Helios-A and Helios-B (after launch renamed Helios 1 and Helios 2) are a pair of probes that were launched into heliocentric orbit to study solar processes. 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 Helios project set a maximum speed record for spacecraft of 252,792 km/h (157,078 mph; 70,220 m/s). Helios-B performed the closest flyby of the Sun so far, a record only broken in October 2018 by the Parker Solar Probe. The probes are no longer functional, but as of 2024 remain in elliptical orbits around the Sun.
Construction The Helios project was a joint venture of West Germany's space agency DLR (70 percent share) and NASA (30 percent share). The Helios 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.
Structure The two Helios 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. Most of the equipment and instrumentation is mounted in this central body. 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. 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). 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. The spacecraft spin around their axes, which are perpendicular to the ecliptic, at 60 rpm.
Systems
Power Electrical power is provided by solar cells attached to the two truncated cones. To keep the solar panels at a temperature below 165 °C (329 °F) when in proximity to the Sun, the solar cells are interspersed with mirrors, covering 50% of the surface and reflecting part of the incident sunlight while dissipating the excess heat. The power supplied by the solar panels is a minimum of 240 watts when the probe is at aphelion. Its voltage is regulated to 28 volts DC. Silver-zinc batteries were used only during launch.
Thermal control
The biggest technical challenge was to avoid heating during orbit while close to the Sun. At 0.3 astronomical units (45,000,000 km; 28,000,000 mi) from the Sun, approximate heat flow is 11 solar constants, (11 times the amount of solar irradiance received while in Earth orbit), or 15 kW per exposed square meter. At that distance, the probe could reach 370 °C (698 °F). The solar cells, and the central compartment of instruments had to be maintained at much lower temperatures. 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). These restrictions required the rejection of 96 percent of the energy received from the Sun. The conical shape of the solar panels was decided on to reduce heating. Tilting the solar panels with respect to sunlight arriving perpendicularly to the axis of the probe, reflects a greater proportion of the solar radiation. "Second surface mirrors" specially developed by NASA cover the entire central body and 50 percent of the solar generators. These are made of fused quartz, with a silver film on the inner face, which is itself covered with a dielectric material. 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. The opening thereof is controlled separately by a bimetal spring whose length varies with temperature and causes the opening or closing of the shutter. Resistors were also used to help maintain a temperature sufficient for certain equipment.
Telecommunications system The telecommunication system uses a radio transceiver, whose power could be adjusted to between 0.5 and 20 watts. Three antennas are mounted on top of each probe. A high-gain antenna (23 dB) of 11° beam width, a medium-gain antenna (3 dB for transmission and 6.3 dB for reception) emits a signal in all directions of the ecliptic plane at the height of 15°, and a low-gain dipole antenna (0.3 dB transmission and 0.8 dB for reception). 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. Synchronizing the rotation speed is performed using data supplied by a Sun sensor. The maximum data rate obtained with the large antenna gain was 4096 bits per second upstream. The reception and transmission of signals were supported by the Deep Space Network antennas on Earth.
Altitude control
To maintain orientation during the mission, the spacecraft rotated continuously at 60 RPM around its main axis. The orientation control system manages the speed and orientation of the probe's shafts. To determine its orientation, Helios used a crude Sun sensor. Guidance corrections were performed using cold gas thrusters (7.7 kilograms [17 lb] nitrogen) with a boost of 1 Newton. The axis of the probe was permanently maintained keeping it both perpendicular to the direction of the Sun and to the ecliptic plane.
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