_Mariner 10_ returned closeup pictures of the cloud cover around Venus and of Mercury’s sunbaked surface. _Mariner 10_ was the first spacecraft to photograph Mercury, the innermost planet. The spacecraft’s instruments also measured particles, fields, and radiation from these planets. _Mariner 10_ flew by Venus on February 5, 1974, after a three-month, 240-million-kilometer (150-million-mile) journey that took the Spacecraft halfway around the Sun. _Mariner 10_ swung around the planet, taking a variety of measurements and photographs of the clouds that obscure the planet’s face. Using the planet’s gravity to “bend” its flight path, _Mariner 10_ flew on toward encounter with Mercury. On March 29, 1974. _Mariner_ sped across the night side of the little planet closest to the Sun. Only 703 kilometers (436 miles) above the rugged surface, _Mariner_’s cameras captured the first closeup views of the planet’s daylight hemisphere. The pictures show craters, scarps—cliffs nearly 3 kilometers (2 miles) high and stretching as far as 500 kilometers (300 miles) across the surface—basins, and hilly furrowed terrain. After providing our first glimpse of Mercury’s surface, _Mariner_ raced on around the Sun and back out across Venus’ orbit. With some trajectory adjustments using on-board thrusters. _Mariner_ returned to within 48,000 kilometers (30,000 miles) of Mercury on September 21, 176 days after the first encounter, again returning pictures and data. _Mariner_’s orbit brought it back to the planet for a third pass in another 176 days. On-board propellant exhausted, the spacecraft continues its orbit of the Sun and innermost planet. _Mariner 10_ is the first complex spacecraft designed to travel to the inner reaches of the solar system. At closest approach to the Sun, the spacecraft received five times as much light and heat as it did on leaving Earth. Thus the solar panels, which collect and convert solar radiation into electrical energy for the spacecraft’s instruments and controls, were designed to tilt more and more away from the sunlight as _Mariner_ approached the Sun. _Mariner_ could transmit much more information to Earth than earlier flyby spacecraft. This higher data rate enabled the craft to send back more live pictures of the planets as it flew by them. Some information was stored on magnetic tape for later transmission. This capability permitted _Mariner_ to collect data when it was hidden from Earth behind a planet, and send the information when it emerged. Prime contractor for _Mariner 10_ was Hughes Aircraft Company. _Mariner 10_ is from the National Aeronautics and Space Administration. U.S.S. Enterprise [Illustration: 19. The starship _Enterprise_ used in the filming of the “Star Trek” television series.]