As a joint venture between German Aerospace Center (DLR) and NASA, the probes were launched from Cape Canaveral Air Force Station, Florida, on December 10, 1974, and January 15, 1976, respectively.

The exceptions are the masts and antennae used during experiments and small telescopes that measure the zodiacal light and emerge from the central body.

Two conical solar panels extend above and below the central body, giving the assembly the appearance of a diabolo or spool of thread.

Also deployed were two rigid booms carrying sensors and magnetometers, attached on both sides of the central bodies, and two flexible antennae used for the detection of radio waves, which extended perpendicular to the axes of the spacecraft for a design length of 16 metres (52 ft) each.

"Second surface mirrors" specially developed by NASA cover the entire central body and 50 percent of the solar generators.

For additional protection, multi-layer insulation – consisting of 18 layers of 0.25 millimetres (0.0098 in) Mylar or Kapton (depending on location), held apart from each other by small plastic pins intended to prevent the formation of thermal bridges – was used to partially cover the core compartment.

In addition to these passive devices, the probes used an active system of movable louvers arranged in a shutter-like pattern along the bottom and top side of the compartment.

To be directed continuously toward Earth, the high-gain antenna is rotated by a motor at a speed that counterbalances the spin of the probe.

The axis of the probe was permanently maintained keeping it both perpendicular to the direction of the Sun and to the ecliptic plane.

[6] The Helios space probes completed their primary missions by the early 1980s, but continued to send data until 1985.

Measurements were taken every minute, with the exception of flux density, which occurred every 0.1 seconds to highlight irregularities in plasma waves.

[18] The Micrometeoroid analyzer developed by the Max Planck Institute for Nuclear Physics is capable of detecting cosmic dust particles if their mass is greater than 10−15 g. It can determine the mass and energy of a micro-meteorite greater than 10−14 g. These measurements are made by exploiting the fact that micrometeorites vaporize and ionize when they hit a target.

The instrument separates the ions and electrons in the plasma generated by the impacts, and measures the mass and energy of the incident particle.

[21] The Coronal Sounding Experiment developed by the University of Bonn measures the rotation (Faraday effect) of the linear polarized radio beam from the spacecraft when it passes during opposition through the corona of the Sun.

The rocket's test flight had failed when the engine on the upper Centaur stage did not light, but the launch of Helios-A was uneventful.

The probe was placed in a heliocentric orbit of 192 days with a perihelion of 46,500,000 km (28,900,000 mi; 0.311 AU) from the Sun.

When the high-gain antenna was connected, the mission team realized that their emissions interfered with the analyzer particles and the radio receiver.

Before Helios-B was launched, some modifications were made to the spacecraft based on lessons learned from the operations of Helios-A.

On April 17, 1976, Helios-B made its closest pass of the Sun at a record heliocentric speed of 70 kilometres per second (250,000 km/h; 160,000 mph).

By its 14th orbit, Helios-A's degraded solar cells could no longer provide enough power for the simultaneous collection and transmission of data unless the probe was close to its perihelion.

In 1984, the main and backup radio receivers failed, indicating that the high-gain antenna was no longer pointed towards Earth.

[25] Both probes collected important data about solar wind processes and the particles that make up the interplanetary medium and cosmic rays.

The observation of the zodiacal light established some of the properties of interplanetary dust present between 0.1 and 1 AU from the Sun, such as their spatial distribution, color and polarization.

[citation needed] Helios collected data about comets, observing the passage of C/1975 V1 (West) in 1976, C/1978 H1 (Meir) in November 1978 and C/1979 Y1 (Bradfield) in February 1980.

The plasma analyzer showed that the acceleration phenomena of the high-speed solar wind were associated with the presence of coronal holes.

In 1981, during the peak of solar activity, the data collected by Helios-A at a short distance from the Sun helped to complete visual observations of coronal mass ejections performed from the Earth's orbit.

These observations, combined with those made by Pioneer 11 between 1977 and 1980 in a distance of 12–23 AU from the Sun produced a good model of this gradient.