In previous posts, I’ve talked about some of the challenges associated with a proposed manned mission to Mars, particularly the difficulty of landing on the red planet. But getting men to Mars is also difficult, although not impossible.Transit time for a mission to Mars is measured in months instead of days; the fastest transit is currently in the neighbourhood of 180 days or six months. For robotic landers and rovers, this transit time poses no great challenge. As long as batteries can keep the mechanical joints and systems from freezing, a robot doesn’t mind the wait. (Pictured, an artist’s rendition of a nuclear pulse rocket nearing Jupiter.)
A manned mission is another story. Keeping that crew alive and in good physical form complicates a trip to Mars; its not as simple as providing the astronauts with a heat source. Six months is a long time for a crew to sit and wait in a small cramped spacecraft, exposed to radiation and in a zero-gravity environment. There are ways to protect the astronauts with radiation shields and spinning the spacecraft to create enough gravity to prevent or at least limit the effects of muscular atrophy. But the best way to overcome the challenges of getting men to Mars is to simply shorten the transit time. This isn’t a new idea, it’s one NASA has been researching for over 50 years. The favoured method is a nuclear rocket.
The thought of a nuclear powered rocket may send up red flags for some. It’s a scenario worth questioning: how would Russia, China, or any other nuclear-inclined nation react or respond to the US sending nuclear material into its airspace? And in the reverse scenario, would nuclear material launched over US airspace send a wave of panic throughout the country? Understandably, no one wants the equivalent power of a Hiroshima-style bomb floating over their homes.
But nuclear power in space is nothing new, it has just been largely unpublicized or else eclipsed by more exciting and less potentially devastating manned spaceflight.
In the early 1950s, speculative talk between engineers and physicists of possible applications for nuclear power eventually turned to flight; a nuclear reactor was thought to generate sufficient energy to propel a rocket faster than any chemical rocket. (Left, a cutaway of the Apollo Service Module shows the engine and fuel tanks.)
The principle difference between nuclear and chemical rockets is the energy source. Chemical rockets get their energy from burning fuel. An oxidizer and propellant are mixed and heated, the resulting explosion is directed through a nozzle at the engine’s end to produce thrust. Nuclear rockets’ power comes from a nuclear reaction. Through fission (the breaking apart of a heavy nucleus) or fusion (the fusion of two lighter nuclei), the energy released heats a propellant that is directed out of the engine’s nozzle and translates to thrust. In either fission or fusion reactions, substantial energy is a product.
A third type of nuclear propulsion has been proposed for applications in spaceflight – nuclear pulse propulsion. Nuclear pulse propulsion uses the energy of a nuclear explosion to push a spacecraft. Small nuclear bombs detonate behind a rocket to give it speed; the energy expelled form the explosion in the form of a high-density plasma pushes the vehicle. This was the method explored as part of the Orion program in the mid-1960s.
One of the first physicists to begin working on nuclear propulsion was Robert Bussard. In the early 1950s, Bussard combined his data on the unconstrained power of nuclear reactions with available information on chemical rocket propulsion. This hypothetical study suggested that nuclear propulsion was potentially far superior in thrust, range, and payload capacity than chemical propulsion. His findings earned him some notoriety, and in 1955 he joined the Nuclear Propulsion Division at Los Alamos to develop a nuclear rocket engine. (Pictured, the Kiwi-A engine is tested.)
The first step was to develop a small reactor engine, a ‘proof of concept’. Called Kiwi after the flightless bird, Project Rover’s first nuclear propulsion engine was tested in 1959. The engine was successful – the reactor produced moderate thrust but offered reassurance that nuclear propulsion was a method worth pursuing.
In 1960, the quest for nuclear propulsion capability received a boost with a new program, Rover, to develop nuclear propulsion capabilities. It was jointly funded by NASA, the Atomic Energy Commission (AEC), and the US Air Force. Within Rover were smaller programs with separate aims. NERVA – Nuclear Engine for Rocket Vehicle Application – was dedicated to the development of a man-rated nuclear propulsion system. The Phoebus engine was devoted to developing the most powerful engine. The Experiment Flight Engine Prototype – XE-P – was aimed at developing a reusable nuclear engines.
