After the Soviet era, nuclear reactors were no longer launched into space, but today everything is gradually changing. Elon Musk is trying on nuclear energy for the Martian colonies, projects for lunar nuclear power plants are being worked out in Russia - and all despite the fact that in space conditions for solar energy are better than on our planet. What makes the space industry increasingly think about nuclear reactors? Oddly enough, the fact is that nuclear power in space is becoming even more important than on Earth. Let's try to figure out why.
Ninety-nine percent of all the energy that we can use in the Universe around us is the energy of the atomic nucleus, and the remaining one is its various derivatives of the second, third, and so on orders.
What is it about? The aforementioned 99 percent is the energy of the fusion of hydrogen atoms in the interior of the sun (where the light and heat of the sun's rays come from), as well as the decay of a number of radioactive isotopes (where a small amount of heat comes from the interior of the Earth).
The second-order derivative of the fusion of atomic nuclei is sunlight. There are no thermonuclear reactions in the outer layers of the Sun, but they heat up to thousands of degrees and radiate outward, illuminating and warming our world. The energy of atomic nuclei also has derivatives of the third order: for example, firewood obtained from trees that used solar energy. And even of the fourth order: the plants that covered the earth with a green cover in past geological epochs, after their death, gave hydrocarbons, which eventually became oil and gas.
For a couple of million years, people have used only derivatives of atomic energy of the third order. First, firewood, then coal, then oil and gas (as we already wrote, the process with methane is still not over: air transport will switch to it).
The logic of this development was simple: the energy of the wood is too weakly concentrated, which is why the energy comes out of it either expensive or inconvenient to use. Fossil fuel energy is much better concentrated, but it also has drawbacks. Key: limited stocks. Whatever one may say, the volume of ancient plant biomass, which became oil and gas, is far from endless.
This issue is even more relevant in space exploration. A phenomenon is gaining popularity in the world, which the famous engineer Robert Zubrin calls "the tendency towards the dark ages" - that is, the spread of ideas among the masses about the uselessness of space flights. Scientists around the world have no doubts that mankind will either conquer space or die sooner or later.
The risks of living on the same planet are low for thousands of years, but everything changes over a million years. Asteroids have erased all large terrestrial species of the planet more than once, and it would be naive to expect that this will change in the future. Whether our species wants it or not, it will have to make at least the solar system its "backyard" - otherwise a person is unlikely to survive on Earth.
But in space, no tertiary derivatives of atomic energy can be used: either there is no coal and hydrocarbons, or there is no oxygen to burn them. Therefore, while mastering it, humanity faces an important choice: to switch to obtaining energy directly from the derivative of atomic energy (that is, sunlight) or to switch to atomic energy itself.
Scientists and futurists have been trying for decades to understand what the energetic future of humans in space will look like, and so far, two points of view are equally common. According to the first, solar power plants will become the main type of energy. According to this approach, the same Dyson sphere is conceived - a huge array of stations orbiting the Sun and receiving energy from it. The latter they - with the help of lasers or microwave emitters - transmit to the Earth or to the bases of the Moon and Mars.
The second approach to providing energy in space back in the middle of the 20th century was described by Isaac Asimov in the Foundation cycle: NPPs are becoming the main source of energy due to their compactness and low specific material consumption.
So which of them will win in the foreseeable future?
How nuclear energy works in space
Space nuclear reactors are not very similar to the huge power units of Russian ground reactors. The extraterrestrial environment forces them to be made compact: there are not many places under the fairings of rockets that withdraw the payload, and a large mass in space is undesirable, and the lack of fresh water without impurities almost excludes (before the dense colonization of Mars) the use of a water coolant.
When the United States faced these problems at the beginning of the space era, they tried to get around them at the cost of a minimal design change: they used turbines with a different working fluid (for example, inert gas instead of water vapor). It turned out that such systems are cumbersome (the gas has a low heat capacity, which is why the core should become too large), moreover, they are prone to breakdowns.
