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FWD Re: (meteorobs) Doomsday Icarus




THE CLOSE APPROACH OF ASTEROID 1997XF11

Near-Earth asteroid 1997XF11, discovered on December 6 1997 by James Scotti
of the Arizona Spacewatch Program, will come very close to the Earth in
2028.  However, contrary to preliminary reports, there is no danger of its
colliding with the Earth in 2028. Like all Earth-crossing asteroids, XF11
may someday hit our planet, but this seems to be an event for the distant
future, and at present we are more at risk from some unknown asteroid
colliding with the Earth than from XF11 or any other object already
discovered.

The orbit of XF11 will bring it close to the Earth many times.  In October
2002 it will be an excellent target for detailed radar observations, and in
2028 it may even be bright enough to be seen without telescopic aid.

The special interest in this object began when International Astronomical
Union minor planet notice #6837 released by Brian Marsden on March 11
estimated a miss distance of only 50,000 km in its passage near Earth on
October 26 2028.  Marsden wrote in a press release reproduced below:
"Recent orbit computations indicate it is virtually certain that it will
pass within the Moon's distance of the Earth a little more than 30 years
from now.  The chance of an actual collision is small, but one is not
entirely out of the question."

The story was widely reported, with the following AP coverage typical:
WASHINGTON (AP) - An asteroid large enough to cause widespread destruction
may be heading toward a 2028 collision with the Earth and will certainly
pass closer to the planet than any such space object in modern times,
astronomers said Wednesday. ``The chance of an actual collision is small,
but one is not entirely out of the question,'' according to a notice filed
by the International Astronomical Union. ``It has enormous destructive
potential,'' said Steven Maran of the American Astronomical Society, but he
added it will take several more years of observations before experts are
certain of its path. ``It scares me. It really does,'' said Jack G. Hills,
an asteroid specialist at the Los Alamos National Laboratory. ``An object
this big hitting the Earth has the potential of killing many, many people.''

The London Times reported in a lead article by Nigel Hawes: Apocalypse
could be just 30 years away, astronomers said yesterday. They have
identified an asteroid a mile across on a near-collision course with Earth.
It is by far the most alarming object yet identified in the search for
asteroids and comets with Armageddon potential.

Following this announcement several astronomers searched older photographic
records to try to locate previously unrecognized observations of 1997XF11.
Eleanor Helin and her colleagues from the Jet Propulsion Lab soon found
images taken in 1990 that permitted calculation of an improved orbit for
the asteroid. Based on this expanded observation set, Don Yeomans and Paul
Chodas at JPL recalculated the orbit and found that the 2028 close approach
circumstances are: Time: 2028 Oct 26.26732 (06:24 UT) +/- 63 minutes;
closest approach distance = 0.00638 AU = 954340 km +/- 0.00058 AU; relative
velocity at closest approach = 13.914 km/s.  This means that the asteroid
will pass by the Earth at about twice the distance of the Moon, and that
the probability of impact with Earth is effectively zero. The JPL press
release providing this information is included below.

The discovery of 1997XF11 illustrates several aspects of the asteroid
impact hazard.  (1) When astronomers carry out searches, they typically
find even threatening asteroids decades to centuries before their actual
impact with the Earth. If it had turned out that XF11 posed a threat to
Earth in 2028, we would have had three decades to deal with this threat.
(2) With a diameter of about a mile, XF11 is near the threshold for global
disaster. The impact of an object this size with the Earth would release a
million megatons of energy and would probably lead to the death of hundreds
of millions of people.  (3) Most of the asteroids that could strike the
Earth and cause a global catastrophe have not yet been found.  For the year
2028 (or any other year) the chances of an unknown asteroid hitting the
Earth are much greater than the chances of this particular asteroid
hitting.  (4) if an unknown asteroid should hit us, we would likely have no
warning at all.  The first we would know of the danger is when we saw the
flash of light and felt the ground shake.  (5) At the current rate of
discovery, it will take more than a century to find 90% or more of the
objects this large with Earth-crossing orbits.  (6) For better or for
worse, the astronomers who carry out these searches and orbit calculations
work in the public eye.  The idea that a threatening asteroid could be kept
secret (or that anyone would want to keep it secret) is ludicrous

For further information see the NASA asteroid and comet impact hazard
website at:

http://impact.arc.nasadot gov

David Morrison

=========================================

ORIGINAL IAU RELEASE MARCH 11

ONE-MILE-WIDE ASTEROID TO PASS CLOSER THAN THE MOON IN 2028

Recent orbit computations on an asteroid discovered last December indicate
it is virtually certain that it will pass within the moon's distance of the
earth a little more than 30 years from now.  The chance of an actual
collision is small, but one is not entirely out of the question.

The asteroid, known as 1997 XF11, was discovered by Jim Scotti in the
course of the Spacewatch program at the University of Arizona.  This
program utilizes modern electronic technology on a 36-inch telescope at
Kitt Peak that was built 77 years ago.

