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Except for special sun-synchronous, Earth orbit eventually leads to eclipses, so spacecraft in Earth orbit generally have solar panels with electrical energy "buffered" by batteries. And those in orbit or on the surface of Mars need batteries every night.

For some deep space missions solar panels are replaced by RTGs.

Batteries don't work well when they get cold and can be permanently damaged if it's too cold, so they are kept warm when necessary either by thermal energy from an RTG or radioisotope thermal generator, or by resistive heaters powered by the batteries themselves.

Some of the earliest satellites were launched with only batteries and when the batteries drained the spacecraft was dead, but soon after that at least small photovoltaic cells were sufficiently available that they could trickle-charge batteries.

Question: When was the last time that a battery-powered spacecraft was launched without an RTG or solar-electric power?

notes:

  1. For the purposes of this question propulsion systems don't count, so no upper stages, kick-stages, etc. Answers should be considered to be primary payloads. Since I am primarily interested in historical spacecraft secondary payloads like hitch-hiking cubesats or "secondary science probes" deployed by a primary payload spacecraft don't count here. For those I have just asked What secondary science payloads deployed from primary science payloads were strictly battery powered?
  2. About fuel cells; normally if it looks like a duck and quacks like a duck, then some would argue that this is sufficient to call it a duck. But I'm really after historical early spacecraft, so for the purposes of this questions fuel cells don't count.
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    $\begingroup$ Some Soviet Lunas only had a battery. $\endgroup$
    – Kozuch
    Mar 17 at 15:33
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    $\begingroup$ Spaceman and almost every second, third and fourth stage qualify as well. $\endgroup$
    – asdfex
    Mar 17 at 17:45
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    $\begingroup$ Do fuel cells count? - Space Shuttle $\endgroup$
    – Puffin
    Mar 17 at 23:43
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    $\begingroup$ @Puffin I thought about that, but ruled out by "battery-powered". Although, long ago, fuel cells were called "fuel batteries".... $\endgroup$ Mar 18 at 1:07
  • 2
    $\begingroup$ Does Starman count? $\endgroup$
    – PearsonArtPhoto
    Mar 18 at 19:58
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I found Pioneer Venus 2; December 9, 1978. Here we have an inversion of expectation; the primary mission was a set of hands-on Venus atmospheric studies. The bus had solar power but the probes in this case where the primary payload and were dropped into the atmosphere on battery power.

Pioneer Venus 1 (already present in orbit) was used as the relay.

Since I haven't found a future atmospheric probe (not intended for soft landing) launched on its own power but rather riding tag-along on a larger probe; I don't expect to find any newer answers. It is interesting to note that this was only a few years before the space shuttle that carried Galileo was launched.

NASA summary can be found in the expected place, but I'm mainly relying on the images from the Wikipedia article.

Excerpts from NASA mission summary:

Pioneer Venus 2, the sistership to Pioneer Venus 1, consisted of the main spacecraft, a Large Probe (698 pounds or 316.5 kilograms), and three identical Small Probes, all of which were designed to collect data during entry into the atmosphere of Venus. The probes were shaped like cones and were not expected to survive impact with the surface.

Pioneer Venus 2 released the Large Probe (5 feet or 1.5 meters in diameter) on Nov. 16, 1978, while about 7 million miles (11.1 million kilometers) from the planet.

Four days later, the bus released the three Small Probes—the North Probe, Day Probe and the Night Probe—while about 6 million miles (9.3 million kilometers) from Venus.

Each probe took about 53 to 56 minutes to reach the surface. Amazingly, two of the three Small Probes survived the hard impact. The so-called Day Probe transmitted data from the surface for 67 minutes, 37 seconds, before succumbing to the high temperatures, pressures and power depletion.

The main spacecraft, meanwhile, burned up in the atmosphere at an altitude of about 75 miles (120 kilometers)—about 1.5 hours after the probes—and provided key data on higher regions.

Thus the probes, not the main spacecraft, are the primary payload; and thus they used batteries to achieve their primary mission. The use of battery power supply is obvious because you cannot use solar powers in the atmosphere of Venus and again because the images show no solar panels on the components expected to survive entry into Venus's atmosphere.

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    $\begingroup$ Wow, very interesting! $\endgroup$
    – uhoh
    Mar 18 at 3:15
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    $\begingroup$ Thanks for your edits! $\endgroup$
    – uhoh
    Mar 18 at 20:23

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