POSITION PAPER ON THE CURRENT STATUS OF NEAR-EARTH ASTEROID RESEARCH
Alan Hale, Southwest Institute for Space Research
March 1, 1999
The growing awareness that the "small bodies" of the solar system, i.e., asteroids and comets, present threats to the earth has been one of the more provocative astronomical developments during the 20th Century. This idea, which might have only been some vague element in a science fiction story several decades ago, is now a relatively strong fixture in the public consciousness, as exemplified by the popularity of the 1998 motion pictures Deep Impact and Armageddon. The growing consensus among the scientific community that a major impact event at the end of the Cretaceous Period 65 million years ago was responsible for the demise of much of the world's living species, including the dinosaurs, and the intense excitement which accompanied the an- nouncement in early 1998 that the recently-discovered asteroid 1997 XF11 might be passing extremely close to the earth in 2028 illustrate that the threat is real. Although the probability that a major impact event will occur anytime during the lifetime of people presently living is quite remote, it is nevertheless not zero, and over a long enough timescale the odds of such an event occurring are virtually certain.
The first-known asteroids were discovered during the early 19th Century, with several hundred being found by the end of that century. With one exception, all of these objects orbit the sun in the main "asteroid belt" located between Mars and Jupiter. That one exception was an object discovered in 1898 and subsequently named Eros; this was the first known "near-Earth" asteroid, and it occasionally makes relatively close approaches to the earth (the closest so far being 14 million miles in 1975). Eros is presently the subject of investigation of the Near-Earth Asteroid Rendezvous (NEAR) mission that was launched in 1996.
Throughout the early decades of the 20th Century additional near-Earth asteroids were occasionally discovered, primarily as a by-product of various photographic sky survey programs. The first-known "potentially hazardous asteroid" (PHA) -- defined as an object 150 meters or more in diameter which can come to within 4 1/2 million miles of the Earth's orbit -- was discovered by Karl Reinmuth at the Konigstuhl Observatory in Heidelberg, Germany, in 1932 and subsequently named Apollo. This was also the first-known asteroid which clearly passes within the earth's orbit, and the asteroids which do so are now collectively known as "Apollo-type" asteroids.
The discovery rate of near-Earth asteroids began to climb during the 1970s, primarily as a result of deliberate but rudimentary search programs initiated at Palomar Observatory in California by Eugene Shoemaker, Eleanor Helin, and Tom Gehrels. Throughout the early 1980s and into the mid-1990s highly successful photographic search programs were conducted at Palomar independently by Shoemaker and Helin; perhaps the most dramatic discovery from this era was Comet Shoemaker-Levy 9, which grabbed the world's attention when it collided with Jupiter in July 1994. Meanwhile, near the end of the 1980s Gehrels initiated the Spacewatch program at Kitt Peak Observatory in Arizona, which was the first near-Earth asteroid search effort to utilize the emerging CCD technology.
Several near-Earth asteroid search programs, many of which utilize CCD technology and automated motion-detection software, have been initiated within the past few years, and as a result the discovery rate of near-Earth asteroids has skyrocketed. As of today almost six hundred near-Earth asteroids are known, and as of this writing (early March 1999) 164 are classified as PHAs.
The two most successful programs utilize complementary approaches. The aforementioned Spacewatch program at Kitt Peak Observatory in Arizona examines relatively small regions of the sky but does so very "deeply," i.e., it detects very faint objects. As a result it is able to detect some of the smaller near-Earth asteroids as well as the larger ones whent they are fairly distant. (1997 XF11, one of the larger known PHAs, was discovered by Spacewatch during December 1997.) The Lincoln Near-Earth Asteroid Research (LINEAR) program, which utilizes DoD-developed surveillance technology and which is based at White Sands Missile Range in New Mexico, examines a much wider area of the sky but doesn't examine it as "deeply" as does Spacewatch. Since going fully on-line in early 1998 most of the subsequent near-Earth asteroid discoveries have been made by LINEAR.
Other search programs currently operating include the Near-Earth Asteroid Tracking (NEAT) program directed by Eleanor Helin and based at Haleakala in Hawaii; the Lowell Observatory Near-Earth Object Search (LONEOS) program based in Flagstaff, Arizona; and the Observatoire de la Cote d'Azur -- Duetsches Zentrum fur Luft und Raumfahrt Asteroid Survey (ODAS) program in France. A few near-Earth asteroids have recently been discovered by amateur astronomers running their own search programs; among those who have been successful are Roy Tucker in Arizona and Takao Kobayashi in Japan.
