In a move that could see a new generation of deep-space CubeSats, NASA has greenlighted a project by the Rochester Institute of Technology to develop a nuclear power source that is a tenth the size of those in current use for planetary missions.
Most satellites in service today are powered by solar panels that turn sunlight into electricity by absorbing photons to create a potential imbalance in the panel cells' materials to generate an electrical current. These panels do their job very well, but in deep space beyond the orbit of Mars or in harsh conditions, such as in the Martian dust storms or the long nights on the Moon, sunlight simply cannot produce the needed energy.
As an alternative, many deep-space craft carry Multi-Mission Radioisotope Thermal Generators (MMRTGs), which use a temperature gradient to generate electricity. In other words, the radioisotope produces heat and thermocouples convert the heat directly into electricity. It's a principle that's familiar to engineers and is widely used on Earth for things like kerosene-powered radios and camp stoves that can also charge mobile devices.
The problem with MMRTGs is that they are relatively bulky. The pair used on NASA's Perseverance Mars rover, for example, are each 25 in (64 cm) in diameter, 26 in (66 cm) long, and weigh in at 99 lb (45 kg). They each contain 10.6 lb (4.8 kg) of plutonium dioxide plugs for fuel to supply heat to the solid-state thermocouples as the radioactive elements decay.
As a result, these MMRTGs are reserved for very large spacecraft, with Perseverance being as large as an SUV. This is because the system used has only so much mass specific power, which is a measure of how many watts of power can be produced per unit of a machine. A family car has a mass specific power of 50 to 100 W/kg, while a fighter plane has about 10,000 W/kg.
By contrast, an MMRTG has a ratio of about 30 W/kg.
By looking at the thermodynamics of the size, weight, and power (SWaP) of a possible RTG, the NASA project hopes to reduce this ratio by an order of magnitude to only 3 W/kg, with a decrease in volume that is equally great.
It does this by using a new principle that is essentially a solar panel working in reverse. When a solar panel absorbs light, part of it is turned into electricity and most of it into heat. The new radioisotope power source works on the idea of the thermoradiative cell, where heat in the form of infrared light strikes a panel with elements made from indium, arsenic, antinomy, and phosphorus in various combinations. This produces a potential difference with a reversed polarity from that found in solar cells.
Long story short, the thermoradiative cell generates electricity from heat and dumps the waste energy in the form of infrared photons. Not only does this work in reverse from a solar panel, but with much greater efficiency. The result is a new ThermoRadiative Generator (TRG)
If this new technology can be made practical, it would mean that future missions to Jupiter and beyond, or to the perpetually shadowed craters of the lunar polar regions, could use spacecraft the size of CubeSats with small generators giving them all the power they need. This means that the concept Flagship Uranus mission, for example, could be accompanied by a small fleet of CubeSats that could aid in exploration by providing more points of view or acting as communication relays with atmospheric probes.
Source: NASA