The telecommunication system uses a radio transceiver, whose power could be adjusted to between 0.5 and 20 watts. Resistors were also used to help maintain a temperature sufficient for certain equipment. The opening thereof is controlled separately by a bimetal spring whose length varies with temperature and causes the opening or closing of the shutter. 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. 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. These are made of fused quartz, with a silver film on the inner face, which is itself covered with a dielectric material. "Second surface mirrors" specially developed by NASA cover the entire central body and 50 percent of the solar generators. Tilting the solar panels with respect to sunlight arriving perpendicularly to the axis of the probe, reflects a greater proportion of the solar radiation. The conical shape of the solar panels was decided on to reduce heating. These restrictions required the rejection of 96 percent of the energy received from the Sun. The solar cells could not exceed 165 ☌ (329 ☏), while the central compartment had to be maintained between −10 and 20 ☌ (14 and 68 ☏). The solar cells, and the central compartment of instruments had to be maintained at much lower temperatures.
At that distance, the probe could reach 370 ☌ (698 ☏). 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 22.4 kW per exposed square meter. The biggest technical challenge was to avoid heating during orbit while close to the Sun. Silver-zinc batteries were used only during launch. The power supplied by the solar panels is a minimum of 240 watts when the probe is at aphelion. To keep the solar panels at a temperature below 165 ☌ (329 ☏) 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 spacecraft spin around their axes, which are perpendicular to the ecliptic, at 60 rpm.Įlectrical power is provided by solar cells attached to the two truncated cones. 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. Once in orbit, the telecommunications antennae unfolded on top of the probes and increased the heights to 4.2 metres (14 ft). Two conical solar panels extend above and below the central body, giving the assembly the appearance of a diabolo or spool of thread.Īt launch, each probe was 2.12 metres (6 ft 11 in) tall with a maximum diameter of 2.77 metres (9 ft 1 in). The exceptions are the masts and antennae used during experiments and small telescopes that measure the zodiacal light and emerge from the central body. Most of the equipment and instrumentation is mounted in this central body. 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.
Their scientific payloads have a mass of 73.2 kilograms (161 lb) on Helios-A and 76.5 kilograms (169 lb) on Helios-B.
Helios-A has a mass of 370 kilograms (820 lb), and Helios-B has a mass of 376.5 kilograms (830 lb). As built by the main contractor, Messerschmitt-Bölkow-Blohm, they were the first space probes built outside the United States and the Soviet Union to leave Earth orbit. The Helios project was a joint venture of West Germany's space agency DLR (70 percent share) and NASA (30 percent share). The probes are no longer functional, but as of 2023 remain in elliptical orbits around the Sun. Helios-B performed the closest flyby of the Sun of any spacecraft until that time. The Helios project set a maximum speed record for spacecraft of 252,792 km/h (157,078 mph 70,220 m/s). 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. 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.
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