If the container is open, how are they planning to retrieve the sample without losing it on reentry?
This, I'm guessing.
Also, I found these parts interesting:
>"Regardless of what’s on board, Osiris-Rex will still leave the vicinity of the asteroid in March — that’s the earliest possible departure given the relative locations of Earth and Bennu. The samples won’t make it back until 2023, seven years after the spacecraft rocketed away from Cape Canaveral."
>Because of the sudden turn of events, scientists won’t know how much the sample capsule holds until it’s back on Earth. They initially planned to spin the spacecraft to measure the contents, but that maneuver was canceled since it could spill even more debris.
>“I think we’re going to have to wait until we get home to know precisely how much we have,” Lauretta told reporters. “As you can imagine, that’s hard. ... But the good news is we see a lot of material.”
In that light, this quote confuses me:
> scientists won’t know how much the sample capsule holds until it’s back on Earth.
They wanted to spin the spacecraft to measure the mass, but they can't risk losing more samples from an open container.
But why can't they spin the spacecraft after the sample is secure in the return capsule, but before reaching Earth? Is it precisely in the center of mass?
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.
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.
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.
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.
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.
They might be able to, but there’s no reason to. The purpose of the sample mass measurement was to determine if they needed to try again, however that’s not an option with a jammed container.