Helios (spacecraft)

 Helios-A  and  Helios-B  (also known as Helios 1 and Helios 2), are a pair of probes launched into heliocentric orbit for the purpose of studying solar processes. A joint venture of West Germany's space agency DFVLR (70% share) and NASA (30%), the probes were launched from Cape Canaveral, Florida, on Dec. 10, 1974, and Jan. 15, 1976, respectively. Built by Messerschmitt-Bölkow-Blohm as the main contractor they were the first spaceprobes built outside the United States or Soviet Union.

The probes are notable for having set a maximum speed record among spacecraft at 252,792 km/h (157,078 mi/h or 43.63 mi/s or 70.22 km/s or 0.000234c). Helios 2 flew three million kilometers closer to the Sun than Helios 1, achieving perihelion on 17 April 1976 at a record distance of 0.29 AU (or 43.432 million kilometers), slightly inside the orbit of Mercury. Helios 2 was sent into orbit 13 months after the launch of Helios 1. The Helios space probes completed their primary missions by the early 1980s, but they continued to send data up to 1985. The probes are no longer functional but still remain in their elliptical orbit around the Sun.

Features spacecraft
The spacecraft is stabilized by Helios probe have no rotation and orbital maneuverability. The thermal control system largely helped define its architecture.

Structure
Both Helios probes have characteristics similar to some details. The probes have a total mass of 370 kg (Helios 1) and 376.5 kg (Helios 2); Of this total charge, composed of eight scientific instruments, it is 73.2 kg (Helios 1) and 76.5 kg (Helios 2). The central body is a cylinder side of a diameter of 1,75 m and a height of 0.55 meters. Most of the equipment and instrumentation is mounted on this central body. Exceptions are the mats and antennas used in scientific experiments and small telescopes that measure the zodiacal light that emerge as the central body. Two conically solar panels extend above and below the cylinder giving the assembly the appearance of a packing yarn. The probe before implantation in high orbit is 2.12 meters and reaching a maximum diameter of 2.77 meters. Once in orbit, a telecommunications antenna is implemented on top of the probe and increases the total height of 4.20 meters. Are also deployed in orbit two rigid bars carrying the sensors, and magnetometers attached on both sides of the central body and two flexible antennas used for the detection of radio waves perpendicular to the preceding one and having a length of 32 meters from a far end.

Power
Electrical power is provided by solar cells attached to the two truncated cones. To keep close to the sun, the solar panels at a temperature below 165 °C (329 ºF), solar cells are interspersed with mirrors, covering 50% of the surface and return the incident rays while dissipating the excess heat. The energy supplied by the solar panels is a minimum of 240 watts when the probe is in the farthest part of its orbit from the sun. The power whose voltage is regulated to 28 volts DC is stored on a silver-zinc battery of 8 Ah.

Thermal control
The biggest technical challenge that has faced the probe designers is the heat to which the latter is subject when it is near the sun. At 0.3 astronomical units from the Sun which undergoes heat flow is 11 solar constant (11 times the amount of received heat in the Earth's orbit) or 22,4 kW per square meter exposed. The temperature can then reach 370 °C. The solar cells and the central compartment of the equipment and instruments are located must be maintained at much lower temperatures. The solar cell should not exceed 165 °C, while the central compartment should be maintained between -10 °C and + 20 °C (or 14 °F and 68 °F). These restrictions require to reject 96% of the heat received from the sun. The conical shape of the solar panels is one of the measures taken to reduce the flow of heat. By tilting the solar panels with respect to sunlight arriving perpendicularly to the axis of the probe, reflected a greater proportion of the solar radiation. Furthermore, as discussed above, the solar panels are covered 50% of its surface mirrors developed by NASA dubbed Second Surface Mirrors (SSM). Thereof whose made of molten silicon, with a silver film on the inner face, which is itself covered with a dielectric material. The central compartment sides are entirely covered by these mirrors. An insulating material partially covering the core compartment to provide additional protection. It consists of 18 layers of 0.25 mm Mylar or Kapton (depending on location) isolated from each other by small plastic pins intended to prevent the formation of thermal bridges. In addition to these passive devices, the probe uses an active system based on a system of movable louvers in a star lining the bottom and top side of the compartment. The opening thereof is controlled separately by a bimetal spring whose length varies depending on the temperature and causes the opening or closing of the shutter. Resistors are also used to keep a temperature sufficient for certain equipment.

