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TABLE I.—Current Observational Status of Apollo and Amor Objects (qs 1.15 AU)”
Object Discoverer, observatory Status Date observed Next
*The order in which objects are listed is that of decreasing aphelion distance, making this table directly comparable with table II of
and (2) how to manage examination of the plates to maximize the chance of the object being promptly recognized. Whether or not objects are found, one would like at least to be able to specify what volume of space was searched, its location with respect to Earth and Sun, and the brightness (size) limit to which discovery should have been complete. Because trailing of images decreases the brightness limit of detection, some interpretable decision needs to be made as to how to compensate for apparent motion of possible objects in any particular field. Very fast, wide-field optical systems are now available that make rapid coverage of vast areas of the sky entirely feasible. Even rapid, unknown rates of apparent motion (which depend on the character of the heliocentric orbit, but more critically on the geocentric distance) are a less serious problem than they were formerly with slower optical systems. But when and how should the second plate be taken for best detection of a moving object? The most critical difficulty comes in examination of the photographic records. Blink examination of rich star fields can take days and even weeks per field, without necessarily reaching to the plate limit. Objects displaced by considerable amounts between exposures can still be missed unless the motion during a single exposure is enough to attract attention. Examination should be carried out at the same rate at which observational material is obtained if there is to be feedback into the observational program to secure continuing observations of any interesting objects that might be found. Without possibility of following up on discoveries, much of the value of the search effort is lost. In fact, no more sophisticated examination than a reasonably careful survey of the plate with a hand magnifier was involved in discovery of some of the known objects of the Apollo class. Others were evident on the most cursory naked-eye inspection. The critical point is that whatever information is to be obtained from the plates must be abstracted promptly (i.e., the day after observation), and further observations must be obtained immediately (within a night or two, depending on the rate of motion and on the equipment available for followup). Full details of the discovery of Icarus and of the struggle to keep it under observation during the first critical weeks have been published by Richardson (1965). Similar sagas can be related regarding the difficult and critical first weeks of observation of most of those objects of table I for which reasonably reliable orbits have eventually been determined. In each success story, and in some failures, someone gave close support with computations of orbits and ephemerides, and someone was in a position to continue with observations and measurements of position as the new objects faded beyond the reach of the instruments with which they were discovered. Names that recur repeatedly are L. E. Cunningham, H. M. Jeffers, Seth B. Nicholson, Paul Herget, and in recent years B. G. Marsden.” It is this immediate effort, and not so much the care that
goes into calculation of the definitive orbit, months or years after the discovery, that really determines whether the new object can be reobserved at future apparitions. Even though Apollo and Adonis have usually been considered lost, very considerable effort was expended in keeping Apollo under observation for nearly 3 weeks in 1932 and Adonis for 2 months in 1936. The situation with Hermes in 1937 was so extreme that every observation save one (14 in all, obtained at four observatories) was made quite accidentally with small patrol cameras. At its closest approach to Earth, to about 0.005 AU on October 30, 1937, Hermes was 8 mag and moving 5 deg/hr. Many attempts were made to obtain observations, but, because of the very rapid motion, only one was successful. A number of plates are known to cover the proper fields, but Hermes was traveling too fast to leave a trace. In 9 days it had traveled completely across the sky—comparison with the problems of meteor photography may not be entirely out of order. The orbit, from a 4 day arc, was determined essentially by the geocentric parallax, to which the Johannesburg observations were critically important, as, indeed, they have been for many other, more ordinary asteroids. In relation to current proposals to conduct a planned search for closeapproach asteroids, it may be worthwhile to mention the survey conducted under the direction of C. W. Tombaugh (1961; Tombaugh et al., 1959) from December 1953 to October 1958 for small natural satellites of Earth. Tombaugh's project was, in fact, inspired by his experiences, together with H. L. Giclas, at the Lowell Observatory in 1943 in the attempt to recover the lost asteroid Adonis during a close approach to