Asteroid laser ablation

February, 2013, the Chelyabinsk Meteor exploded at a height of 30 kilometers over western Russia. The meteor, which weighed around 6.8 kilotonnes (15×10^⁶ lb), was estimated to be traveling 18 km/s (40,000 mph) and entered Earth's atmosphere at an angle of 20 degrees.[6] The explosion was between 20-30 times stronger than the bomb dropped on Hiroshima; the resulting shock wave broke windows on the ground and injured around 1,500 people. Due to the meteor's relatively shallow angle it exploded high in Earth atmosphere. However, had the meteor reached Earth's surface or exploded lower in the atmosphere the results could have been catastrophic.

Despite NASA's efforts to detect Near Earth Objects (NEO's), the Chelyabinsk Meteor went undetected. In recent years, NASA, in partnership with the European Space Agency, have been increasing their efforts to track all NEO's with the potential to cross Earth's orbit.[7] On their website, NASA has a public list of all known NEO's which present a potential impact risk.[8] However, the list remains incomplete and the question of what to do in the event of an imminent impact remains unanswered.

Laser ablation is a promising method because it allows an asteroid to be redirected without breaking the asteroid into smaller pieces, each of which may pose its own threat to Earth. The nuclear impactor is another proposed method for deflecting asteroids, but is less promising than laser ablation for both political and technical reasons:

• Blowing up an asteroid could create multiple smaller asteroid fragments, which could each be as destructive as the larger asteroid.
• Detonating an atomic bomb high up in Earth's atmosphere could produce unforeseen repercussions.
• The Outer Space Treaty is a cold war treaty signed in 1967 which prohibits weapons of mass destruction from being placed in space. It effectively blocks any research group from experimentally verifying the nuclear impactor method.

Laser ablation is already being experimentally tested in labs as a method for asteroid deflection and there are plans to begin testing on the International Space Station (ISS), and in low Earth orbit.

Short acting laser ablation is used, to verify and explore, the effectiveness of the powerful thermal X-ray pulse that would be emitted upon the detonation of an asteroid stand-off nuclear explosive device. Investigations to this end were conducted in 2015 by exposing common meteorite fragments to tuned laser pulses, provided by Sandia National Laboratory.[9]

A stand-on system consists of a small laser array which is sent to the target asteroid. The system would be limited to small or medium size asteroids and would require detection relatively far in advance. For a short notice threat a stand-on system would be ineffective, due to the time required to build and send the system to the target. Additionally, a new system would be required for each target. In the short term a stand-on system could be affordable and feasible; however, in the long term a larger system able to deflect asteroids of all sizes is more practical.[1]

A stand-off system is a large laser array which would orbit the Earth or possibly the moon. It would range from approximately the size of the ISS to around 10 times larger. The system would be able to deflect even the largest asteroids, which can be hundreds of kilometers across,[11] and also ideally be able to target multiple asteroids at once if necessary. Although this system would be the most effective against a wide variety of threats, its size, and consequentially its cost, make it an unrealistic option for the near future. The implementation of this type of system would likely require the cooperation and collaboration of multiple governments and agencies.[10]

A stronger laser can create a greater thrust on an asteroid. Researchers at UC Santa Barbara have experimentally simulated the time it would take to redirect a moderately sized asteroid using different strength lasers. The strongest lasers tested could hypothetically require under a year to redirect an asteroid a safe distance from the Earth, while the weakest lasers could take up to 10 years.[10]

• The advantage of a weak laser is that requires less energy to power and, as a result, costs less than a higher powered laser.
• The benefit of a strong laser is that it doesn't depend upon our ability to predict impacts years in advance. Asteroids are difficult to track and impacts are even more difficult to predict; a strong laser ensures greater protection.

Choosing the optimal laser strength is a question of balancing the cost, energy use, and desired level of protection.

Typically such systems require substantial amounts of power. For space-based systems, this might require either some form of nuclear power, or power from a Space-Based Solar Power satellite. Many proponents of Space-Based Solar Power imagine one of the benefits of such an infrastructure includes the ability to divert asteroids and comets are alter their trajectory for exploitation via asteroid mining, as well as for laser-sail based interstellar propulsion.

Asteroids vary greatly in their composition and shape. An asteroid's composition can range from entirely metallic, to entirely rocky, to a mix of rock and metals.[11] The composition must be taken into account since each material behaves differently when ablated. Initial trials at the University of Strathclyde have shown that laser ablation could be more effective on dense metallic asteroids, because of the shape made by the ejecting material.[4]

In simulations asteroids have been assumed to be spherical; however, in reality most asteroids are irregularly shaped. One of the next steps for research on laser ablation is simulating how a thrust may affect the trajectory of irregularly shaped asteroids.