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The Asteroid Threat: Deflection Options

Part 5 of a series

Who’s In Charge?

So what are the means by which we can confidently deflect an oncoming asteroid before it creates havoc on Earth, and who would be in charge of such an effort?

The latter question has been addressed, but not definitively, from the U.S. perspective by the Obama Administration. In an October 2010 letter to the leaders of the House Science, Space and Technology Committee and Senate Commerce, Science and Transportation Committee, presidential science advisor John Holdren underscored that the Federal Emergency Management Agency (FEMA) would be responsible for alerting the public to a potentially damaging incoming asteroid “once an initiating notice has been received from NASA regarding a potential NEO threat,” with the Department of State responsible for international notifications. FEMA would also take responsibility for disaster relief.

Dominic Checkmate nuclear-test

Dominic Checkmate nuclear test, in which a low yield nuclear warhead was detonated at an altitude of 91 miles above the Earth. Nuclear weapons are among the mitigation options for deflecting asteroids on a collision course with the planet. DoD photo

With respect to the government’s strategy for potential near-Earth object mitigation and deflection actions, however, Holdren’s letter was more circumspect. “The U.S. government’s assessment of potential mitigation/deflection options is at an early stage of development and not yet ready for implementation, particularly in light of the wide range of possible scenarios and challenges involved,” Holdren wrote. Given what Holdren labeled the need for “significantly more analysis and simulation… to understand aspects such as the feasibility and effectiveness of different mitigation and deflection approaches; the technological capabilities that would be required for such efforts; relative costs and benefits; and corresponding operational concepts and plans,” the science advisor recommended that “NASA take the lead in conducting these foundational analysis and simulation activities as a key preparatory step.”

Holdren said NASA should focus on assessing “the technologies that may be applicable to NEO mitigation/deflection (along with preliminary research and development activities concerning such technologies and capabilities).” He added that NASA needed to work in close coordination with DoD and DHS/FEMA, with the Defense Department’s expertise needed “to support the evaluation of any protection options involving military capabilities and resources, as well as the development of a concept of operations for certain mission scenarios.” And noting the global nature of the asteroid threat, Holdren wrote, “It will be important to engage other nations and multilateral forums as appropriate to explore potential opportunities for international cooperation in this arena.”


Asteroid Deflection Options

Various asteroid and comet deflection options have been discussed for years within the space community. A number of options involving either an explosive device meant to deflect and/or fragment the asteroid or efforts to “push” an object off its orbital path primarily through the use of various energy sources were initially considered by NASA in a 2007 analysis.

Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks.

The NASA report, responding to a congressional directive in the NASA Authorization Act of 2005 section regarding the George E. Brown, Jr. Near-Earth Object Survey (named for the late chairman of the House Science and Technology Committee) described the options as follows:


Impulsive Mitigation Alternatives

  • Conventional Explosion (surface): Detonate on impact
  • Conventional Explosive (subsurface): Drive explosive device into Potentially Hazardous Object (PHO), detonate

    Graphic of Nuclear deflection option

    Graphic demonstrating a mission concept to destroy an asteroid with a nuclear weapon, which would follow up a kinetic energy impactor and detonate in the crater created by the impactor. NASA/Bong Wie/Iowa State University

  • Nuclear Explosive (standoff): Detonate on flyby via proximity fuse
  • Nuclear Explosive (delayed): Land on surface, detonate at optimal time
  • Kinetic Impact: High velocity impact with a massive object, such as a spacecraft or even another near-Earth object

Slow Push Mitigation Alternatives

  • Focused Solar: Use large mirror to focus solar energy on a spot, heat surface, “boil off” material
  • Pulsed Laser: Rendezvous, position spacecraft near PHO and focus laser on surface, material “boiled off” surface provides small force
  • Mass Driver: Rendezvous, land, attach, mine material and eject material from PHO at high velocity

    deflection options gravity tractor

    A gravity tractor, where a heavy unmanned spacecraft would fly in close proximity to an asteroid and change its course slowly under the influence of its gravitational attraction, would require a long lead time in order to be effective. Illustration: Dan Durda/FIAAA/B612 Foundation

  • Gravity Tractor: Rendezvous with PHO and fly in close proximity for extended period.  For example, a large heavy unmanned spacecraft hovering over an asteroid would gradually through gravitational attraction pull the PHO into a non-threatening orbit.
  • Asteroid Tug: Rendezvous with PHO, attach tug to PHO, push
  • Enhanced Yarkovsky Effect: Change albedo (reflecting power of surface) of a rotating PHO; radiation for sun-heated material will provide small force as body rotates


At the time, NASA reported the following conclusions to Congress about these asteroid diversion scenarios:

  • Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study.  Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO.  They also carry higher development and operations risks.
  • Non-nuclear kinetic impactors are the most mature approach and could be used in some deflection/mitigation scenarios, especially for NEOs that consist of a single, small body.

    ESA Don Quixote deflection impactor

    In 2005-2006, the European Space Agency studied a proposed mission to test asteroid deflection. The first spacecraft (Sancho) would orbit the object for months, studying it, then back off to a safe distance for the later arrival of a kinetic impactor (Hidalgo), after which Sancho would reestablish orbit to check the results of the impact. ESA image

  • “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.
  • 30-80 percent of potentially hazardous NEOs are in orbits that are beyond the capability of current or planned launch systems. Therefore, planetary gravity assist swing by trajectories or on-orbit assembly of modular propulsion systems may be needed to augment launch vehicle performance, if these objects need to be deflected.

Responding to the NASA report, former astronaut Russell “Rusty” Schweickart, the chairman emeritus of the B612 Foundation, told Congress that it had “serious technical flaws.” Among them, he said, was that NASA focused on impact scenarios involving huge asteroids under “extraordinary challenging” circumstances. This logic, he said, resulted “in a preference for a deflection concept delivering extraordinary capability, i.e. nuclear explosives.” Schweickart argued that unlike 2 percent of the asteroid impact scenarios NASA considered, the remaining 98 percent of Potential Hazardous Objects, including asteroids the size of the one that exploded over Siberia in 1908, present the most likely impact threat, and “over 99 percent of them can be deflected using non-nuclear means.”

Thomas Jones handicaps the most likely asteroid direction missions thusly: “With about 10 years warning if it is a small object you can park a gravity tractor next to it to just tug on it with gravity over the course of several months or a year, and that can change its velocity enough to make it miss its appointment with Earth. The second technique is kinetic impact. It’s exactly the same technique we used to slam the Deep Impact probe into the comet in 2005, except in this case you take a larger bullet with you and you slam it into an asteroid and maybe you slam it several times so that you change its momentum, thus its velocity and make it miss its appointment. And finally, for larger objects where that technique would not be effective because the bullet has to be much bigger, the large objects would be diverted by a nuclear explosive. This would happen in one or two percent of the asteroid impact cases that you can statistically look at. The asteroid is too big.  There’s not enough warning. Then you would use a nuclear explosive. And what that does is fry one side of the asteroid’s surface. It vaporizes the dirt on that side, and makes a puff of gas that pushes the asteroid in a direction that would make it miss its appointment with the Earth. You’ve got an array of techniques to use and you would apply the proper one depending on the size and the warning time.”


Edward Goldstein has more than 20 years’ experience in the U.S. space community. From...