# Is there a point in a planets gravity that complex(human intelligence) life can exist but space flight is impossible?

Title but I've always wondered if there's a scale of a planets gravity and possible advancement of a species. If the gravity is too high what does it limit for technology? If the gravity is too high can complex life even form?

• Nov 23, 2022 at 23:03
• You ask two questions here. The first, "what strength of gravity prohibits spaceflight", is answered in Alexander's comment's links. The second, I'm not sure we can really answer, but you could take a look at the planets discussed in those links and ask yourself: "could complex life form on these worlds?"
– BMF
Nov 24, 2022 at 1:56

# Nuclear rockets can lift you off the planet even at very high gravities.

Chemical rockets have trouble with high gravity planets because the weight of the fuel makes you slower and you need to carry more fuel to overcome that and the numbers don't work out well for chemistry. Nuclear power is about two million times more energy dense than chemical power, so it can overcome even very high gravity.

We don't use it because rockets often explode, but we could.

Big Planet

A planet of mass $$M$$ and radius $$r$$ has surface gravity proportional to $$\frac{M}{r^2}$$. Meaning if you stretch the Earth into a ball twice the distance from pole to pole, the inhabitants feel four times as light $$-$$ and are instantly killed by stretching.

To fix this, you can stretch the Earth and then replace it with an Earth four times the mass. The inhabitants feel the same as before $$-$$ Or they would if they were still alive.

But why do this at all? Because it increases the escape velocity of course! This makes it harder to do space flight.

The escape velocity of the planet is proportional to $$\sqrt{\frac{M}{r}}$$. So the big planet with twice the radius and four times the mass has escape velocity $$\sqrt{\frac{4M}{2r}} =\sqrt{\frac{2M}{r}} = \sqrt 2 \sqrt{\frac{M}{r}}$$ which is $$\sqrt 2 \simeq 1.41$$ times as much as the small planet.

Make your planet a hundred times wider and it is ten times harder to get into space. Complex intelligence can still develop just fine. It won't even notice a difference before it tries to go into space. Only then will it notice rockets need an AWFUL lot of fuel.

You don't need gravity to stop space travel. Perhaps the planet is too close to their sun meaning that there is an increased radiation level. They might have tried once and failed with the entire project dying. Or they could be trapped inside a dark nebula (basically a huge cloud) which means that they never see the sky and have never imagined leaving (inspiration from Douglas Adams)

• How would high radiation prevent a civilization from leaving for space?
– BMF
Nov 25, 2022 at 21:42
• @BMF it makes it harder to put people in orbit so the civilization might take longer to develop a space program, if ever at all. Nov 26, 2022 at 18:58