There is a small but significant chance that an asteroid will strike Earth in 2030 with a force up to 100 times the Hiroshima bomb, an international team of astronomers concluded Friday.
The International Astronomical Union and space scientists at NASA's Jet Propulsion Laboratory said there is a 1-in-500 chance that a newly discovered asteroid-like object called 2000 SG344 will hit Earth on Sept. 21, 2030. The object could be anything from a discarded rocket booster to a sizable asteroid.
The announcement, posted on the Internet by the International Astronomical Union, is the first formal public prediction of a potential collision with a piece of the cosmic debris that litters the solar system.
The warning arises from a special astronomical review process designed after a false alarm two years ago to eliminate premature predictions of celestial calamities.
On a newly devised 10-point scale for grading potential impact hazards, the object is just at the threshold of concern. It was given the lowest rating of 1 because of its relatively small size and the uncertain odds. The rating means the object merits careful monitoring.
The object was discovered trailing in Earth's orbit around the sun by astronomers using the 3.6-meter Canada-France-Hawaii telescope on the island of Hawaii. The probability that it might hit Earth in 2030 was first determined earlier this week by JPL researcher Paul Chodas. It was then verified over the last 72 hours by technical reviewers in Italy, Finland and the United States, organized by the International Astronomical Union.
"This is a first for us," said space scientist David Morrison at NASA's Ames Research Center, who is chairman of the Astronomical Union's working group on such collision hazards. "We have never before had a prediction at this high level of probability. In the past we have talked about 1 in 10,000 or 1 in a million."
In making their concerns public Friday, the astronomers walked a delicate line between prudent secrecy and public disclosure, weighing a chance of ridicule against their demands of public responsibility.
Two years ago, asteroid watchers at the Minor Planets Center in Cambridge, Mass., triggered worldwide alarm by announcing--and then almost immediately retracting--news that a mile-wide asteroid called XF-11 might hit Earth in 2028.
To avoid any repetition of that embarrassing incident, astronomers at NASA and the Astronomical Union recently devised a formal system in which any claims of a potential Earth impact would be verified quickly by a special technical review panel before being made public.
Although the new object is much smaller than XF-11, the probability of impact calculated so far is twice as high as the chances initially given that XF-11 would collide with Earth.
For the time being, no one knows yet just how large the SG344 object may be, its composition or the likelihood it would survive its fiery entry into the planet's atmosphere.
On one hand, the object might be a discarded Saturn rocket booster, lost in space since the days of the Apollo moon program and virtually certain to burn up on entry, said Brian Marsden, director of the Minor Planets Center at the Smithsonian Astrophysical Observatory in Cambridge. NASA records show that nine Saturn V rockets were launched toward the moon in the Apollo program. In each case, spent rocket boosters ended up in uncharted orbits around the sun.
"Its orbit suggests it might even be an old rocket we sent up to launch a satellite decades ago that has come back to haunt us," Marsden said. "At least, we can't exclude the possibility."
But it is equally likely that the object is an asteroid--between 100 and 230 feet in diameter--about the size of a multistory office building.
If it is an asteroid--as several experts believe is most likely--it could be a flying gravel pit of loosely compacted rubble that might easily disintegrate as it skims into the atmosphere. Or it could be a lethal ball of solid stone and iron that could explode on impact with an estimated energy of two megatons, experts said.
Without additional information, there is no way to narrow the range of uncertainty.
"That is our problem--we don't know what it is," said Donald Yeomans, manager of NASA's Near Earth Object Program Office at JPL. "All we know is how bright it appears to be, and so--based on that--we have to guess to come up with a diameter."
There may be as many as 100,000 asteroids of such size, considered relatively small in the cosmic scheme of things, scattered in the spaces between the planets.