Space Exploration

Ever since the space race began in the late 1950s, people have benefited from technology developments that were born in this industry:  plastics, computers, advances in food preservation and packaging, to name a few.  This is still true today as scientists conduct many novel scientific experiments in space. It is, and may very well continue to be, also true of nuclear technology and the space program.

space food
Space food samples.

Astronauts have dined on irradiated food for years and it has now been endorsed by every major food and health organization.  As probes are sent deeper into space it has become necessary to find other ways of propulsion besides chemical energy (with its limitations on mass) and solar energy (as they get further away from the Sun there is insufficient light).

Electric-ion engine
Electric-ion engine.

Increasingly, modern aircraft and space vehicles use composite materials to reduce weight and conserve fuel.  In many cases these materials are treated with ionizing radiation to improve their properties.  Moreover, failures of materials such as the vertical stabilizer on Canada’s CF-18 aircraft have been studied with neutrons from research reactors.

As we move to the future, how will these technologies benefit society?

Nuclear Rockets

In the not too distant future nuclear rockets may become a reality. Although Project Prometheus, a program to develop nuclear propulsion for the Jupiter Icy Moons Orbiter (JIMO) was cancelled in 2005, NASA is investigating a number of technologies for nuclear rocket propulsion and nuclear power for bases on the Moon and Mars.

The system will use electricity generated by a nuclear reactor to power an electric ion drive system for a new generation of spacecraft which one day will be capable of taking man to the outer reaches of the solar system. It sounds like science fiction but the concept of nuclear rockets is actually 50 years old, starting with Project Pluto in 1957. Between 1957 and 1972 several nuclear rocket designs were proposed, but only a few were partially tested. Cost, engineering difficulties and public opinion led to the abandonment of most designs.

Today, the idea of using nuclear rockets is again front and centre. Many experts agree that nuclear rockets may be the only way to carry large payloads to Mars and the outer planets. So far, space exploration has involved the use of chemical rockets to propel most spacecraft through space. One of the problems with chemical rocket systems is weight. Chemical fuels are heavy and add weight to the launch vehicle which puts limitations on the size of the payload that can be launched into space.

In 1998, NASA launched Deep Space 1 and Deep Space 2 which successfully demonstrated that an electrostatic ion engine could propel a spacecraft. Deep Space 1 did a flyby of the Braille asteroid and followed Comet Borelly, sending back data and photographs. Deep Space 2 successfully reached Mars but the surface probes failed to function. In both cases, the electrostatic ion engines performed better than expected.

The engine works by bombarding a gas with a beam of electrons. This knocks electrons off the atoms of the gas, creating a positively charged ion. High voltage metal grids at the back of the engine chamber accelerate the positive ions toward the grid. As they pass the grid, they reach speeds of over 30 km/s and are focused into an ion beam before being exhausted out the back of the engine. Finally, a neutralizer collects excess electrons and injects them into the ion beam to prevent a build-up of negative charge on the spacecraft. Smaller but similar ion propulsion systems are used on satellites and space probes to make course corrections. Both Deep Space 1 and 2 used solar panels to convert sunlight into the electricity needed for the ion engine.

Prometheus 1 spacecraft
Prometheus 1 spacecraft.

Prometheus 1 would have used a nuclear reactor to generate the electricity required for the engine. The mission objectives were be to explore three of Jupiter’s moons: Callisto, Ganymede, and Europa, and to demonstrate that nuclear electric propulsion flight system technologies would enable a range of revolutionary planetary and solar system missions.

While an electric ion engine has smaller thrust than a conventional chemical engine, it can operate for a significantly longer period of time. This technology will allow spacecraft to travel greater distances at greater speeds, carry larger payloads, and also allow spacecraft to be more manoeuvrable. There are advantages to using a nuclear reactor to generate the electricity needed. Nuclear reactors are capable of generating far more electricity than solar panels and the excess electricity generated can, on future missions, be used to maintain life support, computer, and communication systems as well as power large centrifuges to create artificial gravity for manned missions to Mars and beyond.

