SAN FRANCISCO—Imagine this scenario: Astronomers discover a previously unknown asteroid. Though it is millions of miles away, initial calculations suggest that it will, in about 15 years, pass dangerously close to Earth. And though its size is modest—about 100 meters at its widest—it is more than big enough to destroy a major city.
If this were a conventional Hollywood thriller, the plot might focus on how nuclear weapons would be deployed and launched to destroy the menacing asteroid. But for former U.S. astronauts Rusty Schweickart and Edward Lu, any such mission to save the Earth would be far more complex. To create the greatest chance of success, they say, it should begin with ambitious science diplomacy and technology research and development long before the asteroid is discovered.
In this illustration, a small, relatively simple "gravity-tractor" spacecraft is positioned in front of an asteroid in space, using gravity to "tow" it into a different orbit. [Image courtesy of Dan Durda / B612 Foundation / FIAAA]
In a symposium at the annual meeting of the AAAS Pacific Division, Schweickart and Lu suggested that novel technology is available that would allow humans to closely track such an asteroid and to redirect its orbit. What's lacking, they said, is political recognition that asteroids will periodically threaten Earth in the future—and that the time to plan and prepare is now.
"What do we do when we find one with our name on it?" asked Schweickart, the pilot during the first manned test of the lunar module during the Apollo 9 mission in 1969. Because so many nations could be affected, and because their interests and abilities are so diverse, that is "a big, difficult, geo-political issue."
"It's going to be very important to build public confidence when, 20 years from now, we discover a Near-Earth Object where there's one-in-10 chance that it will hit the Earth," he added. "That's going to send a panic around the world. At that point, it will be very important to persuade the public that these scientists know what they're doing and can succeed."
Rusty Schweickart, Edward Lu and Alan Harris
Schweickart and Lu were joined by Alan W. Harris, senior research scientist at the Space Science Institute , in a half-day symposium, "Near-Earth Objects: A Threat and Response Update." Their presentations offered a window onto a small community of space scientists who are gradually winning the attention of government leaders worldwide and at the United Nations.
They have proposed ambitious efforts to track and respond to threatening "Near-Earth Objects," or NEOs. The centerpiece of their strategy: A relatively simple, unmanned spacecraft that would fly to a suspect NEO and position itself close enough to exert a small pull of gravity; over a period of time, it would "tow" the object into a non-threatening orbit. NASA's Jet Propulsion Laboratory last fall concluded that the plan is viable.
The AAAS Pacific Division held its 90th annual meeting from 14-19 August at the California Academy of Sciences and San Francisco State University, drawing more than 475 scientists, engineers, teachers, students, journalists, and others. Marking the 150th anniversary of the publication of Charles Darwin's On the Origin of Species, the meeting focused on how nature changes through evolution, and how humans must build a sustainable relationship with nature to ensure the future health of all life.
But the events at the meeting included a diverse range of issues: the archaeology of shipwrecks; ecological awareness in American landscape art; the use of molecular biology kits in classrooms; the impact of climate change on San Francisco Bay; and communicating science to the public.
An Asteroid Impact "Every Few Hundred Years"?
Schweickart and Lu serve on the board of directors of the B612 Foundation, named for the asteroid that served as the home of the little prince in Le Petit Prince, the world literary classic by Antoine de Saint-Exupéry. The Foundation, a California-based non-profit, has set a challenging mission: "to significantly alter the orbit of an asteroid, in a controlled manner, by 2015."
That's not to say that an asteroid is expected by 2015. In fact, of the thousands of catalogued asteroids, comets and other near-Earth objects, or NEOs, none are on a known earth-intersecting orbit. But the scientists and others in B612 look to the number of NEOs in the solar system—and to history—and conclude that an-earth threatening asteroid is inevitable in our future.
The scientific consensus holds that a massive asteroid strike and resulting climate and atmospheric changes are at least partly to blame for the demise of dinosaurs 65 million years ago.
It's been only 101 years since an asteroid—about 40 meters wide, weighing about 200 million pounds, and traveling perhaps 33,500 miles per hour—exploded in the skies above the Siberian forest northwest of Lake Baikal, over the Podkamennaya Tunguska River. By some accounts , it may have been 250-400 times more powerful than the nuclear explosion that destroyed Hiroshima. Harris, at the 19 August symposium, said the Tunguska asteroid blast leveled an area the size of Washington, D.C.
A century later, in October 2008, sky-watchers discovered a small object that appeared to be headed toward Earth. There was scant warning: Over the course of 19 hours, Schweickart said, there were worldwide efforts to track the object dubbed TC3. Calculations placed the impact zone in Central Africa, and specifically in Sudan.
2008 TC3 was a small asteroid—two to five meters in diameter—and it hit just as predicted. When it caused no damage, scientists celebrated the first instance in which such a body was discovered in space and tracked to impact, with pieces of the object collected by students.
Asteroid Apophis (circled) was discovered on 19 June 2004. Image credit: UH/IA
It's not unusual for small meteoroids to hit Earth. But in 2004, astronomers found an asteroid they christened Apophis , after an ancient Egyptian god of destruction and darkness, and the discovery set off alarms. Apophis is about 240 meters across, and owing to the difficulty of projecting an asteroid's course, it appeared at first that there was a small probability that it would hit the Earth in 2029. Now astronomers see no chance of an impact then, but Schweickart said that there is a 1 in 45,000 chance of impact seven years later, on Easter Sunday in 2036.
Harris sees a larger point in the Tunguska asteroid and in risks posed by Apophis. "Damaging events may be a lot more frequent than we thought—not every 1000 years, but every few hundred years," he said.
