Mercury, November-December 2003, pg 15 ARE ASTRONOMERS CRYING WOLF? By generating scary headlines, NEO searches have become a victim of their own success. David Morrison ------------------------- Asteroid on Impact Trajectory with Earth! Astronomers Issue Warning! Space Rock Discovered Two Days after Passing Earth! Headlines like these appear every few months. Usually the scare is withdrawn within a day or two, although the media don't always report the revised orbits. What is happening here? Has the asteroid impact danger increased? The primary reason for these media flaps is that the Spaceguard Survey is discovering many more asteroids that come close to the Earth. These asteroids have always been there, but previously they passed by unseen. The increased number of "near misses" is evidence of the success of Spaceguard, which has already found more than 60% of the near Earth Asteroids (NEAs) larger than 1 km in diameter -- that is, large enough to threaten global environmental damage if they hit. The second reason for the headlines is that astronomers are posting their orbital computations on the Internet. Both the JPL Sentry system and the NEODys system at the University of Pisa update all asteroid orbits daily, and the results are there for anyone (including a reporter on a slow news day) to see. You can see these results and much additional asteroid information at http://neo.jpl.nasa.gov One of the most recent examples was NEA 2003 QQ47, found by the MIT LINEAR telescopes on August 24. As is the case with many newly discovered NEAs, the initial orbit was highly uncertain and included several low-probability cases of possible future impacts. At one point, with only 6 days of observations reported, the formal odds of an impact in 2014 briefly rose slightly above one-in-a-million, and then went virtually to zero as more data were reported. This is standard operating procedure for dealing with newly discovered NEAs. In this case, however, the government-supported UK NEO Information Center decided this asteroid deserved special attention, and on September 2 they issued a press release. The story was widely reported as an actual impact threat, especially in the UK. When the inevitable refinement of the orbit came as new observations were made, some in the press accused the astronomers of crying wolf. QQ47 is only the most recent example of such misunderstandings. The first modern impact scare was associated with asteroid 1997 XF1. In March 1998, Brian Marsden, Director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics, posted a Press Information Sheet in which he stated that "the chance of an actual collision is small, but it is not entirely out of the question". The story of this "prediction" appeared worldwide. Within a few hours more careful orbital calculations showed that the odds of hitting were extremely small, and when new observations became available a day later, the chances of impact went to zero. For many in the media, it seemed that astronomers had screwed up. The XF11 episode demonstrated the need for both rapid calculation of impact odds and better coordination among scientists before they "go public". Astronomers reacted by bringing in the NEO Working Group of the International Astronomical Union for coordination, and in 1999 we adopted the Torino Scale for communicating the impact risk to the public (see http://impact.arc.nasa.gov.) The Torino Scale uses a color-coded set of warnings that reflect both the probability of impact and the size of the impactor. While it has helped in communications, the scale suffers from the fact that only the lowest level warnings (scale values of 0 or 1) have been exercised. Impacts are so rare that there have been no serious warnings and indeed none are expected, unlike the Richter Earthquake Scale, which is used more often. Other asteroids that made the headlines were 1999 AN10, 2000 SG344, 2002 MN, and 2002 NT7. Each of these is described in the News Archive section of the NASA Impact Hazard website http://impact.arc.nasa.gov. No two situations were alike, but each led to scare headlines. This recurrent problem has led some astronomers to suggest that we should not post preliminary orbits on public websites. However, this information is needed by others (including many dedicated amateurs) who make follow-up observations of the most interesting NEAs. In addition, most of us feel that withholding information would subject us to even greater criticism from the media and the public. The impact hazard is real, and many people mistrust governments and scientists to deal with such information. The best policy is openness, together with an effort to educate the media and the public as to what a "one-in-a-million" chance of impact really means. Are we actually any safer today as a result of the Spaceguard Survey? I believe we are. Each NEA that is discovered represents one fewer unknown object out there that can hit the Earth. In 2008, when we will have discovered 90% of the NEAs large than 1 km, we will have reduced the risk by about the same percentage. These stories are not