On Monday afternoon I went to a few of the sessions describing next steps in our efforts to locate all of the asteroids and comets that could possibly harm Earth, the near-Earth objects. Lindley Johnson, from NASA Headquarters, presented on "The State and Future Direction of NASA's NEO Program." In 1998, NASA was charged by Congress with the goal of discovering 90% of the asteroids above one kilometer in diameter within the next 10 years, which came to be known as the "Spaceguard goal." (A useful backgrounder by David Morrison on what exactly the Spaceguard goal is can be found here.) Various surveys, conducted by both professionals and amateurs, have combined efforts to help us reach this goal. As of October 1, 788 one-kilometer-and-larger objects had been found, including 64 near-Earth comets, and the discovery rate appears to be approaching an asymptote, indicating that we probably predicted the number of objects out there correctly, and have indeed found most of them; people seem to accept that the Spaceguard goal has either already been reached or will be reached by the end of 2008.
These largest bodies represent the greatest risk to the survival of life on Earth. However, much smaller bodies can still bring great destruction -- perhaps not to the whole planet, but certainly to large regions. To reduce 90% of the remaining risk (with risk quantified by the number of human casualties predicted to result from an impact), we need to complete a similar survey to discover the asteroids that are 140 meters in diameter and larger. The NASA Authorization Act of 2005 contained a charge to find 90% of these objects within the subsequent 15 years. There are now major facilities being developed with the capability to do enormous and sensitive sky surveys and find these objects; an example was Pan-STARRS, which, once its four telescopes are online, could find as many as 500 to 1,000 new objects per month. One problem with these more-sensitive surveys is that the faint objects they find will be out of reach of most amateur astronomers, and it's amateurs who are doing the bulk of the follow-up tracking and characterization of recently discovered near-Earth objects to establish their orbits; in lieu of an army of amateurs with telescopes capable of detecting such tiny objects, the giant surveys will have to do their own followup work.
Steven Chesley, from JPL, gave a thought-provoking presentation on what these next-generation surveys (that's what they're being called) will mean for the public. He pointed out that discovery is just the first step; we then have to survey this new catalog for impact risks. And with a massively increased discovery rate, there's going to be a corresponding massive increase in the number of warnings. To date, we've seen a couple of dozen warnings. Most rated a "1" on the Torino Scale; there have been only two objects that reached "2", and a single one (Apophis, formerly known as 2004 MN4) that briefly reached "4" before being reclassified to zero. He posted a table showing the number of warnings at each scale number that we're likely to get. The first column shows warnings to date; the second shows warnings that are predicted to arise from the population of asteroids detected in Spaceguard surveys; and the third column shows how many the next generation surveys will generate.
Torino scale number
Found to date
Spaceguard (90% D>1,000 km)
Next generation (90% D>140m)
The reason that the numbers don't move monotonically with increasing Torino Scale number is the way the Torino Scale categories are defined and because the next generation surveys will yield many mid-sized and small bodies that result in lower-level warnings.)
There will be many, many, many more alarming detections of bodies whose orbits could possibly intersect Earth's. Chesley pointed out that we need to think ahead about how to communicate these warnings to the public. While it's tempting, he said, to try to hide some of the scarier warnings from the public until people can pin down their orbits and very likely find that they don't, in fact, pose a risk, it's not workable; "openness is the only reasonable approach," he said, because the more serious the case, the greater the imperative for a public announcement that will get more telescopes turned to pin down the object's orbit. He recommended that scientists get out in front of the story, and emphasize the transient nature of the hazard -- the fact that these hazards are only hazards in potential, and that further study usually reduces the hazard to zero; but that further study is necessary.
People can get used to warnings like this. Where I grew up, in Texas, we'd regularly get warnings about thunderstorms that could possibly create tornados. Tornados are very destructive but only locally, so most people responded to the warnings with vigilance but not panic; and in a few hours, the risk usually went away, either because there were no tornados, or the ones that developed hit some other unlucky neighborhood a few hundred miles away. In the next decade, we may see asteroid impact warnings in the same light. But I wonder whether frequent asteroid warnings will cause people to have more respect for what's going on in the sky, or whether people will come to regard asteroid hunters as Chicken Littles. Get out in front of the story, and emphasize the transient nature of the risk; be just alarmist enough to make sure these surveys get the public support they need, but not so much that, in the absence of death coming from the skies, people get inured to the threat.
Finally, there was a plea from Don Yeomans for continued support of the Arecibo radio telescope for radar followup of potentially hazardous asteroids. He pointed out that radar data, when added to optical data, can increase the average interval of predictability from 80 to 370 years, greatly improving impact predictions. He opined that radar followup of many near-Earth objects are "nearly equivalent, in their science content, to spacecraft flyby missions." In particular, he said that where optical tracking of an asteroid is not good enough, it may be necessary to send a mission to tag it for easier tracking (much like the missions proposed as a part of our Apophis asteroid tagging competition); but if we maintain our radar capability, such missions may not be necessary. Unfortunately, Arecibo is under threat of closure with no reprieve in sight.