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

Recent orbit computations on an asteroid discovered last December indicated that it was virtually certain that it would pass within the moon's distance of the earth a little more than 30 years from now. Refined computations, based on prediscovery images from 1990, show that the miss distance is now a rather comfortable 600 thousand miles.

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 asteroids" (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.

There was still some uncertainty to the computation. On the one hand, it was possible that 1997 XF11 would come scarcely closer than the moon. On the other hand, the object could come significantly closer than 30 thousand miles. Further observations were necessary in order to refine the figures. It was also possible that prediscovery observations of 1997 XF11 could 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.

Prediscovery images of 1997 XF11 were located by Ken Lawrence (JPL) on films taken at Palomar in 1990 during the Planet-Crossing Asteroid Survey by Eleanor Helin, Lawrence and Brian Roman using the 0.46-m Schmidt. The observations, on 1990 Mar. 22 and 23, were consistent with each other and allowed a much improved orbit to be derived for 1997 XF11. These latest computations show that the nominal miss distance will be 600 thousand miles on 2028 Oct. 26 around 2:30 a.m. Eastern Daylight Time. Although the chances for an actual collision with the earth in 2028 can now be discounted, the orbit of 1997 XF11 will continue to come close to the orbit of the earth for the forseeable future. Furhtur future notable close approaches to the earth will occur in 2002 (5.9 million miles), 2078 (7.5 million miles), 2090 (4.2 million miles) and 2095 (1.3 million miles).

The predicted 2028 approach distance of 600 thousand miles will be the closest predicted for any PHA up to that time. In 2086 the substantially smaller asteroid (2340) Hathor will come to a distance of 550 thousand miles from the earth.

A list of the closest known minor-planet approaches to the earth is available, as is a list of closest known approaches by comets. Also available is a list of predicted approaches to the earth within the next 33 years.

Brian G. Marsden (1998 March 11, updated March 12)

An orbit diagram for 1997 XF11 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 portion of the orbit of 1997 XF11 that lies below the ecliptic plane is shaded. The positions of the planetary bodies, marked by colored dots, are for 2028 Oct. 26--although 600 thousand miles separate them, at the scale of this diagram the dots for the earth and 1997 XF11 overlap.

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