The universe is trying to kill us. In Death From the Skies! by our own Bad Astronomer, Phil Plait, counts the ways the world will end. Start worrying in a few million years about a cosmic dust collision, when the sun hits the closest spiral arm of our galaxy. Take your chances with an exploding star. Or manage to escape these threats, and you just get an extra 10^35 years before all matter decays anyway.
EventDamageOdds of fatality per lifetimePreventable?
asteroid impactLocal for a small rock, global for a big one1 in 700,000Almost 100% preventable. Identify potential impactors, then blow them up or push them out of the way
Collapse of power grid, potential ozone depletion0*
Non-preventable, but mitigatable
Build robust power grids
supernovaOzone depletion, radiation1 in 10,000,000Not preventable
gamma-ray burstOzone depletion, radiation, setting planet on fire1 in 14,000,000Not preventable
black holeDestruction of Earth1 in 1,000,000,000,000Not preventable
Humanity wiped out by aliens, space bugs give us runny noses?
Preventable, assuming we colonize the galaxy first.
Otherwise, forget it.
death of the sunEarth cooked to a crisp0 †Not preventable, but we have a long time to go yet
galactic doomIce ages, radiation, eaten by supermassive black hole0 †Not preventable, but again, none of these will happen on a human timescale
death of the universeDecay of all matter, collapse of false vacuum0 †Not preventable, but dwarfs any timescale we can imagine
* Fatalities are very unlikely from a solar event, but they can still cause extensive damage.
† These events all take billions of years (at least!) to unfold, so the chances of them happening during your lifetime are zero, but are inevitable over longer times.
Table reprinted by arrangement with Viking, a member of Penguin Group (USA) Inc., from DEATH FROM THE SKIES! © Philip Plait, Ph.D., 2008