How to handle an incoming asteroid? What does Don Quijote have to do with it? Movies like "Deep Impact" and "Armageddon" make it seem so easy. But in reality there are many technological, legal and political obstacles to cope with. From 15-19 April, 2013, the International Academy of Astronautics (IAA) will hold its third Planetary Defence Conference “Gathering for Impact!” in Flagstaff, Arizona, USA. This location, just a couple of miles away from the Barringer Crater, seems the perfect place for the international community to discuss many aspects of the near-Earth object (NEO) threats and opportunites.
National Aeronautics and Space Administration (NASA) has chartered a Science Definition Team to study what should be done to find near-Earth objects less than 1 kilometer in size. Although further research had to be done, this team concluded in a 2007 report that while impacts by these smaller objects would not be expected to cause global devastation, impacts on land and the tsunamis resulting from ocean impacts could still cause massive regional damage and still pose a significant long-term hazard.
NASA has developed a Wide-field Infrared Survey Explorer (WISE) to observe our solar system's population of potentially hazardous asteroids. Last update is a recent study about 99942 Apophis, a 325-meter diameter near-Earth asteroid that has been the focus of considerable attention after it was found in December 2004 to have a significant probability of Earth impact in April 2029. Out of many options to encounter an incoming asteroid - a protecting shield, a magnifying lens on solar energy, a conventional rocket engine, ion beam shepherd, etc. - I would like to highlight three.
A method using a large heavy unmanned spacecraft hovering over an asteroid to gravitationally pull the latter into a non-threatening orbit. The spacecraft and the asteroid mutually attract one another. If the spacecraft counters the force towards the asteroid by, e.g., an ion thruster, the net effect is that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. A technical obstacle will be to fuel the unmanned spacecraft for many years. If there are a few years left until the asteroids impact, it's a method worth trying.
According to former astronaut Rusty Schweickart, the gravitational tractor method is controversial because during the process of changing an asteroid's trajectory the point on Earth where it could most likely hit would be slowly shifted across different countries. It means that the threat for the entire planet would be minimized at the cost of some specific states' security. In Schweickart's opinion, choosing the way the asteroid should be "dragged" would be a tough diplomatic decision. In my opinion the decision to let an asteroid hit the Earth should not be a diplomatic or political issue at all, the matter where the likely impact: Kalahari desert, Philadelphia or Pyongyang. The long-term global hazards of an impact will potentially damage all mankind.
Kinetic impactors have been proposed to prevent an asteroid impact by intentionally ramming it to alter its course away from Earth (so-called billiard ball method). A mission concept of this type, named “Don Quijote” was developed in 2005 in the European Space Agency (ESA) Concurrent Design Facility (CDF). The Don Quijote mission proposed to deflect the asteroid Apophis using two separate probes, one that would ram the asteroid at over 48,000 kilometers per hour, and another that would orbit Apophis to monitor the effects of the impact, see video.
Technically, a nuclear bomb could obliterate a smaller asteroid, but it's not these smaller entities that pose a threat to Earth's safety. The asteroids that would be really worrisome -- those larger than 1,312 feet (400 meters) -- wouldn't be easily wiped out by such a bomb. Sure, great hunks of one might break off, but not enough to neutralize the danger. A 2007 NASA report indicated that planting a nuclear bomb on or under the surface of an asteroid would most likely cause it to fracture into several pieces -- and large pieces of an even larger asteroid can still be pretty dangerous if they're hurtling toward the Earth. Besides, apart from satellites, there's more space debris flying around the earth orbit and throughout outer space to worry about.
Nuclear weapons have been explicitly outlawed in space since the Partial Test Ban Treaty was negotiated in 1963. Sending a nuclear weapon into space to hit an asteroid would require modifying the treaty. According to United Nations General Assembly resolution 1884 (XVIII) of 17 October 1963, it is not allowed to place in orbit around the Earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station them in outer space in any other manner. This principle relates to outer space as a whole, i.e., including the Moon and other celestial bodies. In conjunction with the 1963 Moscow Treaty, which was the product of the same period of détente, and which prohibited, inter alia, any nuclear weapons test explosions or any other nuclear explosions beyond the limits of the atmosphere “including outer space”, the Outer Space Treaty established an immense denuclearized area surrounding the planet Earth. The use of nuclear devices is still an international issue and will need to be addressed by the United Nations Committee on the Peaceful Uses of Outer Space. The 1996 Comprehensive Nuclear-Test-Ban Treaty technically bans nuclear weapons in space too.
In conclusion, there are many options to talk about and discuss during the forthcoming Planetary Defence Conference. Maybe a combination of options is imaginable too, like the kinetic impact approach and/or the gravitation method in combination with a shield to protect the Earth from incoming asteroids. I hope nuclear weapons in outer space will not be necessary at all and if so, used as a last resort. In the end, human mankind does not want to end up like the dinosaurs.