A key science instrument that will be carried aboard NASA’s Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) spacecraft being prepared for launch in March 2016 is experiencing a leak in the vacuum container carrying its main sensors. The sensors are part of an instrument called the Seismic Experiment for Interior Structure (SEIS), which is provided by the French Space Agency (CNES).
The seismometer is the prime science payload that will help answer questions about the interior structure and processes within the deep Martian interior. The SEIS instrument has three high-sensitivity seismometers enclosed in a sealed sphere. The seismometers need to operate in a vacuum in order to provide exquisite sensitivity to ground motions as small as the width of an atom. After the final sealing of the sphere, a small leak was detected, that would have prevented meeting the science requirements once delivered to the surface of Mars. [More at link]