Writing Prompt: The Mistake

As I read about all these exoplanets orbiting stars in neighboring solar systems, I imagine what an undertaking a journey to a distant planet would be for humanity.  Can you imagine the effort that would go into creating a spaceship that could travel for many years or even generations at light speed or near light speed?  That would be truly awesome.  It would be the highest human achievement.  I would love to see us launch such an expedition.

Spacecraft Image - NASA / Public domain

However, that is not today's writing prompt.  No, today's writing prompt is to imagine that we launched an amazing interstellar journey towards a distant planet and then realized that we made a crucial mistake about some aspect of the destination.  I mean we are only human.  What if we launched a ship to visit and settle on a distant planet and we missed a crucial detail about the planet?  Today's science fiction writing prompt is -

What do we, or our explorers, do then?