DPS 2012: Future impact risks
Posted by Emily Lakdawalla
24-10-2012 13:14 CDT
Continuing my writeup of notes from last week's Division for Planetary Sciences meeting: presentations on the risks of future asteroid impacts. This issue is an interesting moving target, because the more we survey the skies, the more previously unknown asteroids we find. Usually they're not impact risks, so it becomes less likely that an undiscovered asteroid is on its way to hitting us. At the same time, those surveys inevitably yield the discoveries of objects that do have very low probabilities of hitting us. A couple of talks addressed different sides of this problem.
Richard Wainscot presented the current status of the Pan-STARRS1 discovery survey for near-Earth objects. NEO surveying represents only a small fraction of Pan-STARRS telescope time, although, he said, it would be increasing to 11% by November 2012. He talked about how dependent Pan-STARRS is on other people (mostly amateurs) to follow up their discoveries. Their surveys produce "tracklets" that they turn over as quickly as possible to the Minor Planet Center, and then it's pretty much up to volunteers to attempt to recover the new objects on subsequent nights in order to obtain enough sightings to produce an orbit. Of course, amateurs have smaller telescopes so are more likely to recover brighter objects than dimmer ones. As a result, while Pan-STARRS can find fast, faint objects, these objects are often lost because no followup observations are achieved. Wainscoat said that Pan-STARRS' median magnitude for its discoveries is 22.5, while other surveys (which, I assume, are doing their own followup) achieve median magnitudes of 23.1. Pan-STARRS is uniquely best at finding large, distant, faint objects, because it is better at spotting the faintest objects, and if they are distant, they are slow-moving and so easier to recover.
Alan Harris' presentation posited an interesting question regarding the actuarial risk that we face from asteroid impact. The question: is reduction of risk really worth the cost of large surveys? When the modern effort to survey for potentially hazardous asteroids began, we didn't know where asteroids were, only that they were out there and that an unknown one could present a hazard. Harris showed that the majority of the actuarial risk due to impacts is from undiscovered large objects. Near-Earth object surveys have found (we think) 98% of the largest objects that present the most risk, reducing the actuarial risk due to asteroid impacts from 250 fatalities per year to 64 per year. Based on past discovery rates and projecting forward through proposed future projects, over the next 16 years, we should achieve 90% completion of discovery of asteroids larger than 140 meters in diameter. The effect of this 16 years of work -- at a cost of roughly a billion dollars -- will be to reduce the actuarial risk to 33 fatalities per year. If you see asteroid surveys as a form of insurance, then you're spending about two million dollars per fatality avoided. From the point of view of insurance, this is a relatively expensive effort. Harris' point: "The hazard stuff might sell the program," but in fact, the benefit is questionable; the real value of survey programs is in the science they produce. "The scientific value of deep surveys is such a treasure trove that it's worth it right there."
I'll pause here in my retelling of the presentations at this meeting to note the parallel to the appalling verdict reached by an Italian judge last week. In a case that has been watched closely by geoscientists since it began three years ago, six scientists and a public official were convicted of manslaughter for their "false reassurances" that there was no increased risk of earthquake in the village of L'Aquila despite a recent swarm of earthquake activity. The seven had met at the village to discuss the risk, and determined that there was no reason to suspect that the risk was any more than it usually is. The public official made an unfortunate statement to the public to the effect that they should go home and drink wine to deal with their fear. Sadly, a magnitude 6.3 earthquake happened a week later, killing more than 300. The deaths were caused by the collapse of buildings not properly built or retrofitted to survive the known level of seismic activity in the region. The six scientists and the public official were sentenced to six years in prison. Here, in Reno, there were experts in the field discussing the risk of asteroid impact, and determined that the risk is no higher than it ever was; in fact, that it's less than it used to be. If we discover a hundred-meter asteroid tomorrow that obliterates a city the day after tomorrow, are Harris and his peers culpable for "false reassurances?" It would make just about as much sense. A lot of geobloggers have been writing about the court case; I recommend Chris Rowan's take on it.
Many of those fatalities could have been prevented through the kinds of things that California and Japan have done: the enactment and enforcement of building codes that prevent collapses and subsequent fires, and public earthquake drills to prepare the populace to handle the unpredictable but inevitable events. Citizens are expected to make their own preparations. We should deal with the inherently unpredictable nature of asteroid risk in the same way. This was the subject of the next talk in the session.
Steve Chesley is with the Jet Propulsion Laboratory's Near Earth Asteroid program, and presented an analysis of the unpredictable risk posed by a specific asteroid, 2011 AG5. He quibbled with Harris' talk, saying that Harris' analysis assumes that when asteroids are discovered, you will know that either they do, or do not, pose an impact hazard. In fact, while you can usually determine quickly when an asteroid does not pose a hazard, you are rarely able to determine that an asteroid definitely does. 2011 AG5 is one such asteroid. It has a 0.2% chance of striking Earth on February 5, 2040, if it passes through a "keyhole", a particular region in space near Earth on February 23, 2023, that will deflect its orbit to one that is on an impact trajectory. We don't yet know its orbit well enough to know if it will pass through the 365-kilometer-wide keyhole.
Chesley asked, "How urgent is this situation? Do we need to start hiring a project manager [for a mission to deflect it], or can we wait for more observations and hopefully eliminate the prospect altogether? The cheapest asteroid deflection campaign is the one you don't have to do at all, because your observations improve the orbit and show it not to be a risk." The catch is, it will be much easier to deflect the asteroid before the 2023 close pass by Earth, because if it is found to be passing through that keyhole, its path only needs to be shifted 365 kilometers. If we wait until after 2023 and find that it is on an impact trajectory, we will now have less time and need to deflect it by a larger distance -- Earth's diameter, 13,000 kilometers. So the question becomes: can we do followup observations in time to delay a decision about preparing a deflection mission until after those observations are performed? There aren't any further good observation opportunities in 2012. Late in 2013, there will be a much closer pass. The first radar opportunity -- which would really precisely determine the orbit -- is not until that "keyhole" passage in 2023.
Chesley outlined what he thought was an effective way to handle the risk from 2011 AG5. We wait until 2013 for followup observations. There is a 90% chance that those observations will show the asteroid to be on a course that will not impact Earth. If AG5 actually is on an impact trajectory, these followup observations won't be able to prove it either way, but the impact probability would jump to 10% or more. If that happens, Chesley said, we should start a mission, one that would launch in 2018 for a rendezvous with it in 2020. Once a spacecraft arrives, radio tracking will allow us to collapse the uncertainty immediately: we will know for sure whether it's on course to hit us or not. Hopefully it won't be. If it is, our spacecraft will hopefully be able to shift the asteroid's motion just enough for it to miss that 365-kilometer keyhole. If the first try doesn't succeed, we will still have time to try a second, although more difficult, deflection mission.
An ounce of prevention is worth a pound of cure. It goes as well for risk we face on the ground as for risk we face from the sky.