Helios (spacecraft)

Structure

The two Helios probes look very similar. They have a total mass of 370 kg (Helios 1) and 376.5 kg (Helios 2); the scientific payload, composed of eight instruments, is 73.2 kg on Helios 1 and 76.5 kg on Helios 2. The central body is a sixteen-sided prism 1.75 m in diameter and 0.55 meters high. Most of the equipment and instrumentation is mounted in this central body. Exceptions are the masts and antennas used in scientific experiments and small telescopes that measure the zodiacal light that 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 the probe was 2.12 meters tall and reaches a maximum diameter of 2.77 meters. Once in orbit, a telecommunications antenna unfolded on top of the probe and increased the total height to 4.20 meters. Also deployed on orbit are two rigid booms carrying sensors and magnetometers, attached on both sides of the central body, and two flexible antennas used for the detection of radio waves, which extended perpendicular to the axis of the spacecraft for a design length of 16 metres each.

The spacecraft spins around its axis (which is by design perpendicular to the ecliptic) at 60rpm.

Power

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 ºF) in proximity of 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 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). These are made of fused quartz, 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. For additional protection, multi-layer insulation, consisting of 18 layers of 0.25 mm 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 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 uses 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 of an adapter that connected the probe to the launch vehicle. 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 Deep Space Network network antennas on Earth.

Attitude 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 on-board controller is capable of handling 256 commands. The mass memory can store 500 kb and is mainly used when the probe is in superior conjunction relative to the Earth (i.e. the Sun comes between the Earth and the spacecraft). The conjunction may last up to 65 days. This was a very large memory for space probes of the time; the Viking probe had only 64 kilobits.

Scientific instrumentation

Both probes Helios had ten scientific instruments:

Helios 1

Helios 1 was launched December 10, 1974 from Cape Canaveral, on the first operational flight of the Titan 3E-Centaur rocket. The rocket's previous test flight had failed when the engine on the Centaur did not light (see the main Titan IIIE article), but the launch of the Helios 1 was uneventful.

The probe was placed in a heliocentric orbit of 192 days with a perigee 46.5 million kilometres (0.31 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 should be done with reduced power, but this requires using large diameter terrestrial receivers already stretched by other space missions in progress.

During the first perihelion in late February 1975, the spacecraft came closer to the Sun than any spacecraft had before. The temperature of some components reached more than 100 °C, while the solar panels reached 127 °C, without affecting probe operations. However, during the second pass on September 21, temperatures reached 132 °C and the operation of certain instruments was affected by heat and radiation.

Helios 2

Some lessons were learned from the first Helios operation before the second Helios probe was launched. 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 detectors were improved so that they could detect gamma ray bursts (the existence of gamma ray bursts had been made public in 1973), so that they could be used in conjunction with Earth-orbiting satellites to triangulate the location of the bursts. And, since Helios 1 had verified that the probe temperatures did not exceed 20 °C below the design requirement at perihelion, it was decided to orbit even closer: the thermal insulation was enhanced so that the satellite could support 15% greater heat flux.

Tight schedule constraints pressed on the Helios 2 launch in early 1976 - facilities damaged by the launch of the Viking 2 spacecraft in September 1975 had to be rehabilitated, while the Viking landing on Mars in summer 1976 would require the Deep Space Network antennas that would be necessary for Helios 2 perihelion science. Helios 2 was launched through the window available on January 10, 1976 by another Titan IIIE/Centaur. The probe was placed in an orbit with an 187-day period and a perihelion 43.5 million km (0.29 AU). The orientation of Helios 2 with respect to the ecliptic was reversed 180° versus Helios 1, so that the micrometeorite detectors could have 360° coverage. On April 17, 1976 Helios 2 made its closest pass of the Sun, at a record heliocentric speed of 70 km/s; the highest measured temperature was 20 °C higher than for Helios 1.

The primary mission for each probe spanned 18 months, but they operated much longer. On March 3, 1980, four years after its launch, the radio transceiver on Helios 2 failed. On January 7, 1981 a stop command was sent to prevent possible radio interference in future missions. Helios 1 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 the 14th orbit, Helios 1's solar cells had degraded to the point that they did not provide enough power for the spacecraft to simultaneously collect and transmit data, unless the probe was close to its perigee. In 1984, the main and backup radio receivers both failed, indicating that the high-gain antenna was no longer pointed toward Earth; the last telemetry data was received February 10, 1986.

Results

Both probes 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 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 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 comets, 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.

Current status

As of 2016, the probes are no longer functional, but still remain in their elliptical orbits around the Sun.