After a parenthesis of several years, space missions have recovered their interest on the use of nuclear fission power.
Nuclear energy sources are especially suitable, and even necessary, for some outer space missions, since these sources are compact and have a long life
According to the United Nations' Committee on the Peaceful Uses of Outer Space, nuclear energy sources are especially suitable, and even necessary, for some outer space missions, since these sources are compact and have a long life. This includes both radioisotopes and fission reactors.
1. Thermoelectric generators
Up until now, the main source of energy for U.S. space missions were Radioisotope Thermoelectric Generators (RTG). RTGs, which use mainly plutonium-238 and, to a lesser degree, americium-241, are safe and reliable, do not require maintenance and can provide heat for decades under very hard conditions, especially in situations where solar energy is not feasible.
The importance of these energy sources was quite well ilustrated by the Rosetta mission from the European Space Agency, which sent the Philae probe to comet 67P/Churymov - Gerasimenko in 2014. The probe, equipped with solar batteries and plannels, landed on the comet at an angle where the hills hid the sun's light. The result was that it was unable to use solar energy and only managed to send 64 hours worth of data before its battery ran out.
To date, 24 U.S. space vehicles have been propelled by a RTC. This includes space missions Apolo, Pioneer, Viking, Galileo, Ulysses and New Horizons, as well as many exploration proves and military and civil satellites.
The most recent plutonium RTG is a 290 watt system known as GPHS RTG. The New Horizons space ship that travelled near Pluto in 2015 has a GPHS RTG with 240 watts and 30 volts.
Russia has developed several RTGs with polonium-210, and the Chang'e 3 Chinese lunar probe uses RTG with plutonium-238.
2. Radioactive heater units
Apart from RTGs, satellites and space shuttles are also equipped with Radioactive Heater Units (RHU). Their production is only one watt and they use mainly about 2.7 g of plutonium-238. U.S.A. has utilized approximately 240 RHUs. The Chinese lunar probe Chang'e 3 also uses several RHUs.
For the exploration of Mars, the Center for Space Nuclear Research at the Idaho National Laboratory (CSNR) is developing a Hopper, a vehicle that moves in hops propelled by a radioisitope thermoelectric generator. When stationed, the hopper analyzes its surroundings while slowly absorbing carbon dioxide from the atmosphere and freezing it. Meanwhile, a beryllium core stores the necessary heat energy for the explosive vaporization that will lead to the next hop. When it prepares to hop, nuclear heat quickly vporizes the carbon dioxide, creating a very strong reaction stream that propels the vehicle up to 100 meters up in the air. Hoppers can carry up to 200 kg of weight and explore areas that are not accesible to probes. According CSNR, in a few years there could be several dozen hoppers on the surface of Mars. Among other things, Hoppers can collect samples of Martian rock and deposit them on a ship that will them bring them to Earth to be analyzed.
In December 2014, the Glenn Center at NASA announced that it was making progress on its 4 kWt / 1 kWe KiloPower project
Fission heater systems
For situations that require more power, fission systems have an economic advantage over RTGs. The latest initiative from the U.S.A. in the new reactor generators was a joint NASA-DOE program to develop the SP-100 2 MWt fast reactor unit with thermoelectric system that provides up to 100 kWh of multiuse electric supply for space orbit missions, and can also serve as an electric station on the surface of the Moon or Mars.
In December 2014, the Glenn Center at NASA announced that it was making progress on its 4 kWt / 1 kWe KiloPower project, which uses enriched uranium with a pipe system and a Stirling engine to generate electricity. Critical experiments will be carried out in 2017 on the core, with the corresponding safety supervision.
Fission systems for propulsion in space
Nuclear thermal propulsion systems
For the propulsion of spaceships after launch, certain experience has been acquired with nuclear thermal propulsion systems (NTR). Nuclear fisison heats a hydrogen propulsor, and the hydrogen is stored as liquid in refrigerated tanks, The hot gas is expelled through a nozzle to provide propulsion. This is more efficient than chemical reaction.
Currently, the focus is on nuclear electricity systems, nuclear reactors that work as a heat source for ion electric motors with the pupose of expelling plasma through a nozzle and that way propel the ship in space.
NASA developed its Evolutionary Xenon Thruster (NEXT) and its Annular Engine according to this principle. NEXT is a ion high-power ion propulsion system designed to reduce mission costs and travel time. The Annular Engine can even exceed the performance capacities of NEXT and other electric propulsion designs.
Promises for the future
The main purpose of all these advances is to facilitate an important change in the capacity of space missions. With the use of nuclear technology, space travel will be much faster and safer than it is now possible, and exploration missions will be more efficient and will function for a longer period. In the future it might also be possible to conduct missions in Mars with humans.