Systems & aerospace technology firm Prismatic for test flights in 2019. Titan Aerospace acquired by Google aimed to develop the Solar UAV, however the project seems to be abandoned Sky-Sailor (aimed at Martian flight) Various solar airship projects, such as Lockheed Martin's "High Altitude Airship" Space Solar powered spacecraft Solar energy is often used to supply power for satellites and spacecraft operating in the inner solar system since it can supply energy for a long time without excess fuel mass. A Communications satellite contains multiple radio transmitters which operate continually during its life. It would be uneconomic to operate such a vehicle (which may be on-orbit for years) from primary batteries or fuel cells, and refuelling in orbit is not practical. Solar power is not generally used to adjust the satellite's position, however, and the useful life of a communications satellite will be limited by the on-board station-keeping fuel supply. Solar propelled spacecraft A few spacecraft operating within the orbit of Mars have used solar power as an energy source for their propulsion system. All current solar powered spacecraft use solar panels in conjunction with electric propulsion, typically ion drives as this gives a very high exhaust velocity, and reduces the propellant over that of a rocket by more than a factor of ten. Since propellant is usually the biggest mass on many spacecraft, this reduces launch costs. Other proposals for solar spacecraft include solar thermal heating of propellant, typically hydrogen or sometimes water is proposed. An electrodynamic tether can be used to change a satellite's orientation or adjust its orbit. Another concept for solar propulsion in space is the light sail; this doesn't require conversion of light to electrical energy, instead relying directly on the tiny but persistent radiation pressure of light. Planetary exploration Perhaps the most successful solar-propelled vehicles have been the "rovers" used to explore surfaces of the Moon and Mars. The 1977 Lunokhod programme and the 1997 Mars Pathfinder used solar power to propel remote controlled vehicles. The operating life of these rovers far exceeded the limits of endurance that would have been imposed, had they been operated on conventional fuels. The two Mars Exploration Rovers also used solar power. Electric vehicle with solar assist A Swiss project, called "Solartaxi", circumnavigated the world. This was the first time in history an electric vehicle (not self sufficient solar vehicle) had gone around the world, covering 50000 km