The three D’s of planetary defense
by Jeff Foust
|“The point is that discovery surveys are job one for protecting Earth from asteroid impacts,” said JPL’s Chesley.|
Those search efforts have, so far, failed to turn up any objects that have the Earth definitively in its crosshairs. The biggest alarm in recent years came in late 2004, when asteroid 2004 MN4 (since renamed (99942) Apophis) had, based on the orbital information available, about a 2.7-percent probability of colliding with the Earth on April 13, 2029. It failed to make headlines because of the timing (around Christmas and the Asian tsunami that took place soon thereafter); by the time people would have paid attention other observations had reduced the odds of a 2029 impact to zero. (See “Sounding an alarm, cautiously”, The Space Review, May 31, 2005) Still, Apophis does have about a 1-in-45,000 chance of colliding with the Earth in 2036, depending on if it passes through a narrow “keyhole” during its 2029 close approach to the Earth; JPL’s impact risks page lists, as of March 18th, 137 NEOs, including Apophis, that have a non-zero (albeit extremely small) chance of colliding with the Earth at some point in the next 100 years.
That makes the detection and tracking of NEOs critical. “If it’s part of your job to keep the Earth safe from asteroid attacks, there are three important things you always have to remember,” quipped Steve Chesley, a planetary astronomer at JPL, during a press conference at the annual conference of the American Association for the Advancement of Science (AAAS) last month in San Francisco. “First is, you have to find them as early as possible. And the other ones I can never remember. The point is that discovery surveys are job one for protecting Earth from asteroid impacts.”
In a talk during a later session of the AAAS conference, Chesley noted that, in general, impact probabilities will increase as new observations are added, then quickly drop to zero once enough observations are made—assuming that the object is not, in fact, actually on a collision course. Because of that, Chesley said, “you want to delay your efforts [to deflect an asteroid] as long as possible in the hope that the observations move the asteroid off the Earth, in a sense, rather than having to send a spacecraft to do the work.”
While current search systems, like LINEAR in New Mexico and Spaceguard in Arizona, are doing a good job finding large NEOs, they are not as well-suited for detecting the far larger numbers of smaller NEOs believed to exist. Current estimates suggest that there may be up to 100,000 NEOs 140 meters in diameter or larger, only a tiny fraction of which have been discovered to date. To deal with this issue, Congress, as part of a NASA authorization bill it passed in late 2005, required NASA to perform a study to investigate ways to discover at least 90 percent of these NEOs by 2020.
The study, a summary of which was released by NASA earlier this month, examined a wide variety of ways to do it, from part-time use of existing and planned ground-based telescopes to an array of dedicated telescopes on the ground and in space. By sharing two planned ground-based survey telescopes, the Panoramic Survey Telescope & Rapid Response System (PanSTARRS) and Large Synoptic Survey Telescope (LSST), it would be possible to discover about 83 percent of the potentially hazardous objects 140 meters or larger in diameter by 2020; the 90-percent threshold would be reached in 2026. The 2020 deadline set by Congress could be achieved if those telescopes were augmented with either a dedicated version of the LSST, or a space-based infrared telescope in a Venus-like orbit. In the latter case, this system would find 97 percent of the objects by 2020.
The problem with these enhanced systems, however, is their cost. The NASA study estimated that, in fiscal year 2006 dollars, the system that used the shared telescopes plus a dedicated LSST would cost $835 million through 2020. The system with the shared telescopes and the space-based telescope, not surprisingly, was even more expensive: $1 billion through 2017, the date when it would achieve the 90-percent threshold. That’s big money for a program that currently gets only about $4 million a year for current survey efforts.
|If an impact of blast breaks up an asteroid, said Lu, “Essentially what you’re doing is creating orbital debris on a solar system scale… What would be the uproar if we were to break up an asteroid?”|
“NASA recommends that the program continue as currently planned, and we will also take advantage of opportunities using potential dual-use telescopes and spacecraft—and partner with other agencies as feasible—to attempt to achieve the legislated goal within 15 years,” the report concluded. “However, due to current budget constraints, NASA cannot initiate a new program at this time.”
For many years—dating back at least to the Project Icarus study in the late 1960s (see “Giant bombs on giant rockets: Project Icarus”, The Space Review, July 5, 2004)—the preferred method for dealing with a NEO found to be on an impact course with the Earth was to hit it with one or more nuclear weapons. This was based on the assumption that asteroids were big hunks of rock and/or metal that could be simply pushed by the force of a nuclear blast.
In recent years, however, research into asteroids, including spacecraft missions to or past several asteroids like Eros and Itokawa, have challenged this approach. Small bodies in particular are less likely to be integral chunks of rock than “rubble piles”: agglomerations of smaller objects loosely held together by self-gravity. Attempting to deflect these objects with a nuclear blast could have, in the words of NASA astronaut Ed Lu, “unintended consequences”.
