This Week in Rocket History: the second of two joint American-German missions to explore the “main solar processes and solar-terrestrial relationships” launched in 1976.

The two Helios spacecraft were built by the Federal Republic of Germany (West Germany), which made them the first spacecraft not built in the United States or the USSR to leave Earth orbit. Each Helios spacecraft had ten instruments from Germany and the United States. The seven major instruments were detectors for different particles for everything from electrons to zodiacal light to Cherenkov radiation. Each probe also had three different magnetometers.

Most of the funding was provided by West Germany. The major contribution of the U.S., besides a few of the instruments, was the Titan IIIE, also called Titan Centaur, launch vehicles for the two missions. Helios A was launched in 1974, with Helios B to follow two years later after the launches of the Viking 1 and 2 Mars landers, also on Titan IIIE rockets, during the critical 1975 Mars window. Another constraint was the need to use the large Deep Space Network dishes to support the Mars landings in 1976.

The Helios probes were spin-stabilized, mainly to reduce heat. They weighed only 370 kilograms each because of the capability of the rocket and the low solar orbit necessary for the mission. The main antenna was on a special platform that spun in the opposite direction to keep it pointing the correct way. The spacecraft’s solar panels were designed to be perpendicular to the Sun to reflect heat and the spacecraft was also fitted with quartz “second surface” mirrors to reflect a total of 96% of the heat away from the spacecraft. Finally, the spacecraft had movable radiators on the top and bottom of the spacecraft body to further shed heat.

Between the two Helios launches, engineers took the time to improve Helios B based on the early information from Helios A. The spacecraft’s maneuvering engines were upgraded and its instruments were modified to let it detect new things such as gamma-ray bursts.

Helios B launched on January 15th, 1976, on the fifth Titan IIIE from SLC-41, the same launchpad that the modern Atlas V rocket launches from today. The launch was nominal, unlike the first Titan IIIE, which was detonated twelve minutes into flight after its Centaur upper stage malfunctioned.

After launch, Helios B’s first perihelion was on April 17th, 1976, at a distance of 0.29 astronomical units, which was three million kilometers closer than Helios A. This record stood until the Parker Solar Probe in 2021.

Helios B also observed the comets C/1975 V1 West, C/1978 H1 Meier, and C/1979 Y1 Bradfield1. Its capability to detect gamma-ray bursts was to get a longer baseline from gamma-ray burst detecting satellites in Earth orbit, which allowed for a much finer location of the bursts, down to two arcminutes compared to the solely Earth-based measurements that were much worse, provided basically circumstantial evidence, and didn’t allow for any analysis on the events. This increased precision allowed the scientists of the day to determine that the gamma-ray bursts that they were observing were distinct from all other types of celestial X-ray objects, bursters, and gamma-ray sources known at the time. Scientists have since learned that the same events that cause gamma-ray bursts also make gravitational waves: the merger of neutron stars.

Helios B’s downlink transmitter failed in March 1980, preventing it from sending back any more data. It was shut down a year later to prevent it from interfering with other spacecraft.

More Information

PDF: Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016 (NASA)

“The trip to hot space,” Günter Sandscheper, 1974 December 26, New Scientist

PDF: Gamma-Ray Burst Observations from Helios-2 (NASA)

All in the family: Kin of gravitational wave source discovered (EurekAlert)

Helios-B info page (NASA)