''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 The German Aerospace Center (german: Deutsches Zentrum für Luft- und Raumfahrt e.V., abbreviated DLR, literally ''German Center for Air- and Space-flight'') is the national center for aerospace, energy and transportation research of Germany ...

(DLR) and

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

, the probes were launched from

Cape Canaveral Air Force Station Cape Canaveral Space Force Station (CCSFS) is an installation of the United States Space Force's Space Launch Delta 45, located on Cape Canaveral in Brevard County, Florida. Headquartered at the nearby Patrick Space Force Base, the statio ...

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

, 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 The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

of any spacecraft until that time. The probes are no longer functional, but as of 2022 remain in elliptical orbits around the Sun.

Construction

The Helios project was a joint venture of

West Germany West Germany is the colloquial term used to indicate the Federal Republic of Germany (FRG; german: Bundesrepublik Deutschland , BRD) between its formation on 23 May 1949 and the German reunification through the accession of East Germany on 3 O ...

's space agency DLR (70 percent share) and NASA (30 percent share). As built by the main contractor, Messerschmitt-Bölkow-Blohm, they were the first space probes built outside the United States and the

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...

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 The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction ...

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 The diabolo ( ; commonly misspelled ''diablo'') is a juggling or circus prop consisting of an axle () and two cups (hourglass/egg timer shaped) or discs derived from the Chinese yo-yo. This object is spun using a string attached to two hand ...

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. The spacecraft spin around their axes, which are perpendicular to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

, at 60

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

Systems

Power

Electrical power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions o ...

is provided by

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

s 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 240

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...

s when the probe is at

aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

. Its voltage is regulated to 28

volts The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defini ...

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 22.4 kW per exposed square meter. At that distance, the probe could reach . The

solar cells A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

, 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 Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ( ...

. "Second surface mirrors" specially developed by

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 Multi-layer insulation (MLI) is thermal insulation composed of multiple layers of thin sheets and is often used on spacecraft and cryogenics. Also referred to as superinsulation, MLI is one of the main items of the spacecraft thermal design, pri ...

consisting of 18 layers of

Mylar BoPET (biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and a ...

or Kapton (depending on location), held apart from each other by small plastic pins intended to prevent the formation of

thermal bridge A thermal bridge, also called a cold bridge, heat bridge, or thermal bypass, is an area or component of an object which has higher thermal conductivity than the surrounding materials, creating a path of least resistance for heat transfer.Gorse, Chr ...

swas 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 Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, 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 A sun sensor is a navigational instrument used by spacecraft to detect the position of the sun. Sun sensors are used for attitude control, solar array pointing, gyro updating, and fail-safe recovery. In addition to spacecraft, sun sensors fin ...

. 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 The NASA Deep Space Network (DSN) is a worldwide network of American spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA's interplanetary ...

antennas on Earth.

Attitude 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 A sun sensor is a navigational instrument used by spacecraft to detect the position of the sun. Sun sensors are used for attitude control, solar array pointing, gyro updating, and fail-safe recovery. In addition to spacecraft, sun sensors fin ...

. Guidance corrections were performed using cold gas thrusters (7.7 kg

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

) 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.

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 In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. When two objects always appear close to the ecliptic—such as t ...

relative to the

(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 An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

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

of any spacecraft until

Parker Solar Probe The Parker Solar Probe (PSP; previously Solar Probe, Solar Probe Plus or Solar Probe+) is a NASA space probe launched in 2018 with the mission of making observations of the outer corona of the Sun. It will approach to within 9.86 solar radii ...

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 The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

plasma. Developed by the

Max Planck Institute for Aeronomy The Max Planck Institute for Solar System Research (abbreviation: MPS; german: Max-Planck-Institut für Sonnensystemforschung) is a research institute in astronomy and astrophysics located in Göttingen, Germany, where it relocated in February 2 ...

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 Helios - helios2

Flux-gate magnetometer

The

flux-gate magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...

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 0.4 nT 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 225 nT.

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 The University of Iowa (UI, U of I, UIowa, or simply Iowa) is a public research university in Iowa City, Iowa, United States. Founded in 1847, it is the oldest and largest university in the state. The University of Iowa is organized into 12 col ...

uses two 15 m antennas forming an elecric 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 the

University of Kiel Kiel University, officially the Christian-Albrecht University of Kiel, (german: Christian-Albrechts-Universität zu Kiel, abbreviated CAU, known informally as Christiana Albertina) is a university in the city of Kiel, Germany. It was founded in ...

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 A semiconductor detector in ionizing radiation detection physics is a device that uses a semiconductor (usually silicon or germanium) to measure the effect of incident charged particles or photons. Semiconductor detectors find broad applicat ...

, 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 800

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...

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 An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...

from the Sun.

Low energy electron and proton spectrometer

Developed by the

, 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

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ...

s developed by the

Max Planck Institute for Astronomy The Max-Planck-Institut für Astronomie (Max Planck Institute for Astronomy, MPIA) is a research institute of the Max Planck Society (MPG). It is located in Heidelberg, Baden-Württemberg, Germany near the top of the Königstuhl, adjacent to the ...

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. Helios spacecraft 2

Micrometeoroid analyzer

The Micrometeoroid analyzer developed by the

Max Planck Institute for Nuclear Physics The Max-Planck-Institut für Kernphysik ("MPI for Nuclear Physics" or MPIK for short) is a research institute in Heidelberg, Germany. The institute is one of the 80 institutes of the Max-Planck-Gesellschaft (Max Planck Society), an independent, n ...

is capable of detecting

cosmic dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...

