Tracking Asteroids
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by Jeremy Tatum
A project supported by The Planetary
Society
From the May/June 1988 issue of The Planetary Report
Astrometry, the practice of measuring stars' positions, may sound like some tedious activity of nineteenth-century astronomy, far removed from today's exciting frontiers of black holes, quasars, pulsars and planetary exploration. But there is something deeply satisfying and exciting about striving to measure precisely the positions of asteroids and comets, to calcu- late their orbits, and to predict where they will go next. Keeping precise track of interplanetary bodies is vital to today's and tomorrow's explorers of the solar system.
The tremendous successes of the spacecraft trips to Comets Giacobini-Zinner and Halley depended greatly on ground-based astrometry. Future exploration of comets and asteroids, such as NASA's proposed Comet Rendezvous Asteroid Flyby (CRAF) mission, will also need accurate measurements of their targets. The Planetary Society has provided us with a generous grant to assist with this work, so I'd like to share with members some of the excitement we feel about our project.
Let's begin with a brief review of the types of objects we're observing. There are 130 or more short-period comets, those whose orbits are known and whose return can be predicted years in advance. In addition, every year some dozen or more long-period comets are discovered. They move across the sky for a few weeks and then disappear, not to return again for tens of thousands of years. Most of the approximately 3,700 minor planets or asteroids have fairly well known orbits in the asteroid belt between Mars and Jupiter. A few, called the Trojans, are in almost the same orbit as Jupiter, 60 degrees ahead of or behind the giant planet.
An important and exciting few have eccentric orbits that sometimes take them close to Earth. Because these Earth approachers seem to move so rapidly across the sky, we often call them "FMOs," short for fast-moving objects. These asteroids with well-known orbits have a permanent number attached to them, and most of them have names; 46 Hestia and 433 Eros are just two exam- ples. They are often called "numbered asteroids."
Every month astronomers observe additional very faint asteroids that they cannot identify. Many thousands of these "unnumbered asteroids" exist, quite a few of them FMOS. They receive a temporary designation-usuafly a year plus two letters, such as 1987 HR-and most of them are lost shortly after discovery. But a few are observed over a sufficiently long time span-maybe a few weeks-to enable astronomers to compute at least rough orbits.
Sometimes two apparently distinct objects seen in different years prove to be the same object, and then we can find precise orbits. If an unnumbered asteroid is well observed at three or more apparitions, it may become the proud possessor of a permanent number or even a name. We are particularly interested in recovering unnumbered objects and following them long enough so that they can receive a permanent number. Some 20,000 asteroids have at least rough orbits.
With so many objects to keep track of-most exceedingly faint and hidden in the background of millions of stars-how can we find any given asteroid? Or if we spot an asteroid in a pho- tograph, how can we identify it or tell whether it might be a new one never spotted before? First we must under- stand a little bit about orbits.
Each asteroid's orbit is approximately an ellipse, with the Sun at one focus. Each orbit is characterized by six orbital elements: the first two numbers describing the size and shape of the orbit, the next three angles describing the orientation of the orbit in space, the last telling the instant in its orbit when the minor planet is closest to the Sun.
We can calculate the orbital elements from a set of astrometric measurements of the object--that is, from a number of observations of the precise positions of the object at different times. This is not an easy calculation. Nevertheless, astronomers have calculated the six orbital elements for almost 10,000 asteroids, and these 60,000 numbers require little space on the disk of a modem computer.
From the orbital elements we can then compute an ephemeris, an hour-by-hour, day-by-day, or week-by-week prediction of exactly where the object will appear against the starry background of the night sky. The most difficult part of this calculation is the need to allow for Earth's position in its journey around the Sun, a calculation made particularly difficult by the perturbing effects of the Moon. Nevertheless, a modem computer can make fairly short work of it and can include certain refinements such as al- lowance for the position of the observer on Earth's surface or for the time re- quired for light to travel from the asteroid to Earth.
We have stored in the University of Victoria's computer the orbital elements as well as a program for computing the ephemerides of all the asteroids. At the beginning of each month we decide which asteroids we might be interested in observing. We simply type numbers on the computer keyboard, type the single word ELLIPSE, and presto! Within seconds the computer calculates for us an ephemeris, at half-day intervals, for the entire month for all the asteroids requested.
Our telescope, a 10-inch-diameter Schmidt that is quite tiny by modem standards, is a wide-field astronomical camera. Still, the stellar images are so sharp and we can use so many of them for comparison stars that the system has proved very effective for astrometric work.
We have a few extra tricks. We some- times use a colored filter to increase the contrast between a faint asteroid and the sky's light pollution. We also have a device that is especially useful for fast-moving objects. The image of an FMO appears as a thin line on the photograph, while the stars appear as sharp dots. With our electronically controlled system, we can automatically move the crosshair (spun for us by a real, specially trained spider!) of our guiding telescope at the right speed and direction predicted for the asteroid's or comet's motion. The stars then appear as short streaks, while the asteroid or comet is a steady bright dot. This enables us to photograph objects so faint that they would not be detectable at all if the image were allowed to drift across the photograph during the exposure.
