1997XF11 AND ITS POSSIBLE HAZARD TO THE EARTH                                      

 --- Status Summary From:


Article Posted: June 12, 1998

The following is a brief summary, signed by a substantial fraction of
the international experts in celestial dynamics, concerning asteroid
1997XF11 and its possible hazard to the Earth (see news item of March
16, 1998:  http://web99.arc.nasa.gov/impact/news_detail.cfm?ID=63).

The bottom line is that this asteroid poses no danger for at least the
next century, but since it does make several close passes by the
Earth, it will be an interesting target for future studies from both
the scientific and hazard perspectives.


Asteroid 1997XF11 is among the near-Earth objects (NEOs) with the
smallest known minimum orbital intersection distance (MOID); 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