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Does that also mean you can't land on Phobos (and stay on the "ground" untethered)?

rovingnothing29 3y ago
So Phobos is space Australia.
idlewords 3y ago
You can land on Phobos just fine, and unlike on Deimos there's no danger of accidentally jumping off into space (Phobos escape velocity is about 41 km/h). All the 'sphere of influence' stuff means is that orbits are too perturbed by unevenness in the Martian gravitational field and can't stay stable. Deimos is much smaller, but because it's further from Mars there is a region right above the surface (below 700 meters or so) where you could have stable prograde orbits.
- kzrdude 3y ago
  Since both have gravitational acceleration below one thousandth of a g, I guess that counts as microgravity? Visiting those would be very interesting as an experience of gravity. I wonder if it would even be properly perceived as gravity, you'd have trouble orienting yourself and standing "up".
  - idlewords 3y ago
    It's a good question. There's been very little data on partial gravity, but what there is shows that a pronounced physical sense of "down" and "up" doesn't kick in until about 1/5g [1]. That said, a horizon and big rock underfoot are powerful visual cues, and it's anyone's guess what it would feel like to walk on a small celestial body like that. There's one good way to find out! [1] https://www.sciencedirect.com/science/article/abs/pii/S030439401201244X https://www.sciencedirect.com/science/article/abs/pii/S03043...
    - skykooler 3y ago
      Does that mean that on the Moon (1/6g) you would have trouble telling which way is "up" if you closed your eyes?
      - antonvs 3y ago
        According to https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0106207 https://journals.plos.org/plosone/article?id=10.1371/journal... : > "a gravitational field of about 0.15 g is necessary to provide effective orientation information" That's slightly less than lunar gravity (0.165 g). But, it's about the minimum amount needed to be able to use gravity to detect orientation, and is not necessarily enough to detect it reliably. That's thought to be one reason astronauts fell so often on the moon, although there were certainly other factors as well. The paper also observes: > Benson [30] showed that 0.22 g was not adequate to provide a vertical reference during experiments in the International Microgravity Laboratory (IML-1) on board Spacelab and Clément et al. [31] showed that in microgravity, 0.5 g provided by a centrifuge was enough to produce a perceived tilt of 90°. These values suggest that indeed it is likely to be the case that even after adaptation to a microgravity environment, forces in excess of 0.15–0.3 g are required to provide a behaviourally useful gravitational reference. To answer your question, since vision is an important part of our orientation mechanism (also analyzed in the linked paper), it's likely that astronauts on the moon would have more difficulty with their eyes closed. Not necessarily that they'd have no sense of up at all, but that it wouldn't necessarily be accurate, so if they tried to walk with their eyes closed, they'd be much more likely to fall.
      - idlewords 3y ago
        It's possible. There seems to be a lot of individual variability around this threshold, although as usual the problem is a severe lack of data on the effects of partial gravity on anything. Alan Shephard and Ed Mitchell reported that the 7 degree tilt of their lunar module interfered with their sleep, which suggests it was noticeable. But it would be hard to point to something in their cabin arrangements that didn't interfere with sleep.
    - Pxtl 3y ago
- b112 3y ago
  and unlike on Deimos there's no danger of accidentally jumping off into space (Phobos escape velocity is about 41 km/h) That, sir, sounds like a challenge! I accept!
- marmakoide 3y ago
  So a big spring would be enough to send one out from Phobos, that's neat. You can literally boink people to space.
caf 3y ago
If it were close enough that Mars pulling you away from Phobos was strong than Phobos pulling you towards it, Phobos also couldn't hold itself together - it would crumble into a ring.

So Phobos is space Australia.

You can land on Phobos just fine, and unlike on Deimos there's no danger of accidentally jumping off into space (Phobos escape velocity is about 41 km/h). All the 'sphere of influence' stuff means is that orbits are too perturbed by unevenness in the Martian gravitational field and can't stay stable. Deimos is much smaller, but because it's further from Mars there is a region right above the surface (below 700 meters or so) where you could have stable prograde orbits.

kzrdude 3y ago
Since both have gravitational acceleration below one thousandth of a g, I guess that counts as microgravity? Visiting those would be very interesting as an experience of gravity. I wonder if it would even be properly perceived as gravity, you'd have trouble orienting yourself and standing "up".
- idlewords 3y ago
  It's a good question. There's been very little data on partial gravity, but what there is shows that a pronounced physical sense of "down" and "up" doesn't kick in until about 1/5g [1]. That said, a horizon and big rock underfoot are powerful visual cues, and it's anyone's guess what it would feel like to walk on a small celestial body like that. There's one good way to find out! [1] https://www.sciencedirect.com/science/article/abs/pii/S030439401201244X https://www.sciencedirect.com/science/article/abs/pii/S03043...
  - skykooler 3y ago
    Does that mean that on the Moon (1/6g) you would have trouble telling which way is "up" if you closed your eyes?
    - antonvs 3y ago
      According to https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0106207 https://journals.plos.org/plosone/article?id=10.1371/journal... : > "a gravitational field of about 0.15 g is necessary to provide effective orientation information" That's slightly less than lunar gravity (0.165 g). But, it's about the minimum amount needed to be able to use gravity to detect orientation, and is not necessarily enough to detect it reliably. That's thought to be one reason astronauts fell so often on the moon, although there were certainly other factors as well. The paper also observes: > Benson [30] showed that 0.22 g was not adequate to provide a vertical reference during experiments in the International Microgravity Laboratory (IML-1) on board Spacelab and Clément et al. [31] showed that in microgravity, 0.5 g provided by a centrifuge was enough to produce a perceived tilt of 90°. These values suggest that indeed it is likely to be the case that even after adaptation to a microgravity environment, forces in excess of 0.15–0.3 g are required to provide a behaviourally useful gravitational reference. To answer your question, since vision is an important part of our orientation mechanism (also analyzed in the linked paper), it's likely that astronauts on the moon would have more difficulty with their eyes closed. Not necessarily that they'd have no sense of up at all, but that it wouldn't necessarily be accurate, so if they tried to walk with their eyes closed, they'd be much more likely to fall.
    - idlewords 3y ago
      It's possible. There seems to be a lot of individual variability around this threshold, although as usual the problem is a severe lack of data on the effects of partial gravity on anything. Alan Shephard and Ed Mitchell reported that the 7 degree tilt of their lunar module interfered with their sleep, which suggests it was noticeable. But it would be hard to point to something in their cabin arrangements that didn't interfere with sleep.
  - Pxtl 3y ago
b112 3y ago
and unlike on Deimos there's no danger of accidentally jumping off into space (Phobos escape velocity is about 41 km/h) That, sir, sounds like a challenge! I accept!
marmakoide 3y ago
So a big spring would be enough to send one out from Phobos, that's neat. You can literally boink people to space.

caf 3y ago

If it were close enough that Mars pulling you away from Phobos was strong than Phobos pulling you towards it, Phobos also couldn't hold itself together - it would crumble into a ring.