Helios (spacecraft)

The two Helios probes look very 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 which 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 upon achieving orbit 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.[8]

The spacecraft spin around their axes, which are perpendicular to the ecliptic, at 60 rpm.

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, and the energy is stored on an 8 Ah silver-zinc battery. The batteries were only used during launch.

Launch configuration diagram

A technician stands next to one of the twin Helios spacecraft

The biggest technical challenge faced by the designers was the heat that the probe was subject to when 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 received heat in Earth orbit), or 22.4 kW per exposed square meter. Under these conditions, the temperature of the probe can then reach 370 °C (698 °F). The solar cells, and the central compartment of instruments had to be maintained at much lower temperatures. The temperature of 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 heat received from the Sun. The conical shape of the solar panels is one of the measures that was taken to reduce the flow of heat. By tilting the solar panels with respect to sunlight arriving perpendicularly to the axis of the probe, a greater proportion of the solar radiation is reflected. Furthermore, "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.[9]

The telecommunication system uses a radio transceiver, whose power could be adjusted between 0.5 and 20 watts. Three antennas were overlaid on top of each probe. A high-gain antenna (23 dB), used to emit a top brush of 5.5° on either side of elliptical and 14° wide, 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 dipole antenna (0.3 dB transmission and 0.8 dB for reception). The low-gain horn antenna was located under the center of the probe to allow for an adapter that connected the probe to the launch vehicle. To be directed continuously toward Earth, the high-gain antenna is kept in rotation by a motor at a speed that counterbalances exactly the body of the probe. Synchronizing the 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 network antennas on Earth.

To maintain orientation during the mission, the spacecraft rotated continuously at 60 RPM around its main axis. The orientation control system then started making corrections to 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 kg nitrogen) with a boost of 1 Newton. The axis of the probe was permanently maintained, both perpendicular to the direction of the Sun and the ecliptic plane.

The onboard controllers were capable of handling 256 commands. The mass memory could store 500 kb, (this was a very large memory for space probes of the time), and was mainly used when the probes were in superior conjunction relative to the Earth (i.e. the Sun comes between the Earth and the spacecraft). The conjunction could last up to 65 days.

Both Helios probes had ten scientific instruments.[10]

A Helios probe being encapsulated for launch

Both probes collected important data about the processes that cause solar wind and acceleration of the particles that make up the interplanetary medium and cosmic rays. These observations were made over a ten-year period from solar minimum in 1976 to a solar maximum in the early 1980s.

The observation of the zodiacal light has established some of the properties of interplanetary dust present between 0.1 AU and 1 AU from the Sun, such as their spatial distribution, color and polarization. It has been established that the powder was more sensitive to gravitational forces and electromagnetic forces. The amount of dust was observed up to 10 times around the Earth. Heterogeneous distribution was generally expected due to the passage of comets, but observations have not confirmed this. The probe instruments detected dust near the Sun showing that, despite the sunshine is still present in distance 0.09 AU.

Helios also allowed an increase in data to be collected on 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. During the last probe, instruments observed a disturbance wind solar which translated later by a break in the comet's tail. The plasma analyzer showed that the acceleration phenomena of the high-speed solar wind were associated with the presence of coronal holes. This instrument also detected, for the first time, the helium ions isolated in the solar wind. In 1981, during the peak of solar activity, the data collected by Helios-A at a short distance from the Sun helped complete visual observations of coronal mass ejections performed from the Earth's orbit. Data collected by magnetometers two probes Helios supplemented with interplanetary probes Pioneer and Voyager were used to determine the direction of the magnetic field at staggered distances from the Sun.

The radio and plasma wave detectors were used to detect radio explosions and shock waves associated with solar flares, usually during solar maximum. The cosmic ray detectors studied how the Sun and interplanetary medium influenced the spread of the same rays, of solar or galactic origin. The gradient of cosmic rays, as a function of distance from the Sun, was measured. These observations, combined with those made by Pioneer 11 between 1977 and 1980 on the outside of the solar system (12–23 AU from the Sun) produced good modelling of this gradient. The GRBs Helios-B detector identified 18 events during the first three years of operation of the instrument, whose source can, for some, be identified with the help of searches made by satellites orbiting the Earth. Some features of the inner solar corona were measured during occultations. For this purpose, either a radio signal was sent from the spacecraft to Earth or the ground station sent a signal that was returned by the probe. Changes in signal propagation resulting from the solar corona crossing provided information on density fluctuations.

As of 2020, the probes are no longer functional, but still remain in their elliptical orbits around the Sun.[5][6][1][7]

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Wikimedia Commons has media related to Helios (spacecraft).

• Helios-A at NSSDC Master Catalog
• Helios-B at NSSDC Master Catalog
• Helios-A Mission Profile by NASA's Solar System Exploration
• Helios-B Mission Profile by NASA's Solar System Exploration
• Titan/Centaur D-1T TC-2, Helios-A, Flight Data Report
• Titan/Centaur D-1T TC-5, Helios-B, Flight Data Report
• Helios-A and -B by Honeysuckle Creek Tracking Station
• Helios webpage by Max-Planck-Institut für Sonnensystemforschung