Helios-A and Helios-B (after launch renamed and ) 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 December10, 1974, and January15, 1976, respectively.
The Helios project set a maximum speed record for spacecraft of . 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 , and Helios-B has a mass of . Their scientific payloads have a mass of on Helios-A and on Helios-B. The central bodies are sixteen-sided prisms in diameter and 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 tall with a maximum diameter of . Once in orbit, the telecommunications antennae unfolded on top of the probes and increased the heights to . 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 each.Helios. Bernd Leitenberger. Retrieved May 20, 2016.
The spacecraft spin around their axes, which are perpendicular to the ecliptic, at 60rpm.
Systems Power Electrical power is provided by solar cells attached to the two truncated cones. To keep the solar panels at a temperature below 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 240watts when the probe is at aphelion. Its voltage is regulated to 28volts 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 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 .
The solar cells, and the central compartment of instruments had to be maintained at much lower temperatures. The solar cells could not exceed , while the central compartment had to be maintained between . 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 insulationconsisting of 18 layers of Mylar or Kapton (depending on location), held apart from each other by small plastic pins intended to prevent the formation of thermal bridgeswas 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 ( nitrogen) with a boost of 1Newton. The axis of the probe was permanently maintained keeping it both perpendicular to the direction of the Sun and to the ecliptic plane.
On-board computer and data storage 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). A conjunction could last up to 65 days.
Mission profile Helios-A and Helios-B were launched on December 10, 1974, and January 15, 1976, respectively. Helios-B flew closer to the Sun than Helios-A, achieving perihelion on April17, 1976, at a record distance of , closer than the orbit of Mercury. Helios-B was sent into orbit 13 months after the launch of Helios-A. Helios-B performed the closest flyby of the Sun of any spacecraft until Parker Solar Probe in 2018, from the Sun.
The Helios space probes completed their primary missions by the early 1980s, but continued to send data until 1985.
Scientific instruments and investigations Both Helios probes had ten scientific instruments and two passive science investigations using the spacecraft telecommuniction system and the spacecraft orbit.
Plasma experiment investigation Measures the velocity and distribution of solar wind plasma. Developed by the Max Planck Institute for Aeronomy for the study of low-energy particles. Data collected included the density, speed, and temperature of the solar wind. Measurements were taken every minute, with the exception of flux density, which occurred every 0.1 seconds to highlight irregularities in plasma waves. Instruments used included: Electron detector Detector for protons and heavy particles An analyzer for protons and alpha particles with energies between 231eV and 16,000eV
Flux-gate magnetometer The flux-gate magnetometer measures the field strength and direction of low frequency magnetic fields in the Sun's environment. It was developed by the University of Braunschweig, Germany. It measures three-vector components of solar wind and its magnetic field with high precision. The intensity is measured with an accuracy to within 0.4nT when below 102.4nT, and within 1.2nT at intensities below 409.6nT. Two sample rates are available: search every two seconds or eight readings per second.
Flux-gate magnetometer 2 Measures variations of the field strength and direction of low frequency magnetic fields in the Sol environment. Developed by the Goddard Space Flight Center of NASA; measures variations of the three-vector components of solar wind and its magnetic field with an accuracy to within 0.1nT at about 25nT, within 0.3nT at about 75nT, and within 0.9nT at an intensity of 225nT.
Search coil magnetometer The search coil magnetometer complements the flux-gate magnetometer by measuring the magnetic fields between 0 and 3 kHz. Also developed by the University of Braunschweig, it detects fluctuations in the magnetic field in the 5Hz to 3000Hz range. The spectral resolution is performed on the probe's rotation axis.
Plasma wave investigation The Plasma Wave Investigation developed by the University of Iowa uses two 15 m antennas forming an electric dipole for the study of electrostatic and electromagnetic waves in the solar wind plasma in frequencies between 10 Hz and 3 MHz.
Cosmic radiation investigation The Cosmic Radiation Investigation developed by Kiel University sought to determine the intensity, direction, and energy of the protons and heavy constituent particles in radiation to determine the distribution of cosmic rays. The three detectors (semiconductor detector, scintillation counter, and Cherenkov detector) were encapsulated in an anti-coincidence detector.
Cosmic ray instrument The Cosmic Ray Instrument developed at the Goddard Space Flight Center measures the characteristics of protons with energies between 0.1 and 800MeV and electrons with energies between 0.05 and 5MeV. It uses three telescopes, which cover the ecliptic plane. A proportional counter studies the X-rays from the Sun.
Low energy electron and proton spectrometer Developed by the Max Planck Institute for Aeronomy, the low energy electron and proton spectrometer uses spectrometers to measure particle characteristics (protons) with energies between 20 keV and 2 MeV and electrons and positrons with an energy between 80 keV and 1 MeV.
Zodiacal light photometer The Zodiacal light instrument includes three photometers developed by the Max Planck Institute for Astronomy to measure the intensity and polarization of the zodiac light in white light and in the 550nm and 400 nm wavelength bands, using three telescopes whose optical axes form angles of 15, 30, and 90° to the ecliptic. From these observations, information is obtained about the spatial distribution of interplanetary dust and the size and nature of the dust particles.
Micrometeoroid analyzer The Micrometeoroid analyzer developed by the Max Planck Institute for Nuclear Physics is capable of detecting cosmic dust particles if their mass is greater than 10−15g. It can determine the mass and energy of a micro-meteorite greater than 10−14g. These measurements are made by exploiting the fact that micrometeorites vaporize and ionize when they hit a target. The instrument separates the ions and electrons in the plasma generated by the impacts, and measures the mass and energy of the incident particle. A low-resolution mass spectrometer determines the composition of impacting cosmic dust particles with a mass greater than 10−13g.Helios B – Micrometeoroid Detector and Analyzer. NASA NSSDC Master Catalog. Retrieved May 20, 2016.
Celestial mechanic experiment The Celestial Mechanic Experiment developed by the University of Hamburg uses the Helios orbit specifics to clarify astronomical measurements: flattening of the Sun; verification of predicted general relativity effects; determining the mass of the planet Mercury; the Earth–Moon mass ratio; and the integrated electron density between the Helios spacecraft and the data receiving station on Earth.
Coronal sounding experiment