Telecommunications system
The telecommunication system using a radio transceiver, whose power can be adjusted between 0.5 and 20 watts. Three antennas are overlaid on top of the probe: A large antenna gain (23 dB) emits a top brush of 5.5° on either side of elliptical and 14° wide, a medium antenna gain (3 dB for the transmission and reception of 6.3 dB) emits a signal in all directions of the ecliptic plane at a height of 15° and a dipole omnidirectional antenna (0.3 dB in the transmission and - reception 0.8 db). The low gain horn antenna is located under the center of the probe because adapter that connects the probe in your launcher. With omni-directional antenna that provides a 360° coverage. To be constantly pointed toward Earth, the biggest 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 Sun sensor The maximum flow rate obtained with the large antenna gain is 4096 bits per second upstream. The reception and transmission of signals are supported by the network antennas on Earth to Deep Space Network continually NASA mission start then partially after.

Altitude control
To maintain its orientation during the mission, the spacecraft rotates continuously at 60 rpm around its main axis. The orientation control system eventually makes corrections to the speed and orientation of the probe shaft. To determine its orientation using a rude Sun sensor, a sun sensor and the end of a star seen that latch to the star Canopus. guidance corrections are performed using cold gas thrusters 3 (7.7 kg nitrogen) with a boost of 1 Newton. The axis of the probe is kept permanently, both perpendicular to the direction of the sun and perpendicular to the ecliptic plane.

Board computer and data storage
The onboard computer 256 is capable of handling commands. The mass memory can store 500 kB and is mainly used when the probe is in conjunction superior relative to the Earth (ie the Sun comes between the Earth and the spacecraft). The conjunction may last up to 65 days.

Scientific instrumentation
Both probes Helios exceed ten scientific instruments:


 * Plasma Experiment Investigation: developed by Max Planck Institute study of low-energy particles with three types of sensors: an analyzer proton and alpha particles when it is between 231 eV and 16 keV, a detector for protons and heavy particles and an electron detector. The instrument identifies all significant solar wind parameters: density, speed, temperature. Measurements are taken every minute, but 1/10 s to the flux density to highlight irregularities and plasma waves.
 * Flux-gate Magnetometer: developed by the University of Braunschweig, Germany measuring three vector components of the magnetic field. The intensity is measured with an accuracy of 0.4 nT when below 102.4 nT to 1.2 nT at a lower intensity than 409.6 nT. Two sample rates are available: search every 2 seconds, or 8 readings per second.
 * Flux-gate Magnetometer 2: developed by the Goddard Space Flight Center of NASA, with an accuracy of 0.1 nT about 25 nT to 0.3 nT about 75 nT to 0.9 nT at an intensity of 225 nT.
 * Search Coil Magnetometer: developed by the University of Braunschweig as fluctuations in the magnetic field at 5 Hz frequency range - 3 kHz. The spectral resolution is performed on the probe's rotation axis.
 * Plasma Wave Investigation: developed by the University of Iowa studied electrostatic and electromagnetic waves in the frequencies between 10 Hz and 2 MHz.
 * Cosmic Radiation Investigation: developed by the University of Kiel uses a detector semiconductor, one scintillator and Cherenkov counter encapsulated in an anti-coincidence detector to determine the intensity, direction and energy of the protons and heavy constituent particles this radiation.
 * Low-Energy Electron and Ion Spectrometer: developed at Goddard Space Flight Center uses three telescopes to measure particle characteristics protons with energies between 0.1 and 800 MeV and electrons with an energy between 0 5:05 MeV. A detector is also studying the X-rays of the sun. The three telescopes are installed to cover the ecliptic plane.
 * Zodiacal Light Photometer: counts the number of electrons and energy. instrument's field of view is 20°, and can process stream comprising from 1 10-4 electrons per square centimeter. Three photometers developed by the Centre Heidelberg measure the intensity and polarization of the zodiac light to ultraviolet light and white using three telescope whose optical axis forms an angle of 15, respectively 30 and 90+ more ecliptic. It can be inferred from the powder distribution, grain size and nature.
 * Micrometeoroid Analyser: developed by Max Planck Institute can detect if the mass is greater than 10-15 g to determine the mass and energy of 10-14 g and in some cases the composition from 10-13 g. These measurements are made by taking advantage of the fact that micrometeorites hit a target vaporize and ionize. The instrument separates the ions and electrons in the plasma generated, measure the electric charge and deducts the mass and energy of the incident particle. A small mass spectrometer determines the composition of small ions.
 * Celestial Mechanic Experiment: developed by University of Hamburg uses the Helios orbit specifics to clarify some astronomical measurements: flattening of the Sun, verification of the effects predicted by the theory of general relativity in orbit and spread the radio signal, improving the anniversary of the inner planets, planet Mercury mass, mass ratio of the Earth-Moon, integrated electron density between the ship and the ground station.
 * Faraday Effect Experiment: developed by University of Bonn, operates this physical phenomenon affecting electromagnetic waves that pass through the corona to determine the density of electrons and the intensity of the magnetic field in the space region.

