I don't see the point of using something as complex as a unix kernel for an embedded real time system. There exists an open source RTOS with a lot of space flight history called RTEMS, www.rtems.org. That might be a good choice.

Also I'm not sure they actually are using Linux to control the rockets since that would pretty much make them impossible to qualify for human space travel (i.e. DO-178B qualification).

EDIT: Looking at their jobs site it seems they are using VxWorks. This is more reasonable ...