Powering Space Missions
Radioisotope Thermoelectric Generators (RTG) are a nuclear technology attached to a spacecraft that supplies power and heating. When the radioactive isotope plutonium-238 in the RTG decays, it gives off heat, which can be used to generate electricity using a thermocouple device. This process is known as thermoelectric conversion. The decay heat warms one end of the thermocouple, and the cold environment of space cools the other. This process produces an electric current that then powers the spacecraft. Excess decay heat is also pumped through the spacecraft’s systems in order to warm up its instruments and subsystems, allowing it to operate in cold environments.
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Radioisotopic Thermoelectric Generators have been involved in more than 25 space missions providing power in deep space for Voyager 1 and 2, several Apollo missions, Galileo, Nimbus and LES. RTGs have enabled major scientific accomplishments such as:
- the Cassini spacecraft and Huygens probe’s exploration of Saturn and Titan, one of its moons, since 2004
- the landing of the Curiosity rover on Mars in 2014
- the flyby images of Pluto from the New Horizons mission in 2015
According to NASA, eight generations of RTGs have been used in U.S. spacecraft since 1961. Currently, they use the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) in the vacuum of space or in planetary atmospheres.
NASA is also working on a new RTG technology, called the Advanced Stirling Radioisotope Generator (ASRG). The ASRG, like the MMRTG, converts decay heat from plutonium-238 into electricity, but does not use thermocouples. Instead, the decay heat causes gas to expand and oscillate a piston, similar to a car engine. This moves a magnet back and forth through a coil of wire over 100 times per second, generating electricity for the spacecraft. The amount of electricity generated is more than that of MMRTGs, at approximately 130 watts, with much less plutonium-238 (approximately 8 lbs less). This is a result of the more efficient Stirling cycle conversion. If more power is needed for a mission, ASRGs can also be used together to generate more power. There are no missions scheduled as of yet that would use ASRGs, but they are being designed for 14-year mission lives.
Nuclear technology in space exploration is not limited to the use of decay heat from radioisotopes for power. There are many instruments used to detect radiation and determine the composition of distant stars or another planet’s rocks, atmosphere, and soil, among many other things. One of these instruments is called the Alpha Particle X-Ray Spectrometer (APXS). This instrument determines the composition of rocks and soils using alpha particles and X-rays. To do this, alpha particles are generated in the APXS and directed at a target. The alpha particles interact with the materials in the target, which then emits X-rays of certain energies. Each nuclide emits its own unique X-ray energy “fingerprint”, enabling the APXS to determine the makeup of the target.
While there is no current space application for the large energy generation capability of nuclear reactors, future applications include manned exploration of much of the solar system and reduced trip times between planets. Radiation exposure during space exploration can be dangerously high even for short periods of time, making the use of nuclear-powered rockets is almost requisite for interplanetary visits. Additionally, nuclear reactors can be used for electricity production in inter-planetary missions with large power requirements, such as manned missions and missions with a large scientific payload.How are nuclear rockets different from traditional chemical rockets?
Chemical rockets operate on the principle of combustion. Fuel is mixed with oxygen and ignited to produce heat and combustion products. These combustion products expand rapidly and are pushed through a nozzle. This process causes an equal and opposite force which accelerates the spacecraft. Nuclear thermal rockets operate similarly to chemical rockets; however, heat is provided through a fission chain reaction, rather than the combustion of fuel. The propellant used in nuclear thermal rockets is most commonly liquid hydrogen, as it enables the rocket to accelerate as efficiently as possible. Nuclear thermal rockets are nearly twice as efficient as the best chemical rockets, enabling them to reach higher speeds with similar thrust.
Nuclear electric rockets operate on an entirely different principle to provide thrust. Ions are accelerated to very high speeds using electrostatic or electromagnetic forces powered by a nuclear reactor. Acceleration in these engines is small compared to nuclear thermal and chemical rockets, but the top speed and range is much better than that from any other type of propulsion due to its even higher efficiency.
Several spacecraft have used radioisotope thermoelectric power, nuclear instruments, and in one case even used a nuclear reactor. Below is a list of some of the better known NASA missions and programs that have used nuclear technology.
Voyager I and II
Voyager I and II are identical spacecraft that were designed to explore the outer planets. They were launched in 1977 and are still operating today. In fact, the Voyagers completed their 4-planet mission and have a new mission – explore interstellar space. The Voyagers were designed to explore the outer reaches of our solar system and needed a power supply that could support them in the dark and cold environment, thus they each fitted with an RPS system. The Voyagers also carried instruments to detect the various types of space radiation mentioned in the section “Space radiation protection”. Link to this section, here?
