http://www.thespacereview.com/article/90/1
Near Earth asteroids: the third option by Jeff Foust The Space Review January 26, 2004 For years, the debate about the future of human spaceflight has focused on two destinations: the Moon and Mars. Proponents of a human return to the Moon have argued that such a mission is far more feasible than sending people to Mars and could have numerous scientific and even commercial benefits. However, they have also had to fight against the "been there, done that" mindset that argues that since we went to the Moon over three decades ago, a return would not be that effective in stimulating the public's interest in space exploration. Mars advocates, usually marching under the banner of the Mars Society, have tried to point out that a human mission could be done for far less than what was proposed in the old Space Exploration Initiative and could answer fundamental questions, such as whether life ever existed on the planet. The mixed track record of robotic Mars missions, though, makes it difficult to shake the perception that a human mission would be extremely risky. (See "Choosing our destination" in this issue.) President Bush's new space initiative appears to have resolved this debate, at least for the near term, by emphasizing a human return to the Moon, while offering Mars advocates a consolation prize of eventual Mars expeditions at some point beyond 2020. While the Bush administration may be set on this new initiative, there is no guarantee that this vision will ever become reality. Public opinion is, at best, mixed on this plan, with many concerned about the high costs-real or perceived-such a plan entails. With some members of Congress, including some conservatives, expressing concern about rising budget deficits, this new plan will be subject to considerable Congressional scrutiny in the weeks to come. Even if it survives this near-term challenge, a new president taking office in 2009, let alone 2005, could dramatically reshape-or stop-the plan with a minimum of wasted effort even if it is carried out as planned over the next five years. (See "Looking beyond vision", January 19, 2004.) The plan, and the overall debate, has been predicated on the belief that the only two destinations for humans beyond Earth orbit for the foreseeable future are the Moon and Mars. It turns out this is not necessarily the case. A small group of people, including planetary scientists and former astronauts, has argued for a different destination for manned expeditions: one or more of the many asteroids that pass near the Earth. Such missions could be affordable (relatively speaking), visit new destinations, carry out important research, and also set the groundwork for more ambitious missions, like Mars. Fear, greed, and curiosity As of the end of last week, there were 2,682 known near Earth objects (NEOs), all but 49 of which are asteroids. (The remainder are comets in short-period orbits that cross or approach the Earth's orbit.) Of those 2,682 NEOs, 693 are asteroids with diameters of one kilometer or larger. While astronomers believe that they have identified about 70 percent of the population of those large NEOs, the number of smaller NEOs may be vastly larger: over 100,000 with diameters exceeding 100 meters. In addition to the incomplete catalogs of objects, there are large gaps in our understanding of these bodies. Groundbased telescopes have allowed planetary scientists to crudely classify these objects based on their spectra, and radar observations of some close-passing NEOs have revealed their shapes. Only a handful of asteroids, near Earth or otherwise, have been studied in detail by spacecraft, such as NASA's NEAR Shoemaker mission that orbited and eventually landed on the NEO Eros. Japan last year launched Hayabusa, the first mission designed to take samples of an asteroid and return them to Earth. Why study NEOs at all? Dan Durda, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, and a proponent of human missions to asteroids, provided a rationale that can be summed up in three words: fear, greed, curiosity. Fear comes form the fact that NEOs have, and will continue to, collide with the Earth. Right now the emphasis is on simply finding NEOs and determining which ones could pose a risk to the Earth in the future. If any do pose a major hazard, then attention will have to turn to mitigating that threat, most likely by deflecting the object's orbit. That will require significant knowledge of the asteroid and how to operate on it: detonating nuclear weapons, a popular proposal for deflecting asteroids, might instead simply break apart an object that is a loosely-bound rubble pile. "I don't want to invoke Chicken Little," said Durda in a recent interview, "but it is a real threat." Curiosity comes from the scientific study of NEOs, including work that can only be done on the asteroid itself, preferably by people. Studying these objects, said Durda, offers a window into the origins of the solar system, since these objects are virtually unaltered since the formation of the solar system. Greed is linked to the wealth of resources, from water ice and other volatiles to platinum-group metals, found on asteroids. Scientists and science-fiction authors have long talked about mining asteroids: while there's no need for these resources on Earth for the indefinite future, such spacebased resources may be useful, if not vital, to any long-term settlement in space. Durda notes that NEOs, the most easily accessible asteroids, are hundreds of times richer in unprocessed materials than the Moon. "They're literally gold mines in the sky," he said. By contrast, the lunar regolith "has a composition similar to mining slag." Planning a mission While there are a number of good reasons for visiting NEOs, what makes the case for such missions-human in particular-so compelling is the accessibility of these bodies. The proximity of these objects and their small size sharply reduce the delta-v-the change in velocity-and thus the amount of propellant needed to reach them. In many cases, the total delta-v for a NEO mission is less than a mission to the Moon. At a September 2002 conference on mitigating asteroid impact hazards in Arlington, Virginia, Durda described an example of a mission to one NEO, 1991 VG. A round-trip mission lasting just 60 days would require a total delta-v of 6.1 kilometers per second, approximately the same as a one-way mission to the Moon. Extending the mission duration to 90 days decreased the delta-v to 4.9 km/sec. These factors put manned NEO missions almost entirely within the capacities and experience of