University Times

At 5:08 p.m. Eastern time on March 18, a 100-foot-wide space rock buzzed within 26,500 miles of Earth. That’s just 11 percent of the distance between our planet and the moon.

It was the closest recorded near miss (or near hit) between Earth and an asteroid.

Small by asteroidal standards, the rock probably would have burned up — in a spectacular fireball — had it entered our atmosphere, NASA officials said. Asteroids need to be at least 150 feet across to stand a good chance of penetrating the Earth’s thick atmosphere and striking the planet, although smaller ones do get through.

(Last September, for example, a basketball-sized rock punched through the roof of a New Orleans house, passing through two floors before shattering in the ground beneath the house. Tests conducted at Tulane University on fragments of the charred, sandy-colored intruder revealed it to have been a meteorite.)

According to Bruce W. Hapke, a professor in Pitt’s Department of Geology and Planetary Science, the alarming aspects of last month’s close encounter with the 100-foot-wide asteroid were that:

• Astronomers received little warning of the object’s approach, and

• There is no agreed-upon protocol for alerting government officials and the public about incoming Near Earth Objects (NEOs), let alone a proven defense against them.

“The ones that have come close to hitting us have only been spotted within a day or two of coming close to the Earth, and in some cases they had already gone by us before we were even aware of them,” Hapke said.

In collaboration with the U.S. Air Force, NASA in 1998 started its Spaceguard survey program, aimed at discovering and plotting the orbits of NEOs with diameters greater than 1 kilometer. The goal is to track 90 percent of such asteroids (known as “civilization enders” among astronomers) by 2008. Thus far, NASA has tracked more than 2,650. None has been found to be on a collision course with Earth.

But an estimated 1 million NEOs ranging in size from 300 feet to 18.5 miles in diameter are believed to be out there.

“Even an asteroid just 30 feet in diameter coming in at 15 miles per second could knock out a city the size of Pittsburgh if it penetrated our atmosphere and happened to hit an inhabited area,” Hapke pointed out. “It would have the impact of a small atomic bomb.”

Such an object appeared to be hurtling toward Earth on Jan. 13, 2004. That evening, a British amateur astronomer logged onto the web site of the Minor Planets Center at the Harvard-Smithsonian Center for Astrophysics and noticed the posted orbit of a space rock named AL00667; the orbit indicated that the asteroid would increase in brightness 40 times over in the next 24 hours.

“Like bright headlights on a dark road, this could mean only one thing: AL00667 was heading straight toward us,” noted the web site of The Planetary Society, the world’s largest nongovernmental space advocacy group.

Several professional astronomers, both independently and after being alerted via e-mails and chat room exchanges, also grew alarmed by the apparent collision course with AL00667. Scientists at NASA’s Jet Propulsion Laboratory hurriedly calculated that the asteroid had a one-in-four chance of striking the Earth’s Northern Hemisphere within 36 hours.

No one was sure, though, how or when to warn government authorities. The chairperson of the International Astronomical Union’s Working Group on Near Earth Objects contemplated telephoning the White House but decided instead to wait until more precise observations of AL00667 could be made.

Those observations would reveal that the rock (later re-designated 2002AS1) was, in fact, much bigger — 1,500 feet wide — and farther away from Earth than first calculated. But while it zipped past our planet with 8 million miles to spare, 2002AS1 left controversy in its wake.

Pitt geology and planetary sciences chairperson William Harbert said: “A lot of people, including me, were shocked that this situation would be taken up not by some federal agency but by a tag team of individual scientists, working more or less independently. The magnitude of the hazard posed by near-Earth asteroids and comets is such that we need an integrated approach.”

The need for one organization to take charge when a hazardous NEO is discovered has been discussed at various scientific meetings over the years. But, for all the talk, no anti-NEO defense plan has yet been formulated.

In September 2002, 77 scientists from around the world attending the Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids concluded that NASA should take charge of warning about encroaching NEOs.

Two months ago, some 120 scientists and policy-makers attended the Planetary Defense Conference, sponsored by the American Institute of Aeronautics and Astronautics. In the conference’s keynote address, Congressman Dana Rohrabacher (R-Calif.) compared apathy about NEOs with public indifference to terrorism prior to 9/11. Will it likewise take a catastrophe to alert people to the NEO impact threat? asked Rohrabacher, who recently introduced legislation to authorize $20 million per year for each of the next two fiscal years for NASA to search for small NEOs.

