Chasing Asteroidsby Ray BergOn any given clear night, there is a small group of amateurs squinting and peering through their eyepieces for a glimpse of a tiny piece of space debris orbiting aimlessly through the solar system. Dedicated hunters, they derive their pleasure at the telescope with locating and identifying the various "mini-worlds" called asteroids. Most of these tiny rocky bodies, usually less than 100 miles in diameter, travel between the orbits of Mars and Jupiter. A few venture within the vicinity of the earth and the possibility of an earth/asteroid collision has been the subject of numerous articles in the media recently. Over 7000 have been named, along with a number in order of discovery. Thus, 88 Thisbe was the 88th asteroid to have its orbital elements defined, allowing the discoverer the honor of naming it, in this case, Thisbe. But, for the average amateur, what is the lure of looking for asteroids when so many other interesting objects such as planets, star clusters and nebulae can be found and enjoyed with modest instrumentation? After all, any asteroid looks like just another star in the eyepiece of amateur scopes. Most of them are devoid of any color other than white, with the exception of 4 Vesta which has a definite pale orange hue. Part of the answer may be the quest of finding and positively identifying that tiny glimmer of light followed by the comprehension of its nature and possible past: a small chunk of rocky debris, left over from the primordial material from which the major planets were formed, with most likely an irregular non-circular shape, cratered and likely partially shattered from collisions with other asteroids. Maybe they are also searched for just because they are there! What are the chances of finding one?Actually, quite a few asteroids are within range of even the most humble amateur equipment, particularly if the search is conducted at opposition. Opposition is the period when a solar system body is directly opposite the sun in the sky as seen from the earth. At this time, the object has reached its period of best visibility and brightness for the season. The brightest asteroid at opposition is 4 Vesta, which can reach magnitude 5.5 and be seen with the unaided eye under dark sky conditions. Approximately 40 asteroids can be 8th magnitude or brighter at opposition and therefore observable with 7X50 binoculars or a 60mm department store telescope. A 4-inch scope can reveal hundreds down to magnitude 11, and numbers into the thousands are available to the amateur with medium size apertures of 8-inch and above.Getting PositionsA number of sources are available for finding positions of currently observable asteroids. Sky and Telescope and Astronomy magazines occasionally include star charts with plotted positions of asteroids along with predicted magnitude. The Observer’s Handbook of the Royal Astronomical Society of Canada provides a table of coordinates of about a dozen and a half asteroids with an opposition magnitude of magnitude 9.9 or greater. Computer software, such as MEGASTAR and GUIDE, can provide a printout of a star chart with plotted asteroid positions. Another very good source is the newsletter Minor Planet Observer which includes not only charted positions of current asteroids but also listings of appulses (an apparent close approach of one celestial body to another) of asteroids with bright stars and with deep-sky objects plus a lot of other useful information. Finally, the Constellation Of The Month feature on this web site provides information on any easy to observe asteroid travelling through the current month’s constellation.Plotting PositionsIf you have star charts from the magazines or computer with plotted positions, you are ready to observe. Figure 1 shows a portion of a sky map made with MEGASTAR software, which has also plotted the positions of two asteroids, 4 Vesta and 8 Flora, for part of March, 1998. In this scenario, 8th magnitude Vesta and 10th magnitude Flora race above Lambda, Mu and Xi2 Ceti into Aries. The apparent faster speed of Flora is due to its current distance of 195 million miles from earth compared to the more remote 286 million miles of Vesta. Note that on March 3, Flora passes very near the 5th magnitude star 38 Arietis (an appulse of 2.5 arc minutes). This affords an easy opportunity to find the asteroid, following the techniques described further on in this article.
