Seminars About Long-term Thinking

Asteroid threat report

Schweickart filled the hall with some 240 at the Presidio Officers Club and gave a dazzling lecture. He left the next day for Washington DC to lobby Congress to apply its will to making the Earth safe for the very long term.

"For life to survive in planetary systems," said Schweickart, "it has to figure out how to deal with massive asteroid impacts. Who knows how many advanced life forms in the universe have failed or passed that test. Humanity is just now on that cusp. We have the knowledge and the ability---if not yet the will---to prevent future large-scale extinction events from asteroids."

Data-rich, graphics-rich, and huge in conceptual scale, it was the most long-now Seminar yet---"The Asteroid Threat Over the Next 100,000 Years."

Impact craters everywhere---Moon, Mercury, Mars; even asteroids have craters from other asteroids. (And occasional comets---1% of the source of impacts is from comets, as we saw very recently when a fragmented comet carpet-bombed Jupiter.) "How many asteroids are in the asteroid belt between Mars and Jupiter? Pick a number. But if it doesn't have at least nine zeroes after it, it's wrong."

Rusty's presentation was full of lore. Most asteroids are stone, but they are light---they absorb impact like Styrofoam. There are 100,000 shooting stars every night on Earth---small asteroids. Meteorites are COLD when they land, not hot (the deep cold of space is preserved, while the air-friction hot surface ablates away.) Air bursts from large asteroids cause more damage than ground strikes, and ocean impacts can cause tremendously destructive tsunamis. With really large impacts, the planets exchange material---Mars rocks on Earth, Earth rocks on Mars.

When the last major-extinction impact occurred in Yucatan 65 million years ago, 10% of the stupendous blast debris exploded clear away from the Earth, while the other 90% rained down incandescent with re-entry all over the Earth for 90 minutes of burning sky---everything flammable on Earth burned up, and one meter of the oceans boiled off. (That was a 10-kilometer asteroid; when a 200-kilometer asteroid hits, it boils off the whole oceans). Then there was no sunlight for several years. 65% of all species on Earth ceased to exist, including the super-dominant dinosaurs. They failed the asteroid test.

Rusty showed a diagram with all important asteroid data in it. The power-law distribution (many small, few large) is so perfect you can directly correlate frequency of impact with size of asteroid and energy released in megatons and the damage that would result. (Calibration: the largest nuclear weapon tested was a Soviet bomb of 58 megatons---6,000 times the force of the Hiroshima bomb. The 1908 air-burst of an asteroid over Tunguska, Siberia, made an 11-megaton explosion that flattened 2,100 square kilometers of forest.)

"The asteroid threat over the next 100,000 years is right there in the chart," said Rusty. In the next hundred millennia:

• There's a 10% chance of an asteroid causing planet-scale damage with 100,000 megatons of energy released.
• There's a 50-50 chance of a 500-meter asteroid that could destroy an area the size of Texas with a 6,000 megaton explosion---100 times the USSR's biggest bomb.
• There will be about TEN 200-meter asteroid impacts, good for 400 megatons.
• There will be about A HUNDRED 70-meter-diameter asteroids, each causing 15 megatons of damage (i.e. worse than the Tunguska explosion, which would have wiped out all of London if it had hit there instead of the remote wilderness).

NASA's Space Guard program has been looking for PHA's---Potentially Hazardous Asteroids---that are 1 kilometer or more in diameter. There are about 1,100 of them---700 have been detected and tracked and found innocent of threat; that leaves some 400 still unknown. Since asteroids orbit with the Earth around the Sun, any collision path is gradually convergent. Advance knowledge of collision with a very large asteroid can range from 40 years to 2 years, with the kind of attention now being paid.

Extreme case: "1950DA" is an asteroid that has been tracked since 1950. Its orbit is so precisely known that we can forecast there is a 1-in-300 chance it will collide with Earth on March 16, 2880---876 years from this month. Orbital mechanics are destiny.

One of the sponsors of the SALT series, Leighton Read, noted that the asteroid threat is a rare instance where we really CAN predict the future, a very long way out and in great detail, at least statistically.

Furthermore, this is a rare instance where we really can do something about the future. Threatening asteroids can not only be detected, they can be deflected.

Rusty said there are two main approaches to deflection---sudden impulse (like a nuclear explosion), and slow, guided redirection. He favors the second, and lead-authored an article in last November's SCIENTIFIC AMERICAN on the design and use of an "asteroid tugboat." He noted that some of those favoring nuclear deflectors have an agenda of weaponizing space and see this as a dual-use back door to something now thoroughly forbidden by treaties.

Rusty heads the B612 Foundation (named for the Little Prince's asteroid), which is lobbying to add asteroid deflection to NASA's Prometheus mission to Jupiter's icy moons, planned for around 2015. The tugboat would have an ion or plasma drive (highly efficient) powered by a nuclear reactor.

Asteroids spin, and they are extremely massive. Moving them where you want is tricky. The evening before the talk, Danny Hillis jarred Rusty by suggesting a "tractor beam"---use the gravitational attraction between the tug and the asteroid to "pull" the spinning asteroid without have to touch it. It turned out that one of Rusty's colleagues had come up with exactly the same idea several weeks ago. To make an asteroid miss the Earth, all you have to do is add or subtract 40 seconds from a 2-year orbit---1/200th of a mph of a rock traveling at some 66,000 mph. Big rock, though.