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.
Other relevent materials:
Summary on the Hazard Associated with Asteroid 1997XF11:http://impact.arc.nasa.gov/news/1998/jun/12.htm
Original IAU Release , March 11, 1998:"http://impact.arc.nasa.gov/news/1998/mar/16a.html
Jet Propulsion Laboratory For Immediate Release, March 12, 1998:http://impact.arc.nasa.gov/news/1998/mar/16b.html
XF11 Earth Close-Approach Update:http://impact.arc.nasa.gov/news/1998/mar/16c.html
-- David Morrison