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How big would a planet have to be before its gravity made space travel impossible for the inhabitants?
Alex McDowell
London, UK
When lifting off in a space rocket, on a planet with strong gravity, the problem would be the weight of the fuel. If gravity were more than about twice that of Earth’s, lifting off using the chemical rockets we currently have would be practically impossible.
On planets with stronger gravity, inhabitants may overcome the problem with nuclear-powered rockets or antimatter drive units; if 1 gram of antimatter annihilates a gram of matter, it releases the energy of three Hiroshima-sized bombs.
How about firing ships into space Jules Verne-style – for example, accelerating craft by magnetic levitation along several kilometres of evacuated tube, before opening the end to release the ship? Or maybe outside help could be the answer: visitors from other worlds could install space elevators, which would climb cables tethered on one end to a point on Earth’s surface and the other to a counterweight in orbit over that point.
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A double-mass Earth would have a thicker atmosphere with more hydrogen, and would probably be a water world
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As for how big a planet would need to be to make space travel impossible, this would depend on the planet’s density. Earth’s core is mainly nickel and iron, which is dense. If it were silica or alumina, it would have about a third of the density of nickel/iron. Hence a planet made mainly of silica or alumina throughout would need to be bigger to have the same gravity as Earth.
For a planet of uniform density, the surface gravity is proportional to its radius.
Hillary Shaw
Newport, Shropshire, UK
If Earth were just twice its mass, we would struggle to access space with chemical propellants. The weight of propellant increases exponentially as it has to be lifted.
We could use electromagnetic or nuclear propulsion, but other things would change. A double-mass Earth would have a thicker atmosphere, making the rocket’s job harder still. Worse, that atmosphere would contain more hydrogen, perhaps making Earth a Hycean planet or mini-Neptune. It would have lower mountains and probably be a water world, lacking dry land or free oxygen. Life would be oceanic; its energy source might be sulphur at oceanic vents, or it might photosynthesise near the surface, splitting water into hydrogen and oxygen, then recombining them within cells. Intelligent beings might look like jellyfish or octopuses.
But what technology would they develop? Electrical or nuclear technology seems unlikely given how conductive an ocean world would be. And without fire, there probably wouldn’t be any metal smelting. Such intelligent life might be rather inward-looking spatially, developing much in the arts, geography, geology, biology, but less in physics and chemistry.
It might never escape its ocean home, perhaps never even flying in the hydrogen atmosphere (what would provide lift?), and it might imagine its planet and cloudy, thick atmosphere above was the entire universe, heated and lit by some external, unknowable (divine?) source.
Eric Kvaalen
Les Essarts-le-Roi, France
Depends what you mean by “impossible”. To escape the planet, you have to give your spaceship a certain minimum of what’s called “delta-v”: basically, change of velocity. The minimum required is the escape velocity of the planet, which is proportional to its diameter and to the square root of its average density.
The problem is that the size of rocket needed to achieve a particular delta-v goes up exponentially with delta-v, for a given kind of fuel. That’s why the Saturn V rocket had to be so big, just to send a small spacecraft to the moon. So the size of the rocket needed goes up exponentially with the diameter of the planet, for a given planet density.
Commonly, the size of rocket goes up by about a factor of 10 for every 10 kilometre per second of delta-v. Jupiter has an escape velocity of 60 km/s, compared with ours of 11 km/s. That extra 49 km/s means the rocket would need to be about 100,000 times bigger than the Saturn V!
Earth is rather special in that it is about as big as it can be while still allowing us to reach outer space. If the planet has a very low moon, then you need only enough delta-v to get to its height, then the moon can give you a boost to escape the planet.
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