Madhu Thangavelu’s commentary “Putting Surplus Nukes to Good Use” [April 12, page 41] needs a thoughtful response. While his comments seem to be quite reasonable, there are serious flaws in his thinking. There are a relatively few of us who have been working this problem for the past five to 10 years, and the work, while extensive, has not been broadcast to the wider public, so the lack of familiarity with it is understandable. Nonetheless, when one is talking about nuclear weapons in space, one needs to be very careful and well informed.

Generally speaking, using old nukes, or any nukes, to deflect an asteroid is not a good idea. That said, there are exceptions where the use of a nuclear explosive, in “stand-off” mode, cannot currently be ruled out. But a bit more explanation is required.

The general sense of those who have actually looked at the deflection challenge is that nuclear devices should be used only when other non-nuclear techniques are inadequate, and in any case only with the full support of the international community. When, then, is kinetic impact (the other impulsive technique for deflection — crashing a spacecraft into an asteroid to change its velocity) inadequate? Generally speaking, when the size of the near-Earth object to be deflected is greater than 300 meters across or when the discovery of a smaller one with very little time to impact is the challenge.

The frequency at which the Earth is hit by a near-Earth object greater than 300 meters in diameter is about once per 100,000 years. Should we have nukes sitting around, under U.N. supervision or not, for 100,000 years waiting for such a challenge? Which is the greater risk, an accidental or intentional misuse of the nukes during that average interval or the asteroid impact itself? Will we not have developed, in the interval to the next greater-than-300-meter impact threat, new technologies with comparable effectiveness?

Furthermore, as we get the new breed of search telescopes into operation and actively looking for near-Earth objects larger than 140 meters in diameter, it will be the largest of the remaining unknown population of asteroids that will be discovered first, and the smallest (and most numerous) that will be the last to be found. Therefore, in only 10 years or less after the new telescopes begin operation, we will have discovered more than 90 percent of all 300-meter objects, and perhaps in 15 years that will be more than 95 percent. Thus the likelihood of not knowing a century ahead of time of such a pending impact essentially vanishes. In 100 years, there is no doubt whatsoever that we could tailor a new device, nuclear or not, to do the job. Old nuclear weapons are not worth saving for this purpose.

As to the asteroid Apophis, the only way for it to impact the Earth on April 13, 2036, for which the probability (based on current tracking) is one-in-235,000, is for it to pass through a small region in space near Earth referred to as a “keyhole” seven years earlier as it passes close by the planet. Given that there are two additional tracking opportunities between now and then (in 2012 and 2021), it is very likely (over 99 percent) that the risk of this will drop to zero. If it does not, we will have eight years to perform a very simple gravity tractor deflection of Apophis (altering its path via the gravitational pull of a spacecraft placed nearby). The deflection required to cause it to miss the keyhole in 2029 is very small.

So no, saving old nukes for this purpose is not a good idea. In many cases, this argument is used by nations that are simply looking for a rational-sounding reason to maintain their inventory. Again, while all the above is true, we cannot rule out the very improbable but non-zero possibility of needing a nuclear device sometime in the next 10 to 15 years. However, there is also no possibility of them going away in that timeframe in any event.

Rusty Schweickart
, Calif.

The writer, a former Apollo 9 astronaut, is chairman of asteroid deflection-focused B612 Foundation, former chairman of the Association of Space Explorers Committee on Near-Earth Objects, and current co-chairman of NASA’s Task Force on Planetary Defense.