The Cassini spacecraft, launched October 15, 1997, was designed to explore Saturn and its moons. RPS technology was chosen as the electrical power supply because of Saturn’s distance from the Sun. At the beginning of its mission, the spacecraft had about 890 watts of electrical power but by the end had only about 630 watts of electrical power, roughly equivalent to that used by a small air conditioner unit. Cassini is still in operation today, working on a second extended mission called the Cassini Solstice Mission.
Spirit and Opportunity
The Spirit and Opportunity Mars rovers were sent to Mars in June and July 2003 to explore the possibility of past and/or present signs of water on Mars. The rovers do not use RPSs for electrical power. Instead, they run on solar power. However, the rovers each have one APXS to detect the composition of the rocks and soil on Mars’s surface. Although not the only instrument on the rovers, the APXS brings them closer to achieving their mission goal.
The Mars Science Laboratory was launched in November 26, 2011 and is designed to explore the possibility of past life on Mars. In order to operate for a long duration, the lab – a mobile rover named Curiosity – was supplied with a MMRTG. This ensures that the rover has enough power for at least 687 Earth days and enough heat (approximately 110 watts total electrical power) for operation of several instruments, a robotic arm, wheels, a computer, and radio. Mars is extremely cold, so in order for the rover to stay warm enough to function properly, excess decay heat from the MMRTG circulates warm fluids throughout the rover to regulate its temperature. The rover is also equipped with an APXS, again used to determine the composition of Martian rocks and soils.
The System for Nuclear Auxiliary Power (SNAP) program was a joint program executed by the Department of Energy and NASA with the purpose of developing “compact, lightweight reliable atomic electric devices for use in space, sea, and land use.” Out of the program, only 1 reactor was flown and remains to date the only reactor ever put into space by the United States. The SNAP-10A system weighed under 950 lbs and reached a steady-state power output of 600 watts before shutting down after 43 days due to a separate electrical component failure in the satellite it was powering. The SNAP-10A used thermoelectric conversion to convert heat into electricity and was cooled with a sodium-potassium liquid metal coolant.
The ROVER/NERVA program was a nuclear rocket development program that operated during the 1950’s and 1960’s. Many tests of nuclear rockets were conducted at the Nevada Test Site. About 40 miles NW of Las Vegas, the Nevada Test Site is infamous for the numerous above and belowground tests of nuclear weapons conducted there from the 1950’s to the 1980’s. The test rockets used liquid hydrogen as a propellant and a graphite-uranium matrix as the fuel. Development continued until 1972, when changing national priorities discontinued funding for the nuclear rocket.
Project Prometheus was a program conducted in the mid-2000’s to develop a nuclear reactor powered spacecraft capable of propelling a large unmanned vehicle to Jupiter. The name of the spacecraft to be developed in the program was the Jupiter Icy-Moons Orbiter (JIMO). This nuclear-electric propelled spacecraft was powered by a helium-xenon cooled nuclear reactor and was discontinued in 2005 due to high development costs.
Radioisotope Power Systems run by nuclear decay, which does not require light or heat from the Sun. This makes RPS systems ideal for spacecraft that will be in cold or dark environments. The plutonium-238 in RPS systems is also long-lived in comparison to other types of technology used to power spacecraft, permitting much longer missions.Using nuclear technology in space exploration continues to help further our knowledge of our solar system, including its outer reaches where humans cannot yet travel to or survive. Keeping the Voyager spacecraft, their mission, and their nuclear technology in mind, we will gain more knowledge on interstellar space, still yet to be explored. Although the area around Saturn had been previously explored, the Cassini spacecraft was able to give scientists a better view of the planet and some of its moons. All three Mars rovers are exploring either the possibility of water or of life on Mars. As we can see, nuclear technology has allowed these spacecraft to either complete or work toward completing their primary mission, and in some cases, extended missions. Therefore, with the use of nuclear technology in spacecraft, we are able to learn more about our solar system – and eventually – further out into space.
