One concern about nuclear explosions on incoming asteroids is the ‘buckshot effect’ that one rock splits into a wider damage area. Image Credit: David Hardy

I have just returned from Erice, Italy, which lies on a mountaintop at the northwest corner of Sicily. Erice is a major center for the World Federation of Scientists. From 18-26 August, 2004 the attendees participated in the International Seminars on Planetary Emergencies and Associated Meetings.

I am writing this letter in order to pass on to those professionally involved in the subject of near-Earth objects (NEO), information developed at the seminars which I believe to be of particular interest. The comments below primarily represent my personal concern that with respect to asteroid characterization there is, in the community of NEO researchers, a de facto assumption regarding asteroid deflection methodology which may limit the future availability of critical design information.

Much of the work reported dealt with very creative initiatives directed to the characterization of near-Earth asteroids (NEAs). From current missions underway to others awaiting critical funding, to future proposals, a wealth of knowledge will soon be produced regarding the structural characteristics of these bodies. Most of this work will characterize the seismic integrity and large scale structural character of NEAs. While we will certainly gain some knowledge of the fine scale structure I felt that attention to this aspect of asteroid characteristics was under addressed.

My concern reflects, of course, my own preference for the B612 "gentle push" methodology for asteroid deflection. But this concern in fact applies to virtually all of the "soft" deflection techniques, e.g., mirror deflection, laser deflection, and the direct push recommended by B612.

Gold Olive Branch Left on the Moon by Neil Armstrong. Image Credit: NASA

What I fear there is that, within the community of researchers, there exists a de facto, and perhaps sub-conscious acceptance that the "default" method for future asteroid deflection will be via the use of nuclear explosives. My sense is that, while the current asteroid characterization work is both valuable and unquestionably needed, it comes up short of the detailed surface information needed by the "soft" deflection concepts.

For example, the single most critical unknown currently facing the B612 design is how to anchor a spacecraft to the asteroid surface after landing. In particular, in order to properly deflect an asteroid we must be able to push on it, not only perpendicular to the surface, but at angles down to, and hopefully including, the local horizontal.

The question then is what is the nature of the regolith? What is its cohesion? Can one anchor to it and expect that a force of several newtons parallel to the surface can be reliably applied? For the ablation techniques which count on vaporization of the surface materials the detailed thermal characteristics (and probably chemical composition) will have to be known.

While all of the current characterization work is truly commendable, I believe it is extremely important that future characterization research be broadened to include the detailed characteristics which will enable techniques other than the use of nuclear explosives or direct impact (which is of very limited effectiveness) to move ahead with valid designs.

The rocks inside a crater on the Asteroid Eros, as imaged before impact with the NEAR spacecraft. Numerous small impacts on the asteroid show brown boulders visible interior to the less exposed (white) lip of the crater. False-color for emphasis.
Image Credit: NASA/Eros

I strongly urge that those doing this critical research become familiar with current thinking in the deflection field so that we are not left in the future with the terrible situation where the only deflection technology for which adequate design information exists is nuclear explosions in space.

We should never be placed, by a lack of accessible knowledge, into a position of having to accept the Faustian bargain of nuclear weapons standing by forever in order to assure that the world can avoid a NEO impact.

Related Web Pages

Toutatis Orbit Simulation
Great Impact: Part I: The Benefits of Hard Bodies
Great Impact: Part II: Much Ado about Nothing?
Great Impact: Part III: The Large and the Small
Great Impact: Part IV: On A Collision Course with Earth
Great Impact: Part V: Encore
Impact Hazards Website
NASA/JPL Near Earth Object Program
Asteroid 4179 Toutatis: JPL Near-Earth Objects
Asteroid 4179 Toutatis Earth Crossing: Washington St.
IAU Minor Planet Center