Estimating the NEO Population and Impact Risk:
Past, Present and Future
A. W. Harris
Space Science Institute, Boulder, Colorado, USA
Abstract:
As Near-Earth Object (NEO) surveys continue, we improve our
knowledge of the population of NEOs and become better able to estimate both
the total population versus size and the fraction that remains undiscovered.
As of January 19, 2009, the present surveys had discovered 765 Near-Earth
Asteroids (NEAs) larger than 1 km in diameter out of an estimated total
population of 940, or about 81 % of that population. Since most of the
risk resides in the largest impactors, finding that fraction of the
largest bodies, and that none of them has a significant chance of
impacting in the next century or so, "retires" more than 90 % of
the impact risk, including most of the risk of a globally catastrophic
impact event. It appears that ground damage extends to considerably
smaller impactor sizes than was previously inferred by modeling them
as equivalent to nuclear airbursts. This increases the expected frequency
of damaging events, although it only modestly increases the "fatality rate",
since the smallest events are not very damaging. In the mid-size range,
from ~150 m to ~1 km, the main risk is from tsunami generated by an ocean
impact. The detailed analyses of the 2003 NASA SDT report estimated
a "persons affected" rate of ~182 per year associated with impact tsunami.
They did mention that for earthquake-generated tsunami, the actual death
rate is typically only 10 % or less of the population in the inundation zone,
but did not take full account of that in their risk analysis.
Here we re-evaluate the impact hazard, using our new population and completion
estimates, and revised "kill curves" including the airburst damage down
to smaller size and lower tsunami fatality rate. We estimate that the a
priori impact risk (not allowing for any discovered NEAs)
is (was) ~40/year for local/regional land impacts, ~6/year from
Key words:
Very Long Baseline Interferometry (VLBI), Near Earth Objects (NEOs), impact and deflection of NEOs, a mission for scientific investigation of comets, planning for mitigation.