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stickmanDave t1_j5xc4kn wrote

Reply to comment by janoc 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

If it was coming in on a trajectory to decelerate and end up at the space station, then failing to decelerate would result in it passing nowhere near the space station.

Remember, the space station isn't a stationary target. It's tooling a long at 7km/second or so. Everything has to go exactly according to plan for the two to end up at the same place at the same time.

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janoc t1_j5xxjvp wrote

You have completely missed my point. The orbital mechanic is a completely different issue and certainly can be calculated so that the two objects meet - we are doing rendezvous and dockings routinely.

The point is that even if you do all of this, carry all that extra fuel (and equipment!) required to decelerate and enter the orbit permitting to dock with the space station - would you want to take the risk?

It is not about "failing to decelerate" and hitting the station as some sort of space projectile. The problem is it would be a spacecraft that has likely not been tested to do this before - and will likely never do this after (deep space missions are usually one-offs). What if something goes wrong during the final approach and puts the station at risk?

We have seen what could happen when the Russian Mir got punctured by some ill-thought maneuvering. Only quick thinking and some crazy heroics by the crew has saved the station. And that was a spacecraft actually designed to dock with the station, equipped for that and one that has just undocked, so it was known to be in working order. Unlike something coming from deep space after who knows how many years - and in who knows what state.

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gridsandorchids t1_j5z56m5 wrote

He's talking about something going wrong in the final approach to dock, which also shows how complicated the whole concept is. Once you get to a matching orbit, and close enough on a stellar scale, it's still highly delicate and specialized to actually safely recover it.

MIR had a Progress craft smash into it and almost kill the station and inhabitants and it was just docked and then pushed out a bit and brought back in.

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