# Why haven't there been more space missions that use low-energy trajectories?

The Wikipedia article on low-energy transfers lists only 4 spacecraft over the past decades that at some point of the mission navigated low-energy trajectories, yet there has been extensive research on space mission applications since the 1990s (as a means to reach Lagrange points, reach the Moon, escape Earth's system, navigate the Jovian system, etc).

What is the main reason that there aren't more (proposed) missions that make use of low-energy astrodynamics?

• "This course would result in the probe being captured into temporary lunar orbit using zero delta-v, but required five months instead of the usual three days for a Hohmann transfer." - from the Wikipedia page. Seems like it takes a longer period of time. More time is more potential for failure, which is my guess. Great question though. – Magic Octopus Urn Oct 7 '19 at 15:39
• I think you are asking about low energy transfers without electric propulsion. There are however solutions that use three body effects in conjunction with electric propuslion. See for example answers to Going from LEO to lunar using only low-thrust ion propulsion - can it be done? and also Could a cubesat be self propelled to the moon from LEO?. – uhoh Oct 8 '19 at 4:50
• I think it's an issue of limited scope, being this trajectory only works for earth-moon, combined with increased LV capability. If a falcon heavy can put 20+ mT on a three day TLI, why would you ever choose anything else? I guess maybe - can we design a mission architecture in which this transfer trades the best? – mothman Oct 9 '19 at 7:30

The main reason is the limited range of practical use cases for low energy transfers.

The only low-energy transfers that can be executed in a manageable amount of time are those moving spacecraft between closely linked Lagrangian points, in particular the L1 and L2 points.

In our neighbourhood, these locations are the Earth-Moon EML1 and EML2 points, and the Earth-Sun SEL1 and SEL2 points. It's also possible to navigate between the EML points and SEL points, as well as to a temporary high altitude lunar orbit.

The problem is, those locations contain ...nothing. It's empty space. Even in the cases where they are suitable as locations for spacecraft like solar observatories, there's usually no need to move between them.

If you can reach one of those locations, you can reach them all through low-energy trajectories. But the main cost is still reaching them in the first place, which is approximately equal for all of them.

They are not usable for interplanetary travel.

In practice, they will only be used for "extended mission" type of objectives, where an existing spacecraft with very little propellant left and already in an L-point/high lunar orbit/high Earth orbit can be redirected to do a lunar flyby or escape into a near solar orbit.

• This and this answer are excellent, highly readable, and provide a lot of no-nonsense insight into a complicated topic. +1 – uhoh Oct 1 '20 at 23:44
• @uhoh HopDavid's crusade against the ITN hype is what brought me to this site in the first place. – SE - stop firing the good guys Oct 1 '20 at 23:45
• A few years ago I mentioned it as a possible solution somewhere in this site, perhaps in a comment, and my suggestion was promptly trounced by them, which made the whole n-body thing all the more intriguing. – uhoh Oct 1 '20 at 23:50