Astronomer David Tholen spotted it last year in the early evening of June 19, using the University of Arizona's Bok telescope. It was a new "near-Earth object," a fugitive asteroid wandering through space to pass close to Earth.
Tholen's team took three pictures that night and three the next night, but storm clouds and the moon blocked further observations. They reported their fixes to the Minor Planet Center in Cambridge, Mass., and moved on.
Six months later, Tholen's object was spotted again in Australia as asteroid "2004 MN4." In the space of five days straddling Christmas, startled astronomers refined their calculations as the probability of the 1,000-foot-wide stone missile hitting Earth rose from one chance in 170 to one in 38.
They had never measured anything as potentially dangerous to Earth. Impact would come on Friday the 13th in April 2029.
The holidays and the tsunami in South Asia pushed 2004 MN4 out of the news, and in the meantime additional observations showed that the asteroid would miss, but only by 15,000 to 25,000 miles -- about one-tenth the distance to the moon. Asteroid 2004 MN4 was no false alarm. Instead, it has provided the world with the best evidence yet that a catastrophic encounter with a rogue visitor from space is not only possible but probably inevitable.
It also demonstrated the tenacity of the small band of professionals and amateurs who track potential impact asteroids, and highlighted the shortcomings of an international system that pays scant attention to their work.
"I used to say the total number of people interested in this was no more than one shift at a McDonald's restaurant," said David Morrison, an astronomer at NASA's Ames Research Center and a student of near-Earth objects for nearly three decades. "Now it's maybe two shifts." Awareness of the apocalyptic potential of near-Earth objects has been slow to develop. It took years for Nobel laureate Luis Alvarez and his son Walter to win acceptance for their 1980 research showing that a near-Earth object impact quite likely caused the extinction of the dinosaurs 65 million years ago.
"The human brain wouldn't grasp reality until it had somewhat more direct evidence," said Colorado-based planetary scientist Clark R. Chapman of the Southwest Research Institute, another longtime expert on near-Earth objects. "Alvarez provided that."
The vast majority of near-Earth objects are asteroids -- huge rocks or chunks of iron that travel around the sun in eccentric orbits that cross Earth's path periodically. The rest are comets -- ancient piles of dust, stones and ice that come in from the edges of the solar system.
"The good news is that comets represent 1 percent of the danger," said Donald K. Yeomans, who manages NASA's Near-Earth Object Program at the Jet Propulsion Laboratory. "The bad news is that should we find one, there's not a lot we can do about it. . . . We detect them only nine months from impact."
Asteroids, by contrast, generally offer decades or even centuries of warning -- unless they are too small to detect, in which case there is no warning at all. But today's technology enables astronomers to get a fix on any asteroid capable of causing a global "extinction event" -- six miles in diameter or bigger.
Asteroid 2004 MN4 is a "regional" hazard -- big enough to flatten Texas or a couple of European countries with an impact equivalent to 10,000 megatons of dynamite -- more than all the nuclear weapons in the world. Even though it will be a near miss in 2029, that will not be the last word.
"You don't know what the gravitational effect of the Earth will be," said Brian G. Marsden, who oversees the hunt for near-Earth objects as director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics.
"In 2029, the [close encounter with] Earth will increase the size of the orbit, and the object could get into a resonance with the Earth," he added. "You could get orbit matchups every five years or nine years, or something in between." In fact, 2004 MN4 could come close again in 2034, 2035, 2036, 2037, 2038 or later.