The Mars Rover has a lightweight integrated structure/insulation that has been environmentally tested and qualified for the 1996 launch for the Pathfinder mission to Mars. The basic structure with insulation, called the Warm Electronics Box (WEB), accounts for only 10% of the total Rover mass. The WEB is a thermal isolating composite structure with co-cured thermal control surfaces and an ultralightweight hydrophobic solid silica aerogel which minimizes conduction and radiation. This design provides excellent thermal insulation at low gas pressures and meets the structural requirements for spacecraft launch loads and for a 60 g impact landing at Mars without damage to the insulation or structure. The Rover design has been thermally qualified in a series of both static and transient thermal-vacuum testing in vacuum and 8 torr wind environments simulating all predicted environmental conditions. The Mars Rover has completed all environmental qualification and acceptance testing, and is undergoing system integration testing with the Pathfinder lander.