The use of nuclear power in deep space exploration

There are few envir­on­ments as hos­tile as space, and get­ting humans to sur­vive there is (and always has been) a chal­lenge. But wheth­er explor­a­tion flights are manned or unmanned, one of the major prob­lems is always how to power the space­craft or probe. Up there, the word vacu­um has nev­er been more aptly named, and to carry out space activ­it­ies – power­ing, pro­pelling or com­mu­nic­at­ing – you either have to find a source of energy loc­ally or take it with you from Earth.

These two pos­sib­il­it­ies lead to the two sources of energy cur­rently used in space: sol­ar energy, which enables part of the sun’s light to be con­ver­ted into elec­tri­city on site using photo­vol­ta­ic pan­els; and the use of nuc­le­ar reac­tions to power and, in the future, pro­pel space­craft whose oper­at­ing con­di­tions do not allow sol­ar energy to be used (or not properly).

There are also places on Earth that are so far from civil­isa­tion, and with such extreme envir­on­ments, that it is very dif­fi­cult to provide a stable and reli­able source of energy. Fair­way Rock, for example, is a tiny island of less than 1 km² loc­ated in the icy Ber­ing Strait off the coast of Alaska, between the United States and Rus­sia. It was on this tiny rock, near the Arc­tic Circle, that the US Navy first installed a Radioiso­tope Ther­mo­elec­tric Gen­er­at­or (RTG) in 1966 to provide elec­tri­city for its “envir­on­ment­al mon­it­or­ing” facilities.

Today, this device is the key to the power sup­ply of many space explor­a­tion probes, the most fam­ous of which are the Pion­eer and Voy­ager probes (which are cur­rently leav­ing the Sol­ar Sys­tem) or the Cas­sini-Huy­gens probe which explored Sat­urn and its moons between 2004 and 2017. Although RTGs use radio­act­ive mater­i­als such as Plutoni­um-238 or Amer­i­ci­um-241, they are not con­sidered “nuc­le­ar react­ors” in the sense that they do not cause sus­tained nuc­le­ar fis­sion reac­tions, as in con­ven­tion­al nuc­le­ar power plants. Instead, the nat­ur­al heat­ing of a block of radio­act­ive mater­i­al is used. And it is this heat, induced by the radio­activ­ity, that is con­ver­ted into elec­tri­city with the help of thermocouples.

Although the effi­ciency of this con­ver­sion is quite low (around 10%), it has the advant­age of pro­du­cing elec­tri­city in a stable and con­tinu­ous man­ner for dec­ades. Radio­act­ive mater­i­als are char­ac­ter­ised by their ‘half-life’, which is the time after which their radio­activ­ity (and there­fore their energy out­put) has halved. For Plutoni­um-238 (the most widely used in today’s RTGs), its half-life is ~88 years, which ensures a suf­fi­cient sup­ply of energy for space mis­sions whose dur­a­tions typ­ic­ally extend over one or two decades.

But the length of time the sys­tem provides elec­tri­city is not the only advant­age of this tech­no­logy. The Sun emits light con­tinu­ously, allow­ing a mis­sion to con­tin­ue vir­tu­ally forever as long as it is illu­min­ated by light. But this is pre­cisely where the prob­lem lies. Depend­ing on the para­met­ers of the mis­sion and the envir­on­ment of the sci­entif­ic equip­ment, the sup­ply of sol­ar energy is not always guaranteed.

The rovers used to explore the sur­face of Mars have to deal with the loc­al weath­er conditions.

On Mars, for example, the rovers used to explore the sur­face have to deal with loc­al weath­er con­di­tions. The Red Plan­et has an atmo­sphere which, although much less dense than Earth’s (the pres­sure is ~200 times lower), does not pre­vent the some­times very fast winds from rais­ing the fine ochre dust that scat­ters its sur­face. It will then rede­pos­it itself on the sol­ar pan­els of human robots, some­times almost com­pletely block­ing out the avail­able sunlight.

In Decem­ber 2022, the Insight mis­sion to study the intern­al struc­ture of Mars ended after four years of activ­ity, its sol­ar pan­els almost com­pletely covered in dust. This is why the Curi­os­ity and Per­sever­ance rovers, sent to Mars in 2012 and 2021 respect­ively, are equipped with an RTG. In addi­tion to pro­du­cing elec­tri­city, it also allows cer­tain sens­it­ive parts of the elec­tron­ics to be heated, which do not cope well with the aver­age Mar­tian tem­per­at­ure of around ‑60°C.

But even in envir­on­ments with no atmo­sphere (and there­fore no dust), RTGs may be neces­sary. Dur­ing the Apollo mis­sions to the Moon, RTGs were used to sup­ply power to sci­entif­ic instru­ments placed near the land­ing mod­ules. The reas­on: some of this equip­ment was inten­ded to be used con­tinu­ously for more than 10 years. But a day on the Moon is much longer than on Earth. A day (between sun­rise and sun­set) lasts almost two Earth weeks. The lun­ar night lasts just as long. Sol­ar devices would not have been able to sup­ply elec­tri­city to the instru­ments for a fort­night every month, which is why RTGs are useful.

Finally, even in space, where the Sun shines con­tinu­ously, it is neces­sary to think care­fully about the type of tech­no­logy that will be used to provide elec­tri­city to the explor­a­tion probes. Like any source of light, the fur­ther away we are from the Sun, the less light it gives us. And this decrease in lumin­os­ity is very rap­id. It decreases as the square of the dis­tance. If we move 3 times fur­ther away, we receive 9 times less light. If you move 10 times fur­ther away, you receive 10² = 100 times less light.