human spaceflight today. Durda envisions missions to nearby NEOs lasting 90-120 days, using hardware based on modules developed for the ISS with "a modest investment in new technologies." The duration of a NEO mission is considerably less than the six months crews are now spending on the ISS, although the station crews have the advantage of regular resupply from Earth. The low surface gravity of asteroids-a one-kilometer body has just a few thousandths the gravity of the Earth-will also remind astronauts of working on the station. "All the EVA experience we have gained on ISS will be applicable here," Durda said. The new technologies required for human NEO missions are, in large part, items also needed for lunar or Martian missions: improved power systems, including nuclear power; spacesuits that can handle dusty environments, and radiation protection. The extremely low gravity of asteroids would introduce some unique issues, such as the need to anchor ships or even people to the surfaces of these objects. However, Thomas Jones, a former shuttle astronaut with a PhD in planetary science who has worked with Durda on human NEO mission plans, points out in an interview that an asteroid mission would also makes things simpler. "You would not need a beefy lander, just a mobile platform to ferry EVA astronauts over to the asteroid and back," he said. The cost of a human NEO mission hasn't been estimated in any detail, Jones said. He guesses that a NEO program would cost less than the roughly $30 billion that has been spent on the space station program to date. "This amount would be spent over ten years to do not just one, but a series of asteroid round trips," he said. The same hardware developed for those missions could also be used for lunar missions, he added, with the addition of a lander. Drawbacks While human missions to NEOs sound compelling and straightforward, they are not without a number of challenges. The asteroid environment will pose a number of challenges, from its near-absence of gravity to rapid rotation rates that create day-night cycles that last just a few hours. "The dynamics of moving around an object will be counterintuitive," warned Durda. "You can't just hop from point A to point B." Another issue is selling the mission to the public. Virtually everyone is familiar with the Moon and Mars (although the familiarity may be grounded more in fantasy than fact) and thus can grasp, to some degree, the idea of sending people to these worlds. However, outside of a few, largely forgettable movies like Deep Impact and Armageddon, most of the general public is likely unfamiliar with the idea of sending people to asteroids, near Earth or otherwise. In the body of public opinion and knowledge, it would be difficult for a small asteroid, perhaps still carrying an alphanumeric designation like 2000 SG344, to compete with the Moon and Mars. Durda, speaking at the 2002 asteroid mitigation conference, said that he thinks the public would embrace a human NEO mission, regardless of how obscure the destination, because of the spirit of adventure involved with any human exploration. "I guarantee to you that the first time humans go beyond LEO to a small rock, the public is going to eat it up," he said. Perhaps the biggest challenge a human mission faces, though, is selling the need to send humans in the first place. Some advocates of spacecraft missions to NEOs argue that robotic missions can accomplish all the requirements-scientific, threat mitigation, and utilization-for studying these objects for the foreseeable future. Air Force Brigadier General Simon "Pete" Worden, an advocate of asteroid missions in general, was skeptical about the benefits of human missions during a forum on planetary defense last week in Washington organized by the Homeplanet Defense Institute. While calling the concept of human NEO missions "cool", he warned about the cost and benefit tradeoffs. "My experience with space programs is that whenever you put a person into a mission it is an order of magnitude more cost," he said. "I can't imagine that these kinds of things [asteroid missions] would benefit from some kind of human presence. Durda notes that any human mission would be preceded by robotic reconnaissance, from microsat orbiters to sample return missions. However, human geologists on an asteroid will be far more efficient than their robotic counterparts. "You will never replace the creativity and the intuition of the human mind," he said. "That is the true benefit of putting humans in the environment." Fitting missions into the new plan For the near term, though, the debate over human expeditions to NEOs appears to be academic. Asteroid missions don't figure into the president's new space initiative, at least through 2020. Beyond that, though, advocates believe there is an opening for carrying out these missions as part of preparation for a human Mars expedition. "You can choose either the Moon or asteroids to build up experience for Mars," said Jones. "Asteroids are natural stepping stones for Mars." Jones is optimistic that NEO missions can be worked into the overall Bush initiative. "There is nothing in the President's plan which bypasses the asteroids," said Jones. "The lunar missions lead naturally to expansion of our experience base to the NEOs. The lunar hardware to be developed can be simply augmented with additional propellant and life support to enable months-long asteroid voyages. One of the major benefits of the President's plan is that we will be building hardware and capability to take advantage of asteroidal resources and to learn how to work on them." Durda, in an interview several weeks before Bush unveiled his new plan, argued that, in the end, it was more important for NASA to establish a new destination-driven program for human exploration. "If we decide to go back to the Moon, I am fully behind it," he said. Still, he hoped NASA would look for "something new and exciting, something beyond the Moon but not quite as difficult as going to Mars." With a new plan in place, but still subject to change by this or a future administration, that opportunity to do "something new and exciting" may still present itself sooner rather than later. "A bonanza of new knowledge, the basis for space commercialization, and planetary protection," Jones summarized. "NEOs are a convincing combination." Jeff Foust ([EMAIL PROTECTED]) is the editor and publisher of The Space Review. He also operates the Spacetoday.net web site. He has also recently started a weblog titled Space Politics to track space policy issues. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
