If we're going to protect the Earth from an asteroid, we need to find the dangerous ones whizzing about in the emptiness of space.

Unfortunately, the United States will not complete the survey of large near-Earth objects by 2020 as mandated, but not funded, by Congress in 2005. That's the conclusion of a new National Research Council Report, Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies, released Friday.

The current budget and astronomical tools are just not sufficient to find all near-Earth objects larger than 140 meters (460 feet) across. Better telescopes than we currently have will be needed. While this has been known within the NEO science community, the final report could bring the realization to the policymakers and politicians who control the purse strings.

"There's no longer time to meet the goal by 2020," said Michael A'Hearn, a University of Maryland astronomer and co-author of the report. "There's no way to do the survey in that length of time because the equipment isn't even built yet. We say it is not unreasonable to set a new deadline of 2030 and start funding now. We probably can do the job by then."

Despite the large number of NEO discoveries over the past several years, our current detection instruments like the Catalina Sky Survey, are not up to the task of completing the Congressional mandate, known as the George E. Brown Survey.

"The current instruments, no matter how you operate them are not capable of doing the George Brown survey," A'Hearn noted.

And that's to say nothing of the smaller asteroids, those in the 30- and 50-meter (90 to 165 foot) range, which hit Earth far more often than larger objects. Finding and tracking those little guys will require new telescopes like the Large Synoptic Sky Survey and Panstarrs, neither of which currently has the funding to complete construction.

Scientists have increasingly come to understand that the risk of asteroids and comets hitting Earth is real, but quantifying the risk that humans face from such events is much trickier.

"Our estimates of the risk could easily be wrong by a factor of two or three," A'Hearn said. "I don't think they are wrong by a factor of 10, but the boundaries, again, haven't been explored."

Even the Tunguska asteroid, which exploded over Siberia in 1908, remains something of a mystery. It's unclear even how large the object was, A'Hearn said, which makes it difficult to know how common such an impact is. The rareness of the event makes it very tough to compare the risk from an asteroid strike with that from automobile collisions or other prosaic problems.

Right now, National Research Council scientists estimate the risk of being killed by an NEO impact is comparable to the risk of being one of the 50 or so people who die on an amusement park ride each year. The difference is that a major asteroid would kill many people all at once.

Another area of high uncertainty is the physics of asteroid impacts. Near-Earth objects of different types may require different mitigation strategies.

"The first thing we need to do is understand what the hazard is," A'Hearn said. "That's partly finding them and partly understanding what their effect is. We have to understand in more detail how we'd mitigate against them."

Former astronaut Rusty Schweickart, a tireless campaigner for asteroid risk awareness, said the latest report was the best of its kind, surpassing an earlier NASA report to Congress on near-Earth object risk.

"I can certainly say that Irwin Shapiro, who I know very well personally, did a terrific job in putting this review together," Schweickart said.

Lindley Johnson, program executive at NASA's Near Earth Object Program, which wrote the NASA report agreed.

"It looks to be a very good report," Johnson said. "It had a very strong team of top scientists in the area on the committee. They had the right people and it looks like they looked at all the right things."

Differences begin to emerge between people who study near-Earth objects when mitigation options come up. The new report looks at two main ways of deflecting asteroids, following previous reports. First, the asteroid could be hit with some kind of impactor, either conventional or nuclear. Second, a longer-term, more precise technique like a gravity tractor could be employed.

Schweickart argued, however, that a gravity tractor, which would slowly push an asteroid off a collision path with Earth, should be considered a necessary but not independent part of any Earth defense.

"It's the icing on the cake of stronger deflection needs," Schweickart said. "It's not comparable to and should never be considered the primary means of deflection."

Update 1/25: Typos corrected to include the correct names for the George E. Brown Survey and the National Research Council report.

Image: U.S. Geological Survey/Composite: Tim Warchocki

See Also:

WiSci 2.0: Alexis Madrigal's Twitter, Google Reader feed, and green tech history research site; Wired Science on Twitter and Facebook.**