BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Observing Asteroids

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in June, 1993.]

Asteroids, or "minor planets," are small bodies (by planetary standards) thought to be debris left over from a planet that failed to form due to orbital perturbation resonances. Several thousand asteroids have been identified in orbit around the sun, generally between Mars and Jupiter, forming the so-called "asteroid belt." Many of the orbits are quite eccentric, and there is even a family of asteroids known as the "Earth-Crossers" whose orbits intersect that of Earth. Asteroids range in size from Ceres, the largest with a diameter of 623 miles, down to meteor- and dust-sized particles. There are only about 65 known asteroids with a diameter greater than 100 miles.

As targets for amateur observation, asteroids are generally faint and fast moving. They are too small to show a visible disk, and are difficult to distinguish visually from stars. Their rapid motion usually betrays them, however, and an asteroid can often be identified among a field of stars by simply making an accurate sketch of the field, and then checking back in a few hours to see which "star" moved. More typically, observations on subsequent nights will be required to reveal the asteroid's motion.

There are only 25 asteroids brighter than magnitude 10 at opposition. Vesta, the brightest, reaches magnitude 6 at opposition and can then be seen with the unaided eye under very good sky conditions. Because most are faint, larger telescope apertures are necessary to see them, but there are numerous asteroids within reach of a 6 inch telescope.

The locations of the brighter asteroids are published in the major astronomy magazines from time to time (For example, see "Asteroids Through the Eyepiece" in the June ASTRONOMY, page 53.). More in-depth information on asteroid positions can be obtained from a number of sources, including the Association of Lunar and Planetary Observers and the International Occultation Timing Association.

From time to time an asteroid will occult (eclipse) a visible star. When this occurs, the starlight shadow of the asteroid will sweep a narrow path across the Earth. Anyone in the path at the time will see the star briefly dim or disappear. Because the star is so much farther away than the asteroid, the shadow casts a perfect silhouette and allows the approximate size and shape of the asteroid to be determined from carefully coordinated ground observations. A line of many observers is deployed across the predicted path of the occultation. Each observer accurately (generally +/- 1/10 second) times the beginning and end of the event. From these timings the size and shape of the asteroid are reconstructed. Such observations, by amateur astronomers, are coordinated by the International Occultation Timing Association (IOTA). Occultation timing expeditions offer an exciting and unique opportunity for amateur observing.