Well, let's see, in an ideal world it would have to be the appearance of a bright supernova in the sky, closely followed by the discovery of a comet that would fly very close to Earth, so close that it would be a blazing bright naked eye object at night, casting shadows, and even visible in the daytime sky... these two things would inspire a whole generation of new astronomers and have mankind looking at the sky in wonder again... then the subsequent discovery that the outbound comet would actually impact Mars would change things, and the world would watch fascinated and fearful as Spirit and Oppy sent back images showing the comet steadily growing bigger and brighter in the sky in the weeks and days before impact. Then on Impact Day, Mars would be hit, and images taken by Hubble would show the flash, the shock wave, and the great plume of dust rising up from the impact site, eventually smothering the entire planet in a blanket of dust. On the surface, both Spirit and Oppy survive, just, and when they start sending images back the world has a new understanding of just how dangerous such impacts are, and this leads to renewed interest in and support for space exploration, and space programs to protect and defend Earth from similar assaults...
... but in the
real world I'm thinking:
* Oppy reaches Endeavour and sees stunning scenes...
* A SETI signal is detected
( Ok, maybe sticking my neck out a
little with number 2 there, but I hope for that
every year; I've got to be right eventually!

)