Elon Musk, the billionaire founder and CEO of the private spaceflight company SpaceX, wants to help establish a Mars colony of up to 80,000 people by ferrying explorers to the Red Planet for perhaps $500,000 a trip.
(...) we've put all our eggs in one basket. If we were on many worlds and were to mess up down here, there's a way for the human species to continue. I don't for a moment propose that the Earth is a disposable planet, and we have to put enormous efforts into making sure we don't muss up down here. But there is a chance.
A miniature black hole, dropped into the centre of the Earth (possibly as some sort of weapon).
An asteroid impact sufficiently large to turn the Earth into another asteroid belt.
An asteroid impact sufficiently large to divert the Earth's orbit, sending it falling into the Sun (admittedly, this would have an effect on a Martian colony as well, but not necessarily a fatal one)