The Nuclear Knot for Pluto
It is a truism that nuclear propulsion is not yet developed, but it is important to understand the full import of that fact.

Even optimists doubt that a first nuclear test flight could take place in less than six to eight years, and, as with DS-1 for solar electric propulsion, a nuclear test flight will be required to validate the new technology. Given the difficult launch approval process such a propulsion stage will no doubt require, it could well be longer.

There is no guarantee that such a multi-year development will eventually lead to a flight program, and previous U.S. efforts in developing nuclear technology for spacecraft give no cause for optimism.

From the nuclear thermal NERVA program (1960s) through the nuclear electric SP-100 (1980s) program, nuclear propulsion has always not quite "gotten off the ground." None of this past track record means it is a bad idea, but it does mean that it is technically, and politically, difficult.

Even given a working nuclear propulsion stage, if you want to go somewhere fast, then you must also slow down near the destination in order to obtain sufficient time to make observations.

This is a new problem for mission planners who heretofore only needed to worry about slowing a spacecraft, and typically by far less speed, to go into orbit. This problem is exacerbated in the outer solar system, where illumination levels are relatively low. At Pluto, light levels are 1000 times lower than in the sun-drenched regions near Earth.

Nuclear propulsion does not guarantee unlimited peak speeds either.

A recent study by the Johns Hopkins University Applied Physics Laboratory and Glenn Research Center examined optimistic spacecraft architectures (meaning no one really knows how to build them yet) and found that a nuclear electric system could only cut about 2 years off the the 9.5 year flyout planned for the current PKB mission-New Horizons.

This flight time does not include slowing down to increase the period of the main flyby time and collection of observational data which will make the nuclear option take even longer than found in the study.

The net comparison means that a nuclear option for Pluto will arrive years later (given the development time needed for nuclear propulsion), at significantly more expense (nuclear propulsion is not going to be free), and with greater technical risk than what is on the table now (the New Horizons development effort).

A change to nuclear-based propulsion will lead to a better program of exploration; we must just be careful not to hamstring current efforts during that transition.

The Nuclear Advantage
Nuclear propulsion, once developed and certified for use, holds much promise for many future applications in planetary science. Indeed, there are whole classes of mission that nuclear propulsion will enable once it is developed.

A good analogy can be drawn with solar electric propulsion (SEP) and the recently selected Dawn mission in NASA's competitive Discovery program. Prior to the validation of SEP as a primary deep-space propulsion system with DS-1, NASA was unwilling to select missions like Dawn due to perceived risk of failure.

A DS-1 flight plan could have been flown with a chemical propulsion system, but a mission to orbit the mainbelt asteroids, Dawn's mission, clearly could not. In this case, SEP is an enabling technology because the mission simply cannot be done with existing launch vehicles, spacecraft we can actually build, and ANY form of chemical propulsion. Where the true promise of nuclear electric propulsion (NEP) lies is in similarly otherwise undoable missions in the outer solar system.

Maximizing the scientific return from a wider set of enabled possibilities is what turning the science community loose in a competitive environment is good for. The competitive "New Frontiers" program, coupled with nuclear propulsion promises the same high value return that NASA is beginning to reap with solar electric propulsion and the Discovery program closer to the Sun.

The Power Connection
In-space nuclear power is intimately connected to in-space nuclear propulsion.

In high priority Mars surface missions using rovers, as well as in the outer solar system, light levels are simply too low to provide enough electrical power to do all that the science requires.

Better solar cells will not fix the problem, as the technology is pushing close to the power-generation limits imposed by physics, and nuclear power sources are required for long range rovers on Mars and most missions beyond the main asteroid belt, where radioisotope thermoelectric generators have been the workhorse of all outer planet missions for decades.

One area that needs development is more efficient power converters for radioisotope power supplies, and one solution could be mechanical Stirling converters that show promise for better conversion efficiency.

Coupled with continued NASA investments in lighter and more power-efficient spacecraft subsystems new radioisotope power units married to ion engines can enable the first generation of nuclear propulsion exploration to be launched.

With time and money, it should be possible to make the same amount of electricity with less plutonium, another win-win situation. In some sense, such systems are a stopgap for the application of small fission reactors that can provide far more power. The fission systems have further to go in development, but nuclear space propulsion need not wait as radioisotope electric propulsion will get us started.

The Bottom Line
Nuclear propulsion and nuclear power in space are technologies long-overdue for investment and development. They mean traveling quicker as well as doing more once you have arrived at your destination - but one must keep perspective: they are also not a panacea to all of our current space transportation limitations.

The development of these new resources is also not without risk, technically or politically; and while risk does not mean one should not implement new research programs, the presence of risk does mean that one should not overly count on positive results in a time-critical endeavor. And the exploration of Pluto is such an endeavor.

Advantages must be seized wherever and whenever possible; they are all too few in the space exploration business.

The last real chance to get to Pluto with current technology requires a launch in January 2006 (the curent New Horizons mission plan). Given where we are with NEP, that technology would surely take longer to implement, cost more money, and delay the encounter past that achievable with New Horizons.

With a hammer that promises far larger payloads and missions, such as extensive orbital tours that are not feasible today, nuclear propulsion deserves developing. We need nuclear propulsion and the time and money to get it right. While we are not there yet, NASA has taken a major step in the right direction.

Dr. Ralph McNutt is the Chief Scientist of the Space Department at the Johns Hopkins University Applied Physics Laboratory. He is also the Project Scientist for the MESSENGER Discovery mission to Mercury and a Co-I on the New Horizons concept under study for a Pluto-Kuiper Belt mission. All of the opinions expressed are his own and should not be construed as reflecting the position of the Applied Physics Laboratory or the Johns Hopkins University."

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