Sputnik 1 only worked for 21 days until its battery ran dry, explorer 1 ran for 111 days. It wasn't until Vanguard 1 that we started putting solar panels and recharge methods on satellites. It was successful as Vanguard 1 transmitted its signals for over six years as it orbited the Earth. I want to know why the first few spacecraft in orbit did not have a way to recharge.

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    $\begingroup$ The first few spacecraft had no requirements for that. $\endgroup$ Nov 8, 2022 at 16:33
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    $\begingroup$ The first satellites were put into space to prove it was possible, you don't need solar panels to do that. $\endgroup$
    – GdD
    Nov 8, 2022 at 17:42
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    $\begingroup$ Vanguard 1 didn't have a way to recharge either. It had one transmitter operating from a non-rechargeable mercury battery and another operating directly from solar power. I think Explorer 6 was the first to have rechargeable batteries, and maybe you should look into the battery and solar power technologies available at the time... $\endgroup$ Nov 8, 2022 at 18:39
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    $\begingroup$ You should look at a photo of Vanguard 1. It had no solar panels, it only had 6 very small solar cells mounted on the spherical case. Case diameter was only 152 mm (6.0 in). Solar cells were only about 50 mm (2 in) square. Vanguard "was described by the Soviet Premier, Nikita Khrushchev, as "the grapefruit satellite" ". Sputnik 1 diameter was 580 mm (23 in), more than 4 times bigger. Vanguard had 1.46 kg (3.2 lb), Sputnik 1 83.6 kg (184 lb). $\endgroup$
    – Uwe
    Nov 8, 2022 at 23:21
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    $\begingroup$ @Uwe also, I could only find a schematic for one transmitter, but it's likely Vanguard 1 had all of two transistors total. The two beacons were very simple single-transistor crystal oscillators connected to antennas. The battery-powered one had its crystal thermally coupled to the case, so temperature variations would cause detectable changes in frequency. Even a simple solar powered charging circuit would be more complex than the science payload. $\endgroup$ Nov 9, 2022 at 13:22

1 Answer 1


Functional solar electric systems are a large part a product of space related development so there simply was not an off the shelf solution.

Building a functional rechargeable battery system requires:

  1. Keeping the fragile glass panels intact during launch
  2. Getting the panels to unfold/deploy reliably (Explorer 6)
  3. Keeping the panels from overheating
  4. Keeping the batteries from freezing
  5. Cutting off charge current when batteries are full
  6. Shutting systems down to prevent fully draining battery
  7. Accurately tracking current battery charge
  8. Not weigh more than a non rechargeable system of similar operating life

Also ideally you have a reaction control system that means the solar panels face the sun.

Many of these are trivial problems now, but at a time when relays and vacuum tubes were the known/well understood electrical parts this brings challenges, since both consume substantial amounts of current in operation (tubes due the filament needing to be hot and relays in coil current) so any attempt to use off the shelf hardware would have consumed far more power than the early solar panels could provide. Early transistors were fragile and not well understood, and still physically quite large. Explorer one flew with just 20 transistors. A basic linear regulator needs around the same number, and a current solar charge controller several times that.

Having a complex power management system also brings risks in that any problem in the system is pretty much certain to kill the mission because entire craft goes dead.

These factors mean that when faced with extreme time and weight constraints and political pressure to get something, anything into orbit both US and USSR chose to stick known and reliable non rechargeable batteries into the early payloads and simplest possible power management systems and only switched to solar when the core problems of just getting to orbit had been solved and the initial flights had provided data on the environment they would operate in.

In addition most of the early satellites re-entered after a couple of weeks, so having solar power would not have extended the useful lives much in any case.

edit: Comment from Christopher James Huff links this report which notes Explorer 6 only achieved 3 months use from rechargeable Nickel Cadmium due degradation to the batteries during charge, and the later TIROS 'solved' this by only using 3% of the capacity, allowing simple charge circuit at the cost of flying 97% more battery mass. It is 1966 and the OAO series where basic battery management is used to extend battery life into years, almost a decade after Explorer 1.

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    $\begingroup$ @asdfex - certainly, but means your system efficiency is very low because your panels must be sized small enough to not over voltage the battery but large enough to actually put useful charge in when flat - works ok for garden lights but not great for anything with a variable consumption. Now sorta curious how clever they did try to get with the early craft, since the diode only option is at least reliable and easy to test. $\endgroup$ Nov 13, 2022 at 12:18
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    $\begingroup$ @asdfex nominal voltage of a solar cell is a full illumination and no load (so zero power). And the voltage drops with current draw. But batteries have almost full voltage until they are almost completely drained. So you'd have REALLY bad efficiency on the cells until your batteries are around 80 to 90% discharged... not what you want when every bit of weight counts... $\endgroup$
    – TrySCE2AUX
    Nov 13, 2022 at 14:13
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    $\begingroup$ @asdfex it's really not that easy. You need a chemistry with a suitable temperature range and energy density, ability to operate in a sealed container, which is also tolerant of overcharging and deep draining. They had a very limited range of available battery types, and it was also unknown exactly how well the solar cells would perform in the space environment and how long they'd last with the temperature extremes and radiation. Power systems with rechargeable batteries had to wait for much more sophisticated satellites. $\endgroup$ Nov 13, 2022 at 14:18
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    $\begingroup$ They were. Explorer 6 was the first to use rechargeable batteries, NiCd cells, and only lasted 3 months...Explorer 1 lasted longer with non-rechargeable mercury cells. The TIROS satellites achieved longer lifetimes by using bigger battery packs with very low depth of discharge, at a significant cost in mass. A few years later, the OAO satellites were using NiCd cells with a sophisticated charge control system. trs.jpl.nasa.gov/bitstream/handle/2014/17938/… $\endgroup$ Nov 13, 2022 at 14:54
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    $\begingroup$ @asdfex No, voltage and current is determined by solar cells area, illumination and load. Without load the voltage is two times the voltage at optimal load. If the load is too small or too big you get much less than the peak power at optimal load. Of course peak power and optimal load depends on orientation of the solar cells to the sun. $\endgroup$
    – Uwe
    Nov 14, 2022 at 15:45

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