The Spaceguard Survey, which has the objective of finding 90% of the NEAs larger than 1 km diameter (defined as brighter than absolute magnitude H = 18) by 2009, is now approximately half complete (by number of objects, not by time required). Through the end of June 2000, 410 of these larger NEAs have been discovered. Current estimates of the total population of these larger NEAs are somewhat less than 1000. If we take the recent published estimate by Bottke and colleagues of 900 as representative, then the objective of the Spaceguard Survey would be to discover 90% of 900, or 810, NEAs by 2009. The current known number of 410 thus slightly exceeds this half-way milestone. Of course, the total number of NEAs of 1 km or larger diameter is only an estimate, with considerable uncertainty. However, it is clear that even with this uncertainty, we are at least approaching the half-way mark. Congratulations to all concerned!
Following are several related stories that document and explain the status of the Spaceguard Survey.
Don Yeomans of JPL, the head of the NASA NEO Program Office, has analyzed the recent performance of the various search programs. Yeomans also shows the observatories that are making these NEA discoveries. The MIT LINEAR system operated by Grant Stokes continues to dominate the discovery statistics. Following are the numbers of discoveries of the larger NEAs for the past 5 half-year intervals:
|Date ||98-1 ||98-2 ||99-1 ||99-2 ||00-1 |
|LINEAR ||11 ||29 ||22 ||29 ||41 |
Total 20 39 34 36 53
|All others || 9 ||10 ||12 || 7 ||12 |
|Total ||20 ||39 ||34 ||36 ||53 |
Following are minutes taken during the Near-Earth Object Observers meeting held August 15, 2000 during the International Astronomical Union General Assembly in Manchester England.
Don Yeomans opened the meeting at 11:00 and welcomed the community of NEO observers and several interested IAU attendees. Yeomans noted that the goal of this meeting was to discuss plans to maximize the discovery rate of NEOs among the international community of NEO observers and to investigate the extent to which coordination among the various teams would help reach the Spaceguard goal. The Spaceguard goal is to discover 90% of the near-Earth asteroids (NEAs) larger than one kilometer within 10 years. The assumption is made that a NEA with an absolute magnitude (H) less than 18.0 has a diameter larger than one kilometer. Recent work by Rabinowitz et al. (2000), Bottke et al. (2000), and Harris (2000) suggest that the total population of near-Earth asteroids (NEAs) larger that one kilometer (H < 18.0) is about 700, 900, and 1000 respectively. There has been a dramatic increase in the NEA discovery rate in recent years (over 400 NEAs larger than one km through July 2000). During the same WGNEO meeting, Al Harris presented his analysis suggesting that at the current rate of discovery, the Spaceguard goal of discovering 90% of the NEAs larger than one kilometer in ten years, would be reached not in 2009 but rather in about 2015. Harris noted that to achieve the Spaceguard Goal would require large NEA discoveries at roughly twice the current rate. Hence we may be 40 - 50 % of the way toward meeting the Spaceguard goal in terms of raw numbers, but certainly not in terms of the time interval required.
Short status reports (see Appendix) were given by representatives of the Japanese Spaceguard (S. Isobe), Catalina Sky Survey (S. Larson), LINEAR (G. Stokes), LONEOS (E. Bowell), NEAT (E. Helin), and Spacewatch (R. McMillan). Brian Marsden and Gareth Williams also presented remarks from the perspective of the Minor Planet Center (MPC).
After these short reports, the floor discussion began with Yeomans noting the conclusions resulting from the earlier meeting (Sept. 1999) of the observers at MIT's Lincoln Lab could be summarized as follows:
A. Effective coordination requires knowledge of each search program's capability and capacity.
B. Each program needs to optimize and understand their own efforts before attempting an inter-survey optimization.
C. Each survey program's sky coverage and limiting magnitude needs to be well understood.
D. As a metric for gauging progress among the survey efforts, each survey team needs to compute their search volume covered per unit time.
E. Once each separate survey is internally optimized and once metrics have been established for each survey, then monthly sky coverage could be effectively divided up among the various surveys.
The following remarks represent the impressions of the undersigned (Yeomans) as a result of this meeting. These comments do not necessarily represent a consensus view of the meeting participants.
