(March 11, 5:24 pm) WASHINGTON (AP) - A mile-wide asteroid described as ``the most dangerous one we've found so far'' may be on course for a 2028 collision with Earth and certainly will pass closer than any such 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. But asteroid specialist Jack G. Hills said the speeding space rock, called asteroid 1997 XF11, poses a real danger to Earth. ``This is the first really big one to pass this close,'' said Hills, a Los Alamos National Laboratory scientist. ``This is the most dangerous one we've found so far.'' He added: ``It scares me. It really does. An object this big hitting the Earth has the potential of killing many, many people.'' ``It has enormous destructive potential,'' agreed 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 Asteroid 1997 XF11 was discovered Dec. 6 by the University of Arizona Spacewatch program and was added to a list of 108 asteroids considered to be``potentially hazardous objects.'' Maran noted that no asteroid the size of 1997 XF11 has ever been predicted to pass so close to the Earth. Asteroids are routinely observed and plotted by astronomers around the world because of their potential for great destruction on Earth. An asteroid 6 to 10 miles across collided with the Earth about 65 million years ago and is thought to have caused the extinction of the dinosaurs, along with 75 percent of all other species. Hills said an asteroid the size of 1997 XF11 colliding with the Earth at more than 17,000 miles an hour would explode with an energy of about 320,000 megatons of dynamite. That equals almost 2 million Hiroshima-sized atomic bombs. Such an asteroid hitting the ocean, said Hills, would create a tidal wave hundreds of feet high, causing extreme flooding for thousands of miles of coast line. ``If one like this hit in the Atlantic Ocean, all of the coastal cities would be scoured by the tsunami,'' said Hills. ``Where cities stood, there would be only mudflats.'' If such an asteroid hit on land, he said, it would instantly dig a crater 20 miles across and so clog the sky with dust and vapor that the sun would be darkened ``for weeks, if not months.'' Maran said the best estimate is that the mile-wide 1997 XF11 will pass inside the orbit of the moon, with the most likely separation from the center of the Earth of about 30,000 miles. The Earth has a radius of about 4,000 miles. The estimate, said Maran, has a margin of error of more than 180,000 miles. This means a collision with Earth is theoretically possible, but uncertain at this time, he said. Better estimates of the collision potential will be generated as astronomers plot the course of the asteroid through the heavens over the next few years. Observations made earlier this month by University of Texas astronomers indicated the asteroid would make its nearest approach to the Earth on Oct. 26, 2028, at about 1:30 p.m. EDT. ``There is still some uncertainty to the computation,'' said the bulletin from the International Astronomical Union. The notice said the asteroid, which is on a wide-swinging, independent orbit of the sun, will move out of view to all but the largest telescopes over the next few months. It will become more visible once again in 2000. And in 2002,it is expected to pass within about 6 million miles of Earth on Halloween Eve. Hills said the asteroid is lost from view when it passes behind the sun, but that it will emerge into telescope range about every two years. Astronomers eventually will be able to track the object using radar, he said,and this will enable them to establish a precise orbital path years ahead of the possible impact. Only then, said Hills, will the true risk of collision be known. Experts long concerned about the potential danger of asteroids have said the Earth could be protected by exploding a missile near the speeding rock while it was far away. The intent would be to nudge the asteroid onto a path that would send it safely away from the planet. ``If it is going to hit the Earth, there's no question that we should try to deflect it,'' said Hills. ``It would be money well spent.'' ---------------------------------------------------- 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, 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)