The goal of the Spaceguard Survey is to find 90% or more of the Near Earth Asteroids (NEAs) within a decade. It is also a goal of NASA, stated in the NASA Office of Space Science Strategic Plan, to discover 90% of the NEAs within the next decade. This is a summary, prepared in collaboration with Alan Harris of JPL, to see where we stand in mid-1998 in this effort. We will try to make further updates at 6-month intervals and post them on the NASA Impact Hazard website.
For purposes of this discussion, NEAs with D > 1 km are equated to asteroids with absolute H magnitude less than or equal to 18.0, with perihelion distances less than 1.3 AU. There are approximately 1500-2000 NEAs estimated to exist that fit this definition. Other definitions of NEAs (or ECAs, Earth-Crossing Asteroids, or PHAs, Potentially Hazardous Asteroids) are more restrictive and also require more detailed analysis of their orbits. The present definition of an NEA, however, together with the estimate of 1500-2000 total NEAs, is sufficient to assess the current performance of the survey.
Note that many of the NEAs discovered are smaller than 1 km (H > 18.0). Any survey system will discover as many or more small asteroids as large ones. But we will consider only asteroids larger than 1 km, since these are the most dangerous, and the metric for success of the survey is defined in terms of objects with D > 1 km.
The three most successful searches during the past year have been LINEAR (the Lincoln Laboratory NEA survey of the US Air Force), NEAT (the NEA survey carried out jointly by JPL and the USAF), and Spacewatch (the search carried for more than a decade at the University of Arizona). Together they accounted for more than 80% of the discoveries. LINEAR has dominated the recent growth, going from 0 to 2 to 12 NEAs (D > 1 km) discovered in the last three 6-month periods.
The table below shows 29 NEAs larger than 1 km discovered from July 1997 through June 1998. There was a marked increase in discovery rate over this period, from 8 NEAs discovered in the first 6 months to 21 in the most recent 6 months.
In a ten-year survey expected to detect 90% of this NEA population, we must discover just over 20% in the first year, with the rate declining exponentially thereafter as greater completion is reached and more of the objects found are rediscoveries. Therefore, to achieve the stated Spaceguard goal of finding 90% of the 1500-2000 NEAs in a decade, we must increase the discovery rate by approximately a factor of 13 over the average for the past 12 months, or a factor of 10 over the average of the past 6 months. Searchers have just now pulled within an order of magnitude of the required discovery rate, with another factor of 10 needed to implement fully the Spaceguard Survey.
NEA DISCOVERY SUMMARY (D > 1 km)
JULY 97 THRU JUNE 98
|Discoverer ||12 month ||1997-2 ||1998-1 |
|LINEAR ||14 ||2 ||12 |
|8 ||3 ||5 |
|Spacewatch ||3 ||1 ||2 |
|Other ||4 ||2 ||2 |
|Total ||29 ||8 ||21 |
(Total required for Spaceguard implementation: roughly 400 discoveries per year)