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Projects: Space InformationWe Must Decide to Do It: The Saga of Asteroid 2004 MN4By Rusty Schweickart
This article was originally printed in the July/August 2005 issue of The Planetary Report. Rusty Schweickart is a founder and chairman of the board of the B612 Foundation. He was a NASA astronaut from 1963 through 1987 and flew as lunar module pilot on the Apollo 9 mission in 1963. Schweickart was also the assistant for science and technology to California Governor Jerry Brown in the late 1970s and later served as chairman of the California Energy Commission. The year 2004 ended in an awful week. Most folks were involved in and looking forward to the holiday season, when suddenly it seemed that the world went out of kilter. The main event occurred about 8 a.m. local time on December 26 (Boxing Day), as the India tectonic plate lurched farther under the Burma plate and Earth's crust off the northwest coast of Sumatra broke along a northwest-southeast line. The Burma plate jumped upward by about 10 meters. The magnitude 9.0 earthquake created a massive tsunami that ultimately killed more than 250,000 people, some as far as 8,500 kilometers (5,000 miles) away in South Africa. For the next 2 months, this huge human tragedy dominated the news. But the coincidence of the holiday season and the Indian Ocean tsunami allowed another rare and potentially devastating event, developing at the same time, to go virtually unnoticed. This is the still-unfolding saga of near-Earth asteroid (NEA) 2004 MN4. In June 2004, using the Bok telescope at Kitt Peak, Arizona, Roy Tucker, David Tholen, and Fabrizio Bernardi discovered the asteroid, but weather and other circumstances made it impossible for others to confirm its existence. On December 19, Gordon Garradd of the Siding Spring Survey in Australia rediscovered the asteroid, which was designated 2004 MN4. MN4, as it came to be called, made a particular splash within the scientific community even upon its initial acknowledgment as a potential Earth impactor, entering the list of potentially risky asteroids at a Torino level of 2, the highest risk rating ever assigned to an asteroid. (See JPL's Sentry impact risk table at neo.jpl.nasa.gov/risks and Torino scale explanation at neo.jpl.nasa.gov/torino_scale.html.)
The asteroid, initially thought to be about 500 meters in diameter (subsequently downsized to 400 and then 320 meters), was headed for the vicinity of Earth with an ominous encounter date of Friday, April 13, 2029. Based on observations up through December 23, 2004, it appeared that MN4 would most likely pass outside the orbit of the Moon, but the uncertainty about its orbit also included about a 1 in 300 possibility of an Earth impact. Increasing Risk, But Little AttentionBy December 24, the entire NEO (near-Earth object) community was watching intently as additional tracking information narrowed the uncertainty further. MN4 was determined to be coming even closer to Earth than previously thought. Indeed, the error ellipse (a range of predictions for the asteroid's orbit) had shrunk, and the probability of impact with the Earth had risen to 1 chance in 60, warranting a Torino scale rating of 4. Although the probability of MN4's missing Earth was more than 98 percent, this was nevertheless the most threatening potential impact situation that the NEO astronomers had ever seen -- by far. Those involved in the tracking and calculations were amazed that almost nothing about MN4 appeared in the press. This lack of publicity had its good side: in many prior cases, actual situations had been mischaracterized by much of the press, usually in the alarmist direction. With excitement substituting for sleep, most of us NEO watchers attended closely to new calculations, watching on Christmas Day as the probability of Earth impact rose again, to 1 in 47. On the morning of Boxing Day, it rose yet again, to 1 in 37 -- about the same probability as rolling snake-eyes or boxcars (double 1s or double 6s) in dice. Still, very few in the general public were aware of this unusual risk, and the certain disaster of the Indian Ocean earthquake/ tsunami drew attention even further from the possible disaster of an asteroid impact. The probability of MN4 impacting Earth had risen to unprecedented levels, levels that most of us in the NEO community believed we would never see in our lifetimes. The combination of events that day gave a surreal sense that Mother Nature was bent on reminding us of just who is boss.
