Conclusions

Nearly 50 years have gone by since the Rover Project was started and the NTR engines it spawned were tested. If the vagaries of US politics, US agencies or public opposition does not get in its way, nuclear propulsion has now a chance of becoming the centerpiece of manned and unmanned planetary exploration. No other propulsion technology can—not at least within reasonable mission length and budgets.

So far, this chapter has focused on promising concepts and enabling technologies. However, there are other challenges that need to be faced and overcome before nuclear propulsion can succeed.

Paraphrasing A. Hansson [Hansson, 2001] these are: reducing the mass of the nuclear reactor and engine, including their radiation shields (much progress has been made in this area by the people working at MITEE, but not all issues have been satisfactorily resolved, and the ratio power/mass of any nuclear engine is still much lower than in chemical propulsion systems); dealing with the residual radiation emission after shutting down the nuclear reactor: a GW-class 235U-powered reactor can radiate 0(103 rad/s) at 10 m many months after having been shut down, see Section 7.4.1; this issue depends to a large extent on fuel fission kinetics and information is restricted); and security, in the general sense: although nuclear propulsion fuels are similar to those in nuclear power utilities, and no nuclear explosion can ever be triggered, dirty bomb manufacturing by non-experts, or even fuel refining by experts to obtain fissionable material are potential security issues in the context of the present world situation. The amount of fuels processed for nuclear propulsion can be safely predicted to be negligible compared to that consumed to generate energy; however, some future fuels under discussion have very small critical masses (even 1% of that of 235U), so security should not be dismissed as a minor issue.

In these authors' opinion, one of the outstanding issues is public acceptance of nuclear power in space, witness the 1997 campaign in the US, and in Florida in particular, against the radioisotope thermoelectric generator power source installed on the Cassini probe launched from Cape Canaveral.

Risks and dangers posed by using nuclear power should be neither ignored nor underestimated, and the public needs to be kept informed, and must be. The public must also be educated, in the sense that nuclear power issues should be compared and put into perspective relative to more conventional energy sources. The response given by people in the street to a recent EU survey of opinions about the so-called Chernobyl accident was indeed instructive. Most people interviewed were convinced that hundreds or thousands of people had died in Ukraine following the accident. So far, 31 among the rescue crew attempting to shut down the reactor and the firemen putting down the fire were lost [Del Rossi and Bruno, 2004]. The total number of deaths to date is fewer than 60, according to UN statistics [Kinley, 2006].

This discrepancy between imagined and actual fatalities is telling: even among educated people nuclear power is surrounded by the fear and aura of secrecy that go back to Hiroshima, Nagasaki and to the atmospheric tests during the Cold War. Hardly any people know that the Chernobyl accident was no accident at all, but a deliberate and foolhardy experiment by a single individual to test the spinning-down time of one of the power turbines. Likewise, not many people are aware that natural background radiation here on Earth is capable of biological effects at least 10 times larger than any existing human-made source.

In this light, any positive but exclusively technical conclusion regarding use and convenience of space nuclear propulsion must be cautiously appraised. On its merit, nuclear propulsion is clearly the only practicable technology if exploring our planetary system at reasonable cost and within reasonable mission times is a requirement (regrettably, this may be a strong "if"). This can be simply argued on the basis of energy density, 10 million times greater than that of the best chemical propellants. This factor is by itself assuring that under proper conditions, nuclear propulsion is the natural requisite of interplanetary space missions. Mass, shielding and radiation hazards, now assumed as the unavoidable penalties of nuclear propulsion, are issues in continuous evolution, and actually benefiting from other, sometimes unrelated, technology areas. NASA planning before the Space Exploration Initiative included NP-powered missions to Europa, Pluto, and Venus, and eventually manned missions to Mars. The implication was that this technology was not only considered realizable, but also sufficiently safe, although expensive. SEI stopped all progress in NP, but the technical conclusions reached (e.g., during JIMO mission planning) still stand. In particular, a potential application, independent of SEI and worth investigating, is connected to the asteroid threat. Although the risks posed by near-Earth asteroids (NEAs) and near-Earth objects (NEOs) has been reassessed recently as 1/720,000 [Harris, 2008], the sheer size of the potential catastrophes should, and do, give cause for concern [Chandler, 2008]. Whatever the means of deviating their trajectories, dangerous NEAs should be reached as fast as possible after discovery. It should be noted that there is still no specific program to discover NEAs: an NEA threat might be detected "too late'' to be intercepted with either conventional or electric propulsion. Chemical rockets would not be capable of the DV required, and EP would be too slow. Only NTR would have the right combination of thrust and specific impulse, especially in the case of an NEA closing at high speed [Powell et al., 1997]. Although many scientists would think that a dedicated effort in this area is premature, others suggest that investing in NTR is not [Schweickart, 2008].

