We used to think that space was empty, the sun was a benign source of energy, stars were quietly doing their sun-like thing.
It has slowly dawned on us over my lifetime that the universe is a dangerous place:
- There's quite enough stuff in the space between the stars to make high speed interstellar travel quite dangerous, even though our local area is a low density bubble.
- Violent astronomical events can significantly affect life on Earth if they are close enough.
- Some of these violent events put out most of their energy in a single line, and even distant ones could be significant if they point directly at us, as a supernova in another galaxy recently showed.
- The Sun has convulsions that send a lot of energy concentrated in a line, and the geological record shows that this can have a big effect on the rare occasions that it points at us.
- And there are a lot of high energy cosmic rays (alpha = helium nucleus, beta = proton, gamma = photon). The atmosphere and magnetic field protect us imperfectly and variably, but it is a problem for the conquest of space.
- Some astronomical events produce so many neutrinos that, if close enough, they could kill us all just from their low probability interactions with us, and being on the other side of the Earth from the source wouldn't protect at all.
- And there are comets and asteroids that might hit Earth, and smaller stuff that will affect our space activities. There are whole planets and black holes wandering through interstellar space.
We are subject to a selection effect. We are here observing this dangerous universe, so that places a limit on how badly any of these dangers could have affected Earth in the past. We need to evaluate these risks without using our lucky past history as an indicator of future danger. Maybe it takes a thousand, or a billion, or more, potential Earths in the Universe for one to come as far as ours has in a life-friendly state.
The atmosphere is an imperfect protection. We could improve this by living most of the time in protected environments of various sorts to protect against various brief and extended disasters. It might be just as easy to build such protected environments in space. A large circular structure, rotating to give 1G of gravity, could be protected by a significant envelope of ice.
As a safe haven the Earth has a significant disadvantage: It can't dodge. Sometime in the coming centuries, I expect that living in a large moveable city in space will be seen as safer than the surface of planets. Of course to dodge you first need to detect all these dangers, and that will be a job for humanity as a whole. This seems to me to be the way we will eventually move to other star systems: in large repairable craft that will function comfortably for long periods between the stars.
