Given the odds of some giant space rock crashing into Earth and what it might do when it hits, scientists now estimate that on average 100 people will die each year from a cosmic impact. How much this number scares you depends on how far out you want to look into the crystal ball. Within the next couple of centuries a Tunguska-like blast might match the devastation of the earthquake that just devastated Haiti. Or fast forward 10 million years, and you can expect a titanic crash powerful enough to wipe out a billion people worldwide.
So should you be worried or not? Put another way, to what lengths — and at what cost — should we go to try to protect ourselves from some asteroid or comet "going rogue" in the foreseeable future?
In 1998 Congress felt worried enough about the sky falling that it tasked NASA with finding 90% of all the near-Earth asteroids at least 1 km across within 10 years. (Anything this size would likely trigger global devastation during and after its collision.) Then Congress raised the bar in 2005, mandating that NASA find 90% of all the threatening asteroids with diameters down to 140 meters (460 feet) by 2020.
It looks good on paper, but neither Congress nor NASA have ever anted up the dedicated funds to do those searches. Back in the mid-1990s, NASA scientist David Morrison famously observed that fewer people are watching for asteroids able to collide with Earth than work in a typical McDonald's franchise. And while more observers are warily sweeping the sky these days, including many capable amateur astronomers, they're doing so by coattailing on other space surveys not optimized for the task.
In short, there aren't enough bodies or telescopes in the impact game to meet the 2020 deadline set by Congress. Don't take my word for it — read the 149-page report released today by an A-list panel of experts assembled by the National Research Council. Titled Defending Planet Earth,, it explores both the best ways to track down all the hazardous near-Earth objects (NEOs) and what to do once it becomes clear that one of them is destined to have a run-in with Earth.
The NRC report both summarizes the state of the searches to date and lays out the steps that NASA or the National Science Foundation (which funds most professional astronomy in the U.S.) would need to take to get serious about cosmic threats. "This is a huge milestone," observes small-body specialist Richard Binzel (MIT), who had no role in the committee or its findings. "The asteroid problem is now a grownup, joining the table of other natural disasters for which mitigation strategies are developed."
Pointedly, the NRC panel states that the Congressional target simply can't be met by 2020. If NASA and NSF managers decide they want to complete the survey as soon afterward as possible, then they'll need to fortify observers not only with dedicated ground-based efforts like the proposed Large Synoptic Survey Telescope but also with a infrared-sensing space observatory. Or ground-based telescopes could go it alone, which would keep costs down but delay the completion date until about 2030.
Concerning mitigation strategies, the panel assessed four approaches and found them all viable and complementary. For the smallest and thus localized impact threats, the most cost-effective approach is simply to move people out of harm's way, either by sheltering or evacuating them.
Bigger NEOs, ranging up to 100 m across, would affect too large an area to make civil defense practicable. So a passive space-based defense — using a spacecraft to pull or push the body enough to alter its path — would work better. For still-larger impactors, up to 1 km in size, a salvo of spacecraft might need to strike it head-on to change its course. But both of these methods would make only slight trajectory redirections and thus require many orbits, over decades of lead time, to avert a disastrous collision.
For the biggest threats, or if one of the other methods fails or if the lead time is short, the panel concludes that the "only current, practical means" are nuclear explosions. These wouldn't be used disrupt the incoming body but rather to give it a Really Big Push all at once. (No need to cue Bruce Willis and his Armageddon team — these would be delivered robitically.)
There's more. As noted in its preliminary findings, released last year, the NRC panel emphasizes the crucial role being played by the unique radar capabilities of Arecibo Observatory Puerto Rico — a facility that an NSF review team felt ought to find its funding elsewhere or be shut down.
It's going to take me a few more days to digest all the content in Defending Planet Earth. Click here if you'd like to download it for your own perusal, or click here for a news summary posted by the NRC's news office.