Beating Back Space Invaders

Giant rocks or snowballs in space, while more likely to hit in Hollywood than anywhere else on Earth, remain a threat that policymakers are taking seriously.
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Giant rocks or snowballs in space, while more likely to hit in Hollywood than anywhere else on Earth, remain a threat that policymakers are taking seriously.

Hand-wringing over civilization-ending asteroid impacts has taken a back seat to health care, the economy and this winter’s weather.

Still, catastrophic impacts do happen.

Ask the dinosaurs.

They were wiped off the map for good by an estimated 6-mile-wide impactor that struck the Yucatan peninsula 65 million years ago. (Although some scientists now finger climate change in their extinction.)

Anyone who’s ever walked around Arizona’s mile-wide Barringer crater, made about 50,000 years ago, can attest to the destructive force of our solar system’s space junk.

Only a century ago in Russia, the 1908 Tunguska event flattened 8 million trees, roasted innumerable reindeer, and blasted heat felt by area residents up to 40 miles away. One unfortunate soul 100 kilometers from the blast’s epicenter was knocked unconscious — all by an asteroid estimated to be some 50 meters in diameter, or about half the length of a football field, air-bursting over central Siberia.

In the current tempest that is Washington, D.C., it’s remarkable that such high-risk, low-probability events haven’t gone completely unnoticed by the U.S. Congress.

At Congress’ behest, a National Research Council committee recently issued a report, “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies,” which detailed the committee’s recommendations. Those suggestions range from completing surveys of existing near-Earth objects — loosely defined as objects that cross or come near Earth’s orbit — to how best to counter such “NEO” threats to Earth.

In 2005, Congress authorized the detection, tracking, cataloguing and characterization of 90 percent or more potentially hazardous NEOs larger than 140 meters (about 460 feet) in diameter. Congress singled out those objects that could wreak havoc on Earth, but are thought to hit roughly every 30,000 years. The legislators set the survey’s target completion date for 2020.

“Reaching down to 140 meters at the 90 percent level is not possible with the instruments now funded; I have no idea what Congress will do about this,” said astrophysicist Irwin Shapiro of the Harvard Center for Astrophysics, who chaired the committee that wrote the NRC report.

The report recommended a peer-reviewed research program using space surveys, characterization and mitigation to counter potential impact threat. The committee also recommended setting up an international body tasked with defending the planet if an NEO is found to be on a definite collision course with Earth.

Any response to the report won’t start from scratch. NASA currently allocates $4 million a year toward NEO detection and searches, and has more than 6,000 objects identified, not counting a separate effort to track all the junk humankind has left in orbit.

A few NEOs are remnants of our solar system’s planetary construction 4.5 billion years ago. But most originate as fragments of colliding asteroids in the inner half of the main asteroid belt located between Mars and Jupiter. There, over many millions of years, the gravitational interactions of Mars, Jupiter and Saturn cause a significant number to enter Earth-crossing orbits.

For 3 billion years, Earth has been randomly impacted by such space detritus; some 150 tons of small objects likely strike every day. Most are harmless. Some thousand objects about a foot wide are thought to hit the upper atmosphere annually, while Tunguska-sized impactors may hit Earth regularly every few hundred years.

Some 170 Earth-impact craters have been documented. The largest, the 300 kilometer-wide Vredefort crater in South Africa, is 2 billion years old.

An object with a diameter of 3 kilometers that hits Earth is thought to be the threshold for global catastrophe. Fortunately, these hit roughly only every 10 to 30 million years.

And an Earth-killing impact of 10 kilometers or larger, the stuff of movies like Deep Impact or Armageddon, occurs roughly every 100 million years.

Not every NEO object is an automatic threat. Some of the largest asteroids have even ended up as moons of our nearby neighbors. Mars’ moon of Phobos, for instance, is more than likely a large asteroid captured by Mars’ gravity.

Planetary scientists are still unclear about asteroids’ exact makeup and composition. Some are thought to be iron-rich and very dense. But many of these objects may be very loosely bound piles of rubble circling the sun on timescales that can range from less than a year to decades.

