At the Planetary Defense Conference in Granada, Spain -- in addition to talking a lot about finding near Earth objects (NEOs) -- there has also been a lot of discussion about what to do if we actually detect one headed toward Earth, an area often called mitigation. What we can do depends upon how far in advance we detect the object and figure out if it is going to hit Earth (or likely hit Earth), and how big it is. A lot more options open up when an object is identified far in advance, hence the reason there is so much focus on finding and tracking NEOs.
nless we have a warning time of at least a decade (more for a large asteroid), most scientists and engineers agree that, at this point, the only option is nuclear weapons. The usual concept is to detonate a nuclear weapon a few tens or hundreds of meters from the NEO, which will vaporize some of its surface. That vaporized rock act like a rocket jet, moving the NEO in the opposite direction. This concept needs a lot of work, though scientists feel they understand the physics of the nuclear explosion extremely well. What isn't well understood is the upper surface of a NEO: solid, fluffy, rubble-pile. Each surface will have a different effect. Then, there is the challenge of getting the nuclear weapon to the asteroid -- possibly very quickly -- and detonating it at the right place.
or objects of a few hundred meters or smaller, one may be able to use kinetic impact alone and slam a spacecraft, preferably heavy, into the NEO at very high speed.
f there is more lead time, one can use slower impulse methods ranging from gravity tractors, where we actually use the spacecraft's gravity to slowly tug the asteroid. Many more exotic methods were also discussed, from laser ablation creating jets, to tethers, etc. In all cases, these deflection methods need more work and study.
inally, if we have little warning, days for instance, all we can do is attempt to evacuate the area that will be affected.
t this conference, there was more emphasis on the case of very short time frame small object impacts than there has been in the past. Part of the reason was because of two small impacts that occurred in the last couple years (since the last Planetary Defense Conference). Asteroid 2008 TC3 was discovered less than 2 days before it impacted over Sudan. But, there were enough observations to generate a prediction of where it would hit. It was the first time a natural object had been observed in space before it entered the Earth's atmosphere, and as a bonus, portions of the space rock were recovered. Though it was a very small object that broke up and caused no more damage than scattering meteorite fragments, it demonstrated that current NEO surveys have a chance to observe a NEO "at the last minute" during its so-called death plunge. This type of observation requires particularly quick action, and for an object much larger than 2008 TC3, would ideally allow time for evacuations.
he other impact reported at the conference occurred a couple years ago in Peru -- the Carantas impact. There are indications that it was a relatively small object (2 to 5 meters) that would not have been predicted to make it through the atmosphere in one piece, yet it created a 14 meter crater in a dry river in a field. It occurred at 3800 meters altitude, and the blast wave knocked a man off a bicycle and a bull to the ground.
here is also discussions beginning with emergency management agencies across the world about this issue, but lots more is needed. They would be the ones involved in evacuations, and post-disaster assistance.
he bottom line is still that impact is a low probability any given day, but it definitely will happen eventually. As I've seen at this conference, we can plan for and perhaps even prevent such an impact, but it will take more investment and work.