# Thread: Boats and balloons on other planets

1. Member
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Jun 2010
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## Boats and balloons on other planets

(For the following, let's assume an extrasolar planet with liquid surface water, and an atmosphere of similar nitrogen/oxygen mix to Earth's)

I was talking with some friends and the subject of boats on other planets with stronger or weaker gravity came up. Someone commented that if the gravity on another planet was too strong, your boat designed for Earth would sink.

I thought about it for a while, and personally I concluded that gravity doesn't matter (although there was some disagreement about this). If the planet has, let's say 2 g's of surface gravity, then it pulls on the boat with 2x as much force, but it also pulls on the water with 2x as much force. So the buoyant force from each unit of water volume displaced is 2x as much, so the boat shouldn't float any higher or lower than it does on Earth. A 10,000 kg boat will displace 10,000 kg of water regardless of what planet it's on. It wouldn't sink no matter how strong the gravity was (until it started leaking from the pressure on the hull or being crushed under its own weight, obviously).

Then I started thinking about helium balloons and dirigibles. How would they be affected? A planet with stronger gravity ought to be able to "hold on to more air" (keep it from being lost to space due to solar wind for example) so the air density near or within a few hundred meters of the surface would be more. However I'm guessing this doesn't go in a linear relationship with the gravity. The helium pressure in a balloon is always going to be a little more than the air pressure outside (outside pressure + the tension in the balloon material), so does that mean the ratio of helium density to air density would stay the same, resulting in equal buoyant force for an equal sized balloon on Earth? Or would the force actually increase as the air got more dense?

2. You are right. Gravity makes the water/air heavier as well as the boat/balloon. Relative density is relative. So no sinking.

3. Originally Posted by dgh64
(For the following, let's assume an extrasolar planet with liquid surface water, and an atmosphere of similar nitrogen/oxygen mix to Earth's)

I was talking with some friends and the subject of boats on other planets with stronger or weaker gravity came up. Someone commented that if the gravity on another planet was too strong, your boat designed for Earth would sink.

I thought about it for a while, and personally I concluded that gravity doesn't matter (although there was some disagreement about this). If the planet has, let's say 2 g's of surface gravity, then it pulls on the boat with 2x as much force, but it also pulls on the water with 2x as much force. So the buoyant force from each unit of water volume displaced is 2x as much, so the boat shouldn't float any higher or lower than it does on Earth. A 10,000 kg boat will displace 10,000 kg of water regardless of what planet it's on. It wouldn't sink no matter how strong the gravity was (until it started leaking from the pressure on the hull or being crushed under its own weight, obviously).

Then I started thinking about helium balloons and dirigibles. How would they be affected? A planet with stronger gravity ought to be able to "hold on to more air" (keep it from being lost to space due to solar wind for example) so the air density near or within a few hundred meters of the surface would be more. However I'm guessing this doesn't go in a linear relationship with the gravity. The helium pressure in a balloon is always going to be a little more than the air pressure outside (outside pressure + the tension in the balloon material), so does that mean the ratio of helium density to air density would stay the same, resulting in equal buoyant force for an equal sized balloon on Earth? Or would the force actually increase as the air got more dense?
There is not a fixed relationship between gravity and the amount of air a planet can hold. As a case in point Venus has well over 100 times as much air as Earth, despite being hotter and having slightly less gravity. Sure, it is being stripped by the solar wind but it will take a very long time to blow away that vast amount of atmosphere on Venus. In a thought exercise the gravity, the total mass of air, and the molecular weight of the air can be independent variables. The atmosphere of Venus, which is mostly carbon dioxide, is about 1.5 times as dense as Earth's air would be at the same temperature and pressure.

4. Member
Join Date
Jun 2010
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Originally Posted by Hornblower
There is not a fixed relationship between gravity and the amount of air a planet can hold. As a case in point Venus has well over 100 times as much air as Earth, despite being hotter and having slightly less gravity. Sure, it is being stripped by the solar wind but it will take a very long time to blow away that vast amount of atmosphere on Venus. In a thought exercise the gravity, the total mass of air, and the molecular weight of the air can be independent variables. The atmosphere of Venus, which is mostly carbon dioxide, is about 1.5 times as dense as Earth's air would be at the same temperature and pressure.
I suspected something like that. So, as far as zeppelins are concerned, results will vary depending on a lot of other factors besides gravity.

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