In “NASA’s Asteroid Absurdity” [Commentary, May 20, page 19], Robert Zubrin gets a few things wrong regarding NASA’s gutsy plan to bring a small asteroid into cislunar space for an astronaut visit and to kick-start asteroid mining. Note that NASA’s plan is based on a Jet Propulsion Laboratory (JPL)/Keck Institute study, which is available at

Zubrin claims that to deflect an asteroid from hitting the Earth requires changing the heliocentric periapsis. However, to turn a hit into a miss requires only slowing or speeding an asteroid such that it arrives at the collision point before or after the Earth is there. This usually requires much lower delta-V than changing periapsis. Furthermore, the delta-V required is a function of the time to collision, a point Zubrin neglects entirely.

Zubrin claims that the best way to deflect asteroids is with explosives. This may be the only alternative if there is little time. However, without detailed understanding of the asteroid’s composition it could be a disastrous choice, turning a rifle bullet into a shotgun blast. There are a number of ways to deflect asteroids. For example, given sufficient time a gravity tractor is much more reliable and controllable than a blast and does not require a detailed understanding of asteroid composition.

Zubrin claims the asteroid would likely contain 5 percent water by weight. The amount of water in asteroids varies considerably and there is no easy way to know, a priori, the water content of any particular asteroid. It could be more, perhaps a lot more, or less.

Zubrin implies that the propulsion system of choice is the Variable Specific Impulse Magnetoplasma Rocket, or VASIMR. However, the JPL/Keck study this mission is based on did not use VASIMR in the reference conceptual design.

Zubrin seems to know that the retrieved asteroid would be placed in a near-lunar retrograde orbit. However, while this may be a possibility, the final orbit is a function of the asteroid chosen and will be, I’m quite sure, the subject of rather extensive trade studies.

Zubrin seems to think that the only possible use for asteroidal materials is fuel for a mission to Mars. In fact, there are a number of potential markets that asteroidal materials might service, including radiation shielding for orbital bases outside the Earth’s magnetosphere, fuel for comsat station keeping and samples for collectors/museums (which is surprisingly lucrative, $1,000 per gram or more).  

Zubrin does propose a large number of missions to collect samples from many asteroids. This is a great idea. It may not be cheap though. The OSIRIS-Rex mission to return at least 60 grams of asteroidal material is $800 million plus launch costs. The Hayabusa mission, which retrieved a few grains of asteroidal material, cost $170 million. The JPL/Keck study prices the asteroid retrieval mission at about $2.6 billion. There is, of course, no reason not to do both if funding is available. Considering that understanding asteroid composition is essential to planetary defense, and planetary defense is essential to survival, perhaps we should consider supplying the funding.

This is not a Mars mission; it’s an asteroid mission. For those, like myself, who think that economic activity in cislunar space is the best near-term objective, and free-space settlements co-orbiting with asteroids that supply the materials is the dominant long-term future of mankind, it’s a great mission. It has NASA doing what NASA does best: creating new capabilities for humanity.

Al Globus

Capitola, Calif.