Date: 10 Dec 1974
The Helios 1 solar mission was a cooperative effort between the United States and Germany.
As it looped around the Sun in an orbit that took it from Earth's orbit (1 AU from the Sun) to about 0.3 AU from the Sun and back again, Helios 1 studied the solar wind, magnetic and electric fields, cosmic rays, and dust in interplanetary space. It flew within 47 million km of the Sun at a speed of 238,000 km per hour, the closest any human-made object had been. Its data indicated the presence of 15 times more micrometeorites close to the Sun than there are near Earth.
Credit: Max Planck Institute for Solar System Research