“Δv for velocity, delta for change. In space, this is the measure of the change in velocity required to get from one place to another – thus, a measure of the energy required to do it. Everything is moving already. But to get something from the (moving) surface of the Earth into orbit around it, requires a minimum Δv of ten kilometers per second; to leave the Earth’s orbit and fly to Mars requires a minimum Δv of 3.6 kilometers per second; and to orbit Mars and land on it requires a Δv of about one kilometer per second. The hardest part is leaving Earth behind, for that is by far the deepest gravity well involved. Climbing up that steep curve of spacetime takes tremendous force, shifting the direction of an enormous inertia.
History too has an inertia. In the four dimensions of spacetime, particles (or events) have directionality; mathematicians, trying to show this, draw what they call ‘world lines’ on graphs. In human affairs, individual world lines form a thick tangle, curling out of the darkness of pre-history and stretching through time: a cable the size of Earth itself, spiraling round the sun on a long curved course. That cable of tangled world lines is history. Seeing where it has been, it is clear where it is going – it is a matter of simple extrapolation. For what kind of Δv would it take to escape history, to escape an inertia that powerful, and carve a new course?
The hardest part is leaving Earth behind.”
– Kim Stanley Robinson, Red Mars (1993)