Nonetheless, most asteroids will never pose a serious threat to Earth, but just in case one does, NASA has an active NEO tracking and monitoring effort as part of its Near Earth Object Program. Sample posts on its Web site include “Small Asteroid 2009 VA Whizzes By The Earth” or “Asteroid Impactor Reported over Indonesia” in October. (The latter had a diameter of 10 meters and reportedly “detonated in the atmosphere with an energy of about 50 kilotons.”)

Planetary scientist Don Yeomans, head of the Jet Propulsion Laboratory-based program, monitors the most up-to-date orbital data looking for objects that could hit Earth within the next 100 years. Of the 6,600 objects tracked, Yeomans says none represent a significant threat.

Some 85 percent of all NEOs a kilometer (seven times bigger than the minimum size Congress would have them track) and up have already been detected.

That excludes long-period comets, such as Halley’s, which they estimate represent 1 percent of the total impact threat. Such comets, some taking 200 years or more to orbit the sun, are capable of sneaking up on us. A quick Jupiter flyby can gravitationally sling them into the inner solar system and onto an Earth-impacting trajectory.

“We’ll never have more than a year or two of advance warning on a comet,” said planetary scientist Clark Chapman, at the Southwest Research Institute in Boulder, Colo. “We don’t see comets, even the big ones, until they get to the distance of Saturn or Jupiter. We’re looking at next-century technology before we can really improve warnings for incoming comets.”

Therefore, Yeomans says asteroids remain the bigger concern.

Of the three search programs currently in operation, the NASA-funded Catalina Sky Survey, using three telescopes in Arizona and Australia, finds the most NEOs at the highest rate.

NASA still leads the field in tracking NEOs, but Chapman says that a network of individual researchers worldwide contribute to the NEO database. Chapman, one of the NRC paper’s co-authors, says that while Europe, Japan and Australia contribute to the detection effort, the bulk of such funding stems from the U.S.

To further such detection efforts, the NRC committee report looked at how best to meet Congress’ stated NEO detection goals.

A partially funded ground-based survey telescope operating in the Southern Hemisphere could fill much of the detection effort. The Large Synoptic Survey Telescope, a private and U.S. government collaboration, would continually scan the sky from Chile. The NRC committee concluded that the congressionally authorized survey could be complete by 2030 — if the Chilean scope focuses more on NEO detection and less on astrophysics.

In addition, the German Aerospace Agency has selected AsteroidFinder, a 30-centimeter telescope funded through the development stage, for possible launch into low-Earth orbit in 2013. Its proposed one-year mission would specialize in detecting and characterizing “inner-Earth objects” — difficult to detect objects within Earth’s orbit.

Of late, asteroid-trackers have been most occupied with 99942 Apophis, a 270-meter asteroid spotted in December 2004. On April 13, 2029, Apophis will pass at an altitude — 36,350 kilometers from Earth’s center — that’s below the orbit of some human-launched satellites. It will return again in 2036, when its chances of hitting Earth are an estimated 1 in 250,000.

Click to see the graphic larger.

Click to see the graphic larger.

Those are long odds. Even so, last year, the Russian Space Agency let it be known that it was very interested in Apophis, as well as future NEO detection and mitigation strategies. It even called for international cooperation on these issues.

While the U.S. surely would welcome such cooperation, NASA no longer considers Apophis to be a serious threat.

“There’s a very slim possibility of an impact in 2036,” said Yeomans. He admits if it hit, Apophis could wipe out a couple of big cities, but he remains adamant it will safely miss Earth.

Unlike Hollywood movies where national landmarks seem to magically attract civilization-ending asteroids on final approach, in reality, the geographical odds are in favor of an ocean impact.

Like much in this field, baseline research on ocean impacts was conducted by the military. Cold War research concluded that tsunami-like waves generated by underwater nuclear blasts posed neither a serious threat nor much potential as an offensive weapon. Tsunami expert William Van Dorn of San Diego’s Scripps institution of Oceanography showed waves generated by nuclear blasts would likely break far from shore on the ocean’s continental shelf. This is known as the “Van Dorn” effect.

Whether ocean impactors would create tsunamis is still debated among planetary scientists; H. Jay Melosh at the University of Arizona’s Lunar and Planetary Lab labeled them “an over-rated hazard.”.