Mission
Helios 1 was launched December 10, 1974 from the base of Cape Canaveral by a rocket Titan 3E is the first operational flight. The only test of this launch was a failure due to a stage engine failure Centaur. But the launch of the Helios 1 was uneventful and the probe is placed in a heliocentric orbit of 192 days with a perigee brings only 46.5 million kilometres (0.31 AU) from the sun. However, several problems affecting operation Helios 1. One of the two antennas do not flexible to implement that reduces the sensitivity of the radio plasma apparatus for the reception of low frequency waves. When the high-gain antenna is connected realize that their emissions interfere with the analyzer particles and the radio receiver. To reduce the interference of communications should be done with reduced power, but this requires measuring the return network for use in large land diameter of receive antennas already stretched by other space missions in progress. In late February 1975, the spacecraft is closer to the sun. At the time any spacecraft approached too near the sun. The temperature of the various components amounts to more than 100 °C and the solar panels are measured at 127 °C without the operation of the probe is affected. However, during the second pass by the Sun, which takes place on September 21, temperature reached a maximum of 132 °C and operating certain instruments is affected by heat and radiation.

Some are lessons learned from the first Helios operation before the second probe Helios. Small engines used for attitude control are improved. Changes are made to the implementation mechanism of the flexible antenna and high gain antenna emissions. The X-ray detectors are improved so that they can detect gamma ray bursts detected by U.S. military satellites instruments candle to allow for emissions from these sources triangulation operations with other satellites. Having verified that the probe temperature did not exceed 20 °C nearest the sun, it is decided to take a closer orbit the sun in enhancing the thermal insulation so that the satellite can support increased 15% of the heat flow. The launch of the Helios 2 to be executed early in 1976 under heavy schedule constraints. Shooting damaged by the launch of Viking 2 spacecraft in September 1975 to be rehabilitated and Viking landing on Mars must mobilize during the summer of 1976, the Deep Space Network antennas that will more available for the passage of Helios 2 perigee of its orbit. Finally Helios 2 is released in the narrow window of opportunity available on January 10, 1976 by a rocket Titan/Centaur. The probe is placed in an orbit of 187 days, which went from 43.5 million km (0.29 AU) from the sun. The orientation of the Helios 2 is reversed 180° to that adopted for the first probe to the micrometeorites detectors to perform his remarks with a 360° coverage. The April 17, 1976 Helios 2 made its closest pass of the Sun to a record heliocentric speed of 70 km/s. the highest measured temperature is 20 °C than that suffered Helios 1.

Both probes show a long life. The duration of the primary mission of the two probes is 18 months, but they will operate much longer. March 3, 1980, four years after its launch, the radio transceiver Helios 2 failed and despite several attempts is no longer possible subsequently to recover usable data. On January 7, 1981 a stop command is sent to prevent possible radio interference in future missions. Helios 1 continues its part to function normally, but their data is collected by small diameter antenna to a lower rate. From the 14 th orbit, the degradation of its solar cells can no longer collect data and transmit them at the same time, unless the probe is close to its perigee. In 1984, the main radio receiver and save both fall down the antenna and high gain is not pointed toward Earth. The last telemetry data is received February 10, 1986.

Results
Both probes were collected important data about the processes that cause the solar wind and the acceleration of the particles that make up the interplanetary medium and cosmic rays. These observations were made, both the minimum solar cycle (1976) and at its peak in the early 1970s.

The observation of the zodiacal light has established some of the dust properties interplanetary present between 0.1 AU and 1 AU from the Sun, 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 to collect interesting data on the comet, watching the passage of C/1975V1 West in 1976, C1978H1 Meir in November 1978 and C/1979Y1 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 1 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 10, Pioneer 11, Voyager 1 and Voyager 2 were used to determine the direction of the magnetic field at distances staggered 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 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 modeling of this gradient. The GRBs Helios 2 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 is the ground station sent a signal that was returned by the probe. Changes in signal propagation resulting from the solar corona cross provided information on the density fluctuations, travel speeds of the crown structures 1.7 sunbeam.