With a great sense of relief, tempered by a touch of disbelief, we NEO observers finally relaxed when JPL announced that Jeff Larsen and Anne Descour of the Spacewatch Observatory near Tucson, Arizona had discovered faint traces of MN4 on photographic plates taken in March 2004. Integrating these data with the more recent observations yielded a still smaller error ellipse, but in this case one that excluded Earth. Although it would come close to Earth, MN4 definitely would not hit us -- at least not on April 13, 2029. The Story Behind MN4MN4 is a somewhat unusual NEA in that it spends most of its time inside Earth's orbit. This characteristic puts it in the class of Atens (as opposed to Apollos and Amors), which constitute only about 8 percent of the NEAs discovered. Furthermore, MN4 has an orbit quite similar to Earth's, moving from just outside Earth's orbit to just inside that of Venus, and taking 323 days to circle the Sun. One result of this somewhat Earth-like orbit is that for extended periods, due to glare from the Sun, Earth-based observers can see MN4 only near twilight and sometimes not at all, even though it is relatively close by. Another, more subtle result of this situation is that for several years at a time, MN4 and Earth orbit the Sun relatively close to each other, but then for extended periods (6–7 years), the two are far enough apart that regardless of MN4's position with respect to the Sun, it's too far away to see with our telescopes. We are now about a year from beginning one of those extended periods when we will get little new information to further refine the orbit of MN4. But we already know that it will miss us in 2029, so do we really care about its orbit? As a matter of fact, yes, we do. Our best information indicates that in the fading twilight on April 13, 2029, Londoners will be able to see MN4 with their naked eyes. They will have to look just to the west of due south, about 45 degrees above the horizon, to catch this magnitude 3 object (about the same as a medium-brightness star) as it passes behind Earth, headed toward the just-set Sun. It will dim slightly over the next 40 minutes as it moves almost horizontally to the west, passing closest to Earth in the west-southwest at 21:40 local time. The asteroid will pass over London at less than one tenth the distance to the Moon and 4,000 miles inside the geostationary satellite orbit. There will doubtless be evening parties all across northern Europe celebrating this unique cosmic event. What will be invisible to all of us on that evening is the 28-degree turn that MN4 will take as it whizzes past us. MN4 will end up in quite a different orbit on April 14 from what it had on April 12, shifting from an orbit 323 days long to one of about 428 days. Exactly what its new orbital period will be depends on precisely how far behind Earth it passes on April 13, and the result could, although it is highly unlikely, make all of the 2029 parties in Europe seem highly inappropriate. If, by chance, MN4 passes by Earth so that its new orbit has a period of about 426.125 days, the asteroid and Earth will come back to the identical orbital positions in exactly seven years. MN4, however, will have taken precisely six orbits of the Sun to do so, while Earth took seven. In this situation, called a resonance orbit, two bodies orbit the Sun in periods that are exact multiples (with low values) of each other. That's all well and good, you may say -- so let's plan some more parties. The big "if" in all this is the very low probability that the orbit of MN4 will end up not at 426.1250 days but, in fact, about 30 seconds shorter than that, or 426.1246 days. In that very specific and improbable instance, Earth and MN4 will do their 7/6 dance around the Sun, but instead of an exact repeat of the April 2029 party time, in this case, Earth and MN4 will come together on April 13, 2036 in a cosmic collision the likes of which happen here on Earth about once every 50,000 years. This narrow "window" through which MN4 could pass to bring about such a collision is called a keyhole -- in this instance, the 7/6 keyhole. The likelihood that MN4 will pass through this keyhole is extremely low (about 1 chance in 12,000 at this writing), but it could happen, and the reason we have a program to discover and track near-Earth asteroids is to convert such statistical possibilities into measured certainties.
