Helios 1 was a joint German- American deep space mission to study the main solar processes and solar-terrestrial relationships. Specifically, the spacecraft's instruments were designed to investigate ... phenomena such as solar wind, magnetic and electric fields, cosmic rays, and cosmic dust in regions between Earth's orbit and approximately 0.3 AU from the Sun.
It was the largest bilateral project to date for NASA, with Germany paying about $180 million of the total $260-million cost. Germany provided the spacecraft and NASA the launch vehicles.
After a successful launch, Helios 1 passed within 47 million kilometers of the Sun at a speed of 238,000 kilometers per hour on 15 March 1975, the closest any humanmade object had been to our nearest star. During its mission, the spacecraft spun once every second to evenly distribute the heat coming from the Sun, 90 percent of which was reflected by optical surface mirrors. Its data indicated the presence of fifteen times more micrometeorites close to the Sun than there are near Earth.
Information provided by http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Target&Target...