Because of this, the United States limited itself to simpler radioisotope generators. In them, an actively fissile isotope (such as plutonium-238) emits particles that heat the semiconductor, and in this case, a current begins to flow from heating. Such "nuclear batteries" power all long-range space probes - from Voyagers to Cassini - and even Mars rovers. But we must understand that their capabilities are hundreds of watts, at most kilowatts, so they are not suitable for solving really large-scale issues.
But these limitations are not only disadvantages, but also advantages. Unless, of course, you design nuclear reactors differently. In the USA in 1965 (SNAP-10A), and in the USSR in 1970 (Buk), reactors were launched into space, in which, instead of a turbine, the heat from the decay of atoms was converted into electricity by thermionic converters. The essence of their work is quite simple: many metals, when heated to hundreds of degrees, begin to emit electrons.
Both SNAP-10A and Buk used extremely highly enriched uranium (practically weapons-grade), since space requires weight perfection, and fuel with conventional enrichment - up to a few percent, like in terrestrial reactors - will be too heavy there. In both cases, the heat from the rods with fuel was removed by a mixture of sodium and potassium. On Earth, such a mixture can be dangerous: in the air, it itself begins to burn.
There is nothing to burn in space, but sodium and potassium hardly slow down neutrons, creating an opportunity for very compact reactors. Plus, they heat up to temperatures when the operation of thermionic converters becomes effective - they are also extremely compact and without moving parts, like those of turbines of land-based nuclear power plants. The Americans launched only one such reactor, but he was unlucky: the satellite where he worked was out of order. The USSR launched 31 Buk-type nuclear reactors into space and successfully used them until the end of its history.
Today, promising projects of "space" reactors use such exotic coolants as the lightest metal - lithium. But the thermionic converter remains their key component, and its efficiency has increased from 3% for Buk to 10% for the best promising projects.
Now that we have figured out in general terms how a reactor works in space, it's time to understand how important it is in space exploration.
Base on the Moon: Solar Panels or Reactor?
Today, the issues of creating bases on other bodies of the solar system are actively discussed only in the West, since the Russian space industry has no clear plans of this kind (or does not inform the public about them).
More precisely, since 2007, plans have been circulating in the information space to build a Russian base on the Moon (the first scheduled date for its creation expired in 2015, and in 2020 Dmitry Rogozin announced the possibility of creating such a base together with China). But the fact is that it is extremely difficult to find out anything definite about these plans. After all, Roscosmos does not even have rockets suitable for flying to the Moon (China, by the way, does not), and the state corporation avoids giving specific dates for their creation. While in the United States are now building two similar rockets (SLS and Starship), and one of them should start flying into space next year. Following Western fashion, these plans are "heliocentric".
And just the United States proposes to build a base on the moon at the "peaks of eternal light" - the lunar mountains near the poles of the earth's satellite, where the sun almost never sets. The idea here is that the Moon has a very weak angle of inclination of the axis of rotation, so in local "winter" its North Pole does not move so much "away" from the Sun, and there are simply no winter dips in illumination as on Earth.
A few years ago, the Japanese space agency proposed an even more ambitious plan for the "energy colonization" of Selena, in which its equator should be surrounded by a continuous ribbon of solar panels. When it is night on one side of the moon, energy will flow through the cable (superconducting and running under the array of solar panels) from the day side, and vice versa.
Both plans sound logical until we start getting into the nuances. The research base will need energy all the time. However, NASA's Lunar Reconnaissance Orbiter, a decade ago, discovered that there are simply no circumpolar peaks on the moon that are truly eternally illuminated by the sun. There are points where 94% of the lunar year is light, but there are also eclipses when the Earth passes between the Moon and the Sun or shadows from neighboring mountains block the visibility of the sun. Then these eclipses last up to 43 hours.
Consequently, the lunar base will need a set of solar panels plus a set of lithium batteries that can store the stored energy for a couple of days of operation of the base. In general, this is quite possible - especially as long as the base remains small and with a small number of personnel. The larger it is, the more difficult it is to implement.
But there is also a more serious obstacle to the "peaks of eternal light" scenario. As the proverb, based on a poem by Leo Tolstoy, correctly notes, "it was smooth on paper, but they forgot about the ravines, and walk on them." 94% of the length of the year is illuminated only by the mountains of the ring surrounding the Shackleton crater at the south pole of Selene. The total area of such a unique region is ten square kilometers.