Is an event with a probability of occurrence of 1 in 12,000 worth spending any time or money on? Certainly not, if the consequence of the impact's occurring were negligible. However, in this instance, we're dealing with a substantial 320-meter object, and the most likely consequence of an impact, should one occur, is a tsunami following an impact in the Pacific Ocean. Based on models by Ward and Chesley, the economic cost of an impact tsunami such as would result from an MN4 impact would be about $400 billion, for infrastructure losses alone! Given this cost-probability ratio, it is well worth spending time and money to ensure that we don't suffer such an avoidable calamity. What's Ahead for MN4?So will MN4 pass through this keyhole, or won't it? The answer is that we don't know yet. Although we have been tracking this asteroid since early 2004 and we have more data on it than on most NEAs we've discovered, the data are not accurate enough yet to answer this question. Normally, with the optical tracking that we have on this asteroid, we could predict what will happen to it about 31 years in advance. But in this particular case, the very close pass by Earth in 2029 will dramatically amplify the small unknowns that currently exist in its orbit. An obvious question is "When will we know what's going to happen?" Less obvious but more important questions are "When do we need to know, and will we know by that time?" We don't simply want to know if the asteroid is scheduled to hit us; we want to know far enough in advance that we can do something about it. More specifically, we will want to deflect it to prevent it from hitting us! It may be news to most people that such an audacious thing is possible, but in fact we are just at the point of having technology that will allow us to deflect an asteroid heading toward a collision with Earth. To deflect an incoming asteroid, we need to know early enough that a deflection is needed, and we need a high-efficiency, low-thrust propulsion system to push on the asteroid and slightly modify its orbit. Specifically, we need a couple of decades of warning that an asteroid has our name on it, and we need a spacecraft that can dock with the asteroid and push on it with a couple of pounds of force, continuously, for a year or two. The first requirement, in this instance, is partially met. We know that there is a possibility of impact with 2004 MN4 in 2036, more than three decades away. In fact, we know, via the Spaceguard Survey (impact.arc.nasa.gov), that of the 3,400 near-Earth objects we've discovered so far, only 71 have any chance of hitting Earth in the next 100 years. More important still, we know that the probability of any one of those hitting us is extremely small, and we are tracking them and will have excellent early warning if additional data change those odds. Unfortunately, there are another 300,000 NEOs out there that we don't know anything about yet, and we need to increase our search capabilities so that we have a fighting chance to protect the planet.
Regarding the second requirement to protect Earth from asteroid impacts, we're not quite there yet, but we're getting close. The B612 Foundation (see www.B612Foundation.org) has been working on the challenge of deflecting asteroids from impact with Earth since 2001. We worked through what would be required, recommended a goal of demonstrating such a capability by 2015, and designed a preliminary mission to get the job done. (The demonstration mission that B612 proposes was introduced publicly in "The Asteroid Tugboat," published in Scientific American in November 2003.) From 2002 through early 2005, NASA was developing exactly the propulsion and power technologies that would be required. These key technologies will be needed in any event to enable cost-effective access to deep space. Unfortunately, the Prometheus program, which was developing the key technology of nuclear electric propulsion (NEP), has recently been put on hold in order to focus on nuclear surface power for use on the lunar surface. This is regrettable because NEP appears to be the most effective technology, in most cases, for NEO deflection. Returning to 2004 MN4, the questions resolve to the following: "Will we know enough about MN4 early enough and accurately enough to deflect it using our best technology, if we need to?" The only way to answer this question is to make the assumption that MN4 will hit us in 2036 unless we do something about it, then figure out what we need to know and when, in order to prevent this calamity. Unlike most natural disasters, asteroid impacts come in "sizes" up to and including extinction of some forms of life on Earth, as with the dinosaurs 65 million years ago. What's different here is that unlike virtually all other major natural disasters, we can predict asteroid impacts decades ahead, and we can prevent them. We're not talking about providing a bit of warning so folks can head for their cellars or the high ground, or about making low-interest loans available for reconstruction after the disaster -- we're talking about prevention of the disaster itself. We're just about there, but we need to keep our eye on the ball -- or NEO, in this case. We should continue the development of NEP and use it to demonstrate to ourselves that we can deflect an asteroid. Without question, such a demonstration will teach us a great deal about the process and provide the public with confidence that life can indeed be protected from this natural cosmic hazard. Good News and Bad NewsIf we assume that MN4 has our address and, without intervention, will deliver in 2036, will we be able to make this the first successful exercise in impact prevention? There's no question that very shortly after the 2029 parties are over, we'll know how close our return visitor will come in 2036, but unfortunately, we'll have much too little time left to do anything about it. Furthermore, the amount of energy that it would take to successfully deflect MN4 between 2029 and 2036 would exceed our capability by quite a large margin. The good news, however, is that if we were to deflect MN4 prior to 2029, it would require very little energy to get the job done. In