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It appears that so much material was collected by OSIRIS-REx from the asteroid Bennu, that the cover of the collection head won't close, and some of the sample is floating away.

In reviewing these images, the OSIRIS-REx team noticed both that the head appeared to be full of asteroid particles, and that some of these particles appeared to be escaping slowly from the sample collector, called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) head. They suspect bits of material are passing through small gaps where a mylar flap – the collector’s “lid” – is slightly wedged open by larger rocks.

Because of this development, the OSIRIS-REx team is skipping their plan to measure the mass of the sample, and instead stow it right away in the sample return container.

To preserve the remaining material, the mission team decided to forego the Sample Mass Measurement activity originally scheduled for Saturday, Oct. 24, and canceled a braking burn scheduled for Friday to minimize any acceleration to the spacecraft.

Will this uncertainty in the mass of the sample affect the planning of the trajectory of the return capsule? (For example, requiring extra correction burns.)

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  • $\begingroup$ Val Kilmer has been known to sneak into spacecraft collected sample spaces, has his whereabouts been verified lately? $\endgroup$ – uhoh Oct 24 '20 at 7:49
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    $\begingroup$ They may still be able to measure the mass, perhaps less accurately, once it's in the return capsule. In event the variation of acceleration due to a kilo or less of extra samples in a 1 tonne plus spacecraft is probably less than the normal variability in the performance of the engines. $\endgroup$ – Steve Linton Oct 24 '20 at 21:48
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Will this uncertainty in the mass of the sample affect the planning of the trajectory of the return capsule?

Not by much, if at all. The burn commands to OSIRIS-ReX are commanded delta V rather than timed burns. Using timed burns does result in more and/or larger correction maneuvers than does using commanded delta V burns, even with spacecraft that don't have a payload with uncertain mass.

The mass of the retrieved sample is very small compared to the overall spacecraft mass. The dry mass of OSIRIS-ReX is 880 kg. The amount of material picked up is perhaps 400 grams, less than 0.05% of the spacecraft's dry mass. This is tiny, I suspect less than the uncertainty in the dry mass. The uncertainty in the remaining propellant mass is much larger yet as the thrust from the main engine is typically known to within 5% or so, perhaps 1% on a very good day. Even a 1% variation in thrust will result in a ~12 kg uncertainty in residual propellant mass, about 1.5 orders of magnitude larger than the 0.4 kilograms of retrieved asteroid material. A 5% variation will result in a ~60 kg uncertainty in residual propellant mass, two orders of magnitude larger than the mass of the retrieved asteroid material.

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    $\begingroup$ Thank you for the answer. Do you have a source please for "delta V rather than timed burns"? $\endgroup$ – DrSheldon Oct 26 '20 at 23:26
  • $\begingroup$ fyi I've just asked How exactly do "commanded delta-v burns" work in practice? (e.g. OSIRIS-REx) $\endgroup$ – uhoh Oct 27 '20 at 1:17
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    $\begingroup$ @DrSheldon The eoPortal page on OSIRIS-ReX, for example, emphasis mine: "In this type of sequence, OSIRIS-REx’s momentum wheels turn the spacecraft to point the ACS thrusters toward the desired direction for the burn, and the thrusters fire. After the burn, the momentum wheels turn the spacecraft back to its previous orientation. The total thrust is monitored by an on-board accelerometer that will stop the maneuver once the desired thrust is achieved." $\endgroup$ – David Hammen Oct 27 '20 at 10:04
  • $\begingroup$ The above quote is not quite correct as accelerometers do not stop the maneuver. Accelerometers measure non-gravitational acceleration; that is all they do. It is the on-board guidance, navigation, and control flight software that uses accelerometer readings to start and stop the burn. $\endgroup$ – David Hammen Oct 27 '20 at 10:15
  • $\begingroup$ @DavidHammen: Thank you. Answer accepted. $\endgroup$ – DrSheldon Oct 27 '20 at 13:07

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