Humanity’s last hour has finally come. An asteroid, called Vulcan’s Hammer, is on a collision course with Earth, and large enough to cause global environmental collapse. As far as humans are concerned, Earth will die.
Two plans were put in place.
The first plan plans to eliminate the present danger. A set of nuclear warheads will be refitted and sent against the asteroid, aiming to fracture it, hopefully sending most of the fragments away from the collision course. The plan only offers one chance to deflect the asteroid - the missiles, even with the enhancements, only have enough fuel to reach the asteroid very close to Earth - leaving little time for the fragments to disperse.
At the same time, a backup plan is set in motion. Instead of preventing the catastrophe, this one is tasked with saving as much of humanity as possible. Huge archives are created, put together, with as much of Earth’s science, culture and history as possible. Large amounts of infrastructure are built and staffed, with the ultimate goal of creating Earth’s first interstellar ship - a colony ship, to give mankind a new hope. Even with the available funding and resources, the ship will at best be able to carry a few hundred people - a long shot from the billions of humans living on the planet. Dozens of brand new barely tested technologies are incorporated - automated colony core constructors, computer controlled trucks, interstellar probes, orbital geological probes, solar satellites; technologies both for the ship itself (an experimental fusion plant powering massive ion engines, suspended animation for the passengers, heuristic analytical computers and more) and the fledgling colony, cut off from all support, in territory more hostile than anywhere man has settled before.
As the spaceship is nearing completion, the missiles are launched. The asteroid is hit, but the estimates about its composition were way off. Instead of neatly cleaving in two fragments and small amounts of debris as predicted, the asteroid breaks into five major fragments and a large cloud of dust, comprising large part of the asteroid’s mass - and all of them are projected to hit the Earth. If anything, we’ve made it worse. With this revelation, the final launch schedule of the starship is started, the ship is crewed, loaded with the last few missing supplies, and awaits the moment of collision, comfortably parked in a safe distance - hoping there will be a good ending after all.
In the end, the crew watches in silence as the Earth is hit. The magnitude of destruction is clearly visible using the ship’s telescope - fire storms raging through the atmosphere, burning forests, massive amounts of particulate ejecta, wake radiation and ejecta destroying the ozone layer and disrupting the Van Allen belts. Noone is responding to any attempts at communication. The ship maintains position for a few days, to let the atmospheric ionization subside, but the reality is absolute - Earth is dead. The last surviving humans enter suspended animation and the massive starship starts its acceleration out of the solar system, towards a new home.