Under The Microscope: The possibility that the earth will collide with an asteroid or comet is unavoidable, wrties Prof William Reville Such collisions have happened many times in the past and will occur in the future.
When the diameter of the incoming body is a large fraction of a kilometre or greater, the consequences of the collision are grave. For example, 65 million years ago a 10-kilometre diameter asteroid slammed into the earth causing mass extinction of life and ending the reign of the dinosaurs.
If we discovered tomorrow that this asteroid's tardy twin was also headed towards us, we humans would go the way of the dinosaurs unless we could deflect the asteroid from its path. In my opinion, we should devote a large fraction of space research funding to the development of technologies for the deflection of incoming killer-asteroids.
If we discovered in the morning that a large asteroid was on collision course with the earth, scheduled to hit us six months from now, we could do nothing except fire nuclear warheads at it when it came in range, or, we could send a manned mission to the asteroid and bury warheads in its interior to be detonated when the asteroid astronauts are a safe distance away (hoping to avoid complications such as arose in the film Armageddon). This direct hit seems like a good idea until you think about it for a moment. The explosion will shatter the asteroid into smaller pieces but probably not alter its course.
The asteroid may well still hit the earth but over a much wider area than before.
A better nuclear option would be to use "stand-off" explosions. This means exploding the nuclear charge above the target, spreading the force of the impact over a wider area and melting the asteroid's surface. The molten rock, or ice if we are dealing with a comet, releases gas that pushes like a rocket motor to change the course of the asteroid or the comet. This nuclear option might work, and we have the technology at the moment. However, it is likely to be an option of last resort. It would be far preferable to detect a killer asteroid decades in advance of its date of impact and to nudge it off its collision course while it is still far away from the earth.
An alternative to "nuking" the incoming asteroid is send out a spacecraft to collide with it and thereby deflect it from its path. In order for this to be feasible the asteroid would have to be detected some decades before collision time with the earth. This strategy doesn't require technology significantly more advanced than currently available, but only stands a good chance of success with small asteroids.
Another option for deflecting the path of an incoming asteroid or comet is to mine the body, ejecting the mined debris from the asteroid at a constant angle, thereby propelling the asteroid in the opposite direction. The device for hurtling mined material into space is known as a mass driver and the asteroidal matter could be accelerated into space using magnetic fields.
It is estimated that a one-kilometre asteroid could be deflected into a harmless orbit in about 10 years. However, our current level of technology would only allow us a slim chance to use this device successfully.
Another option is to tie a rocket to the incoming asteroid and push it into a harmless orbit. Such a rocket could not be the conventional chemical rocket used today to launch vehicles into space because this would entail transporting a huge amount of fuel to the asteroid to power the rocket. On some objects it may be possible to mine local material and convert it into fuel.
For example, ice on a comet could be split into hydrogen and oxygen, two gases that can be mixed together to make a powerful rocket fuel. In-situ resource utilisation technologies (ISRU) are being developed with a view to the manufacture of oxygen, water and return-flight rocket fuel from the atmosphere of Mars in order to allow round-trip astronaut missions to that planet. Decades of advanced warning would be needed in order to allow this strategy a moderate chance of success and this assumes that ISRU and other research technologies will continue.
A fifth option under consideration for deflecting the path of an incoming asteroid or comet is to heat one side of it using a giant mirror in space to reflect sunlight onto the hazardous body. Warming one side of the asteroid creates a sideways push that gradually deflects the asteroid from its dangerous course. This option does not require building deflection apparatus on the asteroid itself. The giant mirrors would be assembled in Earth orbit and operated by remote control.
Similar large reflective structures are now being studied, called solar sails. These craft use the sun's energy to push themselves through space and have a similar design to the mirror complexes under consideration. Decades of advance warning would be needed for this option to work but the chances of success are good assuming research continues into solar sail technology.
This whole subject may have a science-fiction feel to it, but it is a serious matter. Already a far-off asteroid - Asteroid (29075) 1950 DA - has been detected heading towards Earth. This 1.1km-diameter asteroid has a 0-0.33 per cent chance of hitting Earth in March 2880. The upper limit of probability could increase or decrease as we learn more about the asteroid in the years ahead.