Curiosity’s nuclear battery

The Curi­osity rover that is the main part of the Mars Science Labor­atory mission is very dif­ferent from its pre­de­cessors Sojourner and the twin rovers Spirit & Oppor­tunity.

L-R: Spirit/Oppor­tunity, Sojourner and Curi­osity.

L-R: The wheels of Sojourner, Spirit/Oppor­tunity and Curi­osity.

Curi­osity is nearly twice as long as Spirit/Oppor­tunity and has more than five times the mass; at 2.1 metres in height it is taller than most of the people that built it.

For me, the most inter­esting dif­fer­ence between Curi­osity and the other Mars rovers is its power source. Both Sojourner and Spirit/Oppor­tunity were powered by solar cells but Curi­osity is powered by a radioiso­tope ther­mo­elec­tric gen­er­ator (RTG), in par­tic­ular the Multi-Mission Radioiso­tope Ther­mo­elec­tric Gen­er­ator (MMRTG) built by Pratt & Whitney’s Rock­et­dyne division.

Curi­osity’s RTG is the large unit attached to the rover’s rear.

The main problem with using solar cells for power is that the cells only work during daylight hours and don’t function well at high lat­it­udes where there is less sunlight; Spirit/Oppor­tunity’s cells only worked at full strength for about four hours per day, pro­du­cing about 900 watt hours (about 3.2 mega­joules) per day at best. Mars is covered in fine dust and dust covering solar panels was a problem for the Spirit and Oppor­tunity rovers, though this dust was occa­sion­ally blown away by high winds.

Spirit’s solar panels before and after a “cleaning event”.

RTGs work via the Seebeck effect, where a dif­fer­ence in tem­per­ature between between the two junc­tions of a ther­mo­couple cause an electric current to be produced. The heat source in an RTG is the decay of a radio­active isotope; in the case of most RTGs this isotope is plutonium-238 in the form of plutonium dioxide. Pu-238 is a nearly pure alpha emitter and there­fore requires only minimal shielding.

A pellet of ²³⁸PuO₂ glows red hot from internal radio­active decay.

The MMRTG uses 32 marshmallow-sized plutonium pellets and will ini­tially produce about 125 watts of elec­trical power (from 2000 watts of thermal power), but this will drop off over time as the plutonium decays. The MMRTG will con­sist­ently produce about 2500 watt hours of elec­tri­city per day compared with Spirit/Oppor­tunity’s average of 600 Wh and this will enable Curi­osity to operate in all seasons and at all times of day.

Curiosity’s MMRTG before installation.