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The use of nuclear power in deep space exploration

Pierre Henriquet
Pierre Henriquet
Doctor in Nuclear Physics and Columnist at Polytechnique Insights
Key takeaways
  • Powering space objects can be done in two ways: by finding a source of energy in space, or by taking energy from Earth.
  • The two power sources currently used in space are solar and nuclear.
  • The Radioisotope Thermoelectric Generator (RGT), using radioactive materials, now provides the power for many space probes.
  • But solar power is not always guaranteed, depending on the mission parameters and the environment of the scientific equipment.
  • In the future, nuclear power could be used for space propulsion, in addition to chemical and electric propulsion.

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).

Over 50 years of nuclear power in space and elsewhere

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.

Red­den­ing of a Plu­to­ni­um Oxide pel­let under the effect of the heat by its own radioac­tiv­i­ty. Cred­it: NASA.

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.

Space power: solar or nuclear?

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.

RTG of the Per­se­ver­ance rover (cen­tral cylin­der in the red insert). Around it are heat dis­si­pa­tion fins. Cred­it: NASA.

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.

RTG deposit­ed by the Apol­lo 14 mis­sion to pow­er the con­trol box for sci­en­tif­ic exper­i­ments (in the back­ground). Cred­it: NASA.

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.

Nuclear propulsion

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.

Novem­ber 7, 2005: New Hori­zons RTG instal­la­tion (black) at the Kennedy Space Cen­ter. Cred­it: NASA.

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”.

Artist’s con­cept of the Demon­stra­tion for Rock­et to Agile Cis­lu­nar Oper­a­tions (DRACO) space­craft, which should demon­strate a nuclear ther­mal rock­et engine. Cred­it NASA.

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.

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