Editing Radioisotope heater unit (section)

== Spacecraft use ==
[[File:RHU parts.jpg|right|thumb|'''RHU''' Photo of a disassembled RHU. RHUs use Pu-238 to generate about 1 watt of heat each.]] Most lunar and Martian surface probes use RHUs for heat, including many probes that use solar panels rather than RTGs to generate electricity. Examples include the [[ALSEP#Apollo_11_.28EASEP.29|seismometer deployed]] on the Moon by [[Apollo 11]] in 1969, which contained 1.2&nbsp;ounces (34 grams) of plutonium-238; [[Mars Pathfinder]]; and the [[Mars Exploration Rovers]] ''[[Spirit (rover)|Spirit]]'' and ''[[Opportunity (rover)|Opportunity]]''.<ref name="NASA Thermal Systems"/> RHUs are especially useful on the Moon because of its lengthy and cold two-week night.

Virtually every [[outer space|deep space]] mission beyond [[Mars]] uses both RHUs and RTGs. Solar [[insolation]] decreases with the square of the distance from the [[Sun]], so additional heat is needed to keep spacecraft components at nominal [[operating temperature]]. Some of this heat is produced electrically because it is easier to control, but electrical heaters are far less efficient than a RHU because RTGs convert only a few percent of their heat to electricity and reject the rest to space.

The ''[[Cassini–Huygens]]'' spacecraft sent to [[Saturn]] contained eighty-two of these units (in addition to three main RTGs for power generation). The associated ''[[Huygens (probe)|Huygens]]'' probe contained thirty-five.

[[ISRO]] included two radioisotope heater units developed by India's [[Department of Atomic Energy]] (DAE) in the propulsion module of [[Chandrayaan-3]] on a trial basis which worked flawlessly.<ref>{{Cite news |last=Laxman |first=Srinivas |date=2023-12-29 |title=Nuclear sector set to power Indian space missions: Isro chief |work=The Times of India |url=https://timesofindia.indiatimes.com/home/science/nuclear-sector-set-to-power-indian-space-missions-isro-chief/articleshow/106359396.cms?from=mdr |access-date=2023-12-29 |issn=0971-8257}}</ref> 

===Isotope===
Radioisotope heater units for NASA missions have used plutonium-238<ref name="NASA Thermal Systems">NASA, [https://rps.nasa.gov/power-and-thermal-systems/thermal-systems/light-weight-radioisotope-heater-unit/ Thermal Systems]. Retrieved 23 June 2022.</ref> as the isotope for heat sources, since the radioactive half-life of 87.7 years means that the decay of the isotope will not limit the mission lifetime. The isotope produces 0.57 watts of thermal power per gram of <sup>238</sup>Pu.<ref>{{cite web|title=Assessment of Plutonium-238 production alternatives|url=https://www.energy.gov/sites/prod/files/NEGTN0NEAC_PU-238_042108.pdf|last=Miotla|first=Dennis|date=April 21, 2008|access-date=September 21, 2020|website=www.energy.gov|page=3}}</ref>

The ESA's [[ExoMars]] [[Rosalind Franklin rover]] will use [[americium-241]] RHUs. The half-life of Am-241 is five times that of <sup>238</sup>Pu, with a concomitant reduction in power-density.

Soviet missions have used other isotopes, such as the [[polonium-210]] heat source used in the [[Lunokhod]] lunar rovers.<ref>Blair, Sean (March 14, 2011)." [https://eandt.theiet.org/content/articles/2011/03/rovers-learning-from-lunokhod/ Rovers learning from Lunokhod]", ''E&T News''. Retrieved 23 June 2022.</ref><ref name=Wang>{{Cite journal|doi=10.1016/j.rser.2019.109572 |hdl=1721.1/129634 |hdl-access=free |title=Critical design features of thermal-based radioisotope generators: A review of the power solution for polar regions and space |year=2020 |last1=Wang |first1=Xiawa |last2=Liang |first2=Renrong |last3=Fisher |first3=Peter |last4=Chan |first4=Walker |last5=Xu |first5=Jun |journal=Renewable and Sustainable Energy Reviews |volume=119 |page=109572 |bibcode=2020RSERv.11909572W |s2cid=209776036 }} </ref> With a half-life of 138.376 days, polonium-210 produces more thermal power per unit mass, but is suitable only for shorter duration missions. [[Strontium-90]] has also been proposed.<ref name=Wang />

===Comparison of RHU with RTG===
While both RHUs and [[Radioisotope Thermoelectric Generator]]s (RTGs) use the decay heat of a radioactive isotope, RHUs are generally much smaller as a result of omitting the [[thermocouple]]s and heat sinks/radiators required to generate electricity from heat. Both RHUs and RTGs feature rugged, heat-resistant casings to safely contain the radioisotope in the event of a launch or re-entry vehicle failure. The total mass of a single one-watt RHU (including shielding) is about 40&nbsp;grams. Similar schemes, such as [[Thermionic converter|thermionic generators]], have also been used.