A week's warning could save an untold number of lives

In ""Too Hard for Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as particle accelerators as big as the sun, or they might be completely unethical, such as lethal experiments involving people. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard for Science?" suggests that nothing might be impossible.

The scientist: John Tonry, professor with the Institute for Astronomy at the University of Hawaii.

The idea: An early warning system against cosmic impacts could give the Earth a week's notice or more before a city-destroying strike, Tonry suggests.

In 2009, a bus-sized asteroid roughly 10 meters long exploded over an isolated part of Indonesia, packing the equivalent of roughly 50,000 tons of TNT, more than three times the strength of the atomic bomb dropped on Hiroshima, he notes.

Although the rate of impacts by large asteroids 140 meters and larger is estimated at just one per 20,000 years or more, the risk of a roughly 50-meter-long object such as the one that might have devastated Tunguska in Siberia in 1908 is about once every millennium while that of a 20- to 30-meter-long asteroid is once every century.

Although no one was reported hurt at the Indonesian blast, Tonry notes such 10-meter-long objects are expected once per decade, underscoring the need for an early warning system against cosmic impacts. Instead of developing a network of telescopes that can find such asteroids years in advance — an effort that can take decades and hundreds of millions of dollars to build — Tonry and his colleagues suggest a network that can give days or weeks of warning. While this is not enough time to deflect an impact given current technologies, it could at least give the authorities enough time to evacuate the area under threat.

The network Tonry and his collaborators propose is called the Asteroid Terrestrial-impact Last Alert System, or ATLAS, and could pinpoint the location and time of an impact to a few miles and a few seconds.

"The difference between asteroid impact prediction and, say, tsunami warning or hurricane prediction is that asteroid impact prediction is pretty darned near exact," Tonry says. "2008TC3 came down over the Sudan after being discovered just one day earlier. The JPL prediction of where it would hit was accurate to about one mile and the time of impact was accurate to about one second."

ATLAS would consist of two observatories separated about 100 kilometers that would scan the visible sky together twice a night. Each would house four relatively small telescopes 25 centimeters in aperture, as well as a camera for each telescope. The distance between the observatories would provide a means for separating nearby and distant moving objects.

ATLAS could give three weeks' warning for 140-meter-long impactors and one week for 50-meter-long objects. Unfortunately, the smaller the object, the less warning there would be — 20-to-30-meter-long asteroids might get two or three days warning, while 10-meter-long objects might get one. Still, that should be enough time to evacuate small towns. "If it's going to hit in Conway, New Hampshire, it's pretty easy for local authorities to inform everybody and get them moved in time to watch the explosion from a safe distance," Tonry says.

The system could detect more than half of impactors larger than 50 meters long and nearly two-thirds of those 140 meters long. Its chances go up the more telescopes ATLAS has, as these help compensate for cloudy weather or lack of coverage in the Southern Hemisphere. However, the network's detection rate can never go higher than roughly 75 percent, Tonry warns, since it cannot spot objects coming from the blinding direction of the sun.

The problem: Although Tonry and his colleagues submitted a $3 million proposal for ATLAS to NASA to operate them for two years, the current budgetary problems the U.S. government faces meant the agency basically declined to fund any 2010 proposals, Tonry says. "I'll resubmit in June."

The solution? "The main challenge ATLAS faces is proving that it's necessary," Tonry says. "Hopefully it won't take a tragedy to do so."

In addition to helping spot incoming cosmic impacts, ATLAS could monitor the sky for a wide range of other phenomena as well — detection of all dwarf planets in the solar system, twice per night monitoring of about most variable stars in the galaxy, novae in most nearby galaxies, the variability of about 100,000 active galactic nuclei and quasars, and more.

"There's also a lot of interest at NASA these days in a manned mission to an asteroid, but the list of suitable asteroids is very, very lean," Tonry adds. "ATLAS would help find more asteroids that a manned mission could reach."

Image of John Tonry from the University of Hawaii.


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About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time he has traveled to all seven continents. Follow him on Twitter @cqchoi.

The views expressed are those of the author and are not necessarily those of Scientific American.