Characterizing an Asteroid
The first question to ask is “Why do we bother?” and the obvious answer is “Because it is there!” and that has been sufficient for astronomers, particularly amateur astronomers from day one. As to what characteristics we wish to derive from “bothering,” the answer is, “those that our data and our imagination will let us.”
Assume that the asteroid is known to be passing close to the Earth. That means that it will be within the earth¹s atmosphere, slowing down, and probably tumbling as it does so. We have an organization known as the IOTA (The International Occultation Timing Association)
Occultations occur when a foreground object, such as an asteroid, passes in front of a more distant object, like a star.
Forecasting the occultations of stars by asteroids is a complicated matter. An occultation occurs when the long, thin shadow of an asteroid, cast by a distant star, traces its narrow course across Earth's surface. Hundreds of occultations are observable every year, but owing to errors in the cataloged positions of asteroids and stars it is easy to make mistakes in predicting the exact path the asteroid shadow will pursue.
Data obtained by timing occultations can provide information on the asteroids' sizes, shapes, and compositions. Asteroids "take on a carnival of forms, resembling lizard heads, kidney beans, molars, peanuts and skulls," notes the planetary scientist Erik Asphaug. It is theorized that many asteroids are not solid objects but rubble piles. Such asteroids cannot be rotating very rapidly, otherwise centrifugal force would have torn them apart, and initial indications are that there is indeed a sharply defined upper limit to asteroid rotation rates, suggesting that at least some asteroids are slag heaps, not boulders. But only a handful have yet been imaged with sufficient accuracy to make this determination, so amateurs can improve the database by using occultations to map the shapes of additional asteroids.
BPAA was asked to assist the IOTA in the asteroid occultation of Zeta Arietes, which was predicted to pass its narrow shadow on Seattle around 9:35 p.m. Oct. 15.
Dave Warman (with Mal Stamper) and Harry Colvin each determined their locations, exactly (with GPS), then timed the occultation using timing
Information from the WWV national source. Dave took video tapes of the vidicon images with the timing pulses superimposed, while Harry used a stopwatch, synchronized to GPS time. Harry’s occultation lasted 2.1 seconds, while Dave recorded at 1.8 seconds.
What does this all mean? (Figure 1.)
Let’s consider two observing stations, Harry and Dave (Aand B), part of a long string of similar stations. Harry records his position A accurately and times the start and stop of the occultation as t1 and t2. Dave does the same at Position B. (tl + t2)/2 is that time at which something approximating the center of pass passes the observation station , and the same holds true for position B. We know the down range distance d1 from GPS, so the average velocity v1 of the asteroid is computed over the interval t1-t4. Knowing the velocity, the down-range dimension of the asteroid at A (t1-t2)/v1 and that at B is (t3-t4)/v1. From this minimal data input, some measure of the asteroid size and shape can be derived. The more data points used, the more refined in the determination. A series of measurements of velocity permit the determination of deceleration, permitting the derivation of Mass of the asteroid. From that and size, density is determined. Density gives one a guess on composition, and spin rate determines the possibility of a “rubble” asteroid.
In other words, you can learn quite a bite by having a team time occultations in a coordinated manner. (The details of the measurements made are supplied by Harry and Dave.)