Chapter 11. Typical Onboard Systems     CONTINUED

Electrical Power Supply and Distribution Subsystems

Future interplanetary missions of the Prometheus Project, including JIMO, the Jupiter Icy Moons Orbiter, are planning to use nuclear fission reactors to produce hundreds of times more electrical power than today's spacecraft, to supply ion propulsion engines, powerful radar sensors and other components.

Photovoltaics

SOLAR CELL

As the term suggests, photovoltaic materials have the ability to convert light directly to electricity. An energy conversion efficiency of about 29% was achieved in July 2000, and gains of a few more percent may be possible over the next decades. Crystalline silicon and gallium arsenide are typical choices of materials for deep-space applications. Gallium arsenide crystals are grown especially for photovoltaic use, but silicon crystals are available in less-expensive standard ingots which are produced mainly for consumption in the microelectronics industry.

When exposed to direct sunlight at 1 AU, a current of about one Ampere at 0.25 volt can be produced by a 6 cm diameter crystalline silicon solar cell. Gallium arsenide is notably tougher and more efficient. Amorphous silicon, less expensive and less efficient than crystalline, is employed in ultra-thin layers for residential and commercial PV applications.

To manufacture spacecraft-grade solar cells, crystalline ingots are grown and then sliced into wafer-thin discs, and metallic conductors are deposited onto each surface: typically a thin grid on the sun-facing side and a flat sheet on the other. Spacecraft solar panels are constructed of these cells trimmed into appropriate shapes and cemented onto a substrate, sometimes with protective glass covers. Electrical connections are made in series-parallel to determine total output voltage. The resulting assemblies are called solar panels, PV panels, or solar arrays. The cement and the substrate must be thermally conductive, because in flight the cells absorb infrared energy and can reach high temperatures, though they are more efficient when kept to lower temperatures.

MGS SOLAR ARRAYS

Farther than about the orbit of Mars, the weaker sunlight available to power a spacecraft would require panels larger than practicable because of the increased launch mass and the difficulty in supporting, deploying, and articulating them. Magellan and Mars Observer were designed to use solar power, as was Deep Space 1, Mars Global Surveyor, Mars Pathfinder, and Lunar Prospector. Topex/Poseidon, the Hubble Space Telescope, and most other Earth-orbiters use solar power.

Solar panels typically have to be articulated to remain at optimum sun point, though they may be off-pointed slightly for periods when it may be desirable to generate less power. Spinning spacecraft may have solar cells on all sides that can face the Sun (see Lunar Prospector). Prolonged exposure to sunlight causes photovoltaics' performance to degrade in the neighborhood of a percent or two per year, and more rapidly when exposed to particle radiation from solar flares.

In addition to generating electrical power, solar arrays have also been used to generate atmospheric drag for aerobraking operations. Magellan did this at Venus, as did MGS at Mars. Gold-colored aerobraking panels at the ends of MGS's solar arrays, visible in the image above, added to the aerodynamic drag for more efficient aerobraking.

Radioisotope Thermoelectric Generators

CUTAWAY VIEW OF CASSINI'S RTGs Click image for additional references.

Plutonium, like all radioactive materials and many non-radioactive materials, can be a health hazard under certain circumstances and in sufficient quantity. RTGs are designed, therefore, with the goal of surviving credible launch accident environments without releasing plutonium. The safety design features of RTGs are tested by the US Department of Energy to verify the survival capabilities of the devices.

Presidential approval is required for the launch of RTGs. Prior to the launch of a spacecraft carrying an RTG a rigorous safety analysis and review is performed by the Department of Energy, and the results of that analysis are evaluated by an independent panel of experts. These analyses and reviews are used by the Office of Science and Technology Policy (OSTP) in the White House to evaluate the overall risk presented by the mission.

CASSINI'S RTG SHADES

Since they remain thermally hot, RTGs present advantages and disadvantages. Cassini employs much of its RTGs' radiant heat inside its thermal blanketing, to warm the spacecraft and propellant tanks. On the other hand, RTGs must be located on the spacecraft in such a way to minimize their impact on infra-red detecting science instruments. Galileo's RTGs are mounted behind shades to hide the near-infrared mapping spectrometer from their radiant heat. Shades are used on Cassini for similar reasons.

RTGs performance degrades in flight about one to two percent per year, slightly faster degradation than for photovoltaics.

Electrical Power Distribution

Here is a discussion about the Cassini spacecraft's electrical power system.

Typically, a shunt-type regulator maintains a constant voltage from the power source. The voltage applied as input to the shunt regulator is generally variable but higher than the spacecraft's required constant bus voltage. The shunt regulator converts excess electrical energy into heat, most of which is radiated away into space via a radiating plate. On spacecraft equipped with articulating solar panels, it is sometimes possible, and desirable for reasons of spacecraft thermal control, to off-point the panels from the sun to reduce the regulator input voltage and thus reduce the amount of heat generated by the regulator.

Electrical Power Storage

Spacecraft which use photovoltaics usually are equipped with rechargeable batteries that receive a charge from the main bus when the solar panels are in the sunlight, and discharge into the bus to maintain its voltage whenever the solar panels are shadowed by the planet or off-pointed during spacecraft maneuvers. Nickel-cadmium (Ni-Cad) batteries are frequently used. After hundreds of charge-discharge cycles, this type of battery degrades in performance, but may be rejuvenated by carefully controlled deep discharge and recharge, an activity called reconditioning.