Power Subsystem

The Power Subsystem generates, stores, controls and distributes electrical power to the other subsystems on board. Most of the power (approximately three-quarters) is required for the communications subsystem. How much power is required on board will vary considerably (from a few hundred to several thousand watts) depending on the mission of the spacecraft and the payload.

To date, only three power systems have proven robust and reliable enough to be able to supply power reliably to a satellite to satisfy both instantaneous and lifetime demands: solar energy systems, chemical energy systems (batteries), and nuclear energy systems. Each of these has particular advantages, disadvantages, and applications.

Solar Power

The majority of satellites produce on-board power by using solar cells (photovoltaic devices) to convert solar radiation into electrical energy. At GEO orbit solar radiation has a considerable power density—approximately 1.45 kW m^–2. Unfortunately, due to the inefficiencies in the photovoltaic process in solar cells only about 15% of this energy can be converted to electrical energy for a solar panel at 90° to the incident radiation—approximately 200 W m^–2.

Large arrays of cells are therefore necessary to produce sufficient energy to power a satellite. Since exposure to radiation gradually destroys the cell, a special protective coating is required on the solar panels. Even with the coating, typically 15% of the cells on a panel would be destroyed after five years and some further 20–40% loss in efficiency results over the operational life of the satellite due to the panels being struck by micro-meteorites. Solar panels are therefore designed with excess cells to ensure that the minimum output power can be provided, even after some cells have been destroyed.

Solar cells can be mounted either on the body of the satellite, or separately on solar panels which are kept oriented toward the Sun to ensure maximum absorption of the incident radiation.

Chemical Power

Of the chemical energy systems available for deployment on satellites, batteries are by far the most common. Almost all communications satellites employ re-chargeable batteries as a back-up secondary power supply to cope with uneven power loads and to provide power when the Sun’s rays cannot reach the solar cells. Batteries are also required to provide power during launch and orbit transfer prior to deployment of the antennas and solar arrays.

Nickel/cadmium, or the more-efficient nickel/hydrogen batteries, are most commonly used because of their high efficiency, although silver/zinc and silver/cadmium batteries are also able to be used. The batteries must be as light as possible and are hermetically sealed to allow them to operate in space.

Nuclear Power

In a nuclear propulsion system, nuclear reactions release energy which is used to heat liquid hydrogen to about 2,430°C (some eight times the temperature of cores in nuclear power plants), which jets out nozzles, producing twice the thrust per mass of propellant than chemical rockets. As an additional advantage, once on station, the nuclear reactor can switch from propulsion system to power source. As a significant disadvantage, nuclear propulsion requires a nuclear rocket used to require weapons-grade uranium. Low-enriched uranium fuels, used in commercial power plants, would be safer, but can become brittle under the high temperatures in the presence of the extremely reactive hydrogen. ¹

The earliest and most widely used nuclear power sources for space missions are radioisotope thermoelectric generators (RTG). For example, NASA’s Galileo mission to Jupiter carried two RTGs that together produced about 570 W of electrical power at launch. Their fuel, plutonium-238 dioxide, is a non-fissile, alpha-emitting isotope with a half-life of 87.7 years. As it decays, it releases heat: one kilogram of pure Pu-238 produces roughly 0.54 kW of thermal power, and the surface of a fuel pellet can reach temperatures above 1050 °C. After 20 years, approximately 85% of the initial thermal output remains. This heat is converted to electricity using thermoelectric couples that exploit the Seebeck effect—the generation of a voltage when two dissimilar materials are maintained at different temperatures. RTGs typically incorporate a shunt regulator to maintain a constant current to the spacecraft.

Alternative concepts have been developed, including the use of Pu-238 General Purpose Heat Source (GPHS) modules in conjunction with alkali-metal thermal-to-electric converters (AMTEC). An AMTEC is a regenerative electrochemical device in which high-pressure sodium ions migrate through a beta-alumina solid electrolyte (BASE), while electrons flow through an external circuit to recombine with the ions on the low-pressure side. The neutral sodium vapor condenses on a radiator and is recycled by capillary action. Demonstrated efficiencies have reached ~19%, with theoretical improvements up to 35–40%. One such system, the Advanced Radioisotope Power System (ARPS), was proposed for missions such as Europa Orbiter, Pluto/Kuiper Express, and Solar Probe. A pair of ARPS units would have delivered ~150 W electric power after 6 years and ~130 W after 14 years at 28 VDC, with a mass of 24 kg.

Safety has always been central to space nuclear power programs. The fuel is used in ceramic form (PuO₂) rather than as pure metal, ensuring that if fractured it breaks into large chunks rather than respirable dust. PuO₂ is also largely insoluble in water, reducing bioavailability. The ceramic bricks are encased in graphite and iridium layers, providing impact resistance and thermal protection during potential launch accidents or re-entry. Any radiation released would be alpha particles, which are easily shielded, but multiple containment barriers are engineered to prevent release under all credible scenarios.

A number of accidents have occurred involving satellites with nuclear power supplies. In the mid-sixties, a US plutonium-powered satellite and a Nimbus weather satellite crashed after a launch failure. In 1978, the Soviet Union’s spy satellite Cosmos 954 crashed over Canada irradiating a large area. In 1996, Russia’s Mars 96 probe, carrying 200g of plutonium, incorrectly fired its engine while in Earth orbit and crashed into the Pacific Ocean.

Regardless of safety issues, nuclear power systems such as RTGs are rarely used for communications satellite since they only generate hundreds of watts at best; a modern geostationary satellite requires kilowatts to tens of kilowatts of electrical power (typically 10–25 kW). Additionally, Pu-238 is scarce, expensive, and politically sensitive.

In recent efforts, the US DoD’s Defense Innovation Unit (DIU) has awarded contracts to Ultra Safe Nuclear and Avalanche Energy to advance nuclear power and propulsion. ²

Ultra Safe Nuclear is developing a football-sized radio-isotope power source called EmberCore. A radio-isotope power source relies on the radioactive decay of Cobalt-60 (with a half-life of five years) to produce power, rather than nuclear fission in which energy is created by splitting nuclei in a controlled chain reaction. EmberCore is proposed to generate ten times more energy than a Uranium-238-based process, yielding over 1 million kilowatt-hours of power. A technology demonstrator is planned to be launched in 2027.

Avalanche Energy makes a lunchbox-sized fusion device called Orbitron, which uses electrostatic fields to trap, around a negatively charged cathode, high-speed ions which collide in fusion reactions, releasing energy. As a propulsion system, Orbitron has the potential to increase speed of orbital transition by 10 kms^-1.

Selection of a Power Subsystem

To date, the most frequently used power system has combined solar photovoltaic cells to provide primary power and batteries to provide secondary power when the satellite is eclipsed. The secondary batteries are fully charged just prior to eclipse. The charging current from the solar cells during irradiation is controlled by a power-conditioning unit—any excess current is dissipated into resistors that then act as heaters to balance the temperature across the platform. Batteries are also required to provide power to critical systems during launch and orbit transfer prior to deployment of the solar arrays. The combination of solar cells and batteries supplies constant electrical power for the operational life of the satellite—something that neither could do separately.

As noted earlier, the main disadvantage of any system that utilises solar power is the increase in mass and volume required on the launch platform to accommodate the solar panels. The limitations of the launch vehicle mean that the solar-powered platforms have a maximum power of approximately 5 kW. Platforms requiring more power, such as radar surveillance satellites, may require the use of nuclear power systems.