reported in the media, but they represent the real accomplishment of the Spaceguard Survey. ---------------------------------------------------- Scientific American November 2003, pp. 54-61 The Asteroid Tugboat By Russell L. Schweickart, Edward T. Lu, Piet Hut and Clark R. Chapman On an average night, more than 100 million pieces of interplanetary debris enter Earth's atmosphere. Luckily, most of these bits of asteroids and comets are no bigger than small pebbles; the total weight of the 100 million objects is only a few tons. And our planet's atmosphere is thick enough to vaporize the vast majority of these intruders. So the debris usually streaks harmlessly overhead, leaving the bright trails popularly known as shooting stars. When bigger objects slam into the atmosphere, however, they explode rather than vaporize. In January 2000, for example, a rock about two to three meters wide exploded over Canada's Yukon Territory with a force equivalent to four or five kilotons of TNT. This kind of event occurs once a year, on average. Less frequently, larger rocks produce even more powerful explosions. In June 1908 a huge fireball was seen descending over the Tunguska region of Siberia. It was followed by an enormous blast that flattened more than 2,000 square kilometers of forest. The consensus among scientists today is that a rocky asteroid an explosion equivalent to 100 megatons or more of TNT. If a large asteroid crashes into the ocean, which happens in about 70 percent of impacts, it could create a tsunami that might kill millions of people by inundating coastal cities. Events of this kind happen once every 40,000 years or so. And an asteroid with a diameter greater than one kilometer would strike Earth with the energy equivalent of 100,000 megatons of TNT, far greater than the combined energy of all the nuclear weapons in existence. Impacts of this size and greater have the potential to wipe out human civilization, and there is a chance of perhaps one in 5,000 that unmanned space tug that would rendezvous with an incoming asteroid, attach to its surface and slowly push the body so that it misses Earth. (Because of the unique characteristics of comets, we do not address them in this proposal. New studies indicate that comets constitute only about 1 percent of the overall impact threat to Earth.) To push the asteroid, the space tug would use nuclear-powered engines that expel jets of plasma, a high-temperature mix of ions and electrons. We believe that a mission to about 60 meters in diameter exploded some six kilometers above the ground with a force of about 10 megatons of TNT. The blast wave devastated an area approximately the size of metropolitan New York City. Recent observations of near-Earth objects -- asteroids and comets whose paths could intersect Earth's orbit'suggest that the chance of a similar event happening in this century is about 10 percent. Asteroids 100 meters across and larger pose an even more ominous threat because they will penetrate deeper into the atmosphere or hit the surface. Such an impact, which has a 2 percent chance of occurring before 2100, would cause such a strike will occur in this century. Can humanity prevent these catastrophes? Over the past decade scientists and engineers have proposed a variety of schemes to deflect an asteroid that is heading toward Earth. Several researchers have advocated detonating a nuclear weapon on or near the asteroid to either break it up or change its course, but the effects of a nuclear blast are difficult to predict, and that uncertainty has led many experts to view this option as a last resort at best. Recently interest has focused on more controlled options for shifting an asteroid's trajectory. For the past two years we have been studying the concept of an demonstrate the asteroid-tug concept could be accomplished by 2015. Why develop such a spacecraft now, before astronomers have identified any asteroids on a collision course with Earth? Because the system should be tested before it is urgently needed. By attempting to deflect an asteroid that is not on, or even close to, a collision trajectory, researchers will acquire the experience necessary to build a reliable defense. Potentially hazardous asteroids have not yet been studied in any detail; because we do not know much about their interior makeup, surface characteristics or structural integrity, we cannot know what will happen when a space tug nudges one. The best way to learn about these crucial aspects is to land a spacecraft on an asteroid and then try to move it. As a bonus, the mission would add to our understanding of asteroids, pioneer the way to asteroid mining, and demonstrate critical technologies for future exploration of the solar system. What is more, NASA is already working on the key technologies needed for the asteroid tug. As part of the Prometheus Project, the space agency is trying to design nuclear reactors that could power ion-propulsion systems for interplanetary spacecraft. NASA plans to integrate these systems into the Jupiter Icy Moons Orbiter (JIMO), a spacecraft that is expected to