"Killer Asteroid" Debate Pits Gravity Tractors Against Bombs, Projectiles

Victoria Jaggard
National Geographic News
March 8, 2007
An unusual type of arms race involving nuclear bombs and supermassive spacecraft has been heating up this week in Washington, D.C.

Each team of players hopes to be the one to design the U.S. government's weapon of choice for deflecting so-called near-Earth objects (NEOs)—comets and asteroids that could be on a collision course with Earth.

The debate has been raging among experts about which solution will be the safest, cheapest, and most reliable.

Ed Lu, a NASA astronaut and physicist, has been developing one of the leading contenders: a "gravitational tractor."

The proposed craft—weighing up to 20 tons (18 metric tons)—could nudge a 656-foot-diameter (200-meter-diameter) asteroid into a new orbit by the sheer force of the ship's gravity.

(Read "'Gravity Tractor,' Super Telescopes Enlisted to Battle Killer Asteroids" [February 17, 2007].)

"We have a controllable method right now" to deflect many potential NEOs, Lu said of the gravity tractor this week during the 2007 Planetary Defense Conference in Washington.

But "it's not ready yet," Lu told National Geographic News. "The technology is familiar, but it's not a case where we have one on the shelf we could send out tomorrow."

Nukes in Space?

Other experts argue that gravity tractors and other "slow push" methods are only viable when scientists have decades of warning before a potential impact.

Jesse Koenig of SpaceDev, Inc., made a case at the conference that the more timely solution is to use kinetic impactors.

Impactors are spacecraft intentionally crashed into an asteroid at high speed. The plan, Koenig said, is a simple, cheap way to solve the problem with only a few years' notice.

"If we needed to actually use it for a real threat, we could use it now," he said.

"We [already] have good targeting algorithms and software," Koenig added, as evidenced by the Deep Impact mission that crashed a probe into comet Tempel 1 to study what comets are made of.

Koenig based his argument on simulations of how well existing spacecraft could deflect 795 of the known potentially hazardous NEOs.

In their study, Koenig and colleague Christopher Chyba of Princeton University found that a single impactor could successfully deflect NEOs up to 820 feet (250 meters) in diameter with as little as five years lead time.

But both the gravity tractor and kinetic impactors will likely only be useful for smaller NEOs, noted Keith Holsapple of the University of Washington.

"Above 500 meters [1,640 feet], nukes are the only option," he said.

So far the Near-Earth Object Program at NASA's Jet Propulsion Laboratory in California has documented nearly 4,500 near-Earth asteroids, 855 of which are 0.62 mile (a kilometer) or larger in diameter.

Detonating a nuclear bomb on the surface of an oncoming asteroid is an option we could use now against larger threats, Holsapple said.

David Dearborn, of Lawrence Livermore National Laboratory in California, said that with long-term warning a nuclear blast could be used to nudge an NEO off course similar to the slow-push methods.

The bomb could also be used to fragment an asteroid if we had only a few years' warning.

But Dearborn cautions that scientists should carefully study the risks before taking any action.

"If you deal with an impactor before you know for sure it's an impactor, you could do a lot of harm," he said.

Models of asteroids' orbits are only estimates, Dearborn explained, and there's always a chance that taking premature action could move an asteroid from a path that might intersect with Earth to one that definitely will.

Dearborn also said that scientists should consider a given asteroid's size, shape, and composition before deciding on a response.

For example, many smaller asteroids are thought to be "rubble piles" made of chunks of rock and metal held together by gravity.

If such a rubble pile also has a solid core, the core could still be coming at Earth even after a nuclear bomb fragmented the outer section, Dearborn noted. A second blast might therefore be required to divert or break apart the core.

"I'm a big fan of knowing your target before you hit it," Dearborn said.

Big Bucks to Divert Big Bang

In the midst of debate over the best solution, the NEO community is also struggling to get enough funding to monitor possible threats.

Right now NASA's NEO program is operating with about four million U.S. dollars annually.

But NASA officials are saying they will need a billion dollars between now and 2020 just to find 90 percent of the 20,000 potentially hazardous asteroids and comets that are believed to exist.

The notion that the odds of a strike are low keeps people from supporting NEO research, experts say.

On average Earth is struck with a large asteroid once every million years. Smaller but still deadly asteroids hit once every 63,000 years.

According to Vern Weyers, former director of flight projects at NASA's Goddard Space Flight Center, the average chance of a person dying due to an NEO is 1 in 20,000.

This is about the same risk of dying in a plane crash, he said, and greater than the risks of dying in a flood, tornado, or due to a venomous bite.

(Explore natural disasters with our interactive feature Forces of Nature.)

The difference is that even though such events occur more frequently, fewer people usually die from them.

"If [an NEO strike] does occur, it can be a catastrophe, all the way up to extinction of life" on Earth, he said.

Michael Simpson, president of International Space University in France, agrees that the risk should be taken more seriously.

"The public perception of what we're talking about is this Chicken Little and Henny Penny idea that the sky is falling—there is a giggle factor," Simpson said.

Most people today see our planet "as a tiny speck of dust in a universe that seems to grow bigger every year. This leads to the conclusion that Earth is a tiny target," he continued.

But in our solar system "Earth dominates its orbit. For several thousand objects, Earth is a major pothole in their paths.

"We need to help convey to a public that is rather skeptical that we are trying to protect them."

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