fact, deflecting MN4 (from a 2036 impact) prior to 2029 would require less than 0.01 percent (1 ten-thousandth) of the energy that it would take after 2029. This should (if we do our homework) be well within not only our NEP/tugboat capability but within the reach of some alternative deflection techniques as well. There is no good news, however, without bad news . . . or so it seems. The reason that deflection would require so much less energy prior to 2029 than after is the amplification effect of swinging by Earth so closely at that time. The corollary of this is that, in order to know prior to 2029 that the asteroid will in fact collide with Earth in 2036, we have to have more accurate knowledge of its orbit than we normally would have -- in fact, thousands of times greater accuracy. Where does this leave us? Let's guess that we'll need to get to MN4 something like 4 or 5 years before 2029 to accomplish the deflection and that it could take us as much as 3 years to get there. Thus, we'll need to launch our tugboat deflection mission by about 2021. The space industry likely would need another 6 or 7 years to plan and put the mission together, so we're talking about committing to the mission by about 2014. Our big question has then worked its way to: "Will we know by 2014 whether or not MN4 will collide with Earth in 2036?" At the moment, our best guess is that unless we do something special in terms of gathering and refining data, the answer to that is probably "no." It looks as though we'll have to determine the specific distance that MN4 will pass behind Earth in 2029 within an accuracy of about 600 meters to know for certain what our situation will be in 2036. But, one may ask, do we really have to know for certain? Well, no. A probability of impact of 1 chance in 10 or higher is likely adequate to justify a deflection decision. However, in this instance, we know that using the best telescopes existing now, and allowing for inevitable uncertainties, we will be able to predict the probability of impact with MN4 to be no higher than about 1 in 150 by 2014, even if we're headed for a direct impact! Radar data that we hope to get in 2012 may help, but probably not enough to allow a clear choice. Would we launch a deflection mission if the chances were 149 out of 150 that the asteroid would miss us? Or even 39 out of 40? Not likely. So what do we do should this unlikely circumstance arise? The unfortunate reality is that there is no one designated within our government to analyze this, or any similar situation. While we are just short of having the technology and knowledge available to protect Earth from this natural hazard, no is charged with the responsibility to provide such protection. In the current situation with MN4, there are critical decisions to be made, options to be evaluated, and actions to be taken. One of those choices is to gather much better information about where the asteroid is going soon enough to do something about it, if necessary. By launching a scientific mission to 2004 MN4, we can do excellent and valuable science, and in addition, we can know whether or not we'll have to deflect the asteroid. How's that? It turns out that if you want to know the orbit (trajectory) of something in space, the most accurate way to do so is to install a radio transponder on it. That's what we do with our spacecraft; it's what enables us to fly cheaply out to Saturn (or wherever) by doing the same orbit-altering trick that MN4 will do using Earth in 2029. We can make these very clever orbit-changing maneuvers, swinging by Venus and/or Earth on the way out to deep space, because we know precisely where the spacecraft is. The trick, then, is to place a radio transponder onto 2004 MN4 in order to know with certainty, by 2014, whether the asteroid is going to be a pest in 2036. Well, that's easy, right? Perhaps it would be if someone were in charge. And on that score there is now hope. The US House of Representatives has included language in NASA's 2006 appropriations bill requiring that it report back within 120 days after the president signs it into law with an assessment of what actions would be necessary to address the potential threat from asteroid and comet impacts. It is hoped that in response to this congressional request, NASA will, for the first time, look not only at discovering NEOs but also at what will be required in order to protect the planet from impacts. In that process, 2004 MN4 should be addressed specifically. How will we deflect it if we need to? By when must the deflection decision be made? Will we have adequate information to make such a decision by that time? Is a scientific mission to 2004 MN4 needed? Is such a mission prudent given the additional knowledge to be gained by the science and exploration equipment aboard? Finally, there is a federal agency charged to look at these questions. We hope this will be done with wide participation and input from interested parties who have been wrestling with these issues. After all this, then, we come to the strange reality that the saga of 2004 MN4 leads right back to today. There's not a thing in the world we could have done on December 26, 2004 to prevent the Indian Ocean tsunami from inundating the coastlines and communities around the Indian Ocean, even if we had known about it ahead of time. Similarly, we have no way of knowing about such earthquakes and tsunamis that lie ahead. We can know, however, whether there is a far worse tragedy headed our way on April 13, 2036. Even more important, there is something we can do about it in the unlikely event that asteroid 2004 MN4 has our name on it: we can prevent the collision. Not only can we do this in the instance of the saga of 2004 MN4, but we also can, and should, do it for all near-Earth asteroids and for all time. The ball (finally) is now in NASA's court. We simply have to decide to do it. Want to learn more? You can learn more about what near-Earth objects are and how we assess the threat they pose, find out about The Planetary Society's projects to discover, track, study, and retire the risk from these bodies, and donate to our NEO programs. |
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