The Spaceguard Survey focuses on surveying near Earth asteroids 1 km or larger in diameter, since these NEAs are both the easiest to discover and track, and the ones that dominate the impact hazard for us on Earth.
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 of where we stand at the end of 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 more small asteroids than 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 four most successful searches during the past 18 months 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), Spacewatch (the search carried out for more than a decade at the University of Arizona), and the new LONEOS (Lowell Observatory NEO Survey) program that just started operation in the second half of 1998. Together these four accounted for almost 90% of the discoveries. LINEAR has dominated the growth, going from 0 to 2 to 10 to 26 NEAs (D > 1 km) discovered in the last four 6-month periods.
The table below shows 62 NEAs larger than 1 km discovered in he 18 months from July 1997 through December 1998. This is an update of the table posted previously, adding data for the second half of 1998. Note one change in the previous numbers: LINEAR now gets credit for only 10, rather than 12, discoveries H < 18.0, in the first half of 1998. As orbits and magnitudes are revised with further observations, estimates of H values can become fainter (or brighter) than 18.0, and computed perihelia values can become greater (or less) than 1.30 for the same NEA, influencing the way these numbers are recorded. However, the relative stability of the numbers indicates that the results are statistically valid. In addition to a new column for 98(2), we have added a new row, for LONEOS.
| NEA DISCOVERY SUMMARY (D > 1 km) JULY 97 THRU DEC 98 |
|Discoverer ||97(2) ||98(1) ||98(2) ||98(1+2) |
|LINEAR ||2 ||10 ||26 ||36 |
|NEAT ||3 ||5 ||2 ||7 |
|Spacewatch ||1 ||2 ||1 ||3 |
|LONEOS ||0 ||0 ||4 ||4 |
|Other ||2 ||2 ||2 ||4 |
|Total ||8 ||19 ||35 ||54 |
The average rate of discovery of large NEOs for 1998 was 54/yr or almost 5/mo. The discovery rate in the second half of 1998 was actually a factor of 5 greater than the same six-month period in 1997.
In a ten-year Spaceguard survey expected to detect 90% of the NEA population with D > 1 km, 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 need an increase by about a factor of 8 over the 1998 average values.
The NEO discovery rate is accelerating due primarily to the continuing success of LINEAR and now the recent addition of LONEOS. If this trend continues, we should be within a factor of a few of the Spaceguard capability by the end of 1999.
Alan Harris and David Morrison (15 Jan 1999)