How big would a planet have to be before its gravity made space travel impossible for the inhabitants? (continued)
James Arnold
London, UK
I feel for the asker of this question. You printed three interesting responses, none of which quite gives an answer to the question!
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First, we need a sensible definition of impossible. Let鈥檚 say that space travel is impossible if the planet鈥檚 escape velocity is equal to the speed of light. Although it seems unlikely that any future technology might be able to launch spaceships at anywhere even close to this speed, it鈥檚 a good limit. How to reach that launch velocity in a practical time frame without killing the occupants with g-forces is a problem we can leave to future engineers!
Then, we need one more assumption, as a previous correspondent, Alex McDowell (11 July), points out: the density of the planet. Let鈥檚 use Earth鈥檚 average density, which is about 5500 kilograms per cubic metre.
Now, we set the escape velocity as the speed of light, fiddle with the equations a bit, and we find that this planet would have a radius of about 170 million kilometres! Interestingly, that is about the same as Earth鈥檚 orbital radius around the sun. Of course, our choice of 鈥渋mpossible鈥� means that, by definition, this object is, in fact, a black hole, so it also isn鈥檛 likely to have rocket scientists tackling this problem.
Linda Phillips
Narrogin, Western Australia
This question is related to another one: how big does a planet need to be to retain its atmosphere?
How to reach that launch velocity without killing the occupants with g-forces is a problem for future engineers
Defining Earth鈥檚 mean gravity as 1.0 g, then gravity of about 1.2 g would be sufficient to prevent space travel, using known rocket technology. Gravity on Mars is 0.38 g by comparison, on the moon 0.17 g.
A planetary body needs gravity of 0.6 g to prevent loss of its atmosphere, hence why atmosphere is almost non-existent on Mars and the moon.
So, for life forms requiring an atmosphere and space travel, the gravity range is 0.6 to 1.2 g.
An ideal gravity would be 0.8 g: strong enough to retain the atmosphere, while more easily facilitating rockets and space travel.
For exoplanets whose gravity has been estimated, some 40 per cent of them have gravity in the desired range, suggesting that, if life forms are plentiful, it isn鈥檛 high gravity preventing interplanetary travel.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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