WASHINGTON -- 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."