The exploration of Mars is (1)_____ carried out by means of satellites. A modern satellite takes about eleven months to travel the 720 million kilometers from the earth to Mars. When (2)_____ arrives, it goes into orbit (3)_____ the planet, circling it every two hours at an (4)_____ of about 300 kilometers above the surface. From there, it can send back to Earth much useful (5)_____ about the planet. There are plans to send more satellites to Mars. The next step will be to send spacecraft which can land on the planet and take samples of materials from the surface. (6)_____ have already designed a nuclear-powered vehicle which will be able to leave the spacecraft and travel up to 290 kilometers over the surface of Mars. This (7)_____ will take samples of soil and rock and report on the weather. Some of these special vehicles are extremely small and light. For example, one fitted with a television camera (8)_____ only seven kilograms and measures 61 centimeters long and 38 centimeters wide. These tiny machines cannot be operated from Earth by remote control (9)_____ radio signals take twenty-two minutes to (10)_____ Mars, so they will have to be programmed with enough "intelligence" to operate largely by themselves.