visit the Jovian moons of Ganymede, Callisto and Europa in the next decade. The same technologies could be applied to the greatest public safety project in history: warding off the doomsday rock that will sooner or later threaten humanity. The B612 Mission The problem of deflecting an asteroid resolves into a timing issue. First, astronomers must detect the asteroid at least a decade before impact to provide time for the actions to take effect. Fortunately, with continued improvement in ongoing asteroid-detection programs, this is a reasonable expectation. To prevent the rock from hitting Earth, the most efficient plan is to either speed up the body by pushing it in the direction of its orbital motion or slow it down by pushing in the opposite direction. Changing the asteroid's velocity alters its orbital period -- the time it takes to go around the sun. Because Earth moves along its orbit at an average speed of 29.8 kilometers per second and its diameter is 12,800 kilometers, our planet takes 215 seconds to move half its diameter. If an asteroid were headed for a bull's-eye collision with Earth, the challenge would be to change the asteroid's orbital period so that it arrives at the rendezvous site at least 215 seconds before or after Earth does, allowing the body to whiz safely by our planet. Applying a soft but prolonged push on the asteroid about 10 years before it is expected to hit Earth, the tug would need to boost the asteroid's velocity by only about one centimeter per second. This change would slightly expand the asteroid's orbit and lengthen the time it takes to travel around the sun. For example, for an asteroid with an orbital period of two years, a one-centimeter-per-second velocity change would increase its period by 45 seconds and create a delay of 225 seconds over 10 years -- enough for the asteroid to miss Earth by a small margin. Alternatively, the space tug could slow down the asteroid, shrinking its orbit and reducing the period by 45 seconds; after 10 years, the asteroid would arrive at the rendezvous site 225 seconds before Earth does. Of course, if the space tug reaches the asteroid when it is closer to striking Earth, it would need to give the body a bigger push. This fact underscores the importance of early and accurate detection of all near-Earth asteroids. To demonstrate this concept and the technologies involved, we have proposed the development of a space tug that could deflect a 200-meter-wide asteroid, which would cause regional devastation if it hit Earth. We have dubbed this test project the B612 mission (B612 is the name of the asteroid in The Little Prince, the well-known children's book by Antoine de St. Exupery). A rocky 200-meter asteroid has a mass of about 10 billion kilograms. Rather than giving the asteroid a brief, powerful shove --which might shatter the body instead of altering its course--the B612 tug would deliver gentle pressure. The force would be only about 2.5 newtons, approximately equivalent to the force required to hold up a glass of milk. But if this light nudge were applied for just over three months, it would be enough to change the asteroid's velocity by 0.2 centimeter per second. Should we be faced with an actual threat by a 200- meter asteroid, our small demonstration mission would either have to be scaled up by a factor of five or more to prevent the body from smashing into Earth, or else we would have to act at least 50 years before impact. Because the force must be provided continuously for an extended period, the space tug's engines would require a significant amount of fuel. An additional large supply of propellant would be needed to get the tug to rendezvous with the asteroid. The average velocity change to get from our planet to a typical near-Earth asteroid is about 15 kilometers per second -- one third more than the velocity change required to escape Earth's gravity. The standard chemical rocket engines, which mix fuel with oxidizer in a combustion chamber, would be hard-pressed to propel a substantial spacecraft (and all the fuel needed to push the asteroid) to these speeds. Such a vehicle would require so much propellant toform the B612 mission that it could not be launched by a single rocket; dozens of heavy-lift rockets would be needed to boost all the components into low Earth orbit. Then the spacecraft would have to be assembled in orbit, which would dras-tically raise the mission's cost and delay the journey to the asteroid. Our goal is to design a space tug that could be launched on a single heavy-lift rocket, such as a Proton, Ariane 5 or Titan 4. Because the tug must have a total mass less than about 20 tons, it needs extremely fuel-efficient engines. The primary measure of rocket efficiency is specific impulse, which is the thrust generated for each unit of fuel consumed per second. The most efficient chemical rockets have a specific impulse of up to 425 seconds when operating in the vacuum of space. (The units of specific impulse are seconds.) But the engines of our asteroid tug must have a specific impulse of 10,000 seconds. This performance is not