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One-Mile-Wide Asteroid To Pass Closer Than The Moon In 2028

http://cfa-www.harvard.edu/iau/pressreleases/1997XF11.html

PRESS INFORMATION SHEET:

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

Produced at the Harvard-Smithsonian Center for Astrophysics (CfA),
Cambridge, Massachusetts, U.S.A.

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Recent orbit computations on an asteroid discovered last December indicate
it 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, as is a current ephemeris.

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.

A list of the closest known minor-planet approaches to the earth is
available (http://cfa-www.harvard.edu/iau/lists/Closest.html), as is a list 
of closest known approaches by comets 
(http://cfa-www.harvard.edu/iau/lists/ClosestComets.html). Also
available is a list of predicted approaches to the earth within the next 25
years (http://cfa-www.harvard.edu/iau/lists/CloseApp.html).

Brian G. Marsden (1998 March 11)

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An orbit diagram for 1997 XF11 (http://cfa-www.harvard.edu/iau/plot/1997XF11.gif)
showing the orbit in relation to the orbits
of Venus, Earth and Mars is available. The view is from the north ecliptic
pole and all objects revolve about the sun in a counter-clockwise direction
(the vernal equinox is off to the right). The ascending node of the orbit of
1997 XF11 is marked with a capital omega and the perihelion point with `P'.
The portion of the orbit that lies below the ecliptic plane is shaded. The
positions of the planetary bodies are for 1998 Feb. 13.

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