Lu, a member of the board of directors of the B612 Foundation, an organization devoted to the study of asteroid deflection and impact threat mitigation, said at the AAAS conference that nuclear weapons or even a kinetic impact, like that used on NASA’s Deep Impact mission, could break apart a rubble pile asteroid, creating complications in tracking and dealing with all the fragments. “If you do break it up into pieces, what’s your Plan B?” said Lu, who likened such a fragmentation to the orbital debris created by a Chinese ASAT test in January and the controversy that followed. “Essentially what you’re doing is creating orbital debris on a solar system scale… What would be the uproar if we were to break up an asteroid?”
“If you have a way of doing a controlled deflection, you should do that first,” Lu said. And Lu and fellow astronaut Stanley Love have come up with an alternative: the “gravity tractor”. The gravity tractor would be a spacecraft that positions itself near a threatening asteroid, and maintains its position using ion thrusters. The hovering spacecraft imparts a feeble but steady gravitational force that tugs at the asteroid, gradually changing its trajectory.
While the gravity tractor would only make a minor change in the asteroid’s orbit, it would be useful for cases like Apophis where only a slight change is needed to prevent a collision, and other cases where there is plenty of warning. Moreover, since the gravity tractor never makes physical contact with the asteroid, there is no need to be concerned about the asteroid’s composition, structure, spin rate, or other factors that pose hazards to an explosion or impact approach. Nor, Lu added, would it require new technology, since ion engines have already been used on a number of spacecraft, such as Deep Space 1.
Interestingly, though, the recently-released NASA report summary isn’t as generous in praise of the gravity tractor or other so-called “slow push” approaches, such as a tug physically attached to the asteroid or a mass driver that propels small pieces of the asteroid away from the main body. “‘Slow push’ mitigation techniques are the most expensive, have the lowest level of technical readiness, and their ability to both travel to and divert a threatening NEO would be limited unless mission durations of many years to decades are possible,” the NASA report concluded.
The report instead fell back on the old approach of nuclear weapons, but with a slight modification: instead of exploding them on or underneath the surface of the asteroid, they would be detonated at some distance above the surface. “While detonation of a nuclear device on or below the surface of a threatening object was found to be 10-100 times more efficient than detonating a nuclear device above the surface, the standoff detonation would be less likely to fragment the target,” the report concluded. “A nuclear standoff mission could be designed knowing only the orbit and approximate mass of the threat, and missions could be carried out incrementally to reach the required amount of deflection.”
Between detection and deflection, though, lies an oft-forgotten step: deciding the course of action to be taken to mitigate the threat posed by the NEO. Who should be responsible for making a decision whose effects impact—figuratively and potentially literally—people around the world?
|Apophis has been, said Schweickart, “a terrific learning experience for all of us, because it has tickled every possibility that there is in this business.”|
Rusty Schweickart, the Apollo-era astronaut who serves as chairman of the board of the B612 Foundation as well as chairman of the NEO committee of the Association of Space Explorers (ASE), believes that any decisionmaking process must be international in nature. He notes that, in the process of deflecting an asteroid initially on a course to hit one country, the risk of an impact might temporarily shift to another country during the course of the deflection. Moreover, such decisions might have to be made long before the probability of an impact becomes one in order to have enough time to deflect the object. That means all the countries involved need to have a say in the decisionmaking process, even if they are not involved with the deflection itself, and with more potentially hazardous NEOs to be discovered in the years to come, more and more countries will face the risks of an impact. “Inherently every nation is going to be involved in making this decision,” said Schweickart at the AAAS conference. “There is one agency, namely the United Nations, which therefore has got get involved in this decisionmaking process.”
Schweickart said he doesn’t know what that decisionmaking process should be, but he is embarking on an effort to help develop one. His ASE NEO committee is planning a series of four workshops, starting in May in Strasbourg, France, to bring together experts to determine what the best approach might be. The goal of this effort is to present to the United Nations in 2009 a proposed treaty that would serve as a protocol should the need arise.
Although Schweickart was clear to state in his AAAS presentation that the UN should be the forum for the decisionmaking process, not the body actually charged with carrying out any deflection, some interpreted his comments to suggest that he believed the UN—an organization not known for its decisive and efficient processes—should be given complete responsibility for planetary defense. “We are not in any way proposing that the United Nations create a bureaucracy or that the United Nations be responsible for taking action,” he said during a public forum that was part of the Planetary Defense Conference in Washington earlier this month. The UN, though, “has to be in a position to make a timely decision” and then contract the actual task out to NASA or another organization or consortium.
This effort was prompted in large part by Apophis, even though this asteroid has only a very slim chance of posing a threat to the Earth in 2036 or beyond. New search programs, even without the dedicated telescopes that are too expensive for NASA, will in the years to come discover more and more objects that might pose a threat to the Earth years or decades from now. “Apophis is an unusual case and we may not see another one for a long time like it,” said Schweickart. “But it’s been a terrific learning experience for all of us, because it has tickled every possibility that there is in this business.”