Most of our exposures are made near the ecliptic (the plane cut by Earth's orbit about the Sun; most known solar system objects lie in or near the ecliptic), so a photograph may show several or even a few dozen asteroids. Of course, it also includes the images of hundreds of thou- sands of stars. The asteroids are exceedingly small and faint and can be seen only with the aid of a high-power microscope. How can we possibly find and correctly identify these tiny images?
Our solution is to take two photographs a couple of hours apart. During this interval, the image of each asteroid will have moved-perhaps a distance comparable to the diameter of the im- age, maybe just a few thousandths of a millimeter. Nevertheless, it has moved. We put the two photographs into a great contraption called a blink comparator, which enables us to look rapidly to and fro from one photograph to the other, two or three times a second. The image of each asteroid then appears to oscillate rapidly to and fro, and this movement can be quickly spotted against the back- ground of numerous but stationary stellar images. The search can be quite tedious, but we experience a special duill each time we spot one of these tiny planets.
Another trick used by some observers is to look at the two photographs through a stereoscopic viewer. The images on the two plates then overlap completely except for any asteroid that has moved between exposures. The asteroid then stands out fairly obviously. It is all very well to find an asteroid, but how do we know which of many thousand it is? We consult the computer again. This time we type in the astronomical coordinates of the center of the photograph, the time it was taken, and the single word PLATE. The computer reads through the entire list of orbits of all the asteroids and works out the position of each object for the time when the photograph was taken. It then calculates to see whether the asteroid would have been in the telescope's field of view at the time, and, if so, it prints out the asteroid's coordinates. As a bonus, it prints an asterisk if the object is within a degree and a half of the center of the field of view. Next comes the part that ought to be tedious-the careful measurement of the position of each asteroid or comet image. We have to compare the position of each object with that of several stars whose positions are known precisely from catalogs. Fortunately, the scale of the photographs closely matches that of a reliable star atlas, so we can quickly identify several comparison stars. The positions of the quarter of a million or more stars of the Smithsonian Astro- physical Observatory Catalog visible from our latitude are stored in the com- puter's memory; all we have to do is tell the computer the catalog numbers of the stars involved, and it will read the posi- tions and other data ftom its enormous memory bank.
Our Magic Measuring Machine is a microscope with a stage that can be moved to left or right or up or down, or rotated at will. We place the photograph on the stage and look at it through a microscope that has a fine crosshair in its eyepiece. We try to bisect the image of each asteroid or comet and the comparison stars with the crosshair by moving the stage bearing the photograph beneath the microscope. Meanwhile, at the other end of the table where we are working is a mysterious metal box with a large display of neon numbers that change rapidly as we proceed. The numbers indicate the precise position of the microscope stage to one thousandth of a millimeter.
Most people know that often in magic it's all done with mirrors, and the sharp- eyed may notice a tiny mirror on the side of the microscope stage. A beam of laser light is reflected from this mirror, and the incident and reflected beams form a system of standing waves. The mysterious metal box is actually connected to the laser, and as the microscope stage moves, the alternating light and dark bands of the standing wave system are electronically counted, each band corresponding to just half a wave- length of the laser light.
The measurement over, the computer makes short work of some more calculations. It checks to ensure that all the star measurements are consistent with their catalog positions and wams us if any of our stars are misidentified, if we have made poor measurements, or if the catalog position is inaccurate. It will make corrections for the proper motions of the stars. The stars are not fixed, but move by an appreciable fraction of an arcsec- ond, or even more, per century, and it is essential to allow for this. The computer corrects for the refraction caused as the starlight passes through the Earth's at- mosphere. But finally, if completely sat- isfied, it produces a position for the ob- ject we're investigating.
The computer may be satisfied, but will the Minor Planet Center in Cambridge, Massachusetts be? We know that the demands made there are, properly, very exacting and we don't want to risk our reputation by submitting observa- tions that are in any way suspect. We know that ours will be scrutinized very carefully in comparison with those sub- mitted to the Center by other highly skilled and experienced observers. A final stage, especially with newly discovered objects such as comets and Earth- approaching asteroids, is to add our observations to those already available and to refine the computation of the object's orbit and then see how far our measured position is from that predicted by the refined orbit. At last, if all is satisfactory, we submit our observation.
Despite many astrometric measurements, we have not so far discovered many new objects, so our first asteroid discovery is especially exciting. David Balam found the faintest of telltale streaks on a photograph made by our colleagues Leopoldo lnfante and Chris Pritchet with the Canada-France-Hawaii Telescope in April 1987. The original image was barely visible with a micro- scope, but it has now been computer-en- hanced with an image processor to pro- duce the rather impressive image ac- companying this article. The object has been named 1987 HR. However, the chances of finding it again are not great, and its overall astronomical significance should not be exaggerated. Some observers routinely pick up lots of faint, previously unknown asteroids, and Leopoldo "discovered" some quarter of a million galaxies on the same photographs on which Dave found the asteroid.
Jeremy Tatum is a Professor of Astronomy at the University
of Victoria in Victoria, British Columbia.
This web page was prepared by Michael Paine for The Planetary Society Australian Volunteer Co-ordinators. Please excuse any OCR typos.
This article was written in 1988. The technology has changed substantially but the task is just as difficult.
See also Predicting a NEO's orbit
Last update 7 November 1998
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