Nuclear Propulsion and What It Means to Space Exploration
written by Brian Rudo on March 05, 2003

Since the beginning of time we have been fascinated with what makes up the world around us. The ancient Greeks first gave us the atom, and work by Renaissance philosophers and the beginning of modern science gave us more and more detailed insight into the structure of atoms and chemical interactions. Rutherford’s discovery of the nucleus and the lightning-fast discoveries that followed it have led to deeper consequences than anyone of any other time period could have imagined. Since the discovery of the fission of an atom’s nucleus and the accompanying release of energy, the world has been poised on the brink of complete destruction.

Everyone knows about nuclear weapons and nuclear power. Many people believe that it is too dangerous to experiment with, and too costly to work with. Many also believe that it is nuclear technology that will save mankind. But the reality of nuclear technology, as in any other technology, is that the results are what we make out of it.

In the 1930s and 1940s, scientists at the Los Alamos National Laboratory successfully attempted to construct a nuclear bomb. This has been arguably the greatest discovery in the history of humankind for its implications to the very nature of the political and societal structure of today. But this was not their only achievement. Los Alamos researchers investigated everything from electricity generation to nuclear-powered aircraft, and it was in many cases not the technological limitations themselves that stopped research, but bureaucratic issues. In the 1940s it was believed that we were on the brink of achieving all of man’s eternal goals: the end of war, the end of hunger, and reaching to the stars.

In reality the implications were much less beneficial to mankind. With the nuclear power disasters of the twentieth century, notably Chernobyl and Three Mile Island, the public has withdrawn from nuclear power. Further problems that have halted nuclear power generation have resulted from the storage of nuclear waste.

Public disfavor with anything nuclear has extended itself into space. When the Cassini probe launched in 1997, its 73 pounds of plutonium sparked protests that called into question any future nuclear project in space. Protesters contended that an error in launch or an encounter with Earth later on in the voyage could result in dangerous radioactivity raining down from the sky. What the protestors failed to realize was the actual risk involved: the increase in radioactivity that would result from the destruction of Cassini would have been equivalent to a 15,000th of a normal lifetime absorption of radioactivity. There is most likely more radioactivity in a tanning booth or dental X-ray.

Nuclear Propulsion

Nuclear Propulsion

Credit: Starship Daedalus

Yet the advantages, if public disfavor can be overcome, are enormous. Non-nuclear spacecraft are not only limited in propulsion speed and range and payload, but also the power available to instrumentation onboard. The 1997 Sojourner rover on Mars stopped functioning after only days because its solar panels had become dust-coated and ineffective, not because of any equipment failure. A nuclear rover under development by NASA for launch in 2009 would be able to travel hundreds of miles and last for months to years on the Martian surface, and its sensors could have orders of magnitudes more power, leading to much more data gained. A spacecraft proposal that would use fission-heated oxygen-hydrogen reactions that would allow cheap daily commuter flights to the moon for thousands of years is under consideration in NASA, and five-year missions to Pluto have been proposed with nuclear fission propulsion. There is no end to what this technology can give us.

So if safety can be assured, within limits of course, and the benefits are so great, what is stopping us from developing these projects and having the entire solar system at our grasp? The answer is mostly political, not technical. Fortunately, NASA has managed to press ahead in its recent nuclear initiative, now named Project Prometheus. NASA will invest billions of dollars over five years to develop and launch a nuclear-powered spacecraft. Unlike past nuclear efforts in space, such as the above-mentioned Cassini, the Prometheus craft will be propelled by nuclear electric propulsion, with power generated from nuclear fission.

There are three main classifications of nuclear propulsion, and three methods of powering them.

Radioisotope decay has been demonstrated feasible and used in many missions for decades. It relies on the natural release of energy when the fuel, for example plutonium, decays into an isotope of uranium, releasing energy. This provides a continuous, safe source of power. This electrical power is then used to accelerate propellant and then eject it at high velocity to propel the craft. The main limitation of this method is the raw amount of power that can be generated. It is mostly useful for long-term, low velocity propulsion. It also is one of the safest methods of nuclear propulsion, as no fission reaction is taking place and there is little danger of complications.

Nuclear fission is the most well known form of nuclear technology. In simplest terms, nuclear fission relies on neutrons bombarding nuclei of uranium or plutonium and forcing the separation of these elements into other elements, releasing energy. This energy output is several orders of magnitude above radioisotope decay and chemical propulsion. The Project Prometheus goal, a 100-kilowatt reactor, is a thousand times the energy output of a moderate-sized solar panel like the ones used on the Pathfinder mission for the Sojourner rover. It is the difference between a desk light and a stadium lighting system.

Nuclear spaceship.

Nuclear spaceship.

Credit: Unknown

The earliest considered form of nuclear propulsion was known as a nuclear rocket, or nuclear explosive propulsion. The nuclear rocket would use nuclear explosions to propel a spacecraft to extremely high velocities. One of the most famous of these proposals was the Titan concept, a giant spacecraft using the world’s nuclear weapons arsenals to propel itself to the stars. Needless to say, this has extreme disadvantages, and never got off the ground. For one, the radiation output would be enormous. It would be folly to launch from Earth, not to mention difficult. If a reactor were used to generate the explosions, the size would need to be thousands of tons – almost an impossibility to get working. This concept has been abandoned due to concerns about radiation exposure and since the Nuclear Test Ban Treaty has been in place.