feasible for standard chemical rockets but is comfortably within the range of electric engines, which use electrical or magnetic fields to accelerate ions out the exhaust nozzle of the rocket. In this way, the engines can achieve much higher exhaust velocities than chemical rockets, which simply burn fuel and allow the expanding hot gases to escape out the nozzle. Ion engines with a specific impulse of 3,000 seconds have successfully flown in space. A promising new engine known as the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) uses radio waves to ionize a gas and accelerate the plasma to even higher exhaust velocities [see "The VASIMR Rocket," by Franklin R. Chang Díaz; SCIENTIFIC AMERICAN, November 2000]. Rather than using a conventional nozzle, the VASIMR employs magnetic fields to direct the expanding stream of ions out of the rocket at specific impulses between 10,000 and 30,000 seconds. Of course, there is a price to be paid for such high performance. Although plasma and ion engines are more efficient than chemical rockets, their thrust is much lower (because the high-temperature exhaust is so tenuous). Several ion engines now under development could achieve specific impulses approaching the target of 10,000 seconds, but with the exception of the VASIMR, most electric engines generate less than 0.1 newton of force. Thus, many such engines would have to be ganged together to reach the desired thrust level of 2.5 newtons. Even when combined, the engines must push on the asteroid for a very long time to alter its orbit. Long-term operation has already been demonstrated, however: the ion engine on the Deep Space 1 spacecraft, launched in October 1998, accumulated 677 days of operating time. To provide the required thrust, the plasma engines would need about 250 kilowatts of electrical power (assuming an engine efficiency of 50 percent). This amount of power is considerably beyond the capability of the solar arrays typically used for small spacecraft. Even the enormous solar arrays of the International Space Station, when completed, will produce less than half this amount (and they will weigh more than 65 tons). Clearly, such an array is infeasible for a spacecraft that must weigh less than 20 tons in total. The only current technology that can steadily supply this much power for several years in a package that weighs just a few tons is nuclear fission. The asteroid tug needs a simple, small and safe nuclear reactor. Fortunately, NASA has already proposed some new designs for spacecraft reactors, and one has undergone preliminary testing. An important safety feature in these new designs is that the nuclear fuel is minimally radioactive until the reactor has produced power for a significant amount of time. Because the reactor would be launched cold -- that is, inactive even a catastrophic launch accident would pose little environmental danger. If the entire uranium core of the SAFE-1000, an advanced space reactor being developed at Los Alamos National Laboratory, were dispersed in a launch explosion, the radiation released into the environment would be only six to 10 curies -- less than the total radiation contained in the walls of New York City's Grand Central Station. Ground controllers would send the command to activate the reactor only after it was safely in space. The Problem of Spin A major challenge for the B612 mission will be maneuvering around the target asteroid, landing on the body and attaching to its surface. In 2000 the NEAR-Shoemaker spacecraft successfully maneuvered into orbit around Eros, the second largest of the known near-Earth asteroids, and even managed an impromptu landing on the 34-kilometer-long body. Japan's Hayabusa spacecraft (formerly Muses-C) is now on its way to near-Earth asteroid 1998 SF36 using ion propulsion. Once there it will lightly touch the asteroid's surface several times to pick up samples that will be returned to Earth. But the asteroid tug would be far larger than either of these spacecraft, and it would have to attach itself firmly to the asteroid because the gravitational attraction at the surface of such a body is only a hundred-thousandth of the gravity on Earth. Researchers are considering several concepts for a mechanism to hold the tug to the asteroid's surface, but the final design will most likely depend on the results of upcoming missions that will study the composition and structure of small asteroids. To speed up or slow down the asteroid, the space tug must keep the direction of thrust parallel to the body's orbital motion. Small asteroids, though, often spin at rates of 10 rotations or more a day. One way to solve this problem would be to stop the rotation before pushing the asteroid. The tug would land on the asteroid's equator (the ring midway between the two poles of the axis of rotation), point its engines horizontally along the equator and fire them until the thrust brought the rotation to a halt. This method could be risky, however, because most rocky asteroids appear to be porous, low-density "rubble piles," collections of many large and small