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Spaceguard Asteroid Survey: How Are We Doing?

Forwarded from David Morrison (dmorrison@arc.nasa.gov)

SPACEGUARD ASTEROID SURVEY: HOW ARE WE DOING?
August 26, 1998

The goal of the Spacewatch 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 today (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 18.5, with
perihelion distances less than 1.3 AU.  There are approximately 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 2000 total NEAs, is sufficient to asses the current performance
of the survey.

Note that many of the NEAs discovered are smaller than 1 km (H > 18.5).
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 85% of the discoveries.  LINEAR has dominated
the recent growth, going from 0 to 4 to 16 NEAs (D > 1 km) in the last
three 6-month periods.

The table below shows 38 NEAs larger than 1 km discovered from July 1997
through June 1998.  There was a marked increase in discovery rate over this
period, from 11 NEAs discovered in the first 6 months to 27 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 2000 NEAs in a decade, we must
increase the discovery rate by approximately a factor of 12 over the
average for the past 12 months, or a factor of 9 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 the Spaceguard Survey.

David Morrison


NEA DISCOVERY SUMMARY (D > 1 km) FOR JULY 97 THRU JUNE 98

Discoverer	12 month	1997-2		1998-1
-----------------------------------------------------------------------
LINEAR		20		 4		16
NEAT		 9		 4		 5
Spacewatch	 4		 1		 3
Other		 5		 2		 3
----------------------------------------------------------------------
Total		38		11		27

(Total required to implement the Spaceguard Survey: >400 discoveries/year)


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