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 Prototype of theHelios spacecraft | |
| Mission type | Solar observation |
|---|---|
| Operator | |
| COSPAR ID | Helios-A:1974-097A Helios-B:1976-003A |
| SATCATno. | Helios-A: 7567 Helios-B: 8582 |
| Website | Helios-A:[1] Helios-B:[2] |
| Mission duration | Helios-A: 10 years, 1 month, 2 days Helios-B: 3 years, 5 months, 2 days |
| Spacecraft properties | |
| Manufacturer | MBB |
| Launch mass | Helios-A: 371.2 kg (818 lb) Helios-B: 374 kg (825 lb) |
| Power | 270 watts (solar array) |
| Start of mission | |
| Launch date | Helios-A: December 10, 1974, 07:11:01 (1974-12-10UTC07:11:01) UTC[1] Helios-B: January 15, 1976, 05:34:00 (1976-01-15UTC05:34) UTC[2] |
| Rocket | Titan IIIE / Centaur |
| Launch site | Cape CanaveralSLC-41 |
| Entered service | Helios-A: January 16, 1975 Helios-B: July 21, 1976 |
| End of mission | |
| Deactivated | Helios-A: February 18, 1985 (1985-02-19) Helios-B: December 23, 1979 |
| Last contact | Helios-A: February 10, 1986 Helios-B: March 3, 1980 |
| Orbital parameters | |
| Reference system | Heliocentric |
| Eccentricity | Helios-A: 0.5218 Helios-B: 0.5456 |
| Perihelion altitude | Helios-A: 0.31 AU Helios-B: 0.29 AU |
| Aphelion altitude | Helios-A: 0.99 AU Helios-B: 0.98 AU |
| Inclination | Helios-A: 0.02° Helios-B: 0° |
| Period | Helios-A: 190.15 days Helios-B: 185.6 days |
| Epoch | Helios-A: January 15, 1975, 19:00 UTC[1] Helios-B: July 20, 1976, 20:00 UTC[2] |
Helios-A andHelios-B (after launch renamedHelios 1 andHelios 2) are a pair ofprobes that were launched intoheliocentric orbit to studysolar processes. As a joint venture betweenGerman Aerospace Center (DLR) andNASA, the probes were launched fromCape 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).[3]Helios-B performed the closest flyby of the Sun of any spacecraft until that time. The probes are no longer functional, but as of 2024 remain inelliptical orbits around the Sun.
The Helios project was a joint venture ofWest Germany's space agency DLR (70 percent share) and NASA (30 percent share). The Helios probes, built by the main contractorMesserschmitt-Bölkow-Blohm, were the first space probes built outside the United States and theSoviet Union to leave Earth orbit.[4]
The twoHelios probes look similar.Helios-A has a mass of 370 kilograms (820 lb), andHelios-B has a mass of 376.5 kilograms (830 lb). Their scientific payloads have a mass of 73.2 kilograms (161 lb) onHelios-A and 76.5 kilograms (169 lb) onHelios-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 thezodiacal light and emerge from the central body. Two conical solar panels extend above and below the central body, giving the assembly the appearance of adiabolo 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.[5]
The spacecraft spin around their axes, which are perpendicular to theecliptic, at 60 rpm.
Electrical power is provided bysolar 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 ataphelion. Its voltage is regulated to 28 voltsDC. Silver-zinc batteries were used only during launch.
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 11solar constants, (11 times the amount ofsolar irradiance received while in Earth orbit), or 15 kW per exposed square meter. At that distance, the probe could reach 370 °C (698 °F).
Thesolar 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 thesolar radiation. "Second surface mirrors" specially developed byNASA 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 orKapton (depending on location), held apart from each other by small plastic pins intended to prevent the formation ofthermal 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.[6]
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 towardEarth, 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 aSun 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 theDeep Space Network antennas on Earth.
To maintain orientation during the mission, the spacecraftrotated 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 crudeSun sensor. Guidance corrections were performed using cold gas thrusters (7.7 kgnitrogen) 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.
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 insuperior conjunction relative to theEarth (i.e. the Sun comes between the Earth and the spacecraft). A conjunction could last up to 65 days.
