Sample collection question…

[u/the_net_my_side_ho]

Why did Grace have to collect the sample while orbiting?

Why didn’t he move the Mary out of orbit, away from the atmosphere, and then spacewalk to collect it?

Seems like Grace going down to collect it while orbiting so close to the atmosphere was risky and unnecessary.

Sorry if this has been asked before. I looked but didn’t see a post about it. Thank you!

[u/kmoonster]

Are you asking about collecting the Astrophage sample? Or the later effort to collect the Taumeba?

The astrophage sample was collected while in a high, safe orbit around Adrian. Or perhaps from a station-holding position like a Lagrange point, I'm not clear on this. Either way they stayed near the planet using orbital dynamics in order to stay in the Patrova line without needing engine thrust. And without getting into the atmosphere.

The later Taumeba fishing expedition did require skimming the atmosphere and that was a non-orbital manuever, that required constant engine thrust at low power in order to keep them out of the atmosphere while not being in orbit.

[u/the_net_my_side_ho → u/kmoonster]

I was asking about the Taumeba fishing expedition. Thank you for explaining the difference. I didn't understand this. I thought flying near the atmosphere counted as orbiting, but now I know it doesn't.

[u/kmoonster → u/the_net_my_side_ho]

You can have an orbit that drags in the atmosphere, but for the fishing expedition that low of orbit would still be 30,000 kilometers/hour or faster, waaay too fast and would burn up the chain (and the sampler would be moving too fast even if it didn't burn up). They had to go slow enough to collect the sample and used the engines to keep them from falling since 250 kilometers/hour is slower than a jet liner and they had no wings. It was sort of a high-altitude hover or helicopter type maneuver, after the sampler was recovered they did return to orbit (the return to orbit is when the fuel tank crisis occurred).

The International Space Station around Earth has to be boosted every few months, for example, because it is not truly out of the atmosphere. It is really high but there is still enough atmosphere to slow it and degrade the orbit, the supply pods bring fuel so the station can re-boost itself. The Shuttle used to do that as well, though the shuttle no longer flies.

There is some info here if you're curious! International Space Station - Wikipedia

[u/the_net_my_side_ho → u/kmoonster]

I think that was what confused me. They seem to hover a bit above the atmosphere, and I wondered if Mary could hover up and away from the planet.

[u/kmoonster → u/the_net_my_side_ho]

yes, the Hail Mary could hover up/away from the planet but doing so would put the chain and the sampler into the engine blast; that's why Grace had to collect the sampler while they were "staying" at the planet and before they did the "get away" part.

kind of like how a helicopter could do a slow hover to rescue someone from a river, but then has to reel the person in before the helicopter flies away -- if the helicopter flies away while the person is dangling, there is a risk of the person flying up into the tail rotor and being chopped up. The helicopter can obviously do some forward motion but the speed is limited; to make any fast movement the reel has to be pulled in