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CountingMyDick t1_j5x2tjs wrote

Reply to comment by fishling 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

To be slightly annoyingly pedantic, the actual return trajectory is most likely nowhere near the ISS, but if they were planning to dock with the ISS, they would presumably cheaply adjust their incoming trajectory to be as close to the ISS orbit as possible while still far away. If they were rather good at it, presumably they could get pretty close to only that 4km/s of total DeltaV to match orbits.

Of course that's still a hell of a lot of DeltaV versus aerobraking.

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cjameshuff t1_j5yfrbi wrote

> they would presumably cheaply adjust their incoming trajectory to be as close to the ISS orbit as possible

That's a pretty major presumption. It can be hard enough just intercepting Earth, requiring that interception to also occur with the probe trajectory aligned with the orbital plane of the ISS would greatly restrict the set of targets we could retrieve samples from.

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UnamedStreamNumber9 t1_j5zb3na wrote

They can be precise enough to know where it’s coming down. That said, Osiris ain’t stopping at the earth. It’s just dropping off a package and heading back out to intercept Apopthis

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KingZarkon t1_j5z8y96 wrote

Isn't it just Newtonian math? I mean, we know the weight of the craft, how much thrust it has etc, so why is it so hard to calculate?

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rabidferret t1_j5zfqvx wrote

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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cjameshuff t1_j600x1g wrote

It's not a matter of it being hard to calculate, it's a matter of the solar system not being physically arranged to conveniently allow it.

A minimum-energy transit will come at Earth roughly aligned with its orbital motion. To match planes with the ISS, the return must happen at one of the two times a year where the ISS orbit is also aligned with that motion, which means the trip must have started on the opposite side of the sun from that point, half a transfer orbit earlier. But we don't control where other solar system objects are or what their motions are. Windows to/from Mars occur every 26 months. If by chance things are properly aligned one year, it will be 60 degrees off the next time, and most Earth-Mars windows will be unusable: it will be 3 launch windows, 6.5 years, before they align again. And they in reality don't line up in such nice whole numbers, so in reality you're going to have a substantial plane correction to make on arrival, even with such limited windows.

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fishling t1_j5xmq9a wrote

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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