[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Summary On The Hazard Associated With Asteroid 1997 XF11

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

SUMMARY ON THE HAZARD ASSOCIATED WITH ASTEROID 1997XF11

* Asteroid 1997XF11 is among the NEOs with the smallest known MOID (Minimum
Orbital Intersection Distance); that is, its orbit approaches that of the
Earth very closely in three-dimensional space.  As such it will repeatedly
come close to the Earth and may well eventually strike our planet.  It is
therefore of great interest, both scientifically and as a long-term threat.
For NEOs like this that make such close planetary encounters, we cannot
reliably calculate detailed orbits for more than a century in the future.
XF11 will undoubtedly be closely tracked over the next decades and
centuries, as well as being an excellent target for scientific studies such
as radar imaging.

* Asteroid 1997XF11 never presented a significant hazard to the Earth at
its close passage in 2028.  Initially any calculated orbit is of course
uncertain, but once a few weeks of observations were available it would
have been clear, had anyone done the calculations, that XF11 could not
strike the Earth in 2028.  While there was considerable uncertainty in the
miss distance, ranging from about 25,000 km up to more than 700,000 km, all
the calculated points of closest approach, projected into the plane of
intersection, missed the Earth by a substantial margin.  Later, as new
observations were made in March and then pre-discovery observations were
located extending the observed arc to several years, the uncertainty in the
position quickly shrank.  However, these observations did not significantly
change the probability of impact in 2028, which was (and is) essentially
zero.

* Part of the initial confusion associated with public and media comments
on XF11 resulted from the fact that the Minor Planet Center (MPC) did not
calculate the impact probability, so their statement that "the chance of an
actual collision [in 2028] is small, but one is not entirely out of the
question" was largely subjective.  When JPL scientists made the first
formal calculation of the impact probability, they realized within an hour
of addressing the issue that the probability of impact by XF11 in 2028 was
essentially zero, a conclusion since verified by several further
calculations using different approaches, and confirmed in IAU Circular 6879.

* Because asteroid 1997XF11 has an unusually small MOID, even among the
one-hundred-odd known Potentially Hazardous Asteroids (PHAs), it warrants
continued attention from a hazard perspective.  XF11 will make several
close passes by the Earth in the next century; however, none of these
appears to represent a hazard, and the risk of impact in 2037, which is the
only post-2028 close approach so far investigated in quantitative detail,
is effectively zero.  Further near-term improvements in the orbit of XF11
(the asteroid is easily radar detectable in 2002) will lead to even more
certain results concerning future impact risks.

* Any discussion of impact hazard from a known NEO should be placed in the
context of the background hazard we all live with due to undiscovered NEOs.
In any year there is a probability of roughly one in 100,000 of the Earth
being hit, with little or no warning, by an unknown object 1 km or greater
in diameter.  In any year the chances may be as high as one in 100 of being
hit by an undiscovered object similar in magnitude to the Tunguska event of
1908.  As a larger and larger fraction of the NEOs are discovered, this
risk from unknown NEOs declines; that is in part the purpose of searching
for these objects.  No known NEO, including XF11, poses a threat of
striking the Earth within the next century that is nearly as high as this
background risk due to unknown objects.

Rick Binzel
Ted Bowell
Clark Chapman
Paul Chodas
Paolo Farinella
Al Harris
Andrea Milani
David Morrison
Steve Ostro
Don Yeomans