In prac­tice, for all mis­sions that go bey­ond the orbit of Jupiter, the amount of light received is so low that the use of sol­ar pan­els is no longer effect­ive. Once again, it is RTG tech­no­logy that over­comes this prob­lem, as was the case, for example, for the Cas­sini-Huy­gens mis­sion to explore Sat­urn and its moons or the fam­ous New Hori­zons mis­sion which, for the first time in the his­tory of astro­naut­ics, obtained resolved images of the sur­face of Pluto after 10 years of travel in the Sol­ar System.

Anoth­er area of space is likely to see the arrival of new nuc­le­ar-based tech­no­lo­gies in the com­ing dec­ade: space propul­sion. In this field, the prin­ciple is always the same: pro­ject as much mat­ter as pos­sible as quickly as pos­sible in one dir­ec­tion to gen­er­ate a force that pro­pels the ves­sel in the oppos­ite dir­ec­tion. This is the fam­ous prin­ciple of Action-Reaction.

At present, this prin­ciple can be roughly divided into two tech­no­lo­gies: chem­ic­al propul­sion and elec­tric (ion­ic) propul­sion, each with advant­ages and dis­ad­vant­ages. Con­ven­tion­al chem­ic­al propul­sion uses pro­pel­lants whose com­bus­tion pro­duces large quant­it­ies of hot gases, which are respons­ible for a very high thrust, but which is very lim­ited in time (around ten minutes at the most). In addi­tion, it requires huge quant­it­ies of fuel to be car­ried on board, which in turn weighs down the space­craft, requir­ing more fuel to be used… to pro­pel this fuel.

In con­trast, ion­ic propul­sion con­sists of accel­er­at­ing an ion­ised gas between elec­tric­ally charged grids. The speed at which the particles leave the space no longer depends on a par­tic­u­lar chem­ic­al reac­tion but on the intens­ity of the elec­tric field that is cre­ated. The speed of the particles is poten­tially much high­er, which allows the amount of fuel used to be drastic­ally reduced. An ion engine uses about 100 grams of fuel per day, where­as the Ariane rock­et con­sumes sev­er­al hun­dred tonnes of pro­pel­lant per second. This ion drive is more effi­cient and can be used con­tinu­ously for sev­er­al weeks or months at a time.

The dis­ad­vant­age is that it gen­er­ates extremely low thrust (a few new­tons at the most), which imposes major con­straints on the types of air­craft on which it can be used. Nuc­le­ar thermal propul­sion, on the oth­er hand, involves using a pro­pel­lant flu­id (hydro­gen, for example) and heat­ing it by passing it through the core of a nuc­le­ar react­or. It can then be expelled at high speed, in large quant­it­ies, to pro­pel the ves­sel. In terms of effi­ciency, nuc­le­ar propul­sion is between chem­ic­al and ion propul­sion, the­or­et­ic­ally allow­ing for very high gas ejec­tion rates, while sup­port­ing very long thrust times.

In 2021, NASA selec­ted three busi­ness con­glom­er­ates to carry out concept stud­ies for nuc­le­ar thermal propul­sion react­ors (BWX Technologies/Lockheed Mar­tin, Gen­er­al Atom­ics Elec­tro­mag­net­ic Systems/X‑energy/Aerojet Rock­et­dyne and Ultra Safe Nuc­le­ar Technologies/Blue Origin/General Elec­tric Hita­chi Nuc­le­ar Energy/General Elec­tric Research/Framatome/Materion). The work is ongo­ing. At the end of Janu­ary 2023, Bill Nel­son, the cur­rent NASA Admin­is­trat­or, announced a col­lab­or­a­tion with the Defense Advanced Research Pro­jects Agency (DARPA) to “devel­op and demon­strate advanced nuc­le­ar thermal propul­sion tech­no­logy as early as 2027”.

Europe, mean­while, pro­duced a report in 2013 called « Mega­hit » that pro­posed a roadmap for the devel­op­ment of space-based nuc­le­ar sys­tems. How­ever, it has not led to any pub­lic­a­tion since. One of the main prob­lems in the devel­op­ment of these nuc­le­ar-based space tech­no­lo­gies is of course the secur­ity and soci­et­al aspect. It is rel­at­ively easy today to build RTGs in which the small radio­act­ive pucks are pro­tec­ted by suc­cess­ive lay­ers that ensure good heat trans­fer while at the same time provid­ing the best pos­sible res­ist­ance to events such as the destruc­tion of the launch vehicle at lift-off or uncon­trolled atmo­spher­ic re-entry. This happened in April 1970 when the Apollo 13 mis­sion returned to Earth in an emer­gency fol­low­ing the explo­sion of part of the space­craft. After with­stand­ing the heat of re-entry without break­ing, the RTG respons­ible for power­ing the instru­ments on the Moon plunged into the Pacific Ocean over the 10 km deep Tonga Trough.

On the oth­er hand, it is dif­fi­cult, giv­en the state of devel­op­ment of future nuc­le­ar-powered engines, to estab­lish how such much lar­ger and more com­plex craft could provide the same level of safety. How­ever, this is a cru­cial step that must be taken if this kind of new space propul­sion tech­no­logy is to be developed and Mars is to be reached in much less time than it takes today.