Also at times called nuclear rocketry is another, more practical method that uses propellant, such as hydrogen, heated to extreme temperatures and ejected at high velocities as in a conventional rocket. Unlike a conventional rocket, the propellant’s energy would come from direct or indirect nuclear energy, and thus be extremely more powerful. This design, usually called nuclear thermal propulsion, has many supporters. It would allow very fast changes in velocities, and would be able to make a trip to Mars in a few days at its most advanced form. It also has use in rover missions and even in a unique concept called a hopper, which would be able to use Martian atmosphere to propel itself from site to site to conduct research. An advantage of this system lies in the fact that although some gases are better, any gas will do for propulsion. This means that a craft on Mars could use the indigenous Martian atmosphere or water ice to refuel, extending its lifetime and reducing cost and increasing efficiency by orders of magnitudes. The United States and the former Soviet Union have both successfully built full-scale functioning rocket engines using nuclear thermal propulsion.

According to NASA, there are still some design issues to be worked out, but it is more likely that the reason the United States’ nuclear thermal program was dropped is political. The project, called NERVA and terminated in 1973, had succeeded early on in building functioning, safe rockets. According to an article on the NASA website, the program was terminated because of “a lack of public interest in human spaceflight, the end of the space race after the Apollo Moon landing, and the growing use of low-cost unmanned, robotic space probes.” Not only that, but most likely since it was cheaper to operate than the much less powerful shuttle, the senators and representatives from the states that produced the shuttle also pitched in on its demise. This passage of events can only be described as tragic. No current major, or even medium-sized, project of any space agency in the world includes nuclear thermal propulsion.

In fact, the only form of nuclear propulsion that is actively being pursued today is the final major form of fission propulsion, nuclear electric propulsion. This form of propulsion is similar to nuclear thermal propulsion, except that instead of heating the fuel to accelerate it, it ionizes the fuel and then sends it through an electric field to propel it at extremely high velocities. There are many ways to do this, and engineers have proposed everything from using the Hall Effect to simple cyclotron designs from the turn of the twentieth century. The major disadvantage to this is the low acceleration that can be achieved through this in current designs. Typically, however, these thrusters would operate over a long period of time and more than make up for the low rate of increase.

The electric propulsion system is favored by Project Prometheus, and is under aggressive development. As technology improves scientists are improving the energy output of these systems, enabling faster engines and more efficient use of fuel. Many experts consider nuclear electric propulsion the most effective form of propulsion in the near future. However, it is not good for large masses over relatively short distances, which makes it a poor choice for future manned missions to Mars.

Project Prometheus' fate?

Project Prometheus' fate?

Credit: Bulfinch's Mythology

Despite the failure of Project Prometheus for manned exploration, the future will bring new opportunities for advancing space travel. Magnetic propulsion, using the solar wind to propel a spacecraft, is showing great promise, although it is unclear whether it will be effective enough for manned missions. Antimatter, which has the highest energy potential of any substance known to man, would provide an incredible propulsion system if production could be made much less expensive. But the true advancement that many scientists expect to see in the near future will lie in fusion propulsion.

Fusion is the opposite of fission. Instead of splitting massive uranium or plutonium, it combines light elements to form slightly heavier elements. Not only is it much more powerful, it is also much safer than fission. Although we have been able to make fusion bombs, we have been unable to sustain a power-producing fusion reactor due to the extremely high temperatures required and other limitations. Many nations around the world are pursuing an international program to develop a working fusion reactor. When this is accomplished, fusion can be adapted to any of the propulsion schemes outlined above. In addition, an interesting combination of antimatter and fusion may lead to a very high-energy propulsion system in the future. This proposal uses an interesting property of antimatter to induce microfusion, which can in turn be used to propel a spacecraft. This is all in the future, however, and not likely to be realized soon.

So, while we should expect to see some form of advancement in nuclear propulsion in the near future, most probably in NASA’s Prometheus nuclear electric propulsion project, we should not expect this to significantly impact space travel as we know it. Manned flight in particular would receive little benefit from the Prometheus project. In fact, Dr. Robert Zubrin, President of the Mars Society and Pioneer Astronautics, believes that the only real benefit for manned exploration would be from the construction of small nuclear reactors for the project. These reactors are ideal for the Mars Direct plan, which ambitiously aims to send humans to Mars within ten years of the program start – for an average of a billion dollars a year if the program were maintained for ten flights to Mars, one per every two-year launch window. This pocket change would produce a safe program that could maintain a manned presence on Mars cheaply and reliably. Unfortunately, it would utilize only conventional propulsion, mostly because nuclear electric propulsion will not hold promise for relatively large spacecraft and relatively short duration flights (See Also: Mars Direct).

Therefore, despite the great promise to propulsion that nuclear technology has showed, it seems that only the most limited benefits are to be had in the near future. Long duration flights, like a probe to another star system, will see extreme benefits from the Prometheus project. Mars rovers and missions will also see a great benefit, but most of that benefit will reside on the staying capacity, endurance, and raw power given to missions once they land, not in any actual propulsion gains. Despite its many benefits and its evident value, Project Prometheus is merely a poor substitute for NERVA and its nuclear thermal propulsion – a system we had working in the 1960s but that was forgotten.

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23) -- The Making of the Atomic Bomb - Rhodes, Richard (Aug 1995 Touchstone Books)