|The Voyager Spacecraft|
Equipment & Instrumentation
The Voyager spacecraft were launched from the NASA Kennedy Space Center at Cape Canaveral in Florida, aboard Titan/Centaur III expendable launch vehicles.
The twins Voyager 1 and Voyager 2 are Mariner-class spacecraft but, while the Mariner spacecraft were traveling sufficiently close to the sun to make use of solar electric panels for power, the Voyagers were outfitted with Radioisotope Thermoelectric Generators.
Each Voyager weighs 1,820 pounds, carries 11 scientific instruments, and has a 12-foot diameter parabolic reflector High Gain Antenna for data transmission, capable of transmitting 115,200 bits per second. The antennae have a dual use as probes of planetary atmospheres because they could monitor signal strengths as a Voyager spacecraft passed behind a planet.
JPL operates the Deep Space Network (DSN), a global spacecraft tracking system that controls the Voyager spacecraft and acquires their data. The DSN receivers are in remote desert locations surrounded by hills in the Mojave Desert in California, near Madrid in Spain, and in Tidbinbilla, near Canberra, in Australia. Thus the signals from the spacecraft are protected from radio interference on Earth.
As the Voyager mission was extended beyond initial expectations, commands from the DSN transmitters traveling at the speed of light, took longer to reach the spacecraft and remote control programming to correct errors meant excruciatingly long delays. To prepare for Voyager 2's flyby of Neptune, the DSN's largest antennas were increased from 210 to 230 feet in diameter.
Uranus receives 400 times less sunlight than we do on Earth, so to obtain high-quality images while moving at high velocities presented a challenge for the Voyager engineers after the encounter with Saturn. In addition, Voyager 2's radio signals were becoming fainter as it moved farther from Earth. Remote control reprogramming of Voyager 2's onboard computers provided the spacecraft with enhanced capabilities.
Left: False-color Voyager 2 composite image of Uranus using orange, short-wavelength methane, and long-wavelength methane filters. The high-altitude hazes are the pinkest, while the bluest areas have the least haze. Middle: NASA's Voyager 2 took this photograph of Saturn on July 21, 1981, when the spacecraft was 33.9 million kilometers (21 million miles) from the planet. Right: This image of Uranus's rings was taken while Voyager 2 was in the shadow of the planet.
The extremely healthy state and, ultimately, the tremendous success of the Voyager spacecraft are due to the foresight and planning of the engineers, who built "redundancy" into the spacecraft to provide backups in the event of equipment failure.
To fix the orientation of Voyager and keep the antenna pointing toward the Earth requires the use of two sensors: a Sun Sensor views the Sun through a hole in the antenna, while a sensor on the side of the spacecraft keeps the star Canopus in sight. Voyager's orientation and velocity are controlled by 16 thrusters around the body of the spacecraft that make use of hydrazine decomposition for power.
The Voyager Imaging System consists of two cameras with attached telescopes: a narrow-angle, long-focal-length camera for high-resolution images and a wide-angle short-focal-length camera for wide-angle coverage. A selection of filters allows color images to be taken.
The Infrared Interferometer Spectrometer (IRIS) combines a Michelson interferometer and a single-channel radiometer in a single device to measure infrared radiation emitted or reflected by the planets, so that the planets' composition and atmospheric temperature could be gauged.
The Photopolarimeter System (PPS) consists of an 8-inch telescope that can send observations through a polarizer and a filter for one of eight bands in the 2200 -7300-A spectral region, and then to a photomultipler tube. The PPS measures the amount of light scatter or reflected from the planets at various wavelengths and angles, to provide information about the surface texture and composition of planets, as well as data about atmospheric scattering properties and densities of the planets and the size distribution and composition of their rings.
Middle: This enhanced false color Voyager 2 image was put together from clear, orange and ultraviolet frames. The color variations reflect possible variations in chemical composition within Saturn's ring system. The C ring appears blue. The picture also shows color differences between the inner B ring and the outer region, where the spokes form, and between the B and the A rings. Right: The graininess of this Voyager 2 image of Neptune's moon Proteus is due to the short camera exposure time.
The Ultraviolet Spectrometer measures atmospheric properties of the planets and ultraviolet radiation emitted from them.
The Cosmic Ray Subsystem (CRS) is the most sensitive of the particle sensors and records the number and energy of the extremely high-energy cosmic ray particles near Voyager.
The Low-Energy Charged Particle (LECP) detector has the broadest energy range of the three sets of particle detectors and is designed to measure low-energy charged particles trapped in the radiation belts of planets.
The Plasma Spectrometer (PLS) measures the lowest-energy particles. It consists of two Faraday-cup detectors, one pointed along the Earth-Voyager line and the other perpendicular to this line. The Earth-pointing detector is designed to measure the velocity, density, and pressure of the plasma ions, while the side-pointing detector measures electron with energies ranging from 5 eV to 1 keV.
The Triaxial Fluxgate Magnetometer (MAG) investigates the patterns of planetary magnetic fields. Voyager has two high-field and two low-field magnetometers that can be used to study the solar wind interaction with planetary magnetospheres, as well as the interplanetary magnetic field out to the boundary of the solar wind and the interstellar magnetic field and beyond.
The 33-foot long Planetary Radio Astronomy (PRA) Antennas measure radio emissions from the planets, as well as the motion of particles in the plasma, and can detect planetary lightning.
The Plasma Wave Subsystem (PWS) shares the PRA antennas, but uses them as a single antenna. Usually the PWS is in scanning mode, but it can be used, simultaneously and especially when Voyager is near a planet, to listen to all the stations on its audio band. The PWS is used to sample plasma behavior in and around planetary magnetospheres.
It is amazing that, while the Voyager spacecraft have been pushed beyond the physical and time limits anticipated, their instruments are still working well and are sufficiently sensitive to continue obtaining data beyond the termination shock and the heliopause. In spite of the vast computer advancements made in the last 25 years, the original onboard computers are able to cope with all the demands of the extended mission placed on them.
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