Helios (spacecraft)

Lore & Background

The Helios project was a joint venture of West Germany's space agency DLR (70 percent share) and NASA (30 percent share). The probes, built by the main contractor Messerschmitt-Bölkow-Blohm, were the first space probes built outside the United States and the Soviet Union to leave Earth orbit. The two probes look similar: Helios-A has a mass of 370 kilograms (820 lb), and Helios-B has a mass of 376.5 kilograms (830 lb). Their scientific payloads have a mass of 73.2 kilograms (161 lb) on Helios-A and 76.5 kilograms (169 lb) on Helios-B. The central bodies are sixteen-sided prisms 1.75 metres (5 ft 9 in) in diameter and 0.55 metres (1 ft 10 in) high. Two conical solar panels extend above and below the central body, giving the assembly the appearance of a diabolo or spool of thread. At launch, each probe was 2.12 metres (6 ft 11 in) tall with a maximum diameter of 2.77 metres (9 ft 1 in). Once in orbit, the telecommunications antennae unfolded on top of the probes and increased the heights to 4.2 metres (14 ft). The spacecraft spin around their axes, which are perpendicular to the ecliptic, at 60 rpm.

Reader's Guide

The Helios probes were significant for their record-breaking speed and close solar flyby, advancing understanding of solar processes. They carried ten scientific instruments and two passive science investigations, including plasma experiments and flux-gate magnetometers. The biggest technical challenge was thermal control: at 0.3 astronomical units from the Sun, the probe could reach 370 °C (698 °F), requiring rejection of 96 percent of received solar energy. The probes completed their primary missions by the early 1980s but continued to send data until 1985. Their legacy includes setting a maximum speed record for spacecraft and performing the closest flyby of the Sun until the Parker Solar Probe in 2018. As of 2024, they remain in elliptical orbits around the Sun, no longer functional.

Did You Know?

• Helios-B flew 3,000,000 km closer to the Sun than Helios-A, achieving perihelion on April 17, 1976, at a record distance of 43.432 million km.
• The onboard mass memory could store 500 kb, a very large memory for space probes of the time.
• The solar cells could not exceed 165 °C (329 °F), while the central compartment had to be maintained between −10 and 20 °C (14 and 68 °F).

A Historic First for Western Europe

The Helios program stands as a landmark in the history of international space exploration. Conceived as a joint venture between West Germany's DLR, which held a 70 percent share, and NASA at 30 percent, the twin probes Helios-A and Helios-B achieved a milestone no other nation had reached: they were the first spacecraft built outside the United States or the Soviet Union to break free of Earth orbit. The main contractor, Messerschmitt-Bölkow-Blohm, delivered the pair for launch from Cape Canaveral Air Force Station in Florida—Helios-A on December 10, 1974, and Helios-B on January 15, 1976. Their scientific purpose was to study solar processes from heliocentric orbit. The program's most celebrated achievements are its records: a maximum spacecraft speed of 252,792 km/h (157,078 mph; 70,220 m/s), and Helios-B's then-closest approach to the Sun, a mark that held until NASA's Parker Solar Probe overtook it in October 2018.

A Diabolo in the Void

The physical design of the Helios probes is both striking and functional. Each spacecraft centers on a sixteen-sided prism, 1.75 metres in diameter and 0.55 metres tall, housing most of the scientific instrumentation. Extending above and below this core are two conical solar panel assemblies that give the whole structure the unmistakable silhouette of a diabolo or spool of thread. At launch, each probe stood 2.12 metres tall with a maximum diameter of 2.77 metres. Once in orbit, the configuration expanded dramatically: telecommunications antennae unfolded on top, raising the height to 4.2 metres, while two rigid sensor booms deployed laterally and two flexible radio-wave antennae stretched out perpendicular to the spacecraft's axis, each reaching a design length of 16 metres. Helios-A weighed 370 kilograms and Helios-B 376.5 kilograms, with scientific payloads of 73.2 and 76.5 kilograms respectively. The probes spin at 60 revolutions per minute around axes held perpendicular to the ecliptic plane.

Surviving Eleven Solar Constants

The single greatest engineering hurdle for the Helios team was thermal management. At a perihelion distance of 0.3 astronomical units, the probes faced roughly 11 times the solar irradiance experienced in Earth orbit—equivalent to 15 kilowatts per exposed square metre—potentially driving temperatures to 370 °C. Yet the solar cells had to stay below 165 °C, and the central instrument compartment needed to remain between −10 and 20 °C, meaning 96 percent of incoming solar energy had to be rejected. The conical geometry of the panels was chosen specifically to reflect more radiation by tilting the surface away from perpendicular incidence. NASA-developed second-surface mirrors of fused quartz, with a silver inner film and dielectric outer coating, covered the entire central body and half the solar generators. An additional 18 layers of 0.25-millimetre Mylar or Kapton, separated by tiny plastic pins to prevent thermal bridging, wrapped the core compartment. Active movable louvers, actuated by bimetal springs that expanded or contracted with temperature, provided further regulation, while resistors kept certain equipment warm enough to function.

Communication, Control, and a Quiet Endurance

Maintaining contact and orientation across vast heliocentric distances required careful systems engineering. Each probe carried three antennas on top: a high-gain dish (23 dB, 11° beam) whose motor-driven rotation counterbalanced the 60 RPM spin to keep it pointed at Earth, a medium-gain antenna covering the ecliptic plane, and a low-gain dipole. The transceiver operated between 0.5 and 20 watts, with a maximum upstream data rate of 4096 bits per second, all relayed through NASA's Deep Space Network ground stations. Orientation was managed by a simple Sun sensor and corrected via cold-gas thrusters burning 7.7 kilograms of nitrogen at 1 Newton of thrust, keeping the spin axis perpetually perpendicular to both the Sun's direction and the ecliptic. The onboard controller handled 256 commands and stored 500 kilobits of data—a substantial capacity for its era. Though no longer functional, both Helios probes continue their elliptical solar orbits as of 2024, silent witnesses to the Sun they were built to study.