boulders, interspersed with pebbles and smaller grains loosely held together by the body's weak gravity. Although this type of structure could withstand a force of several newtons distributed over two to five square meters of its surface, the same cannot be said for the internal stresses created by slowing down and stopping the body's rotation. It seems highly likely that altering the finely balanced gravitational and centripetal forces associated with asteroid rotation would cause significant and possibly destructive rearrangements in other words, asteroid quakes. For this reason, a better alternative might be to allow the asteroid to continue rotating but to torque the spin axis gradually until it is parallel with the body's orbital motion and to keep it there. With the axis properly aligned, the tug would push the spinning asteroid along its orbit like a pinwheel. For the B612 demonstration mission, we plan to choose an asteroid spinning at about four rotations a day (typical of asteroids this size) and torque its spin axis by five to 10 degrees [see illustration on page 57]. Using 2.5 newtons of thrust applied at either the asteroid's north or south pole, the task would require a couple months of steady torquing. Although this result would clearly demonstrate the capability to maneuver an asteroid, an actual deflection would require many months, and perhaps even years, to properly orient the asteroid and accelerate it in the desired direction. Another important challenge would be to deflect the asteroid in such a manner that it does not simply return again several years later on a new collision path. Bodies passing close to Earth are often gravitationally deflected into resonance orbits that have periods that are proportional to Earth's period; as a result, the bodies may periodically return to our planet's vicinity. We must therefore precisely deflect the asteroid onto a trajectory that ensures it will not end up in a resonance orbit. This requirement for precision is one of the best arguments for the asteroid tug concept. The tug provides a carefully controlled maneuver, whereas most of the other deflection schemes yield an approximate, uncontrolled velocity change at best, thereby risking a boomerang scenario. Protecting Our Planet The mission we are proposing would cost about $1 billion -- a bit more than half of 1 percent of NASA's expected spending over the next 10 years -- provided that off-the-shelf power and propulsion systems are used and a single existing launch vehicle can lift the spacecraft. Is this project worth the expense? Although the actual use of an asteroid deflection system would be rarenever in our lifetimes, we hopeits value would be beyond measure. An asteroid collision with Earth would be so potentially devastating that preventing it would be worth almost any cost. By practicing an asteroid deflection, the B612 mission would show whether the asteroid tug concept is feasible and, if so, how it should be refined in the event of a real impact threat. The scientific benefits of the demonstration mission would also be significant. Asteroids are remnants of the early solar system and have much to tell us about the formation of the planets and perhaps even the origins of life. Researchers have already learned a great deal by studying meteorites, the pieces of asteroid debris that survive the fiery plunge through Earth's atmosphere, but a much greater payoff would come from visiting the source of these fragments. In addition, asteroids are believed to contain large amounts of metals, minerals and water ice. Experts on space exploration claim that taking advantage of these resources could dramatically reduce the cost of future interplanetary flights [see "Tapping the Waters of Space," by John S. Lewis; SCIENTIFIC AMERICAN Presents, Spring 1999]. The B612 mission would vividly show that spacecraft could access these materials; using the same maneuvering and docking techniques developed for the asteroid tug, other vehicles could land on asteroids and begin mining operations. And these efforts may eventually pave the way for a manned mission to a near-Earth asteroid. Indeed, many experts contend that sending astronauts to an asteroid would be quicker, less costly and more worthwhile than a human mission to Mars. Most important, the B612 demonstration would fulfill NASA's stated mission, "To protect our home planet ... as only NASA can." A better match could hardly be found. More to Explore Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment. John S. Lewis. Perseus, 1997. Cosmic Pinball: The Science of Comets, Meteors, and Asteroids. Carolyn Sumners and Carlton Allen. McGraw-Hill Trade, 1999. Report of the Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids. Michael J. S. Belton. National Optical Astronomy Observatory, March 2003. Available online at www.noao.edu/meetings/mitigation/report.html More information about the B612 mission can be found at www.b612foundation.org New reports on near-Earth objects are available at neo.jpl.nasa.gov/neo/report.html, impact.arc.nasa.gov/ and neo.jpl.nasa.gov/neo/pha.html |
