By Alan Boyle, Science Editor, NBC News

As of now, there's no comet or asteroid that's definitely due to smash into our planet, but experts say it's high time to figure out how to deal with the uncertainties, misunderstandings and political wrangling that will inevitably arise during the asteroid alerts to come.

Last week's hubbub over the asteroid 2005 YU55, which passed within 200,000 miles of Earth, set the scene for a seminar on near-Earth objects sponsored in Boulder, Colo., by the Secure World Foundation. The public's interest in the harmless flyby was just a foretaste of what could happen when astronomers spot a rock that has a significant chance of hitting Earth.

And it is a question of "when," rather than "if."

Several potential impacts have been flagged over the past decade. In most of those cases, further observations — including observations gained from "pre-discovery" images of the objects in question — have ruled out a collision. But some of the cases are still on NASA's list — including 1999 RQ36, the 560-meter-wide (600-yard-wide) asteroid that's judged to have a 1-in-1,750 chance of hitting Earth sometime in the next 200 years. That rock will be targeted by NASA's Osiris-Rex probe, due for launch in 2016.

Then there's Apophis, the asteroid that sparked a scare in 2004 when its chances of impact were briefly set as high as 1-in-37. Since then, further analysis has reduced the odds to 1-in-250,000 for 2036. A new round of radar observations in 2013 could reduce the chances even further, to essentially zero, or conceivably raise them again.

That's one of the problems for asteroid-trackers: The stated odds of impact are calculated on the basis of how much or how little is known about a near-Earth object's orbit. If the possible track at a particular given time stretches for hundreds of thousands of miles, and Earth happens to lie anywhere on that track, astronomers have to acknowledge there's a chance that Earth will end up getting hit. As the orbital predictions are refined, the stated odds may go up or down. In almost all of the cases to date, an Earth impact is eventually excluded, and the odds go down to zero.

More alerts ahead

As more powerful telescopes come online, there'll be more asteroids that may be added to NASA's impact risk list — and potentially more up-and-down asteroid alerts.

"Those are going to happen every year, or at least every decade," David Morrison, director of the SETI Institute's Carl Sagan Center for the Study of Life in the Universe, said at this week's seminar.

And sometimes the asteroid actually hits: That was the case in 2008, when a boulder-sized asteroid known as 2008 TC3 slammed into Sudan just hours after its discovery. Astronomers knew almost immediately that there'd be an impact, and that there'd be no significant damage in the desert. However, the event demonstrated that near-Earth objects could come at us from right out of the blue.

Astronomers are getting a good handle on tracking the asteroids that are big enough to spark mass extinctions. But they also say thousands of bad-news asteroids that are wider than 100 meters (330 feet) are yet to be detected. Roughly a million yet-to-be-detected asteroids are smaller than that, but still capable of causing damage.

The bottom line is that the most immediate threats from the sky are not likely to be the huge objects portrayed in movies like "Deep Impact" or "Armageddon," but the smaller ones that are nevertheless capable of destroying a city or sparking a tsunami.

"Chances are we wouldn't see it," said Mark Boslough, a researcher at Sandia National Laboratories who recently drew up a computer simulation of the 1908 Tunguska blast. Boslough said the object that caused the Tunguska explosion might have been just 40 meters (130 feet) in diameter, but still flattened 500,000 acres of Siberian forest.

Emergency preparedness

So how can you prepare for something like that? That's what the experts have been trying to figure out: This week's seminar focused on a U.N.-sponsored process to draw up an international emergency preparedness plan for asteroid and comet impacts.

As it stands now, the plan calls for formalizing a network of sky-watchers, most likely including folks from NASA's Near Earth Object Program and the groups that will be part of the European Space Agency's SSA-NEO effort. If something needs to be done about studying or diverting a potentially threatening asteroid, a separate team of planners (known as a Mission Planning and Operations Group) would set up the space mission; a diplomatic group would be charged with signing off on that mission; and the U.N. Security Council would give the final go-ahead.

Why such an involved process? In the movies, NASA just goes ahead and blows up the planet-killer. But in real life, trying to move or break up a truly large asteroid could take years of effort, and temporarily raise the risk for one region of the world while lowering it for a different region. For example, if an asteroid's track is projected to end in a Pacific Ocean impact, do you move the asteroid off track to the east, putting the Americas at greater risk for a while ... or to the west, temporarily putting Asia in the crosshairs. And if something goes wrong with the operation, who would be held at fault? The U.N.-backed effort, which is expected to result in concrete recommendations by 2013, is aimed at anticipating these tricky political issues.

For relatively small asteroids, the advance warning might be on the scale of mere hours, days or weeks, and the response might look a lot like a Katrina-level hurricane evacuation. On the other end of the scale, it could take decades to resolve an Apophis-style situation.

Education effort needed

The long-term scenario would require a lot of education about the observational uncertainty, about the campaign to divert the asteroid, and about the potential effects of impact. For example, based on the Tunguska example, a 50-meter-wide (165-foot-wide) object could wipe out an area the size of a major city. (The Earth Impact Effects Program lets you tinker with the parameters of a cosmic impact and find out how far you'd have to run.)

If it takes until the year 2036 to resolve the Apophis situation, the right time for addressing the issue of near-Earth objects is ... right now. In fact, some folks have already organized school projects on the subject.

"Let it grow up with the kids as they grow older," said retired Apollo astronaut Rusty Schweickart, who helped organize efforts by the Association of Space Explorers and the B612 Foundation to address the asteroid issue.

What do you think? Are near-Earth objects on your list of things to worry about? Or are they on your list of things to look forward to, thanks to the Obama administration's plans to send astronauts to an asteroid by the mid-2020s? Feel free to weigh in with your comments below.

Update for 3:25 p.m. ET Nov. 17: Rusty Schweickart added a little perspective about Apophis in a follow-up email:

"I’d like to remind people that Apophis is No. 7 on the list of objects with non-zero impact probability (ranked by Palermo level), and if you include those seen in the last 60 days, it's No. 8. Some of the others are pretty interesting and, while still having a low probability of impact, are more threatening than Apophis.

"Unfortunately more people have heard of Apophis, given its history, and don't seem to get that its impact probability is now ridiculously low ... and very likely to go to no threat at all when we track it again in 2013. Not so with some of the others ... but they’re not 'known' by the public. Yet."

Also, Harvard instructor David Ropeik, an expert on risk communication (and former msnbc.com contributor), has published his own report on the Boulder seminar as a Big Think blog item, titled "The Sky IS Falling. Should We Worry?"

Update for 8 p.m. ET Nov. 18: Another participant in the Boulder seminar, Carolyn Collins Petersen, explores "The Undiscovered Country of Small Bodies" on her blog, The Spacewriter's Ramblings.

Update for 3 p.m. ET Nov. 19: My colleague from Sky & Telescope, Kelly Beatty, reports on the Boulder seminar in a posting titled "If an Impact Looms, Then What?"

More about asteroids:

I attended the Secure World Foundation's seminar as a participant, and the foundation paid some of my expenses for the trip. The seminar sessions were conducted under the Chatham House Rule for information sharing. The direct quotes used here were cleared by individual speakers after the sessions.

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or following the Cosmic Log Google+ page. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Alan Boyle, Science Editor, NBC News