The Rover program progressed at a steady pace throughout the 1960s. Over the course of the decade as tests yielded increasingly successful results, NASA began planning an outlet for its new propulsion system. Engines were producing increased amounts of thrust and burning for longer durations, and it looked as though further development would yield a rocket capable of delivering men to Mars within a reasonable time frame; the transit time would shrink from months to weeks. In 1968, project managers were optimistic that the first nuclear rockets would be en route to Mars by 1975. (Left, a schematic of the NERVA engine.)
Once Apollo 11 landed on the moon in 1969, however, NASA was hit with significant budget cuts, the effects of which radiated beyond the lunar program. It became clear that a manned mission to Mars was not in line with national goals after the moon landing had been achieved, and the nuclear-powered Mars mission was abandoned. The Rover program was likewise affected, finally terminated in 1973. The vehicles that launched NASA’s first landers Vikings 1 and 2 to Mars in 1976 were traditional chemical rockets.
Nuclear propulsion for Martian missions has been revived many times since the initial efforts of the Rover program were cancelled. Multiple proposals surfaced in the 1970s promising a 30-day transits to Mars if NASA would commit to development and application of nuclear rocket propulsion. In 1987, a proposal from the Lewis Research Centre in Cleveland, Ohio, presented NASA with a thorough survey of nuclear propulsion in an effort to promote a renewed interest in nuclear propulsion in spaceflight calling it a vital technology to Martian exploration. Nuclear propulsion is still touted by many as the best way not only to send a crew to Mars, but to other destinations in the solar system. The chemically-propelled six months to Mars could be spent on a six month nuclear-propelled trip to Saturn’s moon Titan.
While nuclear propulsion has never enjoyed a central role in spaceflight, nuclear energy has been a staple in space exploration for decades.
At the heart of many planetary spacecraft sent on long voyages through the solar system is a radioisotope thermoelectric generator or RTG – a useful tool when spacecraft travel too far from the sun for solar power to be of any use. An RTG gets its power from decaying radioactive material. The heat generated is converted into electricity that can power the spacecraft’s systems and instruments. With a slow rate of decay of the nuclear payload, a spacecraft could live for decades; the twin Voyagers 1 and 2 that have recently reached the heliopause (where our solar system formally ends) have spoken to NASA as long as they have thanks to their RTGs. (Pictured, an artist’s concept of the Voyager spacecraft.)
The same nuclear generators were used on Apollo missions 12 through 17 as an auxiliary power source for the instruments and experiments left on the Moon. But not all RTGs were left out of harms way. When Apollo 13 was aborted, the astronauts were forced to bring their nuclear payload home nestled inside scientific payload; the Lunar Module and its contents splashed down into the Pacific ocean just before the crew reentered the Earth’s atmosphere. The nuclear core of an RTG is somewhere at the bottom of the ocean.
The nuclear material in an RTG, however, is perhaps less worrying than the nuclear material in a rocket. The nuclear material sent into space as a power source is properly packaged in an ultra-durable sort of black box built to withstand direct contact with a rocket explosion.
Rockets, on the other hand, explode. A nuclear propelled rocket may be much more unsettling to even the most devoted fans of spaceflight. But if nuclear propulsion were used for interplanetary travel, it is unlikely to be the propulsion method for launch; chemical rockets would take the planetary spacecraft to orbit where its nuclear engine would take over. For the moment there are no definite plans for a nuclear powered rocket to send a mission to Mars or anywhere else, and all nuclear material sent into space is safely stored for its journey. Perhaps, however, there will soon come a day when nuclear propulsion is the norm in spaceflight, unsettling though it may be.
Suggested Reading/Selected Sources
Stanley Borowski. “Nuclear Propulsion – A Vital Technology for the Exploration of Mars and the Planets Beyond.” NASA. 1987.
“Nuclear Pulse Vehicle Study Condensed Summary Report.” NASA. 1964.
Annie Jacobson, Area 51. Little Brown. 2011.