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Photovoltaic solar panels on spacecraft operating in the inner Solar System

Spacecraft operating in the inner Solar System usually rely on the use of power electronics managed photovoltaic solar panels to derive electricity from sunlight. Outside the orbit of Jupiter, solar radiation is too weak to produce sufficient power within current solar technology and spacecraft mass limitations, so radioisotope thermoelectric generators (RTGs) are instead used as a power source.

The first practical silicon based solar cells were introduced by Russell Shoemaker Ohl, a researcher at Bell Labs in 1940. It was only 1% efficient. In April 25, 1954 in Murray Hill, New Jersey. They demonstrated their solar panel by using it to power a small toy Ferris wheel and a solar powered radio transmitter. They were initially about 6% efficient, but improvements began to raise this number almost immediately. Bell had been interested in the idea as a system to provide power at remote telephone repeater stations, but the cost of the devices was far too high to be practical in this role. Aside from small experimental kits and uses, the cells remained largely unused. This changed with the development of the first US spacecraft, the Vanguard 1 satellite in 1958. Calculations by Dr. Hans Ziegler demonstrated that a system using solar cells recharging a battery pack would provide the required power in a much lighter overall package than using just a battery. The satellite was powered by silicon solar cells with ≈10% conversion efficiency. The success of the Vanguard system inspired Spectrolab, an optics company, to take up the development of solar cells specifically designed for space applications. They had their first major design win on Pioneer 1 in 1958, and would later be the first cells to travel to the Moon, on the Apollo 11 mission's ALSEP package. As satellites grew in size and power, Spectrolab began looking for ways to introduce much more powerful cells. This led them to pioneer the development of multi junction cells that increased efficiency from around 12% for their 1970s silicon cells to about 30% for their current gallium arsenide (GaAs) cells. These types of cells are now used almost universally on all solar powered spacecraft.

Up until the early 1990s, solar arrays used in space primarily used crystalline silicon solar cells. Since the early 1990s, Gallium arsenide based solar cells became favored over silicon because they have a higher efficiency and degrade more slowly than silicon in the space radiation environment. The most efficient solar cells currently in production are now multi junction photovoltaic cells. These use a combination of several layers of indium gallium phosphide, gallium arsenide and germanium to harvest more energy from the solar spectrum. Leading edge multi junction cells are capable of exceeding 39.2% under non concentrated AM1.5G illumination and 47.1% using concentrated AM1.5G illumination.

To date, solar power, other than for propulsion, has been practical for spacecraft operating no farther from the Sun than the orbit of Jupiter. For example, Juno, Magellan, Mars Global Surveyor, and Mars Observer used solar power as does the Earth orbiting, Hubble Space Telescope. The Rosetta space probe, launched 2 March 2004, used its of solar panels as far as the orbit of Jupiter (5.25 AU); previously the furthest use was the Stardust spacecraft at 2 AU. Solar power for propulsion was also used on the European lunar mission SMART 1 with a Hall effect thruster. The Juno mission, launched in 2011, is the first mission to Jupiter (arrived at Jupiter on July 4, 2016) to use solar panels instead of the traditional RTGs that are used by previous outer Solar System missions, making it the furthest spacecraft to use solar panels to date. It has of panels. The InSight lander, Ingenuity helicopter, Tianwen 1 orbiter, and Zhurong rover all currently operating on Mars also utilize solar panels. Another spacecraft of interest was Dawn which went into orbit around 4 Vesta in 2011. It used ion thrusters to get to Ceres. The potential for solar powered spacecraft beyond Jupiter has been studied. The International Space Station also uses solar arrays to power everything on the station. The 262,400 solar cells cover around of space. There are four sets of solar arrays that power the station and the fourth set of arrays were installed in March 2009. 240 kilowatts of electricity can be generated from these solar arrays. That comes to 120 kilowatts average system power, including 50% ISS time in Earth's shadow.

For future missions, it is desirable to reduce solar array mass, and to increase the power generated per unit area. This will reduce overall spacecraft mass, and may make the operation of solar powered spacecraft feasible at larger distances from the sun. Solar array mass could be reduced with thin film photovoltaic cells, flexible blanket substrates, and composite support structures. Solar array efficiency could be improved by using new photovoltaic cell materials and solar concentrators that intensify the incident sunlight. Photovoltaic concentrator solar arrays for primary spacecraft power are devices which intensify the sunlight on the photovoltaics. This design uses a flat lens, called a Fresnel lens, which takes a large area of sunlight and concentrates it onto a smaller spot, allowing a smaller area of solar cell to be used. Solar concentrators put one of these lenses over every solar cell. This focuses light from the large concentrator area down to the smaller cell area. This allows the quantity of expensive solar cells to be reduced by the amount of concentration. Concentrators work best when there is a single source of light and the concentrator can be pointed right at it. This is ideal in space, where the Sun is a single light source. Solar cells are the most expensive part of solar arrays, and arrays are often a very expensive part of the spacecraft. This technology may allow costs to be cut significantly due to the utilization of less material.