Nuclear bomb explosion in space, plutonium probes... it's happened

By Yaël Nazé, FNRS Astronomer at the Institute of Astrophysics and Geophysics, University of Liège

Recently, a chill has spread through international political and military circles. Russia and the United States are accusing each other of wanting to deploy nuclear weapons in space, or even having already done so. Outcries and protests are multiplying, and discussions about the militarization of space are intensifying... People seem to forget that the space domain is quite familiar with nuclear technology, in several forms and for a long time now.


"Nuclear" is a rather vague term as it covers several very different technical realities. Let's start with the first use: the heater. Indeed, a block of radioactive material heats up as the material decays, and this heat can be used to keep the inside of the space probe warm.


This technique is often found in rovers. For instance, the Lunokhod program used 24 lbs (11 kg) of polonium to counter the lunar cold, while Spirit and Opportunity each carried about 0.7 oz (20 grams) of plutonium to combat the Martian chill, and the Yutu used the same isotope on the Moon.

In a slightly more complex vein, we find the RTG (radioisotope thermoelectric generator). This time, the heat released by the radioactive material is used to generate electricity (using thermocouples, an assembly of two metals reacting differently to temperature, generating an electric potential between their ends).

Plutonium in probes

Often, plutonium is used. With its long half-life of 88 years, its productivity decreases by only 0.8% each year. It's an expensive compound, but perfect for long-term missions! This type of generator is mainly used for distant missions—when solar panels are difficult to use, beyond the asteroid belt.

For example, the Voyager probes, which flew by the giant planets, carried 30 lbs (13.5 kg) of plutonium, the large Cassini probe, which explored Saturn, 73 lbs (33 kg), and the small New Horizons probe, which explores the Kuiper Belt, 24 lbs (11 kg).


The only downside of this type of machine: efficiency, limited to a few percent! Moreover, the residual heat must be dissipated to avoid internal overheating of the satellite. Furthermore, decaying material emits ionizing particles, which onboard electronics do not handle well: the radioactive source must be shielded (and not placed at the center of the satellite) to avoid problems. Finally, the electrical production of these generators is quite constant. This seems advantageous, but for missions involving flybys, the demand is high during those and nearly null outside... Not easy to regulate.

Satellites powered by nuclear energy

The second "nuclear" option: the reactor (only available as fission reactors for now). This is the classic reactor of our nuclear power plants... and such reactors have indeed flown in space, several times. Notably for two American tests (SNAP-10A in 1965 and DUFF in 2012) and especially with the series of Russian RORSAT missions (Radar Ocean Reconnaissance SATellite)—about thirty between 1967 and 1988.

These military satellites flew very low to obtain very precise data, but consequently, they experienced significant atmospheric drag. Energy was needed to maintain orbit and not crash to the ground. Solar panels could have worked but, much like sailboat sails, would have "caught the wind" and increased atmospheric drag. Additionally, these satellites experienced eclipses half the time, and a nuclear reactor avoided regular power interruptions.

The main downside of these nuclear engines: fallout. If they fall to Earth, we end up with nuclear pollution. Some may imagine the risk is negligible, a whim advanced by ill-informed detractors, like Elisabeth Teissier in August 1999 during a famous eclipse coinciding with the passing of the Cassini probe. Not really! Several cases of space nuclear pollution are known.

The first is due to the RORSATs. At the end of their mission, their reactor was ejected into a graveyard orbit to ensure it would not re-enter the Earth (or not quickly). But during the ejection of the enriched uranium cores, some of the sodium-potassium cooling mixture was also released: thus, Earth's orbit contains radioactive droplets, up to 2 inches (5 cm) in diameter. Not exactly something operational satellites would want to encounter.

Things worsen when operations are uncontrolled, obviously. For example, the RORSAT satellite called Cosmos 1402 failed to eject its reactor correctly at the end of its mission, and it fell into the Atlantic in 1983. The RORSAT satellite called Cosmos 954 accidentally fell to Earth in 1978, polluting 47,877 square miles (124,000 km²) of the Canadian Arctic.


Thanks to international conventions, the Soviet Union was clearly legally responsible. However, the two countries negotiated an agreement: the Soviets eventually paid three million dollars, half the sum claimed by Canada for cleaning the polluted area.

Another critical case: launch. Another offspring of the RORSAT series polluted the Pacific near Japan, the first Lunokhod dropped 24 lbs (11 kg) of polonium onto its Soviet homeland, while the Russian Mars 96 probe crashed in the Andes between Chile and Bolivia with 7 oz (200 grams) of plutonium. In this latter case, neither Chile nor the Russians reacted, and nothing was evacuated. Too bad for the Andean population, llamas, and condors!

A nuclear bomb in space

Last possibility: nuclear weapons. Certainly, there has never been a nuclear bomb in space and never any nuclear explosion, right? The Space Treaty explicitly forbids it! Granted, but it was signed starting in 1967—long after the first... space nuclear tests. Yes, for their anti-satellite arsenal, militaries have indeed considered the nuclear weapon.


On July 9, 1962, an American nuclear bomb exploded in space. Operation code name: "Starfish Prime". Everything went wonderfully, or so it was believed until closer inspection.

First trouble: an electromagnetic pulse occurs at the moment of the explosion, with a nasty tendency to render all electrical devices in the vicinity inoperative. Both on Earth, beneath the satellite, and in space, for all satellites not hidden by the Earth. Particularly, the spy satellites supposed to observe the explosion from afar perished instantly, as did others just passing by—it's problematic.

Second trouble: the explosion generates a slew of high-energy particles, slowly spreading around the Earth, creating a new radiation belt. A number of satellites cannot avoid crossing it and end up with some electronic issues.


Thus, Telstar and Ariel-1, along with a few others that had nothing to do with it, perished prematurely. The Soviets observed the same thing: the nuclear weapon is frankly impractical in space. Easier to sign a treaty banning it, evidently...

And now? Well, things are clear: we know the advantages and drawbacks of various nuclear uses in space. After that, it's up to each party to make their decisions... but no one should act surprised, space nuclear technology is an old acquaintance!