However, back in 2005, NASA researchers noted that inside the crater, the walls drop quite abruptly downward, which will make it almost impossible to study it with automatic machines. It is not easy to build a base on the outside: under the eternal sunlight, the surface of the Moon heats up to plus 130. Moreover, the slopes of the “peaks of eternal light” are relatively steep there as well.
Naked Science has already written: the space suits for the Moon that exist today are extremely heavy (more than 100 kilograms in the western version) - so much so that an astronaut who has fallen in them can only rise with great difficulty without assistance. In such conditions, even exploring such places is difficult.
How are we going to install solar panels on these steep slopes? How do we run long cables from there to the base? How can we ensure the reliability of their operation if the upper layer of the lunar regolith is eternally heated above the boiling point of water in terrestrial conditions?
Moreover, it must be understood that from the lunar landing site to the base itself, in this case, the path cannot be close: the first two astronauts almost died due to their attempt to land on rough terrain. That is, the landing should be planned in an area quite far from the edge of the crater, otherwise those trying to land on the lunar may not survive.
But that's not all. In 2014, a group of researchers from the Energia Corporation published a special report on the most efficient energy supply to the lunar base. An interesting thing was discovered: to use a purely research base for ten people, it takes up to 100 kilowatts of constant electrical power.
This, in general, is not less than 250 square meters of even the most efficient solar cells - based on gallium arsenide (with an efficiency of 40%). The minimum capacity of storage batteries for the period of eclipses should then be expressed in megawatt-hours. Even with one megawatt-hour, the weight of such a system will be at least five tons.
According to the calculations of the authors of the work, "solar electricity" at the "peaks of eternal light" on the Moon will cost $ 13.25 per kilowatt-hour, and atomic electricity - with a reactor power of 600 kilowatts - $ 12.6 per kilowatt-hour. At the same time, heat from a solar battery will cost 5.0 cents per kilocalorie, and from a nuclear reactor - 1.5 cents. It would seem, why is there warmth in Selene, especially at the peak of eternal light, where the problem is rather how to get rid of this heat?
However, the need for heat is there - and it is very great. First, placing your base in an eternal light is not a good idea. Solar radiation can overheat its outer walls - you need protection with a decent layer of soil. Finally, in the event of a solar storm, reaching the surface at the "peaks of eternal light" is a bad idea, because the level of protons from the Sun will be too high.
In contrast, when placing the base in the shade, radiation will not be a major problem, but some heating will be required. In addition, heat is needed to melt water ice, which is in Shackleton Crater: it can be used to obtain oxygen for breathing and hydrogen for rocket fuel components.
It turns out that with an energy consumption of more than 500 kilowatts, the lunar base has practically no alternative to use an atomic reactor. As the authors of the work note, a base with more than ten employees and obtaining resources from local ice will clearly require hundreds of kilowatts, so a nuclear power plant here looks like the most logical solution.
The situation is even worse if the base is placed not near the Shackleton crater, but at an arbitrary point on the lunar surface, without such unique conditions. The cost of energy storage for a moonlit night and eclipses would then make even the best solar panels a more expensive source of energy than a nuclear reactor.
We are talking about space, so the spending itself is not so important here. Much more serious is something else: the mass and dimensions of the goods that will need to be delivered to the base. A nuclear reactor with a thermionic conversion of heat into electricity is compact: it can be placed under the fairing of even a small rocket.
Solar panels with a capacity of hundreds and even more so a thousand kilowatts cannot boast of this. It will not be possible to organize their production on site, from lunar resources, without a large plant, so that the lunar nuclear power plant, with serious development of the satellite, simply has no alternative.
By the way, the thesis “no one will dare to take a nuclear reactor into space” is not an obstacle here either. First, the reactor, before launch, does not contain complex isotopes that are produced during its life cycle. Secondly, more radiation hazardous substances regularly fly into space: for example, Russian plutonium-238 aboard the Curiosity or American plutonium-238 aboard the Perseverance. The reasons why there are no protests are obvious: in a typical accident, the contamination of the spray area would be too small to affect anything.
