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Asteroid Hunter: The Game

by Perry Pezzolanella

A young man took out his telescope one winter evening back on January 22, 1988 with the goal of finding his first asteroid. He had already found all the planets except Pluto, which at that time was way below the horizon. Therefore, he decided to begin a quest to observe as many asteroids as possible. There were over 10,000 known back then, so there was no limit as to how many could be seen. With his trusty old 60mm Montgomery Ward Tasco refractor, he took a star chart with the position of Amphitrite marked on it and aimed the telescope. He plotted the stars in the field of view and he thought he saw Amphitrite shining as an extra star where it should be, but he needed another clear night to confirm it. If the little star moved, it would prove it was an asteroid. The next night he replotted the stars through his telescope and sure enough it moved! Amphitrite was his first asteroid ever observed and the Asteroid Hunter was born.

Asteroids conjure up many images to different people. Some will picture them as flaming boulders streaking through the sky and killing the dinosaurs. Those who are more science oriented picture them as thousands of small, fractured and cratered worlds that orbit mainly between Mars and Jupiter. Several members of an Upstate New York astronomy club tie the word “asteroid” to a member who has seen over 300 of them and call him the Asteroid Hunter. In either case, an asteroid looks like nothing more than a star, which is what the word asteroid means: star-like. This may make them seem boring and uninteresting to observe, but it can be fun to observe as many as possible and can even become addicting as the Asteroid Hunter has discovered. There is more that can be done by amateurs than simply finding and plotting asteroids. The size, shape, and composition of an asteroid are the data an amateur can seriously contribute to science.

The size and shape of an asteroid can be determined during the occultation of a star by the asteroid. As it blocks the light of the star, it casts a shadow upon the Earth. The observer can record the length of time that the star remains invisible. This procedure must be done very accurately, preferably by using a tape recorder and an audio time signal, and by verbally dictating the exact moment the star disappears and reappears. Several observers have to be located in staggered areas within the predicted path of the shadow and on the outside edge. Those in the middle of the path would most likely see the star eclipsed longer than on the edge because most asteroids are roundish or oblong. Observers outside the predicted path of the shadow are important in case the asteroid has an unknown moon that could occult the star, is bigger than expected, which would cause a wider shadow, or if its orbital calculation is a little off, which would shift the predicted shadow path. The result when all the observations are gathered and plotted is a very accurate silhouette of the asteroid. Several asteroids, including Pallas, have had their shapes and sizes determined by this technique.

Another method of measuring an asteroid is called photometry. This is done by measuring the brightness of the asteroid for several hours and comparing it to nearby stars of known magnitude. When plotted against time to form a curve, any change in brightness indicates that the asteroid is rotating or tumbling and can even be refined to determine if it is rotating or tumbling smoothly or chaotically. Large, regularly repeating dips indicate a rotating brick or cigar-shaped asteroid. It would be brightest when the entire length of the asteroid is presented to Earth and dimmest when the smaller tips rotate into view. Other dips and bumps in the light curve indicate a highly irregular shape. By plotting the light curve of the asteroid for several weeks, it becomes possible to determine the rotation period, magnitude, surface composition, shape, and even the position of the poles. Photometers are available to amateurs, which makes it possible for them to do important work in this field. The MVAS Apollo Observatory houses a 16” Meade Schmidt-Cassegrain telescope perfectly suited for asteroid projects and other research.

Professionals are also studying asteroids using the Hubble Space Telescope and radar. The Hubble Space Telescope has resolved Vesta and Ceres into tiny discs and has proved they are not alike. Ceres is round, 605 miles in diameter, and may be rich in water ice. It has a very bright spot and a large dark area of unknown origin. It rotates in about nine hours. There may be more fresh water in Ceres than on Earth. Ceres may also have a thin atmosphere, a surface covered with frost, and may be as warm as 0ºF. Vesta has a huge impact crater at its south pole and vast plains are flooded with molten material. This is unexpected for an asteroid hardly 360 miles across. Vesta rotates in about 5.3 hours. Bouncing radar off asteroids is a useful tool in resolving their shapes. Toutatis is shaped like a peanut and Kleopatra is shaped like a dog bone. Radar has also detected moons around several asteroids.

Asteroids escaped the melting and differentiation that shaped the planets. They offer a chance to study the chemical composition and evolution that occurred within the solar nebula. There have been thousands of meteorites found on Earth from asteroids, some even from Vesta, but they have been altered by their fiery passage through Earth’s atmosphere and by being exposed to weather. There are different types of asteroids with the three primary classes being: carbonaceous, stony, and metallic. Carbonaceous asteroids are rich in carbon compounds, much like a lump of coal and appear nearly black. Stony asteroids are composed mainly of silicates, nickel, iron, aluminum, and other elements giving them a reddish tint. Metallic asteroids are rich in iron and appear bluish, although the eye would tend to see them as grayish, or bluish-gray. Some asteroids are nothing more than a huge pile of rubble; it was discovered that Mathilde was such when NEAR flew past it on June 27, 1997. It is a carbon asteroid and one of the darkest objects ever photographed by a spacecraft reflecting only 4% of the Sun’s light.

The Asteroid Hunter knows all about his asteroids and approaches each dark of the Moon with eager anticipation of adding to his bounty. It is important to have superior weapons for the hunt, and like in real game hunting, excellent vision, knowledge of the game, and ample patience is required. How does the Asteroid Hunter successfully hunt and bag over 300 asteroids?