Sounding an alarm, cautiously
by Jeff Foust
|If 2004 MN4 passes through a “keyhole” in 2029 just 640 meters across—about twice the diameter of the asteroid itself—the object would enter a trajectory that would result in a collision with the Earth on April 13, 2036.|
However, in a presentation May 20 at the International Space Development Conference, and in a paper published on the foundation’s web site the same day, Schweickart’s attention was focused not on deflecting asteroids but understanding the impact risks of 2004 MN4. While the asteroid poses no impact threat to the Earth in 2029, it will pass within about 30,000 kilometers of the planet—closer than satellites in geosynchronous orbit. That passage will be close enough to allow the Earth’s gravity to significantly alter the asteroid’s orbit. The change in the asteroid’s orbit will depend on exactly how close to the Earth 2004 MN4 approaches, a degree of precision not yet possible with the currently-known orbit of the object.
Schweickart focused on a particular scenario where the 2029 flyby puts 2004 MN4 on a new orbit with a 7:6 resonance with the Earth’s: the asteroid would complete six orbits of the Sun in the time it takes the Earth to complete seven. That would bring the asteroid back into the vicinity of the Earth in 2036. (Other scenarios exist that would bring the asteroid back near the Earth in 2034, 2035, and 2037, but the 2036 opportunity lies closest to the centroid of the error ellipse for the asteroid’s 2029 flyby.) Schweickart added that if 2004 MN4 passes through a “keyhole” in the error ellipse just 640 meters across—about twice the diameter of the asteroid itself—the object would enter a trajectory that would result in a collision with the Earth on April 13, 2036.
Such a collision—admittedly unlikely—would be devastating. The impact location would be somewhere along a path thousands of kilometers long but only 51 kilometers across, extending from the Kurile Islands in the Russian Far East across the North Pacific to the Mexican coast near the southern tip of Baja California. The path then extends across central Mexico and into the southern Gulf of Mexico and Caribbean Sea before terminating in the central Atlantic.
The center of that path lies in the Pacific about 1,100 kilometers from the coast of southern California. An impact there would create a tsunami that would strike the California coast with a height in excess of 10 meters, comparable to hard-hit Banda Aceh, Indonesia, in last December’s tsunami, according to a computer model of the impact calculated by Steven Ward of the University of California Santa Cruz. The rest of the West Coast of the US, as well as Baja California, would also be subjected to a powerful tsunami. A model of the economic effects of impact-induced tsunami estimated that the costs of such an event could exceed $400 billion, based on globally-averaged infrastructure costs. Schweickart notes that the actual cost could be significantly higher given the built-up, valuable nature of the California coast. However, unlike the December 2004 tsunami, loss of life would presumably be minimal since people would have up to years of advance notice to evacuate vulnerable sites.
A map showing the potential impact path of 2004 MN4 in April 2036. (credit: R. Schweickart)
While Schweickart makes a compelling case for the potential hazard of a 2036 collision by 2004 MN4, why think about an event that is more than 30 years in the future, and one that, at the present, he estimates has only a 1-in-10,000 chance of happening? Schweickart believes that, despite the long lead time, now is the time to at least start considering what should be done. He noted that if we wait until after the 2029 flyby—when the asteroid’s trajectory and potential impact risk will be far more certain—any mission to deflect the asteroid will require a large amount of energy to deflect its orbit: a change in velocity (delta-v) of about 0.1 meters per second. However, if such a deflection is attempted prior to the 2029 flyby, far less energy will be needed—a delta-v of as little as one-millionth of a meter per second—since such a mission would only have to shift the asteroid’s path out of the 640-meter-wide keyhole.
|A model of the economic effects of impact-induced tsunami estimated that the costs of such an event could exceed $400 billion, based on globally-averaged infrastructure costs. Schweickart notes that the actual cost could be significantly higher given the built-up, valuable nature of the California coast.|
Working backwards, Schweickart estimates that if an asteroid deflection mission is completed by 2028, it would need to arrive at the asteroid and start work by 2026. This, in turn, requires a launch by 2022 or 2023, depending on the spacecraft’s trajectory to 2004 MN4. Meeting that launch window, he concludes, requires making a decision to begin design and development of that mission by around 2014. This gives astronomers less than a decade to refine the asteroid’s orbit to the point where the odds of an impact are known well enough to support any decision to proceed with the mission.
This, though, raises another problem. While the asteroid will be in view of Earth-based telescopes next year, its orbit will take it out of view, obscured by the Sun, from mid-2006 through late 2012.After 2013 it will similarly be out of view until around 2020. This gives astronomers limited time to refine the asteroid’s orbit and thus the odds of a 2036 impact. By next year the odds of an impact, assuming 2004 MN4 is actually on a collision trajectory, would only be refined to about 1-in-3000, and by 2013 to 1-in-140. Radar observations in 2013 could further reduce the error by a factor of four, but by the time a decision would have to be made to develop a costly, risky deflection mission, the probability of an impact would still lie at only a few percent.