After the discovery observations on December 6, observations made by two
Japanese amateur astronomers during the following two weeks showed that the
minimum distance between the orbits of 1997 XF11 and the earth was very
small.  Given also that the object was quite large as earth-approaching
asteroids go, perhaps one mile across, it was added to the list of
"potentially hazardous objects" (PHAs) that need to be monitored, lest they
are destined to come dangerously close to the earth over the course of the
next several centuries.  There are currently 108 PHAs.

As astronomers continued to gather data on 1997 XF11, it slowly began to
become apparent that there would be a particularly close approach to the
earth in October 2028.  A computation from observations spanning 60 days
suggested that the miss distance would be 500 thousand miles.  This
distance may seem large in human terms, but it was less than had previously
been predicted in advance for any other known asteroid during the
foreseeable future.

Observations made on March 3 and 4 by Peter Shelus with a 30-inch telescope
at the McDonald Observatory in western Texas extended the observed arc of
1997 XF11 to 88 days.  This time, the orbit computation indicated a miss
distance of only 30 thousand miles from the center of the earth; the
earth's radius is about 4 thousand miles.  The time of encounter would be
around 1:30 p.m. Eastern Daylight Time on Thursday, October 26, 2028.  That
evening the object should be visible with the naked eye.  In Europe, where
it would be dark by that time, the object should be a splendid sight as it
moves from northwest to southeast across the sky over a couple of hours.

There is still some uncertainty to the computation.  On the one hand, it is
possible that 1997 XF11 will come scarcely closer than the moon.  On the
other hand, the object could come significantly closer than 30 thousand
miles.  Further observations are necessary in order to refine the figures.
It is also possible that prediscovery observations of 1997 XF11 can be
located on archival photographs.  Particularly favorable opportunities for
recording the object would have occurred in 1990, 1983, 1976, 1971 and
1957.  Ephemerides for these times are available.

It is hoped that continuing observations will be made during the next few
months.  The object is starting to move into the dusk and to fade week by
week.  Nevertheless, it should be quite accessible for a while with large
telescopes, which in addition to helping establish whether a collision in
2028 is possible, could usefully provide more definite information about
the object's size.

Further observations of 1997 XF11 should be possible with moderate-sized
telescopes equipped with electronic sensors early in the year 2000.  A
better opportunity will occur in late 2002, when the object should be
detectable with quite modest telescopes.  On that occasion the closest
approach will be on Halloween, but the miss distance will be a safe 6
million miles.

Dr. Brian G. Marsden, IAU Central Bureau for
Astronomical Telegrams  617 495-7244
marsden@cfa.harvarddot edu
1998 March 11

===================================================

JET PROPULSION LABORATORY
FOR IMMEDIATE RELEASE,  March 12, 1998

ASTEROID WILL MISS EARTH BY "COMFORTABLE DISTANCE" IN 2028

Asteroid 1997 XF11 will pass well beyond the Moon's distance from Earth  in
October 2028 with a zero probability of impacting the planet,  according to
astronomers at NASA's Jet Propulsion Laboratory, Pasadena, CA.

The asteroid "is predicted to pass at a rather comfortable distance of
about 600,000 miles (about 960,000 kilometers) in 2028," reported Dr.
Donald K. Yeomans and Dr. Paul W. Chodas, JPL scientists who specialize in
computing the predicted orbits of comets, asteroids, planets and other
bodies in the solar system.

Data on the asteroid from March 1990 (well before its discovery in December
1997) was integrated into the orbit calculations by Yeomans and Chodas to
arrive at the distance the asteroid will pass Earth. The 1990 observations
of the object were found today in the Palomar Planet Crossing Asteroid
Survey conducted at Caltech's Palomar Observatory, by JPL's Eleanor Helin
and Ken Lawrence and by Brian Roman, formerly of JPL.

====================================================1997

XF11 Earth Close-Approach Update

Using prediscovery observations from March of 1990 made by JPL's Eleanor
Helin, Brian Roman, and Ken Lawrence at Palomar, we have computed a new
orbital solution for 1997 XF11.  The data arc is now much longer so the
solution is much more reliable.  The result of the new solution is that
the close approach distance is significantly increased (from 0.00058 to
0.00638 AU).

The resulting orbit is as follows:

Epoch   1998 Mar 30.0 TDB

e      0.48374055
q      0.74443182 AU
Tp     1997 Jul  1.191519  TDB
Node   214.12726 deg
w      102.47228 deg
i        4.09477 deg
a        1.441752196 AU
M      154.75017350 deg
102 observations over interval 1990 Mar. 23 to 1998 Mar. 4.
RMS residual = 0.644" (Mean Residual = 0.52")

 The close approach circumstances for 2028 are:
 Time: 2028 Oct 26.26732 (06:24 UT) +/- 63 minutes
 Closest approach dist. = 0.00638 AU = 954340 km +/- 0.00058 AU (3-sigma)
 Relative velocity at closest approach = 13.914 km/s

The computed probability of impact remains zero in our model.

The asteroid makes a close Earth-approach in 2002 Oct 31 at a distance of
0.06358 AU = 9,500,000 km +/- 0.0001 AU (3-sigma).  This will be a good
opportunity to make ground-based optical and radar observations, resulting
in an improved orbit solution.

Paul Chodas, Don Yeomans and Alan Chamberlin
JPL/NASA/Caltech