Current survey efforts are posting the sky areas that they covered the night before and this seems to be helping the plan to coordinate the total effort. Because of the vagaries of weather, equipment, personnel support, etc., this a posteriori posting of "where we've been" is considerably easier to provide than an accurate a priori posting of "where we'll look tonight." A truly integrated and coordinated international program of a priori postings of planned search areas will be difficult and, in any case, not realistic until the above-mentioned points A-E are properly addressed. As the separate survey efforts optimize their own techniques, and total accessible sky coverage goes to deeper limiting magnitudes, the issue of follow-up observations becomes more important. The search efforts may soon evolve to a point where targeted follow up (most by relatively small aperture telescopes) will be replaced by relatively automatic inter survey follow-up as the same regions of sky are searched more and more frequently by the various surveys. Gareth Williams noted in his remarks that, to some extent, this automatic follow-up between the various surveys is already taking place.
References: Bottke, W.F. et al. (2000). Science, 288:2190. Harris, A.W. (2000). Personnal communication. Manuscript by Werner, Harris, Ivanov, and Harris in prep. for Icarus. Rabinowitz, D.L. et al. (2000). Nature, 403:165.
Appendix: Short summaries of the status of the survey efforts.
Japanese Spaceguard (S. Isobe): Located at Bisei town Japan, the Japanese Spaceguard program will consist of two optical telescopes for the detection of near-Earth objects and space debris in Earth orbit. The 0.5 m telescope, in operation since February 2000, has a field of view of 2 square degrees (f/1.9) and uses a CCD array consisting of two 2K x 4K CCDs. The one meter telescope, scheduled for operation in September 2000, has a field of view of 3 square degrees (f/3) and will use a mosaic of ten 2K x 4K CCDs.
Catalina Sky Survey (S. Larson): The Catalina program consists of both a northern and southern hemisphere search and follow up capability. In the north, the Catalina Schmidt 0.7m (f/1.6) telescope is used for search while the Mt. Lemmon 1.5 m (f/2.0) is used for follow up observations. In the south, the Siding Spring Uppsala Schmidt 0.6 m (f/3) is currently being used for search while the co-located 1.0 m (f/8) telescope is used for follow up. Upgrades are in progress for the Catalina Schmidt (corrector plate, new computers, dome control), the Uppsala Schmidt (declination drive and control room), and Mt. Lemmon (declination drive, computer controls, coma corrector). Proposed upgrades include a thinned 4K x 4K chip for the Catalina Schmidt, a 0.9 m (f/1.7) optical system redesign for the Uppsala Schmidt and a larger 4K x 4K chip for the Mt. Lemmon telescope.
LINEAR (G. Stokes): While efforts to utilize the U.S. Air Force one meter aperture Ground-based Electro-Optical Deep Space Surveillance (GEODSS) telescopes for discovering NEAs go back several years, it was in March 1998 that the LINEAR program began routine operations using a special 1960 x 2560 CCD camera. This CCD is a thinned, back side illuminated, frame transfer device that allows very fast readouts. In October 1999, a second co-located GEODSS telescope was added to the LINEAR survey and the combination of these two telescopes now accounts for roughly 70% of all NEA discoveries.
LONEOS (E. Bowell): The LONEOS 0.6 m Schmidt telescope (f/1.9) is currently making about 15,000 asteroid detections per lunation. With the recent improvements in computer software (Sextractor) and the new camera (two 2K x 4K thinned backside illuminated CCDs), the current detection rate is about twice what it once was. Ted Bowell noted that while an improvement to the current thermal environment might increase the system efficiency somewhat, the current system has gone about as far as it can so that plans are underway to investigate the use of the USNO 1.5 m telescope in Flagstaff for future NEA searches.
NEAT (E. Helin): NEAT began operations with the 1.0 m GEODSS telescope at Haleakala, Maui, Hi in 1995. In 1999, NEAT was moved to the use of the MSSS 1.2 m telescope at the same location and began operations there in February 2000. The current telescope not only has a larger aperture but is available 18 nights per month whereas the GEODSS telescope was only available about 6 nights/month. In addition, upgrades are already in progress to convert the Palomar 1.2 m Schmidt telescope into a NEA search instrument with operations expected to begin in October 2000. The Maui MSSS 1.2 m telescope uses a 4Kx4K CCD with a field of view of 2.6 sq. degrees whereas the Palomar Schmidt will utilize an array of three 4Kx4K CCDs for a field of view of 3.9 sq. degrees.
