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The 40+ year old Voyager probes have three Radioisotope Thermoelectric Generators (RTGs) currently producing 249 watts, losing around 4 watts per year. They began with 470 Watts.

The Curiosity Rover's Multi-Mission RTG (MMRTG) and was used during its spacecraft's transit to Mars. The MMRTG produces 2000 watts of thermal energy or 125W-100W of electrical energy.

Is a Mars bound spacecraft limited to ~100 watts of electric power for onboard systems during its 200+ day transit? That does not sound like much, although it would run a current laptop.

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  • $\begingroup$ The "What Powers New Horizons" question indicates it also used an RTG that produced around 290W of power used for onboard electronics. Heading away from the sun made solar impractical. space.stackexchange.com/questions/9902/what-powers-new-horizons $\endgroup$
    – Dan Sorensen
    Nov 19, 2021 at 6:06
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    $\begingroup$ "although it would run a current laptop." More like run 3-12 laptops, or 30 if you switch the display off. Modern power-efficient laptop runs on 8 watts, 5 of which go to the screen.(we're talking power-efficient laptop here. not alienware ultra-framerate gaming rig!) $\endgroup$
    – CuteKItty_pleaseStopBArking
    Nov 19, 2021 at 6:12
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    $\begingroup$ Agreed -- I was erring on the high side. A workstation class comes with a 130W power supply, but draw varies on system load. Also agreed that power goes a lot further with no screen. $\endgroup$
    – Dan Sorensen
    Nov 19, 2021 at 6:14
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    $\begingroup$ That 130W is to allow the device to charge a 10-to-20-hour duration battery, in 30 minutes. Also most consumer laptops focus on multimedia and gaming performance rather than efficiency, full pixels ahead and damn the watt-hours! For comparison: A modern high-end cellphone, while operating but not in a phonecall or game, consumes about one watt. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Nov 19, 2021 at 6:18
  • $\begingroup$ Thank you for explaining that further. That makes more sense to me now. :-) $\endgroup$
    – Dan Sorensen
    Nov 20, 2021 at 15:53

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There is not a theoretical limit on the amount of power that can be available during Mars transit.

Power generation systems are normally sized for the operational case which requires the greatest demand over the entire mission. For most robotic Mars missions to the surface, this can be expected to occur during the surface operations phases. The power consumption during cruise is typically low since the majority of the spacecraft systems will usually be off or in a standby mode.

If the mission uses an RTG for power generation on the surface, it makes sense to also use the power it generates to keep the spacecraft alive during the cruise. If you wanted to increase the power generated during cruise this could be done by carrying additional RTGs onboard. Of course, there is a cost associated with this (both financial and in terms of mass/volume resources available).

However, not all surface missions carry RTGs. In this case, solar arrays can be used to generate power for the cruise stage during transit to Mars.

RTGs will provide a constant power output that will gradually decrease with time. The power generated by solar arrays will vary during the transit as the distance from the sun increases. We can estimate this since we know that a). the solar flux at Earth is about 1360 W/m2 b). the inverse square law tells us roughly how much the solar flux will decrease with distance from the sun and c). space based solar arrays can be assumed to have a conversion efficiency of ~28%. Taking this into account we can approximate how much power our cruise stage could generate for every m^2 of solar array area it carries:

enter image description here

Note how the power available at Mars orbit will vary due to the eccentricity of its orbit around the sun (varying between ~1.38 AU and ~1.66 AU).

As mentioned, for most missions the power demand during cruise is not especially high. This might not the case however if electrical propulsion (EP) is used for the transit to Mars, since power is needed to continuously thrust.

As an example, consider ESA's Earth Return Orbiter (ERO) being developed for the Mars Sample Return campaign with NASA. It is planning to use a highly capable EP system for the transit to Mars and back. This link from Airbus (the mission's prime contractor) states that ERO is being designed with a 144 m^2 solar array. Using the chart above we can estimate that this must be able to produce around 50 kW at Earth solar range and about 20 kW at farthest Mars solar range. A fair bit more power than in the examples cited in your question!

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  • $\begingroup$ Thank you for the detailed explanation. You got to the source of my curiosity, which is how there could be excess power capacity for proposed human transit. $\endgroup$
    – Dan Sorensen
    Nov 20, 2021 at 15:49
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No, you are not subject to such limits. At Mars orbit, you can expect about 600 W/m^2 of solar radiation. While solar panels are less effective, they are still significantly more efficient in terms of W/KG while in orbit or free flying.

If that's not enough, something like Kilopower https://en.m.wikipedia.org/wiki/Kilopower might well suit.

The real question is... how much power do you actually need while free flying, and can you afford the mass required?

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  • $\begingroup$ That is helpful! The answer to your last question arose as I wondered how they would have enough power capacity available for eventual humans transit. But that discussion is best saved for another question. $\endgroup$
    – Dan Sorensen
    Nov 20, 2021 at 15:46

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