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rabidferret t1_j5zfqvx wrote

Reply to comment by KingZarkon 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

Measuring a spacecraft's position and trajectory has a margin of uncertainty. Engines are not devices that can produce a known exact amount of thrust for an exact amount of time.

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Beetin t1_j5zutlb wrote

I mean, when you have a nice long trip like these missions, we get REALLY accurate pretty quick, and there are smaller more reliable thrusters we can use to make small course corrections once we get the data on the initial thrust errors.

We've got really good computers compared to even 10 years ago.

For example, the dart mission accurately hit a 530 foot object orbiting another 2500 foot object which was 11 million kilometers from earth (1/10th of the distance from earth to mars). All were travelling at several km/s. While that isn't the type of rendezvous the ISS is looking for :) it shows the extreme accuracy we are able to achieve aligning with objects and doing orbital mechanics.

There are no technological limitations on docking with the ISS, but huge practical disadvantages as talked about above. We aren't going to spend the money designing a return ship that can slow down into a stable orbit near the ISS and then correct into a docking procedure when we can just slam into the atmosphere with a heat shield and get the data back faster, easier, and WAY cheaper.

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cjameshuff t1_j602f2y wrote

Yeah, the issue isn't accuracy. It wouldn't be that difficult to hit the ISS. The solution space for a rendezvous with near-zero relative velocity is rather more restrictive.

For Earth, there's vast areas suitable as landing locations, where it doesn't really matter what direction we approach them from. We just need atmospheric entry to happen at a reasonable angle and velocity.

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