It might seem that except for stars, planets, and asteroids, outer space is entirely empty, but there is far more to it than what we can see. In fact there are particles (smaller than atoms) moving at extreme speeds through the empty vacuum of space where there is almost no material to slow them down. These are not much of a problem here on Earth because the atmosphere stops most particles before they can reach us on the ground. However, these particles are a serious problem for astronauts. When such fast particles collide or pass near an atom it causes ionization, meaning it knocks away that atom’s electrons. This process changes the way it connects to other atoms and can easily break apart molecular bonds, including the DNA that holds our blueprints inside of each cell. Our cells can repair most of these damages if the radiation isn’t too intense, but sometimes the damage is beyond repair.
There are three major sources of space radiation, each with different risks and behavior. Small particles are constantly being ejected from our sun, including negatively charged electrons and many other atoms such as hydrogen or helium that are positively charged (their electrons were stripped away). These charged atoms are called ions, and in the case of hydrogen it is given a special name: “proton”. The Sun is constantly swirling and churning, and it occasionally erupts in certain spots to eject large bursts of particles. If that burst happens to be aimed at the Earth or a spacecraft it will produce a “storm” of particles, called a Solar Particle Event (SPE). These can be especially dangerous for astronauts working outside during a spacewalk.
There are also other particles that come from outside of our solar system, called Galactic Cosmic Rays (GCR). These are often much heavier charged particles, including iron, that were probably shot out of distant supernova star explosions long ago. They are so heavy and energetic that they can shoot through a spacecraft and through the body, where they can cause even more damage to our DNA than many lighter particles.
There is one final large source of radiation in space, again made of those lighter particles (protons and electrons). These can become trapped inside of Earth’s magnetic field and continue to speed around the planet within bands of radiation, also termed the “Van Allen Belts”. These trapped particles are only a problem for astronauts that stay in Earth’s orbit. However, astronauts that leave Earth orbit for exploration missions will actually be hit by more particles overall, since those other sources (GCR and SPE) are often deflected by Earth’s magnetic field.
Radiation can be detected in a variety of ways, but all methods rely on the process of ionization. It can be detected in electronic semiconductor chips or in chambers filled with special gas, where the electrons released by ionization are collected and eventually sent to some measuring device. Other materials produce light when they are ionized, a process known as scintillation, and light sensors can measure this light to determine the amount of radiation energy deposited. Other detectors can take advantage of the ionizing damage that breaks apart molecules, such as CR-39 plastic. In these, a heavy particle track leaves a channel of weakened material so that dipping the plastic in certain chemicals causes those channels to melt away quickly, and the particle track is then visible in a microscope. There are many other methods to observe particles (including cloud chambers that you can build at home), and many more are developed every year.
Determining the amount of energy deposited within a certain amount of material and dividing that energy by its mass can provide a reasonable estimate for the damage caused by ionizing radiation of any kind. This is referred to as “Dose” and it is a good starting point to determine how a person’s health will be affected when they are exposed to radiation. The heavy particles in space radiation are much more damaging to DNA than other radiation types, even for the same deposited energy; so special instruments are needed in space. Devices such as the Radiation Assessment Detector (RAD) and the Tissue Equivalent Proportional Counter (TEPC) are critical for protecting astronauts on the International Space Station. These can deliver a warning to astronauts so that they can take shelter during a solar particle event, and thus prevent very dangerous amounts of radiation dose.
On Earth, barriers of heavy material, such as lead or concrete, are often used to stop most types of radiation. Surrounding a spacecraft with heavy shielding is not a practical approach, though, as each pound of payload currently adds up to $10,000 to the cost of a mission just to get it into orbit. That expense is even greater for exploratory missions, such as to the Moon or Mars, which need even more fuel to escape from Earth’s orbit. One option for these missions is to create a small “storm shelter” inside of a spacecraft, where the walls are thick enough to stop most particles. Astronauts cannot spend all of their time in such small spaces, though, so a warning system is required to tell when a solar particle event is occurring or is on the way.It is also possible to simply design the spacecraft so that most of the living areas are built within a central area, while storage of other necessary equipment and food supplies can be located within the walls. One of the best ways to protect astronauts from space radiation hazards is to minimize the time they must spend in the unprotected zone of outer space during exploratory missions. This requires much faster spacecraft, such as those currently possible with nuclear-powered propulsion.
In the future, perhaps sooner than we think, spacecraft could feature magnetic shielding much like the field produced naturally here on Earth so that harmful charged particles can be deflected entirely. This form of active shielding would likely need a significant amount of electric power, and would therefore almost certainly require a nuclear power source. This may be the ultimate solution for space radiation protection in the effort to expand space exploration for long-term missions, even beyond the Moon and Mars.
Center for Nuclear Science and Technology Information of the American Nuclear Society
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