Hartley 2 at close approach, about 400 miles away, imaged by NASA Deep Impact spacecraft. (NASA)
After the flyby of Comet Hartley 2 on Nov. 14 by the Deep Impact spacecraft on its EPOXI mission, I was asked by a colleague if comets, particularly long-period comets from the outer solar system, pose a worrisome impact threat to Earth. He was thinking in particular of the Comet Shoemaker-Levy 9 impact into Jupiter in 1994, releasing explosive energy hundreds of times that of Earth’s nuclear stockpile. Is such a comet collision a near-term threat to our civilization?
The chances against a large comet striking Earth in the near future are, well, astronomical. Comets make up about 1% of the total impact threat to Earth. (The largest threat is from near-Earth asteroids.) Even near-Earth comets (NECs) are just a few percent of the Near-Earth Object (NEO) population, and are smaller than the typical comets from the outer solar system. (Hartley 2 was just over a kilometer long).
Right now we have no technology to find long-period comets, and they are big (~ 10 km) and come in with such high velocity that we can’t do much except nuke them…and that may not work. SO – we should not do anything about the comet hazard now, count on the odds being in our favor for centuries (extinction events occur about every 100 M years), and say that future technology will be much more capable of dealing with comets.
The most likely impact situation in the next few centuries is a small (just above atmospheric penetration threshold) object that will detonate with a few megatons of TNT force. Right now it would happen without warning. But it would also probably hit over the ocean or sparsely inhabited area (most of Earth). But if we do the modest search mission (space-based telescope), we will find most of these objects in next 10 years, enabling us to decide if we must divert one. Ignorance has “protected” us til now, but knowledge is power.
I would not overplay the threat to civilization. Hurricanes are terrible but don’t threaten civilization. NEOs are bigger disasters than hurricanes if they hit in the wrong place, but only a tiny fraction can threaten civilization. But we can cope with a damaging impact (disaster response), and we can also avoid them altogether with a very modest investment (2% of NASA budget over 10 years).
A good review of the general impact threat to Earth is at the JPL NASA NEO Program site, and in the 2010 National Research Council report, Defending Planet Earth.
Read the recommendations of the NASA Advisory Council’s ad hoc Task Force on Planetary Defense (Oct 2010). (I co-chaired with Apollo 9 astronaut Rusty Schweickart).
The impact hazard is real, but it is manageable with a very modest investment of NASA’s budget over the next 10 years. Consider it asteroid insurance!
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