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A fun Deimos fact is that there exist stable prograde orbits around it, while there are no such orbits around its bigger brother Phobos. Because Phobos orbits c

A fun Deimos fact is that there exist stable prograde orbits around it, while there are no such orbits around its bigger brother Phobos. Because Phobos orbits closer to Mars, its gravitational sphere of influence is smaller than its physical size. A nice paper on the different ways of orbiting the Martian moons and their trade-offs: https://sci-hub.ru/https://doi.org/10.2514/6.2012-5067 https://sci-hub.ru/https://doi.org/10.2514/6.2012-5067

mcv 4y ago
If we're sharing fun facts: apparently it takes the same amount of delta-V to travel to the surface of Deimos as it does to the surface of the Moon. Because of the Moon's much larger gravity. There have been people playing with the possibility of using it as a refuelling station, because it's thought to contain a lot of water.
franky47 4y ago
Does that also mean you can't land on Phobos (and stay on the "ground" untethered)?
- rovingnothing29 4y ago
  So Phobos is space Australia.
- idlewords 4y 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 4y 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 4y 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 4y 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 4y 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.
  - b112 4y 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 4y 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 4y 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.

mcv 4y ago

If we're sharing fun facts: apparently it takes the same amount of delta-V to travel to the surface of Deimos as it does to the surface of the Moon. Because of the Moon's much larger gravity. There have been people playing with the possibility of using it as a refuelling station, because it's thought to contain a lot of water.

franky47 4y ago

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

rovingnothing29 4y ago
So Phobos is space Australia.
idlewords 4y 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 4y 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 4y 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 4y 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 4y 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.
- b112 4y 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 4y 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 4y 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.