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I was reading into the Cassini–Huygens mission and was surprised to learn that the Huygens probe had a battery life of only 3 hours. Wikipedia doesn't seem to mention why this was the case.

What were the prominent factors that constrained battery life?

Maybe it was power drained by constant transmission to the orbiter? Or maybe there was a need to heat elements of the probe against Titan's environment? (Doubtful.) Maybe it was a technological or design constraint (battery densities at the time/available space in the probe's design limited)?

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The Huygens was not capable of transmitting data directly to the Earth, instead Cassini relayed all communication. Therefore mission lifetime was limited by possible communication window from Titan surface to Cassini (which was orbiting Saturn, making only fly-by around Titan and departing the moon soon after the landing). Keeping Huygens alive until Cassini is in favorable position for communication again would be likely unreasonable.

So the battery life was designed accordingly to the planned communication window. Even with more battery life time, there will be no way to send further measured data.

The trajectory of the flyby is shown in The Huygens probe: science, payload and mission overview paper, the communication window ended when Cassini flew behind horizon:
Cassini trajectory during Huygens mission
Similar picture can be found in An overview of the descent and landing of the Huygens probe on Titan paper.

The flyby trajectory during Huygens landing was changed after the Cassiny-Huygens launch to mitigate only then discovered design flaw in handling Doppler shift in communication link. I was not able to find an analysis what would be the longest possible communication window and if three hours was a hard limit or rather compromise given the primary target of the mission was atmospheric science. But the communication time would be limited to relatively short time period by orbital mechanics anyway.

Actually, Huygens was functional some time after Cassini stop listening for data (confirmed by detection from the Earth, nevertheless the signal would be too weak for transferring data directly), so battery depletion was not the reason for end of Huygens mission.

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    $\begingroup$ Ah, I see. I didn't take into account the trajectories of the two craft. For some reason, I guess I figured that since everything was in the Saturn system, communication could continue. $\endgroup$
    – BMF
    Commented Nov 30, 2019 at 22:29
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    $\begingroup$ I have added picture of the flyby, limiting factor was Cassini flying below horizon, not the distance. I have not found any clear statement if communication window was the primary constrain on mission duration, at least in the initial design, or only one of the factors taken into the account, though. $\endgroup$
    – Martin
    Commented Nov 30, 2019 at 23:29
  • $\begingroup$ @BMFForMonica As usual, spaceflight isn't very intuitive. Kerbal Space Program helps :D $\endgroup$
    – Luaan
    Commented Dec 2, 2019 at 9:35
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    $\begingroup$ Is there any information available why going into hibernation until communications can be reestablished was ruled out? Akin to the "cruise phase" of interplanetary probes where most science instruments are turned off until the sequencer turns them back on again close to the target? $\endgroup$
    – Georg Patscheider
    Commented Dec 2, 2019 at 12:50
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    $\begingroup$ @GeorgPatscheider No authoritative answer, but I assume the gain from this would be negligible given the design of mission. Most scientific data was obtained during the descent and landing, there was probably not much new to learn after short time on the surface. Scientific payload would need to be changed or extended to make use of such "second pass". Given the little knowledge of landing place (even solid or liquid) and high uncertainty if probe even survives the landing, payload useful during the descend was likely preferred. $\endgroup$
    – Martin
    Commented Dec 6, 2019 at 22:41
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This book has a table with the energy budget for Huygens:

Data relay system: 214 Wh
Computer: 246 Wh
Electric power system: 5 Wh
Payload: 325 Wh
Losses: 132 Wh
Pre-separation checks: 50 Wh

Total: 972 Wh for the designed mission:

The batteries and all other resources were sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half-hour or more) on Titan's surface.

Then they added a big margin to account for failed components, and arrive at a capacity requirement of 2059 Wh.

Thanks to this margin, the probe ended up functioning for 70 minutes after landing.

According to this timeline, Cassini dropped out of sight of Huygens just over 2 hours after landing, giving another constraint. It would take a while before Cassini came into view again (how long? I don't know, but IIRC Cassini's orbits were on the order of a month), so your design decision is to either size the probe for 2 hours, or for it to survive until communications are reestablished. If that's 1 month, you can't carry enough batteries to survive that long.

There were no electric heaters AFAIK, Huygens carried radioisotope heaters.

The major constraint is mass. Huygens' total mass was about 300 kg, and all the systems, experiments, structure etc. must fit in that mass. You can't increase the mass by much without jeopardizing the launch (IIRC Cassini was already the heaviest interplanetary spacecraft).

Work on Cassini-Huygens took place 1988-1997, which places constraints on the technology you can use: e.g. computers take a lot of power, limited battery chemistries available and proven for 10 years in space.

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    $\begingroup$ Maybe worth noting that the power margin gave it an extra hour of operation on the surface than planned. $\endgroup$
    – Russell Borogove
    Commented Dec 1, 2019 at 4:39
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    $\begingroup$ All the more does my wish to put nuclear reactors on everything deepen. $\endgroup$
    – ikrase
    Commented Dec 1, 2019 at 4:41
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    $\begingroup$ You don't need reactors, just RTGs which are a little safer and lighter. $\endgroup$
    – Burgi
    Commented Dec 3, 2019 at 12:28

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