Radioisotope Thermo Electric Generators

Nuclear technology has been used in space since the beginning of the space program. The first unmanned space vehicle was Sputnik, launched by the Soviet Union in 1957. The United Statesfollowed three months later with its first unmanned flight, Explorer 1. The number of US and Soviet space missions increased dramatically in the next few years. By the end of 1969, more than 1,000 spacecraft were orbiting Earth. Five years later the number of these satellites had climbed to nearly 1,700.

In 1961, the first radioisotope thermo-electric generator (RTG) used in a space mission was launched aboard a US Navy transit navigation satellite. The electrical power output of this RTG, which was called Space Nuclear Auxiliary Power (SNAP-3), was a mere 2.7 watts. But the important story was that it continued to perform for 15 years after launch.

RTGs are electricity generators which convert the heat generated by the decay of plutonium-238, a radioactive isotope, into electricity through the use of thermocouples. Thermocouples convert heat energy directly into electricity which can be used to power electrical systems. To date, NASA has launched over 25 missions equipped with RTGs into space and the former Soviet Union has launched over 40 satellites and probes using similar technologies.

Radioisotope thermo electric generators
Radioisotope thermo electric generator.

RTGs are used in spacecraft for several reasons. First, they are rather simple to construct and very reliable, often providing electricity for years, and in some cases decades, after launch. Second, they are able to provide power for deep space missions to the outer planets and beyond the solar system where there is not enough sunlight to power solar panels. Third, they can provide much more electrical energy than is possible with solar panels, with the result that more equipment and experiments can be powered. RTGs also provide heat energy which helps keep spacecraft from freezing in the depths of space where temperatures can reach minus 200 degrees Celsius.

In 1997, NASA and ESA (European Space Agency) launched the Cassini-Huygens obiter/probe to investigate Saturn and its moons. The Cassini obiter is powered by three RTG units and took seven years to reach Saturn, where it still orbits today, radar mapping and photographing the Saurian system. On January 15, 2005, the Huygens probe successfully landed on Saturn’s largest moon, Titan, and sent back data on atmospheric composition and photographic images of the surface for several hours. Some of the discoveries of the mission include: Titan has hydrocarbon lakes, Saturn has hurricanes, and the rings of Saturn are much older than what was once thought. Without the use of RTGs, missions like Cassini-Huygens would have been impossible and much of what we now know about the outer solar system would still be a mystery.

The launch of the Cassini-Huygens probe did however, cause some controversy. Scientists and environmental groups raised concerns that if a launch failure of the rocket carrying the Cassini-Huygens probe were to take place, it might result in the release of radioactive plutonium-238 into the atmosphere. This possibility does exist but the chances are very remote. RTGs are designed to withstand re-entry and remain intact even after a high speed impact from space.

There was an RTG to be used to power a lunar experiment package inside the lunar module which was jettisoned after helping bring the astronauts home at the end of the aborted Apollo 13 moon mission in 1970. The lunar module re-entered Earth’s atmosphere and disintegrated. The RTG crashed and sank in the South Pacific Ocean, but remained intact and did not release any radiation. Because of its construction, it is expected to remain intact for over 800 years, long enough for the radioactivity to decay to a safe level.

Mars concept vehicle
Mars concept vehicle.

In the future, RTGs will play an even larger role in space exploration. RTGs power the next generation of larger, more sophisticated Mars rover, the Mars Science Laboratory, scheduled for launch in 2011. Presently the New Horizons probe is on its way to give us our first close-up look at Pluto. Pluto is so far away, the New Horizons probe will not arrive there until July 2015. Such a mission is only possible with RTG technology.

If the planned manned mission to Mars takes place in the coming decades, the astronauts will be in space for 30 months and will need to be almost completely self sufficient. RTGs will be used to provide a reliable means of both heat and electricity needed for survival on the Mars mission.

Sources:

NASA, http://www.nasa.gov/.
NASA, Cassini Solstice Mission, http://saturn.jpl.nasa.gov/home/index.cfm.
Steven R. Oleson and Frederick W. Elliott, Electric Propulsion Technology Developed for Prometheus.