I am writing a science fiction story that involves fusion powered space travel, but not constant acceleration like in the Expanse. Instead, the manned spacecraft accelerates at the beginning of the journey but coasts at constant velocity most of the way. The setting is roughly 100 years from now, so technology has developed significantly.

I would like to know how long it would take to travel from the moon to the asteroid belt with this technology, assuming realistic constraints and optimal trajectories.

Additionally, how long might it take to reach Jupiter from the moon?

What plausible future technologies might allow for a shorter journey (without achieving constant acceleration)?

Edit - To clarify, by "optimal trajectories" I mean shortest travel time. Thank you all for the helpful resources.

  • $\begingroup$ If you know a bit about orbital mechanics and want to calculate it, these 2 videos might be of interest: m.youtube.com/… m.youtube.com/… In the first one they show you how to calculate the delta V required for a hohmann transfer and in the second they show you how to calculate the time needed to get there. $\endgroup$ Aug 31, 2023 at 7:07
  • $\begingroup$ You can start here and read the references therein en.wikipedia.org/wiki/Fusion_rocket. $\endgroup$
    – Heopps
    Aug 31, 2023 at 8:04
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    $\begingroup$ Also, could you clarify what do you mean by "optimal trajectories"? If with the shoterst travel time - well, than the traectories will not be Hohmann, and you should read about "Lambert problem" instead. Also, the energy of fusion per mass of the fusion reagents (deiuterium+tritium, or others) is huge, but still limited, and if we want to fly fast than we can reach this treshould rather quickly. But Hohmann transfer would be too slow for sci-fi, I suppose, over a year to reach the main asteroid belt and about 3 years to reach Jupiter, for example. :) $\endgroup$
    – Heopps
    Aug 31, 2023 at 8:20
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    $\begingroup$ Fusion rocket technology is only theoretical, we don't really know how much impulse you could get. You need to make some assumptions about how much thrust you'll get from how much fuel, otherwise there's no basis for an answer. If you are asking for us to make those assumptions then you're asking for speculation, which is off-topic. $\endgroup$
    – GdD
    Aug 31, 2023 at 8:58
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    $\begingroup$ @PM2Ring - I suspect that the constant velocity requirement was inadvertent, probably based on the assumption that this was the most efficient method. After all we travel in cars and airplanes at constant speed which is generally the most efficient way to do it (except for hypermiler hybrid owners who use a pulse-glide technique for higher efficiency). I expect that once they are informed that the most efficient methods will involve constant slowing of the spacecraft they would accept this. $\endgroup$ Sep 2, 2023 at 16:21

1 Answer 1


Assuming fusion engines would enable the high delta-v needed for such flights, according to https://transfercalculator.com/calculator/ a transfer would take between 11 and 16 months depending on "how the stars align", and need between 25 and 50km/s of impulse

Ceres would take between 3 and 7 months, and between 12 and 100 km/s


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