For comparison, there are estimated to be a total of 1100 +/- 100 NEAs larger than 1 km. Thus at the end of 2003 we had found 63 percent of these NEAs.

Recently there appears to have been a modest slow-down in the discovery rate of NEAs larger than 1 km, perhaps reflecting the fact that we have already discovered nearly 2/3 of this population group. For the most recent three complete years (2001, 2002, and 2003), the numbers discovered are: 89, 95, and 67, respectively. We can check this effect by noting that the total discovery rate of all NEAs has not changed much, remaining at about 450/yr. Previously, improvements in the search systems more than compensated for the declining number of unknown asteroids bigger than 1 km waiting to be discovered.

The Spaceguard Goal is 90 percent completeness by the end of 2008. This corresponds to discovery of 990 NEAs brighter than H=18 for the nominal population. The survey passed its halfway mark of 495 in mid-2000 (see NEO News, 08/01/00). The 75 percent objective is 742 NEAs larger than 1 km. If we anticipate 50 discoveries during 2004, then the survey should reach this milestone at the end of this year.

This leaves 4 more years to increase the number of known large NEAs from roughly 750 to 1000. To meet this goal, the current (2003) discovery rate of about 50/yr will need to be maintained by improvements in the system to compensate for a shrinking pool of undiscovered objects. Stay tuned to see if this is accomplished.

The 2003 NASA study of sub-kilometer NEAS (NEO News, 07/23/03) focused on PHAs rather than NEAs. The estimated population of PHAs larger than 1 km is 258 (from the NASA SDT Report, pg 21), leading to a reformulated Spaceguard Goal of discovering 232 large PHAs by the end of 2008. The number of 131 PHAs (discovered as of the end of 2003) is 56 percent of the way to meeting the Spaceguard goal. Please note that conversions between NEAs and PHAs, as well as the total numbers above the 1 km size, vary because the assumptions about such things as the conversion from magnitude to diameter are not exactly the same in different studies.

The Spaceguard Survey is healthy and continuing toward meeting its 90 percent goal sometime between 2008 and 2010. However, the discovery rate of large NEAs has apparently gone thought its peak. Larger aperture survey telescopes now under design (at Lowell Observatory and the University of Hawaii), of course, can be used in the future to accelerate the discovery rate and push into the sub-kilometer size range.

David Morrison

NEO News (now in its tenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov. For additional information, please see the website http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.