Helios 1 was launched on December 10, 1974, and Helios 2 on January 15, 1976, as a scientific collaboration between the United States and West Germany. These two spacecraft were launched into highly elliptical orbits, where their aphelion (greatest 274 distance from the Sun) was about 1 AU (93 million miles), but their perihelion (closest distance to the Sun) was only 0.3 AU (28 million miles), which is less than the distance to Mercury from the Sun.

Each Helios probe had a number of scientific instruments aboard designed to study the space environment between Earth and the Sun. Among other things, they studied particle emissions from the Sun, the strength of the Sun’s magnetic field, zodiacal light, micrometeoroids, and cosmic rays. Their scientific mission ended in the mid-1980s, but the spacecraft are still orbiting the Sun to this day.