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 Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...

determines the composition of impacting cosmic dust particles with a mass greater than 10⁻¹³g.

Celestial mechanic experiment

The Celestial Mechanic Experiment developed by the

University of Hamburg The University of Hamburg (german: link=no, Universität Hamburg, also referred to as UHH) is a public research university in Hamburg, Germany. It was founded on 28 March 1919 by combining the previous General Lecture System ('' Allgemeines Vo ...

uses the ''Helios'' orbit specifics to clarify astronomical measurements: flattening of the Sun; verification of predicted

general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

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 receivig station on Earth.

Coronal sounding experiment

The Coronal Sounding Experiment developed by the

University of Bonn The Rhenish Friedrich Wilhelm University of Bonn (german: Rheinische Friedrich-Wilhelms-Universität Bonn) is a public research university located in Bonn, North Rhine-Westphalia, Germany. It was founded in its present form as the ( en, Rhine ...

measures the rotation (

Faraday Effect The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the ...

) 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.

Mission specifications

Helios-A

''Helios-A'' was launched on December 10, 1974, from Cape Canaveral Air Force Station Launch Complex 41 in

Cape Canaveral, Florida Cape Canaveral ( es, Cabo Cañaveral, link=) is a city in Brevard County, Florida. The population was 9,912 at the 2010 United States Census. It is part of the Palm Bay–Melbourne– Titusville Metropolitan Statistical Area. History After t ...

. This was the first operational flight of the

Titan IIIE The Titan IIIE or Titan 3E, also known as the Titan III-Centaur, was an American expendable launch system. Launched seven times between 1974 and 1977, it enabled several high-profile NASA missions, including the Voyager and Viking planetary prob ...

rocket. The rocket's

test flight Flight testing is a branch of aeronautical engineering that develops specialist equipment required for testing aircraft behaviour and systems. Instrumentation systems are developed using proprietary transducers and data acquisition systems. D ...

had failed when the engine on the upper Centaur stage did not light, but the launch of ''Helios-A'' was uneventful. The probe was placed in a heliocentric orbit of 192 days with a perihelion of 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. During the first

in late February 1975, the spacecraft came closer to the Sun than any previous spacecraft. The temperature of some components reached more than , while the solar panels reached , without affecting probe operations. During the second pass on September 21, however, temperatures reached , which affected the operation of certain instruments.

Helios-B

Before ''Helios-B'' was launched, some modifications were made to the spacecraft based on lessons learned from the operations of ''Helios-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. The

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

detectors were improved so that they could detect

gamma ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten millise ...

s, allowing them to be used in conjunction with Earth-orbiting satellites to triangulate the location of the bursts. As temperatures on ''Helios-A'' were always greater than below the design maximum at perihelion, it was decided that ''Helios-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 the ''Helios-B'' launch in early 1976. Facilities damaged during the launch of the ''

Viking 2 The ''Viking 2'' mission was part of the American Viking program to Mars, and consisted of an orbiter and a lander essentially identical to that of the ''Viking 1'' mission. ''Viking 2'' was operational on Mars for sols ( days; '). The ''Vi ...

'' spacecraft in September 1975 had to be repaired, while the

Viking Vikings ; non, víkingr is the modern name given to seafaring people originally from Scandinavia (present-day Denmark, Norway and Sweden), who from the late 8th to the late 11th centuries raided, pirated, traded and se ...

landing on

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

in summer 1976 made the Deep Space Network antennas that ''Helios-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 . The orientation of ''Helios-B'' with respect to the ecliptic was reversed 180 degrees compared to ''Helios-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 . The maximum recorded temperature was higher than measured by ''Helios-A''.

End of operations

The primary mission of each probe spanned 18 months, but they operated much longer. On March3, 1980, four years after its launch, the radio transceiver on ''Helios-B'' failed. On January7, 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-As 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 last

telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", an ...

data was received on February10, 1986.

Mission results

Both probes collected important data about solar wind processes and the particles that make up the interplanetary medium and

cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ow ...

. These observations were made over a period from

solar minimum Solar minimum is the regular period of least solar activity in the Sun's 11-year solar cycle. During solar minimum, sunspot and solar flare activity diminishes, and often does not occur for days at a time. On average, the solar cycle takes abou ...

in 1976 to a

solar maximum Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle tak ...

in the early 1980s. The observation of the zodiacal light established some of the properties of

interplanetary dust The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System. ...

present between 0.1 and 1 AU from the Sun, such as their spatial distribution, color and polarization. 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. ''Helios'' collected data about 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 event, probe 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 by ''Helios-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 by ''Helios'' magnetometers supplemented data collected by ''

Pioneer Pioneer commonly refers to a settler who migrates to previously uninhabited or sparsely inhabited land. In the United States pioneer commonly refers to an American pioneer, a person in American history who migrated west to join in settling and de ...

'' and '' Voyager'' 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 by ''Pioneer11'' between 1977 and 1980 in a distance of 12–23AU from the Sun produced a good model of this

gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...

. 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.NASA Space Science Data Coordinated Archive
Note that there is no "Epoch end" date given, which is NASA's way of saying it is still in orbit.NASA Space Science Data Coordinated Archive