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fishling t1_j5xmq9a wrote

Reply to comment by CountingMyDick in Why do sample return missions such as OSIRIS-REx use their own reentry vehicles instead of just going to the space station for pickup and return with ISS equipment? by PromptCritical725

You're still assuming that the path is coming up from behind the ISS, in the same direction ISS is moving. If it's moving in the opposite direction, it would have to come to a stop and then accelerate to catch up to the ISS. Or, if it coming in at a right angle, it would have to shed all that extra perpendicular velocity and add all the parallel velocity. Only in the most perfectly aligned case could it be 4 km/s.

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jinxbob t1_j5xtox4 wrote

Choosing whether to enter prograde or retrograde is relatively easy if you're far enough away from earth

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Mispunt t1_j5xnwyd wrote

There isn't really an opposite direction or a perpendicular direction though. If you circularize on the 'right' side you go left around the planet, on the 'left' side right. Edit: once you match the orbital plane of the ISS you can catch up or slow down for a rendezvous by changing orbit height.

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Nemisis_the_2nd t1_j5y7igp wrote

That's really not a big problem. Coming back fro somewhere like Mars, you'd need to alter the tragectory by a fraction of a degree to flip the reentry trajectory 180^0.

From there, you could bleed off speed like in early space missions with rounds of aerobraking.

The original commenter makes a good point about fuel weight, but its also got less to go wrong if you just slam the vehicle into the atmosphere one time.

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