After all technical and societal issues are sorted out and solved, the key condition to transfer nuclear propulsion from technology to space-qualified engines is a steady political will and steady funding. While the US government is on record about supporting development of this technology, ESA in Europe has still to clarify its official posture. ESA is ruled by many of the EU member states, so such indecision simply mirrors reluctance from member states to take a stand. Russia has few or no qualms about nuclear power in space: informed sources have claimed some of its reconnaissance COSMOS satellites orbited in the past were in fact powered by nuclear reactors. Japan, on the contrary, has no intention of doing anything of the sort, even though it must develop new strategic surveillance satellites to reconnoiter over North Korea; because of Hiroshima and Nagasaki, Japan still prefers to rely on miniaturization and electronics powered by solar cells, although there are recent signs this attitude might change (see [Nagata et al., 2008]).

Any effort to develop this key propulsion technology, and especially if the effort should become international, must therefore enjoy a clear and lasting political will. After deciding to go ahead with nuclear power in space, there should not be second thoughts, accepting technical hurdles are a part of life; from the start, conflicting roles of different agencies, or countries should be avoided. In fact, because nuclear energy was managed by military and civilian organizations well before the space age, nuclear and space agencies find in most cases difficult to talk to each other (the Russian nuclear propulsion effort was an exception, but the key people involved, the "three Ks'', were also exceptional). An additional factor in this respect is that a typical aerospace company is smaller, or much smaller, than a company manufacturing nuclear reactors, and so are the business prospects of selling space engines. Faced with a joint nuclear/space program, the standard lawmaker committee is tempted to legislate or "suggest" a joint team, where responsibilities are inevitable shared or diluted, rather than clearly assigned. Such politically over-cautious management was at the root of some significant disasters, notably that of the US SNAP-100 RTG satellite power source [Bennett, 1998]. The opposite example is the US Navy Nuclear Reactor program, managed very successfully for 20 years by a single and clear-headed individual, Admiral Rickover.

Finally, international treaties on nuclear power in space must be given a second look. The scope and text of the UN principles accepted by the 1992 General Assembly seem at this time to be overly restrictive and even preventing in practice the use or deployment of space nuclear propulsion. Born right after the end of the Cold War, during the rush to agree on and to approve what would have been impossible a few years before, the UN principles on nuclear power in space seem now more an obstacle than a tool for protecting humankind from the unwanted effect of nuclear energy. They should be revisited and revised, as suggested in [Lenard, 2005].

At this time humankind is searching for solutions to problems never before so severe or so dramatic: local wars, poverty, terrorism, financial crises seem to focus everybody's attention, as if the oldest questions humans kept asking (Where do we come from? Where are we going? Are there other beings like us? Or at least life? Where?) were forgotten.

In fact, these age-old questions have only been put aside, drowned by the sound and fury. In fact, humankind still wants answers to these questions. More than 70 years after Lise Meitner and Otto Frisch discovered fission and 63 after its use in war, this technology might provide at least one.

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