Unable to fix a leak with the mission’s main science instrument in time to make the 2016 launch window, NASA must wait two years to try again.
“The JPL and CNES teams and their partners have made a heroic effort to prepare the InSight instrument, but have run out of time given the celestial mechanics of a launch to Mars,” said JPL Director Charles Elachi. “It is more important to do it right than take an unacceptable risk.”
Read more from Sky and Telescope.
Or if you are really, really into Mars missions, pop over to the NASA site.