Today is an interesting anniversary: the sixtieth anniversary of solar power in space. On 17 March 1958, the American satellite Vanguard 1 entered orbit, becoming the fourth satellite ever to do so, and the first to use solar power. Vanguard 1 was small enough to be held by a person in one hand – 1.5 kg (3 lbs) in mass and 16 cm (6”) in diameter, or 76 cm (30”) wide including the antenna aerials. Altogether, Vanguard 1 had 6 silicon solar cells which generated about 1 watt in total. For comparison, the power produced by a typical rooftop solar PV system in 2018 is several thousand times greater.
Why did the first satellites use electrical power? Primarily, to send radio signals back to Earth. The first satellite ever to reach orbit, the Soviet satellite Sputnik 1, broadcast radio signals at 20 MHz and 40 MHz which could be detected on Earth, even by amateur radio users. The radio signals provided additional proof that the satellite was really in orbit around the Earth (it could also be seen with large telescopes), and allowed observers to measure its position and trajectory. The power to provide this radio signal came from batteries, which allowed it to broadcast radio signals for 21 days before the batteries went flat.
This radio communication was one-way only. Early satellites could not receive communications from Earth, and were essentially a battery, radio transmitter, and an antenna. The second and third satellites, the Soviet Sputnik 2 and US Explorer 1 also used batteries to power radio signals, which lasted somewhat longer (Explorer 1 had a lower power mode that lasted 105 days). Vanguard 1 had two radio transmitters. One transmitter (at 108 MHz) was powered by a battery, which lasted for 20 days. The second transmitter (at a slightly higher radio frequency, 108.3 MHz) was powered by the (as yet unproven in space) solar cells, and that broadcast for over six years, or over 23,000 orbits of the earth, before going silent.
Satellites and spaceflight were one the first uses of solar power in which it was clearly the best choice – where it was cheaper, more reliable, and/or more practical than other sources of electrical power. At that time, generation of electricity on Earth was most commonly done with a combustion engine. On Earth, oxygen is in the air everywhere and fuel can usually be found, whether it is wood, coal, natural gas, or some other fuel. Photovoltaic cells at that time were incredibly expensive, and their intermittent output was difficult to manage since rechargeable batteries were also very expensive.
In space, solar power had clear advantages. Combustion in space requires that both fuel and oxygen (or another oxidant) be brought up from the Earth in tanks. For example, the Saturn V rocket engines that sent humans to the moon had tanks of liquid hydrogen and liquid oxygen. (Those engines were obviously used to make the rocket move, not to generate electricity). Non-rechargeable batteries could not provide power for long without a lot of “chemical fuel”. In space, above the clouds, solar power was more or less constant the whole time that the panels were not behind the Earth. No fuel needed to be carried up. Satellites typically orbited the earth every few hours, so that when rechargeable batteries did start to be used with solar panels, they were small compared to those needed to last the entire long night on earth.
What happened to Vanguard 1? Although its radio no longer transmits, Vanguard 1 continues to orbit the Earth today, at an altitude between 600 and 4,000 km, going around the Earth approximately every 2 hours. It is now the oldest object made by humans to still be in space. Sputnik 1 and Sputnik 2 reentered the atmosphere after around one year after launch, and Explorer 1 reentered in 1970. Vanguard 1 is in a very stable orbit, and is predicted by NASA to remain in orbit for another 180 years. Several websites track its location: you can see where Vanguard 1 is right now at n2yo and Satflare. As I write this, it is currently over Indonesia.
As a final note, in researching this article I came across some amazing audio recordings of Vanguard 1 transmissions made by amateur radio enthusiast Roy Welch in 1958 and 59. (The signals have been modified to bring them into audible frequencies). The one from 1959 I find quite eerie. Roy Welch wrote that he believed that the variations in frequency were due to the satellite spinning around, bringing the different solar cells into and out of the light. It is quite something to listen to the recording and imagine the tiny Vanguard satellite, moving high above the Earth, broadcasting its radio signal, and slowing turning around in the sunlight.