Helios-A andHelios-B were launched on December 10, 1974, and January 15, 1976, respectively.Helios-B flew 3,000,000 kilometres (1,900,000 mi) closer to the Sun thanHelios-A, achievingperihelion on April 17, 1976, at a record distance of 43.432 million km (26,987,000 mi; 0.29032 AU),[7] closer than the orbit ofMercury.Helios-B was sent into orbit 13 months after the launch ofHelios-A.Helios-B performed the closest flyby of theSun of any spacecraft untilParker Solar Probe in 2018, 0.29 AU (43.432 million km) from the Sun.[7]
The Helios space probes completed their primary missions by the early 1980s, but continued to send data until 1985.
BothHelios probes had ten scientific instruments[8] and two passive science investigations using the spacecraft telecommuniction system and the spacecraft orbit.
Measures the velocity and distribution ofsolar wind plasma. Developed by theMax 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:[9]
Theflux-gate magnetometer measures the field strength and direction of low frequency magnetic fields in the Sun's environment. It was developed by theUniversity 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 0.4 nT when below 102.4 nT, and within 1.2 nT at intensities below 409.6 nT. Two sample rates are available: search every two seconds or eight readings per second.[10]
Measures variations of the field strength and direction of low frequency magnetic fields in the Sol environment. Developed by theGoddard Space Flight Center of NASA; measures variations of the three-vector components of solar wind and its magnetic field with an accuracy to within 0.1 nT at about 25 nT, within 0.3 nT at about 75 nT, and within 0.9 nT at an intensity of 225 nT.[11]
Thesearch 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 themagnetic field in the 5 Hz to 3000 Hz range. Thespectral resolution is performed on the probe's rotation axis.[12]
The Plasma Wave Investigation developed by theUniversity 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.[13][14][15]
The Cosmic Radiation Investigation developed by theUniversity of Kiel 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, andCherenkov detector) were encapsulated in an anti-coincidence detector.[16]
The Cosmic Ray Instrument developed at theGoddard Space Flight Center measures the characteristics of protons with energies between 0.1 and 800 MeV and electrons with energies between 0.05 and 5 MeV. It uses three telescopes, which cover the ecliptic plane. A proportional counter studies theX-rays from the Sun.[17]
Developed by theMax 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.[18]
TheZodiacal light instrument includes threephotometers developed by theMax Planck Institute for Astronomy to measure the intensity and polarization of the zodiac light in white light and in the 550 nm 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.[19]
TheMicrometeoroid analyzer developed by theMax Planck Institute for Nuclear Physics is capable of detectingcosmic dust particles if their mass is greater than 10−15 g. It can determine the mass and energy of a micro-meteorite greater than 10−14 g. 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-resolutionmass spectrometer determines the composition of impacting cosmic dust particles with a mass greater than 10−13 g.[20][21]
The Celestial Mechanic Experiment developed by theUniversity of Hamburg uses theHelios orbit specifics to clarify astronomical measurements: flattening of the Sun; verification of predictedgeneral relativity effects; determining the mass of the planetMercury; the Earth–Moon mass ratio; and the integrated electron density between the Helios spacecraft and the data receiving station on Earth.[22]
The Coronal Sounding Experiment developed by theUniversity of Bonn measures the rotation (Faraday effect) of the linear polarized radio beam from the spacecraft when it passes during opposition through the corona of the Sun. This rotation is a measure of the density of electrons and the intensity of the magnetic field in the traversed region.[23]
Helios-A was launched on December 10, 1974, fromCape Canaveral Air Force Station Launch Complex 41 inCape Canaveral, Florida.[24] This was the first operational flight of theTitan IIIE rocket. The rocket'stest flight had failed when the engine on the upperCentaur stage did not light, but the launch ofHelios-A was uneventful.
The probe was placed in a heliocentric orbit of 192 days with a perihelion of 46,500,000 km (28,900,000 mi; 0.311 AU) from the Sun. Several problems affected operations. One of the two antennas did not deploy correctly, reducing the sensitivity of the radio plasma apparatus to low-frequency waves. When the high-gain antenna was connected, the mission team realized that their emissions interfered with the analyzer particles and the radio receiver. To reduce the interference, communications were carried out using reduced power, but this required using the large diameter terrestrial receivers already in place thanks to other space missions in progress.[25]
During the firstperihelion in late February 1975, the spacecraft came closer to the Sun than any previous spacecraft. The temperature of some components reached more than 100 °C (212 °F), while the solar panels reached 127 °C (261 °F), without affecting probe operations. During the second pass on September 21, however, temperatures reached 132 °C (270 °F), which affected the operation of certain instruments.
