David Morrison Alan Harris Don Yeomans
During 2000 (up to January 8, 2001, to cover the final observing month in full), 125 NEAs of absolute magnitude H < 18.0 were discovered, or ~10.5/month, from the tabulation by Harris. This is up from around 7.5/month the previous year. The total numbers to date (Jan 08) are N(all) = 1254, N(H<18) = 467. Following is a listing of the monthly (by lunation) discoveries of H < 18 and all. The dates correspond to new moon each month:
date N(H<18) N(all) 2000.01 6 19 2000.10 10 28 2000.18 16 40 2000.26 12 30 2000.34 9 25 2000.42 6 21 2000.50 5 7 2000.58 12 29 2000.66 10 35 2000.74 9 43 2000.82 10 24 2000.90 13 43 2000.98 10 27
The above listing includes quite a few relatively bright (large) NEAs. This year's discoveries include 4 NEAs with H between 13 and 14, 8 NEAs with H between 14 and 15, and 18 NEAs with H between 15 and 16. The dozen discoveries of new NEAs brighter than H = 15 are in a size range that was thought (on some models) to be already nearly complete.
The following breakdown by discovery team refers only to the calendar year 2000 ending on December 31. Of the 109 new NEAs brighter than H = 18 listed by Yeomans, 82 (75%) were discovered by the LINEAR program using two USAF telescopes in New Mexico, with 8 (7%) by the second-ranking LONEOS program at Lowell Observatory in Arizona. A number of additional smaller search programs contributed the remaining 18% of discoveries.
Harris has carried out an analysis of the rate of rediscoveries from LINEAR to estimate the total population of NEAs as a function of size. The resulting estimate is slightly more than 800 brighter than H = 18, consistent with several other recent estimates in the 800-900 range. However, Harris notes that such estimates based on rediscovery will always yield a lower bound for the total numbers, since they implicitly assume that the undiscovered population is similar to the discovered population. Thus any groups of NEAs that is more difficult to detect will be underestimated in the total just as they are undercounted in the observed population. Therefore the true total number of NEAs down to 1 km diameter is probably as high as 1000. Also note, however, that the statistically undercounted groups (those with orbits that don't bring them close to the Earth as often) are also probably less important in terms of the impact hazard than are the groups that do often come close. Finally, note that we continue to discover larger NEAs where the standard models indicate that we should have a nearly complete listing. The point of this complicated discussion is that estimating total populations is complex, especially if we are primarily interested in those NEAs that pose a potential impact hazard to the Earth. The calculations by Harris will appear in a future issue of Icarus.
If we adopt an estimate of 1000 NEAs larger than 1 km, then we have now found 47%. If the true number is nearer 900, as indicated by recent estimates, then we now have 52% in our NEA catalogues. Either way, we are clearly beyond the halfway mark to the goal of finding 90% by 2008.