So, until a real threat is detected and the clock begins its countdown to impact, theoretical debate over asteroid-generated tsunamis are at best premature. The day that threat is confirmed, will the human response be decisive … or quixotic?

“Upon detection of an Apophis-like threat,” said Shapiro, “the first thing we’d do is call an international meeting. Hopefully, they wouldn’t break down in bickering and we would know where it would hit and whose ox would be gored.”

Twenty years out from impact, Chapman says, the object’s path across Earth would likely already be known. If its path were on a line that passed through Los Angeles, that would suddenly generate enormous headlines, he notes: A 140-meter asteroid could create an impact radically bigger than the nuclear blast over Hiroshima, effectively wiping out everything from Santa Monica to Pasadena.

If time permitted, researchers would first send out a probe to learn as much as possible about the impactor’s physical properties. Once that’s done, Chapman says the idea would be to design and launch a “kinetic impactor,” or maybe even a couple just in case.

The idea behind a kinetic impact is to hit the NEO in the opposite to its direction of motion, like a locomotive hitting another head on, or its direction of motion, like hitting that locomotive from behind. The idea isn’t to smash it, but make it miss its appointment.

“You’re trying to either slow down or speed up the asteroid in its solar orbit,” said Chapman, “so that it arrives either at Earth’s orbit before Earth gets there, or after Earth has already passed.”

Gravitational attraction is directly proportional to its mass. So, a spacecraft’s proximity to an asteroid can nudge the object out of its original orbit, acting as a so-called “gravity tractor.”

Chapman says NASA’s current launch capabilities would allow for a one-ton spacecraft to be launched, either an impactor or a tractor.

One idea calls for equipping the kinetic impactor observer spacecraft to also be a gravity tractor, says Chapman. That way, even if the kinetic impact maneuver failed, the gravity tractor could tweak the asteroid’s motion away from an Earth-impacting rendezvous.

With 30 years notice of a definitive Apophis-type asteroid impact, Shapiro says the first 20 years might best be spent researching the latest mitigation strategies. Then, the last 10 years could be used to put that plan in motion.

The European Space Agency has done an initial study for Don Quijote, a $500 million demonstration of kinetic impact. The mission, unlike the fanciful title character from Cervantes’ classic novel, would be expected to charge asteroids rather than windmills.

The mission would consist of two separate spacecraft. One, nicknamed Sancho, would study the asteroid’s makeup and orbit before and after kinetic impact. The other, Hidalgo, would actually smack the asteroid to change its velocity.

The idea would be to prove that humans could deflect an asteroid via a kinetic impactor, says planetary scientist Alan Harris of the German Aerospace Center in Berlin.

Harris chaired the independent advisory panel that selected Don Quijote for further study. He says the mission would be aimed at an asteroid that has not already been deemed a threat to Earth.

While the project remains viable, the Europeans have yet to take it further.

“If we discover something dangerous, the money will flow like water,” said Shapiro, who notes that current technology could deflect NEOs of a couple of kilometers in diameter “pretty well.”

But if an NEO is 2 or 3 kilometers in diameter and expected to impact Earth within 20 years time, Chapman says the best option is to go nuclear.

In that case, he suggests sending out a series of warhead-laden spacecraft to literally blast the rock out of its Earth-impacting trajectory.

And if all else fails, there’s always civil defense.

“But to the best of my knowledge, asteroid-related disasters are not on the disaster contingency list of any country or international organization,” said futurist Jim Dator at the University of Hawaii in Manoa.

Even so, he says incorporating asteroid disaster preparedness should be a priority.

Evacuation might very well be the best response for local impacts, says Dator, who was one of the NRC report’s co-authors. “If we’re prepared,” he said, “we will have enough time to get people out. So, we propose planning for these things now. If we wait until that asteroid shows up, it’s too late.”

If we become vigilant, the odds remain in our favor.

The NRC committee estimates that $50 million in annual funding could meet Congress’ NEO detection, cataloguing, and characterization goals while also funding impact mitigation research.

As Shapiro puts it, “I sleep without worrying that my house will burn down. But I do have smoke alarms.”