Solar panels on spacecraft

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Main article: Solar panel

Spacecraft operating in the inner solar system usually rely on the use of photovoltaic solar panels to derive electricity from sunlight. In the outer solar system, where the sunlight is too weak to produce sufficient power, radioisotope thermal generators (RTGs) are used as a power source^[1].

Contents 1 History 2 Chemical Power vs. Solar Power for Propulsion 3 Implementation 4 Spacecraft that have used solar power 5 Future Uses 6 References

[edit] History

The first spacecraft to use solar panels was the US Army satellite Explorer 1 in January 1958. This was largely because of the influence of Dr.Hans Ziegler, who can be regarded as the father of spacecraft solar power. ^[2]. Of his 30 year tenure at Fort Monmouth (1947-1976), Ziegler spent 12 years in the top position as Chief Scientist^[3].

[edit] Chemical Power vs. Solar Power for Propulsion

As opposed to chemical rockets, which are powered by a chemical reaction of the propellant, some spacecraft propulsion methods use electricity derived from solar panels or RTGs to provide the energy necessary to expel mass from the spacecraft in order to generate thrust. These methods typically have a higher specific impulse. With solar power the acceleration that can be produced is very low (much too low for a launch), but it is enduring. Typical burn times are months instead of minutes. An important factor is the power the solar panel produces per kg, as an upper limit of the power the spacecraft has at its disposal per kg spacecraft (including solar panels)^[4]. See also energy needed for propulsion methods.

[edit] Implementation

Solar panels need to have a lot of surface area that can be pointed towards the Sun as the spacecraft moves. More exposed surface area means more electricity can be converted from light energy from the Sun. Since spacecraft have to be small, this limits the amount of power that can be produced ^[1].

Spacecraft are built so that the solar panels can be pivoted as the spacecraft moves. Thus, they can always stay in the direct path of the light rays no matter how the spacecraft is pointed. Spacecraft are usually designed with solar panels that can always be pointed at the Sun, even as the rest of the body of the spacecraft moves around, much as a tank turret can be aimed independently of where the tank is going. A tracking mechanism is often incorporated into the solar arrays to keep the array pointed towards the sun^[1].

Sometimes, satellite scientists purposefully orient the solar panels to "off point," or out of direct alignment from the Sun. This happens if the batteries are completely charged and the amount of electricity needed is lower than the amount of electricity made. Alternatively the extra power can be dissipated by a shunt into space as heat.

[edit] Spacecraft that have used solar power

To date, solar power, other than for propulsion, has been practical for spacecraft operating no farther from the sun than the orbit of Mars. For example, Magellan, Mars Global Surveyor, and Mars Observer used solar power as did the Earth-orbiting, Hubble Space Telescope. The Rosetta space probe, launched March 2, 2004, will use 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.

[edit] Future Uses

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. The same principle is used to start fires with a magnifying glass on a sunny day.

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 allows costs to be cut significantly due to the utilization of less material.

Research is currently underway to develop space-based solar plants — solar power satellites with large arrays of photovoltaic cells that would beam the energy they produce to Earth using microwaves or lasers. Japanese and European space agencies have announced plans to develop such power plants in the first quarter of the 21st century.

[edit] References