How to Protect a Planet

In the not so distant future, NASA is making preparations for its first ever mission to Mars. The organization is hoping to eventually build permanent settlements on the red planet. What they dont know, however, is that another intelligent species got there first. This species, called Zems, found out about the humans planet. They sent their greatest warrior with one apprentice to Earth to stop NASA. Will the aliens stop NASA in time, or will Mars be trampled under the humans greedy feet?