth Approaching

The smallest known asteroids are members of the Earth-approaching groups, since these asteroids can approach the Earth to within a few hundredths of 1 AU (the mean distance between the earth and sun- one AU is c.92,960,000 mi). The smallest routinely observed Earth-approaching asteroids measure less than 0.5 km across. It has been estimated that there are 250 asteroids larger than 100 km in diameter and perhaps 1,000,000 with diameters greater than 1 km.

Mineral Composition

The first mineralogical determination of the surface composition of an asteroid was made in 1969 by Thomas McCord, John B. Adams, and Torrence V. Johnson of the United States, who used spectrophotometry to identify the mineral pyroxene in the surface material of 4 Vesta. 

Rotational Periods

Asteroid rotational periods and shapes are determined primarily by monitoring their changing brightness on time scales of hours to days. Short-period fluctuations in brightness caused by the rotation of an irregularly shaped or spotted body (a spotted body being a spherical object with albedo differences) give rise to a light curve (a graph of brightness versus time) that repeats at regular intervals corresponding to an asteroid's rotation period. They range from 2.3 hours to 48 days, but the majority (more than 80 percent) lie between 4 hours and 20 hours. The largest asteroids (those with diameters greater than about 175 km), however, have a mean rotational period close to 7 hours, whereas this value is about 10 hours for smaller asteroids.

Shapes

Some asteroids, such as 1 Ceres, 2 Pallas, and 4 Vesta, are nearly spherical, whereas others are quite elongated. Still others, as, for example 4769 Castalia (which appears in radar observations by the American astronomer Steven J. Ostro to resolve as two spheres in contact), apparently have bizarre shapes.