Assessing Earth's Vulnerability
Currently, Harris said, researchers have identified an estimated 80% of asteroids bigger than 1 kilometer wide and 40% of those around Apophis' size. Congress has mandated that by NASA should, by 2020, detect and track 90% of NEOs that are 140 meters in diameter or larger. (The U.S. National Research Council is due to issue a report, by the end of the year, on how NASA can meet this goal.)
But as a new generation of telescopes come online, Harris said, the NEO count is likely to increase dramatically. In the decades ahead, he said, we may learn that there are "half-a-million Tunguskas floating around out there."
While the short-term odds remain in Earth's favor, Apophis had the effect of focusing attention and energy. If it were to hit Earth—even a glancing blow on the leading or trailing edge of the planet—it could cause regional devastation. Harris said the impact and possible tsunami could cause thousands of casualties and extensive destruction "within many hundreds to a thousand kilometers of the impact point." Impact by a much bigger asteroid could have global repercussions.
But tracking Apophis and understanding its course are challenges since all measurements are imperfect. Its current location makes it untrackable from Earth, and contact won't be reestablished until 2012. Nevertheless scientists know that it will pass close by Earth at 9 p.m. Greenwich Mean Time on Friday 13 April 2029. There's a slim chance that it could hit earth seven years later in 2036.
Schweickart said the level of risk could be determined by one quirky factor: When Apophis passes Earth in 2029, our planet will exert an unpredictable gravitational effect that could shift the asteroid's trajectory just enough to put it on a more certain course to hit Earth seven years later. The question is whether Apophis, on its close pass, will shoot through a gravitational "keyhole" about 600 meters wide in the space above Earth. If Apophis threads that needle, gravity could shift its orbit onto a potentially catastrophic course.
The combination of risk and uncertainty requires a concerted, coordinated effort to plan ahead, Schweickart and his colleagues said.
The "Gravity-Tractor"—a Space-Age Tow-Truck
While a nuclear strike against the threatening NEO might work, Schweickart called nukes a "blunt instrument" with an uncertain outcome. It would require a very accurate billiard shot across open space, and if the asteroid broke apart, Earth might be at risk from its fragments.
Schweickart and Lu see the "gravity tractor" spacecraft as far more subtle and surgical—and well within the capability of current technology.
Lu envisions partners on Earth responding to an approaching asteroid with a "deflection campaign" beginning 10 to 15 years before the potential impact. "Once you can put a transponder nearby," he explained, "you can know the distance and speed exactly" in a way not possible for scientists on Earth using telescopes.
That would allow scientists to calculate exactly the change in orbit needed to save the Earth. At that point, some sort of a direct spacecraft impact or a nuclear weapon fired from Earth could be used to hammer a NEO into a new orbit, Lu said.
Or, he said, the "gravity-tractor" could be carefully maneuvered into position close to the asteroid, and then serve as a space-age tow truck. The proximity alone would create a weak gravitational relationship between the two orbiting bodies, even if the asteroid was 200 meters wide and the craft weighed only a ton. By firing its thrusters off to the side, the tractor over a period of months could tug the asteroid into a new orbit.
After assessing the new orbit, control teams on earth could decide whether to seek further refinements. Even a small shift, at that distance, would be magnified as the asteroid continued toward Earth—and so would be enough to prevent an impact.
But timing is the key. "You've got to act 10 to 15 years ahead of time to allow a rendezvous to occur and let the deflection event take place and let that have its effect over a period of years," Schweickart explained.
"This is extremely simple control logic—it's not the least bit complicated," he added. "It sounds sexy as hell. You're not touching the asteroid, but you're affecting it. It really is very simple."
The Jet Propulsion Laboratory (JPL) conducted an evaluation at the request of the B612 Foundation, and issued a report  last October concluding that the gravity-tractor proved a "viable concept" in a simulation.
"And, in all likelihood, it would be a viable concept for the majority of impact scenarios, although further study is needed to confirm this," the JPL authors said. (However, they noted, the gravity-tractor might not be sufficiently powerful to counteract huge gravitational "keyholes.")
Schweickart told the AAAS Pacific Division the "gravity-tractor" should be built and tested soon on a non-threatening asteroid.
Can Science and Diplomacy Build Global Consensus?
For the long-term effort of preventing a deadly NEO impact, he said that nations of the world—and their scientists—need to cooperate now to put in place a plan for the future.
In a world preoccupied with economic crisis, war, and environmental problems, that could prove a challenge.
Currently, the United States is the only nation with an active, government-sponsored effort to track the NEO threat. The B612 Foundation and others are working to persuade the U.S. government and science community that the effort needs go beyond merely tracking asteroids to planning for a real threat.
Schweickart last October led a committee of the Association of Space Explorers , the professional organization of astronauts and cosmonauts, that met with United Nations officials  in Vienna, where they presented a report, Asteroid Threats: A Call for Global Response . In June, he made a presentation  to the U.N. Committee on the Peaceful Uses of Outer Space.
Planning for a possible asteroid threat raises a complex set of questions and issues. Who should pay for the research? How would the gravity-tractor be financed, and who would decide where and when it would be tested? What criteria would be used to decide which direction to deflect a threatening asteroid?
Even facing potential catastrophe, Schweickart said, global relations may fall into conflict since the process of deflecting an asteroid unavoidably involves a temporary shifting of risk to people not originally threatened in the process of eliminating the risk to everyone.
He would like to see an international network formed to address the asteroid threat, perhaps modeled on the Intergovernmental Panel on Climate Change. The network of experts, answering to the U.N. Security Council, could study and, in cooperation with the world's space agencies, advocate strategies for mitigating the risk.
"You can't get away from this being an international issue," Schweickart said, "and you want to have the arguments now, when it's hypothetical. You don't want power politics to be driving this at the last minute. You want reason, you want logic."