Power supply systems (power supply, if it's simpler, because even machines need to eat something) are an important part of the spacecraft. They have to work under extreme conditions and be extremely reliable. However, with the ever-growing energy demands of complex spacecraft, we will need new technologies in the future. Missions that will last for decades will require a new generation of power supplies. What options?
The latest mobile phones can barely survive a day without having to be plugged into a power outlet. But the Voyager probe, launched 38 years ago, is still sending us information from beyond the solar system. Voyager probes are capable of efficiently processing 81,000 instructions every second, but on average smartphones are 7,000 times faster.
Your mobile phones are, of course, born to be recharged regularly and are unlikely to go several million kilometers from the nearest outlet. It is not practical to recharge a spacecraft that is 100 million kilometers from the nearest station. Instead, a spacecraft must be capable of storing or generating enough energy to navigate space for decades. And this, as it turned out, is difficult to arrange.
While some onboard systems only occasionally require energy, others must be constantly running. Transponders and receivers must be active at all times, and in the case of a manned flight or space station, life support and lighting systems must also work.
Dr. Rao Surampudi is the Power Technology Program Manager at the Jet Propulsion Laboratory at the California Institute of Technology. For over 30 years he has been developing power supply systems for various NASA spacecraft.
According to Surampudi, spacecraft power systems account for approximately 30% of the transport mass and can be broken down into three important subgroups:
power generation;
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energy storage;
power management and distribution
These systems are critical to the functioning of the spacecraft. They must have a low mass, live long and be “energetically dense,” that is, produce a lot of energy from relatively small volumes. They also need to be quite reliable, because some things in space would be almost unrealistic or impractical to fix.
These systems must not only be able to provide power to all onboard needs, but also do so throughout the entire mission - some of which could last tens or hundreds of years.
“The life expectancy has to be long, because if something goes wrong, you can't fix it,” Surampudi says. "It will take five to seven years to get to Jupiter, more than ten years to Pluto, but leaving the solar system is 20-30 years."
Due to the unique environment in which they operate, the spacecraft's power supply systems must be able to operate in zero gravity and in a vacuum, as well as withstand colossal radiation (usually, electronics do not work in such conditions). "If you land on Venus, temperatures can reach 460 degrees Celsius, but on Jupiter they can drop to -150 degrees."
The spacecraft, which is heading towards the center of our solar system, will receive a lot of solar energy for its photovoltaic panels. Spacecraft solar panels may look like regular solar panels for our homes, but are designed to work more efficiently than at home.
The sudden rise in temperature from close proximity to the sun can also cause solar panels to overheat. This is mitigated by rotating the solar panels away from the Sun, which limits exposure to intense rays.
When a spacecraft enters the orbit of a planet, solar cells become less efficient; they cannot generate much energy due to eclipses and passing through the planet's shadow. A reliable energy storage system is needed.
Atoms respond
One such type of energy storage system is nickel-hydrogen batteries, which can be recharged more than 50,000 times and have a lifespan of over 15 years. Unlike commercial batteries, which do not operate in space, these batteries are hermetically sealed systems that can operate in a vacuum.
When you fly away from the Sun, solar radiation gradually decreases from 1.374 W / m2 around the Earth to 50 W / m2 near Jupiter, while Pluto already amounts to some 1 W / m2. Therefore, when a spacecraft flies out of Jupiter's orbit, scientists turn to atomic systems to provide the spacecraft with energy.
The most common type is the radioisotope thermoelectric generators (RTGs for short), which were used on Voyager, Cassini, and the Curiosity rover. They are solid state devices that have no moving parts. They generate heat during the radioactive decay of elements such as plutonium and have a lifespan of over 30 years.
When the use of an RTG is not possible - for example, if the weight of the shielding required to protect the crew makes the apparatus impractical - and the distance from the Sun precludes the use of solar panels, then fuel cells are turned.
Hydrogen-oxygen fuel cells were used during the Apollo and Gemini space missions. Although hydrogen-oxygen fuel cells cannot be recharged, they have a high specific energy and leave nothing but water for astronauts to drink.
Ongoing research by NASA and JPL will enable future power systems to generate and store more energy using less space and for a longer time. Nonetheless, new spacecraft require more and more reserves as their onboard systems become more complex and hungry for energy.
The high energy requirements are especially true when the spacecraft uses an electric propulsion system like the ion engine, first delivered to Deep Space 1 in 1998 and still successfully used on spacecraft. Electric propulsion systems usually eject fuel with electricity at high speed, but others use electrodynamic ropes that interact with the planet's magnetic fields to move the spacecraft.
Most of the energy systems on Earth will not work in space. Thus, any new power supply system must be thoroughly tested before being installed on a spacecraft. NASA and JPL are using their laboratories to simulate the harsh conditions in which this new technology will operate, bombarding new components and systems with radiation and exposing them to extreme temperatures.
Extra life
Stirling radioisotope generators are currently being prepared for future missions. Based on existing RTGs, these generators are much more efficient than their thermoelectric siblings, and can be much smaller, albeit with a more complex arrangement.
New types of batteries are also being developed for NASA's planned mission to Europa (one of Jupiter's moons). They must operate in a temperature range of -80 to -100 degrees Celsius. The possibility of creating advanced lithium-ion batteries with double the stored energy is being studied. They could allow astronauts to spend twice as long on the moon before the batteries run out.
New solar cells are being developed that can operate in conditions of reduced light intensity and temperatures, that is, the spacecraft can operate on solar energy farther from the Sun.
One day NASA will finally decide to build a permanent base on Mars with people, and maybe on another planet. The agency will need power generation systems that are much more powerful than existing ones.
The moon is rich in helium-3, a rare element on Earth that could be an ideal fuel for nuclear fusion. However, so far such a synthesis is not considered stable or reliable enough to form the basis for the power supply of the spacecraft. In addition, a typical fusion reactor, such as a tokamak, is about the size of a house and will not fit into a spacecraft.
What about nuclear reactors that would be perfect for electrically powered spacecraft and planned missions to land on the Moon and Mars? Instead of bringing a separate power supply system to the colony, the spacecraft's nuclear generator could be used.
Spacecraft with a nuclear-electric type of engine are considered for long-term missions in the future. "An asteroid redirection mission will require powerful solar panels that will provide enough electrical propulsion for the spacecraft to maneuver around the asteroid," Surampudi says. "At some point we were going to launch it on solar energy, but with nuclear power, everything will be much cheaper."
However, we will not see nuclear powered spacecraft for many years. “The technology hasn't matured yet,” Surampudi says. "We need to make sure they are safe after launch." They will have to undergo rigorous testing to show whether it is safe to expose such nuclear installations to the harsh tests of space."
The new energy supply systems will allow the spacecraft to operate longer and travel further, but are still only at the beginning of their development. When tested, they will become critical components for manned missions to Mars and beyond.