July 19, Oxford, UK-Death by asteroids, comets and gamma ray bursts was on the agenda of the conference on Global Catastrophic Risks this morning. First up was NASA senior scientist David Morrison to talk about the Spaceguard Survey and the threat of a catastrophic asteroid strike. Morrison started out by noting that we had just past the centenary of the 5-15 megaton Tunguska airburst over Siberia. In 1998, Congress charged NASA with surveying the skies to detect 90 percent of near earth asteroids (NEAs) greater than 1 kilometer in size in 10 years. An impact by a kilometer-sized asteroid could end civilization. Besides the blast, such an asteroid would inject so much dust into the atmosphere that it would cause global winter that would cause massive crop failure.
According to Morrison, 80 percent of the NEAs that are a kilometer or more in size have been identified. Consequently, Morrison could happily assure the assembled Oxford catastrophe mavens, "We are not going the way of the dinosaurs." Why because the Spaceguard Survey has not turned up any NEAs near the size of the one that likely killed off the dinosaurs 65 million years ago. That asteroid measured between 10 and 15 kilometers and blasted the 180 kilometer-wide Chicxulub crater. Proposals to have the Spaceguard Survey expand to detect NEAs as small a 100-meters are now being considered. Morrison groused that NASA has spent only $ 4 million on Spaceguard and argued that the magnitude of the risk merits a budget of half a billion dolars.
Morrison pointed out that asteroid strikes are the only natural hazard that in principle can be completely eliminated. Thanks to the Spaceguard Survey, humanity will likely have decades of warning before an impending collision. Once alerted, missiles could be used to nudge a threatening asteroid so that it misses the earth. In this regard, Morrison cited an apt quotation from the poet Lord Byron:
"Who knows whether, when a comet shall approach this globe to destroy it, as it often has been and will be destroyed, men will not tear rocks from their foundations by means of steam, and hurl mountains, as the giants are said to have done, against the flaming mass? - And then we shall have traditions of Titans again, and of wars with Heaven."
But before becoming too complacent, keep an eye out for reports on the 210-330 meter asteroid Apophis-there's a 1 in 45,000 chance that it could hit the earth on April 13, 2036. Measurements in the next 3 to 4 years will determine just how big a chance of a collision there is.
Gamma Ray Bursts
Technion physicist Arnon Dar warned of another space hazard--gamma ray bursts (GRBs). GRBs were originally detected by U.S. military satellites that were checking to see if the Soviets were testing nuclear weapons. GRBs are beams of highly energetic photons produced when a gigantic star goes supernova. Dar described a GRB beam hitting the earth would be like a kiloton bomb per square kilometer going off at the top atmosphere. He speculated that some of the earlier mass extinctions, such as the Permian extinction in which perhaps 90 percent of all life died out might have been caused by GRBs.
So are there any stars likely to go supernova nearby? Dar pointed out that the gigantic star Eta Carinae at a distance of 7,500 light years has been extremely unstable of late. Eta Carinae is 100 times more massive than the sun and 5 million times brighter. When it goes it will be a hypernova. Dar then gave us the good news: Eta Carinae's axis is pointed away from the earth, so the GRB beam it will generate when it dies will be aimed far from us. However, don't get too complacent about GRBs. Future of Humanity Institute research fellow Anders Sandberg mentioned that some astronomers are worried that we may be looking down the barrel of gamma ray gun when the WR 104 binary located 8,000 light years away goes supernova.
If asteroids aren't likely to get us, maybe comets will. William Napier from the Center for Astrobiology at Cardiff University in Wales noted that long-period comets-those that originate in the Oort Cloud beyond Pluto-have generally been thought to constitute around 1 percent of the risk of catastrophic collision. Napier argues this is big underestimate. Napier's main concern is dark comets. Dark comets do not sport showy ejection tails like Halley's Comet does, but are much stealthier. Because they are dark, they are very hard to detect. Napier pointed out that the IRAS-Araki-Alcock comet was detected only two weeks before it came within 0.03 astronomical units of the earth (about 230 earth diameters) back in 1983-the closest of any known comet since 1770.
Napier argues that the record of large impact craters suggests that the earth experiences periods of cometary bombardment every 36 million years or so. He attributes the episodes to the sun's periodic passage through galactic plane where contact with molecular clouds dislodges comets from the Oort cloud surrounding the solar system. He believes that the earth is currently in a bombardment episode. "We have comet problem because they are hard to detect which means that we would have months or weeks of warning at most," said Napier.
Next up from the Oxford Global Catastrophic Risks conference are pandemics and nuclear war.