Because it has such high delays? Basically revealing the information which the onion service or exit node encrypted for you only after, potentially, a few trips around the globe?
That makes me think this can be achieved without spacecraft, by just having geographically distributed private keys (even just a few kilometers; you just need the light delays to dominate over processing delays).
And I don't think you need more than two keys: if you wrap it in A(B(A(B(message)))), then party B cannot work on the first layer but first needs to send it to A, party A cannot decrypt the second layer but first needs to sent it back to B, etc. One of the parties could be your recipient, so that would also work with an expensive one-off spacecraft.
> land on the surface a distant body in the solar system
landing is a lot more difficult than staying in Neptune's orbit (notice how many moon-bound spacecraft crashed, even only in recent years!); you'll get the same characteristics by just going out to a desired orbit.
Also note that the amount of delay between Earth and <your orbit, such as Neptune's> will vary wildly