≡ Menu

Defending Earth: Two Space-Based Approaches

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.

Comments on this entry are closed.

  • philw August 9, 2007, 12:57

    Before we build a space based infrastructure, we really need to visit several NEOs and ascertain their chemical and mechanical structure. That way we can deduce which of the variety of methods in our toolbok would work best for whatever type of NEO approaches.

  • Roger August 9, 2007, 21:23

    Having an infrastructure in space makes it easier to “visit” NEOs, and divert their orbits if necessary.

  • Michael Thomas August 10, 2007, 0:56

    Why not use our MEDUSA space age weapon to move it or destroy it ?


  • Ken Murphy August 16, 2007, 20:09

    If we were taking a rational approach to space development then instrument packages at EML1 would be on the agenda over the near term, leading to NEO defense facilities (amongst others) at the Lagrange point over the longer term. EML1 offers an ideal “test run” destination for whatever ends up being the CEV. Drop off an instrument package to scan Sunward for NEOs while there. EML1 is also connected to EML3 by low-dV trajectories, and it makes sense to have an instrument package there as well. Both locations are also good for observing near-Earth cislunar space, and assets at both locations would allow for constant global observation.

    EML1 is also a great place to service (a la Hubble) space assets that are sent out (and brought back) on the InterPlanetary Superhighways. It’s an ideal station for a GEO sat servicing business, much better than LEO. It offers 24/7 access to everywhere on the Moon for essentially the same delta-V. If I were going to build a Mars ship I would build it at EML1. And now NEO defense.

    EML1 should be our next destination after LEO. Science, Security and Commerce, the goals of the VSE, all in one location. What more could one ask for?

  • Darnell Clayton August 21, 2007, 22:52

    Yeah, I know I’m a little late to the game, but I thought this was the best post on the Carnival of Space!

    Hopefully our species can figure out a way to deflect these killer space rocks, because if we do not, then setting up shop on other worlds en mass may be dismal at best (as all those craters on other worlds can hint at).

  • ljk November 28, 2007, 10:39

    Colliding Worlds: Asteroid Research and the Legitimization of War in Space

    Social Studies of Science 2007; 37; 499

    DOI: 10.1177/0306312706075336

    Felicity Mellor

    ABSTRACT Over the past 20 years a small group of astronomers
    and planetary scientists has actively promoted the idea that an
    asteroid might collide with Earth and destroy civilization. Despite
    concerns about placing weapons in space, the asteroid scientists
    repeatedly met with scientists from the Strategic Defense Initiative
    to discuss mitigation technologies.

    This paper examines the narrative context in which asteroids were
    constructed as a threat and astronomy was reconfigured as an
    interventionist science. I argue that conceptualizing asteroids
    through narratives of technological salvation invoked a ‘narrative
    imperative’ that drew the astronomers towards the militaristic
    endings that their stories demanded. Impact-threat science
    thus demonstrates both the ways in which scientific research
    can be framed by fictional narratives and the ideological ends
    that such narratives can serve.

    Full article here:


  • ljk February 22, 2008, 11:45

    An Elegant Proposal for Near Earth Asteroid Deflection

    Written by Nancy Atkinson

    Although the chances of an asteroid hitting Earth appear to be small for any given year, the consequences of such an event would be monumental. The science community has come up with some ideas and proposals for ways to mitigate the threat of an incoming asteroid hitting the Earth. Some proposals suggest almost Hollywood type theatrics of launching nuclear weapons to destroy the asteroid, or slamming a spacecraft into a Near Earth Object to blow it apart. But other ideas employ more simple and elegant propositions to merely alter the trajectory of the space rock. One such plan uses a two-piece solar sail called a solar photon thruster that draws on solar energy and resources from the asteroid itself.

    Physicist Gregory Matloff has been working with NASA’s Marshall Spaceflight Center to study the two-sail solar photon thruster which uses concentrated solar energy. One of the sails, a large parabolic collector sail would constantly face the sun and direct reflected sunlight onto a smaller, moveable second thruster sail that would beam concentrated sunlight against the surface of an asteroid. In theory, the beam would vaporize an area on the surface to create a “jet” of materials that would serve as a propulsion system to alter the trajectory of the Near Earth Object (NEO.)

    Changing the trajectory of a NEO exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time. Since the Earth is approximately 12,750 km in diameter and moves at about 30 km per second in its orbit, it travels a distance of one planetary diameter in about seven minutes. The course of the object would be altered, or either delayed or advanced and cause it to miss the Earth.

    But of course, the arrival time of the impactor must be known very accurately in order to forecast the impact at all, and to determine how to affect its velocity.

    Full article here:


  • Guy Warinner Bremerton, WA January 28, 2009, 14:27

    A possible process to deflect a comet or asteroid
    A Rocket engine
    A Rocket engine is no more than a tube, a plug at one end and a fast burning fuel.
    A possible way to deflect an astral body
    A comet might be easier than a metal asteroid as the initial process might take a longer time to complete than the time to impact.
    First issue: Who and what needs to be involved
    The first issue is to arrive on the astral body with the necessary equipment in time to affect a resolution. To do this it will be necessary to have the equipment and vehicles located in space at several space stations around the earth. To do this it will be necessary for every nation to participate in this endeavor, as it will take the whole of the earth resources to setup and man the station.
    Second Issue: When and how will this work
    This must be done immediately, waiting only puts us in a tighter time constraint. Once the station has been setup in space, it will be necessary to have this station manned at all times with observers, flight crews and the men that will be doing the actual work on the astral body.
    Final issue: How to deflect the Astral Body
    If a large tunnel/hole is cut to the approximate center of the astral body and it has been packed with spent fuel rods and an initial igniter the center of the astral body could be heated up enough to melt the center of the astral body. When enough heat is created, a Rocket Engine Action would take effect blowing out the tunnel/hole that was dug. The rocket engine effect would not destroy the asteroid but could give it enough of a nudge to move its trajectory…