In spaceflight, there are four different categories of electrical power sources; battery, solar, fuel cell, and nuclear. For safety concerns for the crew, nuclear power is automatically ruled out, especially since nuclear is optimal for either very long duration missions or for high electrical demands, neither of which my orbiter requires. The remaining three categories of power all have long service histories in space flight.

    Battery powered systems are the most reliable of the three. However, they are also the lowest yielding power source and are very limited without another power supply available so they can be recharged. Nonetheless, battery technology has improved dramatically since the beginning on spaceflight, from heavy lead acid batteries similar to ones used in automobiles to newer, higher efficiency lithium polymer batteries. There are numerous other chemical compositions that comprise batteries, including alkaline, nickel cadmium (NiCad), nickel metal hydride (NiMH), and lithium ion (Li-ion). These types of batteries are identical to ones used in everyday life here on earth, and each has specific characteristics that make them suitable for specific mission requirements.

    Solar power is a very common power source in space flight, and for good reason, as it is a free and limitless supply of electrical power. The Soyuz and Progress spacecraft used by the Russians to ferry crew and cargo the space station use solar panels. Solar panel technology has improved over the years to be more efficient per unit of surface area, be better adept to the extreme temperature swings of space, and are durable enough to survive impacts from small space debris. Some solar panels are even rollable, allowing for minimum volume occupied during the launch phase of the satellite. However, while in orbit, a satellite is in sunlight only half the time, so panels are always used in conjunction with batteries so that the satellite can continue to function during the night half of its orbit. The panels much yield more electrical power than would otherwise be necessary since it has to sun the satellite's normal systems and recharge its batteries at the same time. Batteries used in this configuration must be capable of many recharge cycles, with electrical capacity deterioration at a minimum.

    The last category of power generation while in orbit are fuel cells. Fuels cells combine liquid oxygen and liquid hydrogen to produce electrical power as well as water and heat. Fuel cells have been used in America's manned space program since early in the Gemini missions when battery technology did not provide enough power orbital missions lasting several days. They yield the most electrical power without having to worry about sunlight or recharging of batteries. However, they are complicated devices, making them slightly less reliable than batteries or solar panels. I personally saw the Space Shuttle Columbia make an emergency landing at the Kennedy Space Center when one of its fuel cells malfunctioned, with the other two fuel cells not being able to supply enough power to complete the mission. Their production of water seems at first to be a prized asset, since water is very limited in space, but most of this water is dumped overboard by the shuttle.

    Since my orbiter will not be as electrically demanding as the Shuttle, I do not see the benefits of fuel cell technology overcoming its shortfalls. While the orbiter is designed to spend years in orbit, a vast majority of this will be while docked to the International Space Station. The time it spends in space while not docked to ISS can be measured in hours, round trip. Thus, the use of solar panels is not needed. In turn, batteries will provide the power supply, using the latest lithium polymer technology for capacity. Their ability to go through hundreds of discharges and recharges will mean that they will probably never have to replaced within the lifespan of the orbiter itself, and their mass savings compared to other types of batteries will keep the overall mass of the orbiter to a minimum,