Planetary Defense Conference attendees also heard reports on the possible use of nuclear-tipped ballistic missile interceptors or other nuclear devices to disperse or deflect incoming asteroids, a la Hollywood movies such as “Armageddon” and “Deep Impact.” The bad news: Nukes probably wouldn’t work against asteroids 1 kilometer wide and larger, which are believed to strike the Earth once every 800,000 years. Also, testing of anti-NEO nuclear devices would be banned under current international treaties forbidding any kind of nuclear tests in space, whether for weapons or peaceful purposes.

Pitt’s Harbert recently co-developed a training course on geohazards (earthquakes, volcanoes, landslides and tsunamis, as well as NEO impacts) for an online university. The course is intended for geographical information systems professionals although, as part of the site license, Harbert’s students at Pitt have free access to it.

“Having analyzed these various geohazards,” Harbert said, “my opinion is that while asteroid and comet impacts are not as immediately threatening as, say, earthquakes, they represent the ultimate in terms of global devastation. There’s no comparison, really. And while the threat is remote, impacts certainly have occurred in the past.”

In 1908, an asteroid or comet exploded over virtually uninhabited Tunguska, Siberia, with the force of 800 Hiroshima bombs, creating an air burst that flattened a section of forest the size of Rhode Island. Had that NEO been delayed by four hours, it would have exploded over St. Petersburg.

And, of course, 65 million years ago an asteroid believed to measure 10 kilometers in diameter smacked into the Earth just under Mexico’s Yucatan Peninsula. It left a crater 120 miles in diameter and (depending on your favorite extinction theory) either killed off or delivered the coup de grace to the dinosaurs.

“This sort of impact appears, from the geological record, to occur about once every 111 million years,” said Harbert, who calculated that the tsunami resulting from such an impact today would drown 15 million to 20 million people along the Gulf Coast and the eastern coast of the United States. Many, many more people — quite possibly, the rest of humanity — would die from starvation and disease during the ensuing global “impact winter,” caused by dirt and other debris being thrust into the atmosphere and blocking sunlight, he said.

David A. Turnshek, a professor in Pitt’s physics and astronomy department, pointed out that Galileo constructed the first astronomical telescope in 1609, “meaning that we’ve only been able to really look out into the solar system in detail for less than 400 years. And already, we’ve seen a major impact on one of the planets in our solar system” — the awe-inspiring collision in 1994 between Jupiter and the comet Shoemaker-Levy 9. Two dozen fragments of the comet (shattered by Jupiter’s powerful tides) slammed into the planet, exploding with such force that dark clouds the size of the Earth formed in Jupiter’s atmosphere.

“A message from Jupiter: Catastrophes happen,” observed the NASA@science web site.

For all of the apocalyptic speculation about killer asteroids and comets, William A. Cassidy — Pitt professor emeritus of geology and planetary sciences, and the University’s pre-eminent meteorite hunter — noted that there has been only one confirmed NEO-related human injury: In the 1940s, a woman in Sylacauga, Ga. was napping on her couch when a softball-sized fragment of a meteorite crashed through the roof of her house, bounced off her Victrola and struck her on the hip, leaving an ugly bruise.

Another reported casualty is “somewhat problematic. We can’t confirm for certain that it happened,” said Cassidy, who in 1976-77 founded the Antarctic Search for Meteorites program (which continues, although Cassidy is no longer its principal investigator).

The incident allegedly occurred when a meteorite fell on Egypt, likewise in the 1940s. “This meteorite, strangely enough, was a fragment of Mars. A newspaper report said that it killed a dog. If so, that’s the only mammalian fatality that we’re aware of as the direct result of a meteorite impact,” Cassidy said with a chuckle.

Regarding the overall risk of NEO impacts, Cassidy said: “I don’t think there’s much of a threat, really. Of course, I could be just whistling past the graveyard. Most stony meteorites — and that’s 90 percent of all meteorites — break up in the atmosphere and fall in relatively small pieces. Some iron meteorites do survive, and they’re the ones that make impact craters on the Earth’s surface. You might get one of them every 4,000 years or so, and they would still affect only a very tiny portion of the planet’s surface. And, of course, three-quarters of the Earth is covered by ocean.”

NASA’s Ames Research Center acknowledges, “We don’t know when the next NEO impact will take place, but we can calculate the odds. Statistically, the greatest danger is from an NEO with about 1 million megatons energy (roughly 2 kilometers in diameter).

“On average, one of these collides with the Earth once or twice per million years, producing a global catastrophe that would kill a substantial (but unknown) fraction of the Earth’s human population.

“Reduced to personal terms, this means that you have about one chance in 20,000 of dying as a result of a collision. Such statistics are interesting, but they don’t tell you, of course, when the next catastrophic impact will take place — next year or a million years from now.”

—Bruce Steele