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Comment by SamBorick | Hacker News Reader

I recommend the scott manly video another commenter linked to for a more full explanation, but:

The sample container that's currently jammed open is on the end of a long arm, while the sample return capsule is mounted on the body of the spacecraft. The only way to measure mass in free-fall is to spin the entire craft and measure where the center of rotation is compered to before the sample capture. The difference is then used to calculate the sample mass.

With the sample at the end of a long arm, the spacecraft doesn't have to rotate very fast to get a large enough difference in rotation to measure the sample. If the sample is in the return capsule, the moment is so much smaller that it's probably below the sensitivity of the instruments they were planning to use.

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Bright side - they have so much material, that weighting it is not really that much of a concern. They were hopeful to get maybe 50 grams of material, they got about 400g, but maybe 100g will be lost. They won't have exact number, but they will have more than expected anyway.

Won't the extra weight require a change in earth re-entry fuel calculation etc. ? Without knowing the exact change in weight, they are going to be firing in the dark, as it were.

It's not enough to make a difference either way around it. The sample return container could be filled to the brim with pure osmium or iridium and would make it through the atmosphere just fine.

The scientists are mostly worried because they want as much of the sample as they can possibly get, and they're a little worried about closing the sample container - if there's a rock that gets stuck somewhere, the container might not seal shut and then they've got a real bad problem they've gotta contend with.

It's 2020, they don't want to jinx it, just shut it up in the container and wait for the time capsule to arrive in 2023.

I don't know that much, but I suppose they will do a burn and then some corrections. If mass difference is big enough to be detectable for reentry, it will be detectable when they are going back to earth.

A key difference is that spinning the full satellite with the arm extended and measuring the center of rotation measures the mass of the samples collected regardless of the thrusters/reaction wheels.

Measuring the trajectory of the reentry vehicle gives a parameter that can be used to estimate the sample mass assuming nominal thrust values, but you can't distinguish between an underperforming or overperforming rocket engine and heavier or lighter samples. This ambiguity doesn't really matter for the purposes of burn correction - you need to get to the correct reentry trajectory no matter what caused the deviation - but your sample mass estimate may be off.

I don't think there's a whole lot of utility in knowing the sample mass, as long as you've got enough (and they've definitely got enough!), it would just help satiate the curiosity of the scientists and engineers who otherwise have to wait a few more years to see their results.

Instead of adjusting fuel you can probably compensate by changing when you do the burn. And there are likely to be midcourse corrections anyways.

That makes me wonder: how is it possible to even determine what the correction should be? Space is vast, how do they determine the position of a microscopic vessel precisely enough to know how much to burn?

Multiple correction burns along the way. For equivalent burns, the later along the trajectory the burn, the smaller its net effect on final position. So it's a series of increasingly better approximations.

Try it in ksp. It's quite illuminating to plan your burns and see their effect graphically and build an intuition for celestial mechanics and astrogation.

The primary instrument is probably a star tracker. It's possible to position yourself within the galaxy to a very high degree of accuracy by looking at changes in relative position of stars and planets which have a known "position" (really orbits).

They should also be able to figure it out via delta-V per unit of thrust when they execute any orbit-change maneuvers to return to Earth, but that's a much less accurate metric (perhaps not accurate enough to differentiate a spare 100 grams of material).

They might not even be able to measure that small of a difference in the spacecraft's acceleration, to be honest. We're talking about a difference in grams of a spacecraft that's somewhere north of 1000kg right now - wet mass was maybe 1529kg at launch [according to NASA; Lockheed said the bus could be up to 2000kg at launch, so I'm not sure who to believe being honest - it's a little hard to believe they'd risk things and short the mission on that much hydrazine], dry mass 860kg, split the difference and it's probably around 1200kg [or up to 1500kg] currently.

It's a little easier to believe they could detect whether or not they got 60g by the last significant digit change in the moment of inertia from their instruments by nulling out the rotation... It's a little harder to believe they could detect a difference between 0 and maybe 500g in the sample return container via their acceleration data. I doubt they even know how much hydrazine and helium is left with all that much precision - at more than hundred grams a second burned, it's easy to believe there's some amount of accounting slop.

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