Earth. In the course of the satellite search, Tombaugh and his coworkers took and examined more than 15 000 photographs. Various cameras, including a 22 cm f/1.6 Schmidt, were employed to search systematically, with appropriate telescope driving rates, for Earth satellites in near-circular orbits ranging through periods from 2 to 24 hr and to limits of 12 to 14 mag. Several promising, but faint, objects were found. Vigorous efforts to reobserve them, however, failed in every case. Although it was felt that most of these objects, originally recognized from weak images near the plate limit, probably were spurious, the possibility remains that some were objects in highly eccentric geocentric orbits, or small asteroids passing close by Earth in heliocentric orbit. In either case it would have been extremely difficult to know in what position to search, and for what rate of motion to compensate. It would be very discouraging indeed to conduct a comparable search at the present time, with the abundance of space “junk” to identify and discount. It would seem that the most productive avenues open at the present time toward discovery and observation of close-approach asteroids are likely to lie in two directions: (1) encouragement of observers who take wide-field plates for various astronomical purposes to look for and report potentially interesting objects to a center at which both observing and computing resources are
available for immediate, but probably somewhat selective, followup and (2) devotion of adequate efforts toward reobservation of those known Apollo and Amor objects, unseen since their discovery apparitions, for which a reasonable search prediction can be made. Wide-field plates taken with fast cameras show so many asteroid trails that some basis for selection must be applied by even the most enthusiastic asteroid hunter. It has been said that for an asteroid to attract the attention of a Palomar Schmidt observer, the trail “must be an inch long, or must be directed the wrong way” (Struve, 1952; the quotation is apparently attributed to Jesse Greenstein). Any object found far from the ecliptic must either have a high inclination or be relatively close to Earth. If there are other asteroids in the field, distinctly different motion will identify potentially interesting objects. The typical minor planet near opposition has a daily motion, in the retrograde direction, in the range from 5' to 15'. Near quadrature the motion will be of comparable magnitude, but in the direct sense. At discovery 1960 UA was between 11 and 12 mag and had a direct motion of about 2 degsday, exemplifying the fact that an object close to Earth may have large direct motion even at opposition. On occasion it may be necessary to infer the direction of motion from characteristics of the trail on a single plate. This was, in fact, the situation at the discovery of Icarus. The usual rule is, in the absence of complicating effects such as variable transparency because of clouds or changing altitude, that the beginning of the trail is denser than the end (Miller, 1965). In estimating the magnitude, necessary for the guidance of other observers, correction should be applied fo, any appreciable trailing. Marsden (1969) has recently redetermined the orbits of several of the known Apollo and Amor asteroids and provided search ephemerides for several for which he considers recovery efforts reasonable. Included are Apollo and Adonis, as well as 1948 EA and 1953 EA. Even though bands as long as 75° across the sky may need to be searched, every encouragement should be given to the effort, at the appropriate times, to reobserve these objects. It seems clear that search efforts in these cases would have a much better chance of contributing useful information than would a comparable amount of time spent in a general effort to discover new objects. While observing with the 122 cm Schmidt on Palomar Mountain late in March in a survey of the western equilateral point for faint Trojans, Tom Gehrels discovered several asteroids that, by their motion, seemed sufficiently interesting to warrant further observation. One of these, of about 16 mag, and with opposition motion of about 40 arcmin/day in the retrograde direction, has turned out to be a new Apollo asteroid (Gehrels, Roemer, and Marsden, 1971). This motion is some two to three times that of a typical asteroid, but by no means sufficiently extreme to make the asteroid an immediate Apollo suspect. The object was, in fact, at a distance of about 0.9 AU from Earth at the time of discovery. The high orbital inclination (22°) and present orientation of the line of nodes preclude approaches to Earth closer than about 0.3 AU, the first Apollo asteroid to which a comparable statement presently applies. The new Apollo, designated 1971 FA, was kept under observation through May 26. There is an extended favorable opportunity for observation in late 1971, and it seems that this new close-approach asteroid will be put in secure status with little additional effort.
Gehrels, T., Roemer, E., and Marsden, B. G. 1971, Minor Planets and Related Objects. VII.