During their 1995 report to the U.S. Congress following the Shoemaker-Levy 9 impacts into Jupiter Gene Shoemaker and his colleagues estimated that roughly 7% of the potentially threatening objects had been discovered by that time. By making allowances for the objects which have been discovered since then that figure is probably somewhere around 15% today. Despite the recent successes by programs such as Spacewatch and LINEAR, then, it is clear that much work remainsto be done. Only when the discovery percentage approaches 100%, and reliable orbits have been calculated for all these objects, can we be reasonably sure of being safe from majorasteroidal impacts for the foreseeable future.
The Shoemaker report recommended that approximately $5 million be allocated per year for the detection and tracking of Earth-threatening objects, but to date this level of funding has not materialized. NASA currently allocates approximately $3 million to near-Earth asteroid research (including the maintenance of the recently formed Near-Earth Object Program Office at the Jet Propulsion Laboratory in California), and the LINEAR program has received approximately $1 million oftotal funding from the U.S. Air Force over the past three years. Some programs, such as Spacewatch, have successfully supplemented their funding with money from private sources, but nevertheless the current level of funding attached to these programs continues to fall short of that recommended.
An additional issue is that these programs, for the most part, do not operate all the time. Spacewatch and LINEAR, for example, only operate about ten nights per month (weather permitting). Part of this is due to limitations caused by moon phases -- detection efforts will be less effective near the time of the full moon due to the brighter background sky -- and part of this is due to time-sharing and budgetary constraints.
An ideal configuration would be a worldwide network of instruments able to survey the sky with the coverage of LINEAR and with the "depth" of Spacewatch, solely dedicated to this task every clear night of the year. (There would be some loss of effectiveness around the times of the full moon, although brighter objects could still be discovered.) Short of this ideal, a prudent course of action would be to continue the current programs, at as high a funding level (from both public and private sources) as can be reasonably managed, and to further programs which can satisfactorily address the following items, which in this author's opinion are the most important remaining issues (in somewhat of a descending order). Many of these items are discussed in more detail in the Shoemaker report.
1. FOLLOW-UP AND RECOVERY. Programs such as Spacewatch and LINEAR are primarily search-and-discovery efforts; they are primarily oriented towards the discovery of previously-undiscovered objects. Follow-up observations, such as positional measurements which are essential to the calculation of a reliable orbit, are usually left to other entities. This type of work may not be as "glamorous" as the discovery efforts but it is these follow-up observations that may extend over the weeks and months following the actual discoveries that allow us any hope of tracking these objects down in the future.
Equally important is the recovery of these objects when they are next observable from the earth's vicinity, which could be one year, two years, or ten years or more (depending upon the characteristics of the orbit) after the original discovery. In many cases an asteroid will be less well placed for observa-tion, and considerably fainter, than it was when discovered. Nevertheless the recovery of these objects during the periods when they are observable is absolutely essential if we wish to calculate orbits that will allow us to keep track of them into the foreseeable future.
2. THE SOUTHERN HEMISPHERE. A low-budget but very productive near-Earth asteroid search program used to be based at Siding Spring Observatory in New South Wales. (This program was actually piggy-backed onto a more general photographic sky survey program.) The Australian government pulled the funding for this program at the end of 1996, and since that time there has been no near-Earth asteroid search efforts from the southern hemisphere. Since these objects can approach from almost any direction and since there are portions of the southern sky which are inaccessible or accessible only with difficulty from mid-northern latitudes, we are currently missing a non-trivial percentage of the objects which might potentially pose a threat to us.
Some recently-discovered near-Earth asteroids will travel southward rapidly after discovery, necessitating follow-up from southerly latitudes. When some objects are in a position for recovery several years after their discovery they may be in a sky location which is only accessible from the southern hemisphere. The Siding Spring program was very productive in this endeavor, and also in the "precovery" of near-Earth asteroids, i.e., the identification of recently-discovered objects on program photographs taken in the past. Today the only follow-up and recovery efforts underway in the southern hemisphere are those being made by a small number of dedicated amateur astronomers, primarily in Australia.