Exploration of Mars and the rest of the solar system
On the fourth planet of our system, oddly enough, solar energy has more prospects than on Earth.Yes, there are 590 watts of solar energy per square meter of Martian energy, and 1000 watts on Earth. But on our planet, there are many clouds in the atmosphere, which are practically absent on Mars. Therefore, a square meter of a standard stationary silicon solar battery with an efficiency of 25% on Earth will produce from 200 kilowatt-hours per year (in central Russia) to 600 kilowatt-hours (in a cloudless desert).
And on Mars, its output will still have the same 600 kilowatt-hours as in the Sahara. At first glance, nothing better could be wished for: solar energy, working stably 12 hours a day, without unpredictability from clouds and excellent - by the standards of a more illuminated Earth! - production.
But there is a nuance: this is true only for small bases. Mars is far away, it has a gravity of 0.38 on Earth, and it is possible to return from there for reasonable money only by getting fuel for the return flight in place. Elon Musk plans to do this using carbon dioxide from the Martian atmosphere and hydrogen from Martian water (there is quite a lot of it in the form of ice). But this requires electrolysis of water and noticeable heat costs to maintain the temperature required for the Sabatier reaction.
Solar electricity is available on Mars only during the day. It is too difficult for him to drag batteries from Earth: a huge mass will be required. That is, automated mini-factories for the production of rocket fuel on the way back can either be powered by solar panels (and produce fuel twice as slow), or take energy from mini-nuclear power plants - and do it twice as fast. In the first case, a mini-plant must be created twice as efficiently: that is, and twice as heavy as all other things being equal. But the question of mass during the exploration of Mars will become much more acute than during the exploration of the Moon.
But it's not just about the fuel. Margarita Marinova, who published a scientific paper on the most effective way to terraform the Red Planet, works for Musk as a senior engineer for Mars exploration. It is not so complicated: it is necessary to obtain SF6 gas and other "super-greenhouse" gases from local rocks, which retain infrared radiation tens of thousands of times more efficiently than carbon dioxide. According to the calculations of the researchers, the fourth planet needs to be heated by only 4 ° C so that the carbon dioxide polar caps there melt, the atmosphere becomes much denser, and the average planetary temperature rises to the level of the Earth (plus 15 ° C).
At this temperature, a hydrological cycle will arise there (because the ice will melt), and plants will be able to convert carbon dioxide into oxygen. Fortunately, as we have already written, terrestrial plants photosynthesize in laboratory conditions similar to the Martian atmosphere. Considering that there is as much land mass on Mars as there is on Earth, those who will terraform it will provide their descendants with a second inhabited planet, and “not far from home”.
Terraforming is achievable even earlier for certain parts of Mars - such as the Marinerian valleys stretching for thousands of kilometers: by isolating part of the valley with airgel domes, it is possible to raise the concentration of SF6 gas in them to such values that at the bottom of the valley there will be quite terrestrial temperatures after a short period of time.
But the production of any supergreenhouse gas requires chemical reactions to absorb heat - and a decent cost of electricity. In other words, the terraforming of the fourth planet can proceed either due to solar energy - but twice as slow - or due to atomic energy (and twice as fast).
At this point, it is worth mentioning that in a recent interview, Musk bluntly noted: in his space projects, he is not going to limit himself in energy sources. With a high probability, it was about atomic energy - after all, with it, both the production of rocket fuel and the transformation of Mars into a second Earth will go incomparably faster.
This is even more true for the rest of the solar system. Ceres has a whole subsurface ocean, a lot of ice, and the fuel costs for traveling to it are about the same as for a flight to the Moon.Exploratory drilling is quite possible there in order to understand: is there life in the local ocean?
But the amount of solar energy there is several times less than on Mars, and no one canceled the night. Therefore, both there and in regions farther from the Sun there is no reasonable alternative to atomic energy. Solar panels will remain in demand for small bases or research probes. But where a base for several people is required or fuel production for return, not to mention real colonization, nuclear power plants will, sooner or later, inevitably prevail.