If we used nuclear weapons to deflect an asteroid, how would we go about it? One thing we don’t want to do is explode a nuclear device that fails to move the target, thus scattering radioactive materials into Earth’s atmosphere in addition to the damage the incoming object would cause. But Marshall Space Flight Center (Huntsville, AL) has been working up alternative scenarios in a study that looks at objects like Apophis, which will pass within the orbit of the Moon in 2029. Let’s take a look at what MSFC is doing, and then ask whether there are better options.
Flight International‘s story on this study reports that a nuclear interceptor could deflect a Near Earth Object (NEO) in the range of 100 to 500 meters if launched two years before impact. Larger NEOs might be deflected with a five year lead time. The idea here isn’t to blast the asteroid into rubble, much of which would doubtless fall to Earth in any case, but to deflect it by a ‘stand-off’ detonation near the object. This could be handled in various ways depending on the sequence and the number of available warheads, and running the numbers shows it might just work.
A tentative design for a spacecraft carrying nuclear warheads involves six 1500 kg interceptors carrying one 1.2 megaton warhead each, delivered by a so-called ‘cradle’ spacecraft that could be launched by an Ares V cargo vehicle. Exploding the warheads at a distance of one-third the NEO’s diameter could turn part of its surface into a plasma that would generate the necessary deflective force. Such a mission would be preceded by a precursor spacecraft designed for close-up studies.
Other options are on the table for asteroid deflection in this study, including a so-called ‘kinetic bullet’ with non-nuclear warhead, and a solar collector, which would set up station-keeping near the NEO and heat surface material through a 100-meter parabolic collector that focuses sunlight into a ‘thruster.’ The resultant heating and evaporation of surface material generates thrust and hence deflection, though presumably over considerably longer time frames.
We need a range of responses because we don’t know the nature of the next problematic NEO. Detected long enough in advance, the object could conceivably be moved using any of the methods listed above. Short notice would doubtless demand the nuclear option, amid hopes that the object hadn’t been detected too late even for that. Clearly, early detection is our best bet for studying the danger and choosing the optimal strategy, reinforcing the need to keep major facilities like Arecibo’s radar, now threatened by de-funding, operational (see Asteroid Watch: Saving Arecibo’s Radar for more on these disturbing developments).
The other obvious need is for a flexible and responsive space infrastructure that can react swiftly to such events. While the Marshall study takes needed preliminary steps, we should also be considering permanent stations able to deliver a warhead as necessary to objects found too late for the use of other methods. Italian space scientist Claudio Maccone has been examining this option for some time. In a 2004 paper, he discussed the unique capabilities of such a space-based solution:
A system of two space bases housing missiles is proposed to achieve the Planetary Defense of the Earth against dangerous asteroids and comets. We show that the layout of the Earth–Moon system with the five relevant Lagrangian (or libration) points in space leads naturally to only one, unmistakable location of these two space bases within the sphere of influence of the Earth. These locations are at the two Lagrangian points L1 (in between the Earth and the Moon) and L3 (in the direction opposite to the Moon from the Earth).
We show that placing bases of missiles at L1 and L3 would cause those missiles to deflect the trajectory of asteroids by hitting them orthogonally to their impact trajectory toward the Earth, so as to maximize their deflection. We show that the confocal conics are the best class of trajectories fulfilling this orthogonal deflection requirement.
Note the key rationale: The Lagrangian bases allow us to maximize asteroid deflection. But there is a second advantage, one that far outweighs the option of targeting an asteroid from anything other than L1 and L3. Maccone again:
… suppose that one missile–asteroid collision occurs, but the deflection in the asteroid’s path is not large enough to bring it off its collision course with the Earth. Since the Earth always lies at the common focus of both the asteroid and missiles’ trajectories, the confocal conics theorem insures that all the elliptical trajectories of subsequent missiles are orthogonal to whatever hyperbolic trajectory the asteroid may have. This basic result means that we may shoot more than one missile in a sequence and have the asteroid deflected from one hyperbola to the next more eccentric one, and so on and so on for as many times as it may be needed until we finally push the asteroid off its collision course with the Earth. We like to call this result the ‘cumulative effect’ of the repeated interception capability, and regard this “march of the dimes” of many smaller deflections totaling up into one, larger deflection as the key to saving humankind from the impact.
Maccone’s two papers on planetary defense should be required reading for those studying the asteroid deflection scenario. The first is “Planetary Defense From Space: Part 1-Keplerian Theory,” Acta Astronautica Vol. 55, Issue 12 (December, 2004), pp. 991-1006. The second is “Planetary Defense from Space: Part 2 – (Simple) Asteroid Deflection Law,” Acta Astronautica Vol. 58, Issue 12 (June, 2006), pp. 662-670. The robust space-based infrastructure to build such stations will be highly controversial to create but could be critical to civilization’s survival.