Minimum usable acceleration for a spacecraft

[Drashner1]

Why would 0.003 g take "years" to spiral out of Earth orbit or any orbit at all or to match velocity with the target? Dawn probe was able to enter and leave the orbit of Vesta and then enter Ceres orbit at 0.000007541 G much quicker than that plus the same ProjectRho site has concept studies of fusion spacecraft with about 0.003 g acceleration with much less than a year time to get even to Jupiter.

I'm not saying that it would, I'm saying that even if it did, it wouldn't matter. Most solar sail missions that I've ever read about involve moving cargo around in support of some other crewed mission or base (since sails are comparatively cheap, reusable and don't consume reaction mass) or being used on probe missions where spending years is likely not a problem (based on our RL experience).

Solar sails are not the fastest idea ever suggested, but when considering the range of tech options that we are currently capable of and/or have some idea of how to actually build (speaking conservatively), nothing else significantly outperforms them AFAIK. I'm not counting various flavors of beamed energy/mass atm (sort of outside the context of this), and we have no actual clue how to build a fusion drive or the like, so not counting that either.

In practice, I strongly suspect a solar sail ship would be much faster and wouldn't need anything like 'years', but I don't have the math or background to be able to figure out how long a solar sail based mission would take across all the 'parts' that such a mission would consist of. So again, being deliberately conservative for the reasons above.

Hope this helps,

Todd