Spacewatch (R. McMillan) The Spacewatch telescopes include the 0.9 m and the 1.8 m. When used with the 4 x (4.6K x 2K) mosaic CCD, the 0.9 m has a field of view of 2.9 sq. degrees. When brought on line at the end of 2000, the 1.8 m telescope will have a field of view of 0.32 sq. degrees and utilize a 2K x 2K CCD. The average rate of discovery of NEAs with H < 18.0 has been about 7 per year since 1995. The Spacewatch telescopes are used primarily for deep searches in limited areas for NEAs and Kuiper-belt objects (KBOs) rather than the wide area NEA searches provided by the other search programs. As a result, Spacewatch finds many of the smaller NEAs, some KBOs and recently, the 17th satellite of Jupiter.
IAU Minor Planet Center, Cambridge MA (G. Williams) Until Oct. 1998, most of the data processing of incoming observations and orbit improvements were performed on one VAX workstation. Starting in Oct. 1998, the first of the Alpha workstations were brought on line for doing the orbit improvements while the VAX still processed the observations. In early 1999, four clustered Alpha workstations were brought on line to process the observations, compute orbits and prepare the MPCs for export. The porting of the entire MPC operations from the VAX to the Alpha systems has been very time consuming and is still ongoing. Since late 1999, several batches of MPCs have contained over 300,000 observations and the expectation is that one of the 2000 MPC batches will break the 500,000 observation mark. Gareth noted a "follow up rating" that was defined as the percentage of submitted observations made in a specified period that can now be identified with previously known objects or linked with other objects. For the first few months of 2000, these follow up ratings were about 95% for NEAT, 70% for Spacewatch, 85% for LONEOS and 90% for Catalina and LINEAR.
The search for NEAs is nearly a century old, but only in recent years has there been a systematic effort to discover and track the larger (diameter greater than 1 km) NEAs. The Spaceguard Survey was first proposed by a NASA working (The Spaceguard Survey Working Group, Chaired by David Morison) in January 1992. However, the NASA commitment to carry out the Spaceguard Survey (to 90% completeness in ten years) was not made until May 1998. Thus the goal against which to measure progress should be 90% completion by the beginning of 2008. At the current rate of discovery, it is estimated that we will reach this goal around 2015. An increase of roughly a factor of 2 in discovery rate would be required to meet the 2008 goal. It is not clear how we might achieve this improvement, however. It may not be possible to probe faint enough with 1 m telescopes, but the alternative construction of new 2 m telescopes would take such a long time that the survey completion date would probably not actually be moved that much closer.
The NASA commitment to Spaceguard can be dated from the Congressional testimony before the House Space and Aeronautics Subcommittee on May 21, 1998, by Carl Pilcher, NASA Director of Solar System Exploration. In his concluding remarks, Pilcher said:
"The issues and challenges posed by NEOs are inherently international, and any comprehensive approach to addressing them must be international as well. Central areas of concern include coordination among NEO observers and orbit calculators around the globe and public notification should an object posing a significant hazard to Earth be discovered. NASA has begun discussing, with the international community, convening an international workshop to address these issues. The workshop will likely be held during the first half of 1999. The goal of this workshop will be to develop international procedures and lines of communication to ensure that the best available accurate information about any potentially hazardous object is assembled and disseminated to the public in the shortest possible time.
"To facilitate coordination among NASA-supported researchers, other agencies and scientists, and the international community, NASA is establishing an NEO Program Office. This Office will coordinate ground-based observations, ensure that calculated orbital elements for NEOs are based on the best available data and support NASA Headquarters in the continuing development of strategies for the exploration and characterization of NEOs. In the unlikely event that a potentially hazardous object is detected, the Office would coordinate the notification of both the observing community and the public of any potentially hazardous objects discovered.
"NASA is committed to achieving the goal of detecting and cataloging 90% of NEOs larger than 1 km in diameter within 10 years, and to characterizing a sample of these objects. We are developing and building instruments, and developing partnerships -- particularly with the Air Force -- which should lead to the necessary detection and cataloging capability being in place in 1-2 years. This capability will also allow us to detect and characterize many NEOs smaller than 1 km.
"In summary, NASA's obligation and commitment is to ensure that we have the information necessary to understand the hazards posed by NEOs."
By David Morrison and Al Harris and Clark Chapman