ObservingThe first aim is to locate the field stars in the asteroid path, so begin your search with the lowest power eyepiece you have. The field of view in the eyepiece should be at least a half to one degree - more is better. Identify each star with those on the chart. Now look for an "out of place" star. If the asteroid is faint, it may be necessary to increase eyepiece magnification. A brighter asteroid (7th or 8th magnitude) will be relatively easy to identify as a "foreigner" and will tend to "sparkle" less, standing as a lone intruder in the starry field. However, fainter asteroids are more difficult to identify as each step down in magnitude below 9 seems to expand the number of stars exponentially.Thus, at the approximate position of a 12th magnitude asteroid, for example, 2 or 3 or even more stars of that brightness may be in close proximity. Therefore, all suspect objects must be carefully plotted and the field rechecked 2 or 3 hours later, or even the next evening (warning here: it may be cloudy tomorrow night!) to see which "star" moved. As a final note, if the predicted magnitude of the asteroid will be fainter than the stars shown on your published charts, then you must make your own chart ahead of time, carefully plotting each star in the target field. This is not difficult but great care must obviously be given to placing the stars correctly on your chart. This latter technique is particularly useful when observing appulses. Follow up viewsOnce located, asteroids can be followed for the rest of the season. At times, one may go through a retrograde loop at opposition time and remain in the same area of the sky for weeks or months. They are particularly interesting to watch when their path takes them past deep-sky objects or through star clusters or concentrated bright star formations. For example, in August of this year, 8th magnitude 1 Ceres will pass through the central part of the Hyades, make a large retrograde loop around the bright star Aldebaran in October, and zoom back through the heart of the Hyades as a 7th magnitude object in November. Another interesting facet can be following brightness changes either through the season or possibly localized magnitude changes with one evening’s time. The latter effect is the result of an irregular shaped asteroid spinning on its axis of rotation, often in just a few hours. Since this rotational brightness change is usually less than a half magnitude, it is difficult to detect visually. However, on occasion the total brightness range is of the order of a full magnitude which is readily apparent. The asteroids 433 Eros and 216 Kleopatra, both at about magnitude 9.5 at opposition, will vary by 1.5 magnitude over a 5 hour period. Estimating the actual value of brightness for local hourly changes and constructing a light curve (magnitude vs. time) graph of the results can be easily done. Figure 2 shows an example of this for a favorable opposition of the asteroid 216 Kleopatra.
Real scientific work?As David Levy has pointed out in one of his books, one of the most interesting things one can do with asteroids is simply to compile a list of successful sightings, much like bird watching. However, for the amateur with a scientific bent, who likes to "take data" and to "measure" everything observed, a number of asteroid projects are available of which results would also be of interest to the professional astronomer community. Most of these involve photoelectric photometry or CCD work and is beyond the scope of this article. However, for the visual observer, one project that could result in useful scientific data is the timing of occultations of stars by asteroids. An occultation is defined as the passage of a nearby celestial body in front of a more remote one. In this case, an asteroid passes in front of a star, as seen from the earth, resulting in partially or completely blocking the star’s light from the earth observer. In the eyepiece, the event will be marked by a sharp decrease in brightness of the star by one or several magnitudes (or it may wink out completely!) for a span of 5 to 30 seconds, as the asteroid crosses the star’s position and then snap back to the original star brightness as the asteroid passes on. Rarely, a second dimming of the star has revealed that the asteroid consists of two individual pieces. Accurate timings of the start and finish of these occultations, when combined with those of other observers along the occultation path, are collected by the International Occultation Timing Association (IOTA) and the data reduced to determine the size and the profile of the asteroid. The February 1998 issue of Sky and Telescope, page 88, illustrates how numerous observer’s timings revealed the profiles of 27 Euterpe and 85 Io. This article also lists asteroid occultation predictions for 1998.The equipment needed for occultation timing is modest and includes a telescope with which the target star can be easily seen (the asteroid does not have to be seen - it could well be invisible to the observer), a tape recorder to record your announcing of "in" and "out" when the star fades and reappears and time signals from a short wave radio. Your latitude and longitude is needed, accurate to within 300 yards, which is easy to determine from a good topographic map obtained from your state or province agency. You should locate the target star well ahead of time and begin a continuous watch 10 minutes before the predicted time of occultation. Don’t look away during this time as the event could take place a couple of minutes earlier or later than predicted due to orbital uncertainties. IOTA provides updated asteroid occultation predictions as well as information on receiving data at its web site at the following URL: Give asteroid chasing a try. Whether simply hunting down these tiny bits of rock or engaging in more serious occultation work, it is challenging and fun! Copyright ©1998 Calumet Astronomical Society. |
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