BeforeHelios-B was launched, some modifications were made to the spacecraft based on lessons learned from the operations ofHelios-A. The small engines used for attitude control were improved. Changes were made to the implementation mechanism of the flexible antenna and high gain antenna emissions. TheX-ray detectors were improved so that they could detectgamma ray bursts, allowing them to be used in conjunction with Earth-orbiting satellites to triangulate the location of the bursts. As temperatures onHelios-A were always greater than 20 °C (36 °F) below the design maximum at perihelion, it was decided thatHelios-B would orbit even closer to the Sun, and the thermal insulation was enhanced to allow the satellite to resist 15 percent higher temperatures.
Tight schedule constraints pressed on theHelios-B launch in early 1976. Facilities damaged during the launch of theViking 2 spacecraft in September 1975 had to be repaired, while theViking landing onMars in summer 1976 made the Deep Space Network antennas thatHelios-B needed to conduct its science while at perihelion unavailable.
Helios-B was launched on January 10, 1976, using a Titan IIIE rocket. The probe was placed in an orbit with a 187-day period and a perihelion of 43,500,000 km (27,000,000 mi; 0.291 AU). The orientation ofHelios-B with respect to the ecliptic was reversed 180 degrees compared toHelios-A so that the micrometeorite detectors could have 360 degree coverage. On April 17, 1976,Helios-B made its closest pass of the Sun at a record heliocentric speed of 70 kilometres per second (250,000 km/h; 160,000 mph). The maximum recorded temperature was 20 °C (36 °F) higher than measured byHelios-A.
The primary mission of each probe spanned 18 months, but they operated much longer. On March 3, 1980, four years after its launch, the radio transceiver onHelios-B failed. On January 7, 1981, a stop command was sent to prevent possible radio interference during future missions.Helios-A continued to function normally, but with the large-diameter DSN antennae not available, data was collected by small diameter antennae at a lower rate. By its 14th orbit,Helios-A's degraded solar cells could no longer provide enough power for the simultaneous collection and transmission of data unless the probe was close to its perihelion. In 1984, the main and backup radio receivers failed, indicating that the high-gain antenna was no longer pointed towards Earth. The lasttelemetry data was received on February 10, 1986.[26]
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Both probes collected important data about solar wind processes and the particles that make up the interplanetary medium andcosmic rays. These observations were made over a period fromsolar minimum in 1976 to asolar maximum in the early 1980s.
The observation of the zodiacal light established some of the properties ofinterplanetary dust present between 0.1 and 1 AU from the Sun, such as their spatial distribution, color andpolarization. The amount of dust was observed to be 10 times that around the Earth.Heterogeneous distribution was generally expected due to the passage of comets, but observations have not confirmed this.[citation needed]
Helios collected data about comets, observing the passage ofC/1975 V1 (West) in 1976,C/1978 H1 (Meir) in November 1978 andC/1979 Y1 (Bradfield) in February 1980. During the last event, the probe[which?] detected disturbances in solar wind later explained 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, helium ions isolated in the solar wind. In 1981, during the peak of solar activity, the data collected byHelios-A at a short distance from the Sun helped to complete visual observations of coronal mass ejections performed from the Earth's orbit. Data collected byHelios magnetometers supplemented data collected byPioneer andVoyager and 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 cosmic ray gradient, as a function of distance from the Sun, was measured. These observations, combined with those made byPioneer 11 between 1977 and 1980 in a distance of 12–23 AU from the Sun produced a good model of thisgradient. 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 remain in orbit around the Sun.[27][28][1][29] In January 2024, a smallNear-Earth asteroid was discovered and given theprovisional designation2024 BY15. It was recognized as the upper stage ofHelios-B in August 2025, and the designation was subsequently deleted by theMinor Planet Center.[30][31]
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