A CNN news report this morning (19 February 2016) tells of Russian plans to modify existing ICBMs to carry warheads to intercept and blow up incoming asteroids. This can be found at:
The story is disturbing for a host of reasons.
First, ICBMs can carry nuclear (thermonuclear) warheads over intercontinental range, for which purpose they can achieve a terminal velocity of 8 kilometers per second. They are designed so that achieving altitudes much higher than about 1000 km is not possible. The simplest way of using an ICBM would be to intercept the incoming asteroid at an altitude of 1000 km, a move of such extreme stupidity that even the Russian Ministry of Defense would hesitate to do it. A multi-megaton explosion in space so close to Earth would not only kill a large fraction of all the satellites operating in Earth orbit, but its EMP would knock out surface electrical power grids over a continent-sized area. As a further bonus, fragments of the incoming asteroid would shower a wide area on the ground, likely inflicting several times as much damage as the intact asteroid would have done. The unanimous conclusion of international Planetary Defense studies (with the participation and concurrence of leading Russian scientific experts) is that blowing up a threatening asteroid is a high-risk, damage-multiplying endeavor that should be avoided at all cost.
Second, redesigning a strategic missile for asteroid interception at a safe distance from Earth would require replacing the payload with an additional upper stage and a much smaller warhead. The largest operational Russian ICBM, the SS-18-6, carries a 20 megaton thermonuclear warhead weighing about 9 tonnes; replacing that warhead with a new upper stage and a smaller (5 megaton?) warhead with a mass of about 2 tonnes would permit interception out to lunar distances. But that raises another question:
Third is the question of how we deal with different kinds of targets. There is no doubt that interception and destruction of a 10-meter diameter asteroid at the distance of the Moon would be safe: the problem is that asteroids of this size are extremely difficult to find. Virtually all of the asteroids of this small size (>99.99% of them) remain undiscovered. They can be found only if they approach Earth very closely. In other words, an incoming 10-m asteroid on a collision course with Earth would almost certainly be unknown to us. Discovery of a new asteroid of this size, even if it occurs by incredible good fortune while the asteroid is still at the distance of the Moon, would occur about one day before impact. The asteroid would have to be discovered and tracked, and the mission would have to be planned and launched, within hours of discovery. The asteroid would typically be traveling at 20 km/s and the interceptor rocket at 2 km/s, so interception would occur 1/10th of the distance to the Moon, an altitude of about 40,000 km, which happens to be altitude of the Geosynchronous Orbit belt of communication satellites. A 5 megaton explosion at that altitude would destroy most of the world’s communications assets. An asteroid that, if it by incredibly bad luck should have hit a city, might have caused thousands of casualties, is destroyed at the cost of world-wide communication capabilities. If we chose to leave it alone (or, more likely, never saw it coming) it is overwhelmingly more probable that it would have fallen in a remote and unpopulated area, probably over the ocean, and inflicted little or no damage. The cure would probably be more lethal than the disease.
What about kilometer-sized asteroids, which constitute a serious threat to areas the size of a country? Asteroids of this size and brightness are much easier to discover and track: of all the Earth-crossing asteroids larger than about 1 km in diameter, we have discovered and tracked more than 95%. Best estimates are that there about 980 such asteroids: of the estimated few dozen that have not yet been discovered, we are finding several new ones each year. We know with surety that none of the ones discovered to date threatens impact with Earth in the next few centuries. But suppose we were to discover a new one this year in an orbit that threatens Earth. It is highly probable that we would have hundreds to thousands of years to prepare for that threat. But the impact could be avoided by minuscule changes in the orbit of the asteroid. As an example, suppose we find a km-sized body that would impact Earth in 300 years. If we could change the orbit enough to miss Earth, we would buy ourselves thousands of years of additional time to deal with it. Changing the asteroid’s orbit enough to displace its position by 10,000 km and guarantee that it would miss Earth 300 years from now requires changing the velocity of the asteroid by a minuscule 0.1 cm per second. This can easily be effected by setting off a large nuclear explosion several km from the asteroid: the vaporized surface rock would exert a mild but entirely adequate vapor “puff” that would very slightly deflect the asteroid and change its speed without running the risk of turning the asteroid into a deadly shower of a thousand 100-meter sized chunks of shrapnel.
In short, this proposed “defense” scheme is sufficiently crazy that we would be well advised to look for other explanations of why Russia would want to suggest it.
Oh, by the way, the United States no longer has any operational ICBMs with multi-megaton “city buster” warheads. These relics of the cold war survive only in Russia and China—and are effective threats only against population centers, not military targets. This means the US doesn’t even have the option of doing something equally stupid with asteroids.