One way to better refine the asteroid’s orbit would be to launch a spacecraft to 2004 MN4 in the next few years and place a radio transponder on it, allowing it to be tracked even when it is out of view of Earth-based optical telescopes. Such a mission, which Schweickart estimates would cost on the order of $300 million (the typical cost of a NASA Discovery-class planetary science mission), could also carry other instruments to perform scientific studies of the asteroid. This additional tracking data from the transponder would refine the probability of an impact to about 10% at the 2014 decision point. “I would suggest that a 1-in-10 chance that you’re going to have a $400-billion-plus consequence if this thing hits the Earth is probably adequate enough justification to decide to go ahead and launch,” he said. “But without the transponder, we’re not going to have adequate enough information.”
At the conference, and in his paper, Schweickart did not call directly for planning an asteroid deflection mission, or even demand a robotic mission to 2004 MN4 to place a transponder there. He said he was well aware that the current 1-in-10,000 odds of an impact mean that there’s a 9,999-in-10,000 chance that this asteroid poses no risk at all in 2036. The problem, he concluded, is that we don’t know for certain what risk the asteroid poses, and are ill-equipped to deal with an impact hazard if this or another NEO poses a threat. The 1-in-10,000 current impact odds are about the same, he noted, that the average American faces in getting into an auto accident on any given day, risks mitigated with insurance. “Right now we’re driving around the solar system uninsured,” he said.
Schweickart also stressed that his analysis of the 2036 impact threat requires additional, independent analysis to verify his conclusions. Thus, the first recommendation he made at the conference was to ask Congress to have the National Research Council (NRC), or another entity, review the impact risk analysis he presented and make a recommendation on a course of action.
|“By the time we get to 2014, there will probably be some other object with an even higher probability that what we found” with 2004 MN4, NASA’s Lindley Johnson said.|
The second recommendation was more general. Right now, Schweickart noted, there is no office of the US government—not NASA, the Defense Department, nor another organization—specifically charged with the responsibility of asteroid impact mitigation and its assorted policy issues. Specifically, if the US was to deflect an incoming asteroid, any impact could no longer be considered an act of God. “What if you move it away from the California coast but put it over Guatemala by accident?” he asked. Thus, he believes that Congress should have the NRC study the issue and recommend where in the government such responsibility should lie.
Schweickart made his presentation during a panel session of the conference devoted to NEO impact threats. Other panelists, while not fully endorsing Schweickart’s recommendations, did agree that NEOs in general can pose a risk and are worthy of further study. Lindley Johnson, manager of NASA’s NEO observation program, said that continuing search programs will likely produce an object with an ever greater risk of impact than 2004 MN4, which Johnson nicknamed “Minnesota 4” because that state’s postal abbreviation is MN. “By the time we get to 2014, there will probably be some other object with an even higher probability that what we found” with 2004 MN4, he said. “We won’t even be thinking about Minnesota 4, since there will be something else with an even higher probability of impact if we continue the search effort and are able to extend it down to objects with diameters in the 100-meter range.”
Another issue not directly addressed by Schweickart’s proposal is the need for regular tracking of already-discovered NEOs. “You have to keep tracking them because of modeling inadequacies, an understanding of all the forces than impact an orbit,” Johnson said. An example is the Yarkovsky effect, a force linked to the solar heating of asteroids and the emission of thermal radiation from them, which Johnson estimated could alter 2004 MN4’s orbit by as much as 20 to 50 meters between now and 2013.
Johnson agrees with Schweickart that a spacecraft mission to place a transponder on 2004 MN4 may be worthwhile. “Not only do we need to put a transponder on it,” he said, “we need to put a full science station on it so we are able to fully characterize it and all the others that are going to come close to us over the next 10, 20, 30 years.”
Also backing the idea of a spacecraft mission to 2004 MN4 is Brian Marsden, director of the Minor Planets Center at the Harvard-Smithsonian Center for Astrophysics, the nexus for coordinating NEO observations and calculating orbits. “It’s really a very good idea to think about a transponder,” he said. “We may find that in 2013 the whole thing disappears, but we do get science out of it.”
Marsden also believes that more funding is warranted for NEO studies in general. In one case, his center was not able to pay a salary for one of his few employees earlier this year because of a funding glitch. “Really not enough money is being put into this effort at the present time,” he said.
|“This is a very real issue,” said Pete Worden, “but it also has a rather large giggle factor.”|
Whether such funding is forthcoming remains to be seen. Earlier this month the House Science Committee approved HR 1022, a bill introduced by Rep. Dana Rohrabacher (R-CA) that would authorize $20 million for NASA in 2006 and 2007 to expand NEO searches to objects as small as 100 meters across. Schweickart said that he has already been talking to members of Congress about his two recommendations, but didn’t offer any more details.
Schweickart made it clear he was not trying to raise alarm about 2004 MN4, but instead trying to kickstart the government to think about the risks of such impacts and the need to plan accordingly. “There’s not a reason in the world for anyone to lose any sleep about this, unless you’re the one assigned responsibility for this,” he said. “Then you should be staying awake a bit.”
Such caution was shared by panel moderator Pete Worden, the retired Air Force general and current University of Arizona astronomy professor who has sought for years to get the government more interested in NEO impact risks. “This is a very real issue,” he said, “but it also has a rather large giggle factor.”