3. GEOGRAPHICAL DIVERSITY. Both Spacewatch and LINEAR, as well as some of the other near-Earth asteroid search programs, are based in the southwestern U.S., which for the most part enjoys excellent weather conditions. This scenario suffers during poor weather, however, because when it is cloudy in one of these locations, it is generally cloudy over most or all of them. This is especially acute during the months of July and August, when the summertime "monsoon" rains cloud out many of the potentially usable nights. As a result, the productivity of the programs is cut down sharply during this time of the year (although this is also partially due to the fact that the ecliptic regions -- those parts of the sky where one is most likely to find solar system objects — traverse the most densely populated star fields of the Milky Way at that time of the year).
Near-Earth asteroid search efforts would be greatly served if appropriate search instruments could be placed at some of the other climatologically favorable locations —- in both the northern and southern hemispheres -- which will have their "unfavorable" times of the year at times different from those in the southwestern U.S.
4. RECORD-KEEPING. Positional data and orbital information for asteroids are maintained by the International Astronomical Union's Minor Planet Center based in Cambridge, Massachusetts. At present the orbital calculations are performed by a scientific staff of only two persons. The recent flood of asteroidal data produced by the LINEAR program has all but overwhelmed these individuals. An augmented staff and enhanced computer facilities will be needed to accommodate the even greater flow of information we might expect in the future.
5. SMALL OBJECTS. Recently declassified data from DoD early warning satellites indicates that small asteroids -- say, a few meters in diameter -- enter the earths atmosphere on a continual and consistent basis (once a month or more). For the most part, these objects explode harmlessly in the upper atmosphere. Occasionally, though, somewhat larger objects may enter; the Tunguska event, which flattened trees for hundreds of square miles in the central Siberian wilderness in 1908, appears to have been due to a small asteroid approximately 80 meters in diameter.
Although objects of this size do not present a global threat and are not classified as PHAs, they can nevertheless cause significant damage over localized regions of the earth's surface, as was demonstrated by Tunguska. Since these objects are so small and dim they can only be detected when they are relatively close to the earth, and thus a long lead time prior to any potential impact events -- the rationale for most of the ongoing search programs -- cannot really be utilized in these cases. The Spacewatch program, with its "deep" capability, has occasionally detected objects in this size range near the earth, but it is probable that the vast majority of such objects are passing by undetected. Search instruments with "deep" capabilities similar to, and greater than, that of Spacewatch may help discover a larger percentage of these objects, but eternal vigilance is probably going to be called for here.
6. COMETS. Comets, being diffuse objects with measurable apparent size (as opposed to the pointlike stellar appearance exhibited by asteroids) are, in general, easier to detect than are asteroids. Almost all of the short-period comets —- i.e., those which return to the inner solar system every few years -- which have orbits that pass close to the earth's orbit have probably been discovered already, and there are no threats from any of these objects within the near future.
The long-period comets, which may only visit the inner solar system once every few thousand years, are a different matter, since we have no knowledge of their existence until they make their approach. A long-period comet which poses a threat to the earth may not be discovered until a fairly short period ahead of time; for example, Comet Hyakutake, which passed 9 1/2 million miles from Earth in March 1996, was discovered less than two months before its closest approach. Furthermore, since these objects have "fallen" all the way in from the outer solar system and can approach the earth from almost any direction, they will very likely produce more energy during the course of an impact event than an asteroid of comparable size. Accordingly, long-period comets may, in the long run, pose the biggest threat to Earth, although fortunately these objects are far less numerous than are near-Earth asteroids. Search instruments with "deep" capabilities similar to those of the Spacewatch program should prove somewhat effective in detecting long-period comets while still relatively far from the sun and Earth, but this is an endeavor where eternal vigilance will continue to be necessary for the foreseeable future.
THE SOUTHWEST INSTITUTE FOR SPACE RESEARCH
The Southwest Institute, located in southern New Mexico, presently has a moderately small telescope which is CCD-capable. Although this instrument is not really one which can compete with Spacewatch and LINEAR in discovery efforts, it is well suited for many of the follow-up and recovery observations. Since a significant part of the Institute's mission involves education, part of its near-Earth asteroid program is expected to involve the direct participation of students -- both on-site and via the Internet —- in collecting and analyzing the observations, thereby creating a "learning-by-doing" environment.
Pending the obtaining of appropriate funding, future work by the Southwest Institute in the near-Earth asteroid arena includes the acquisition of instrumentation large enough to perform detailed physical observations of these objects (for threat and resource assessments), and the development of sites -- especially in the southern hemisphere -- for search programs and other related efforts.
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