Four unmanned U.S. spacecraft have flown past Jupiter and sent data back to Earth. But each mission raised as many questions as it answered. Pioneer 10, which was launched in 1972, was the first probe sent to the outer planets. It passed Jupiter nine months after its launch. Four weeks later it crossed the orbit of Hades, Jupiter's outermost moon.
Pioneer 11 lifted off later that year. Its first mission was to collect data on the vast interplanetary space between Mars and Jupiter. After it had passed the asteroid belt between the two planets it continued the research on Jupiter begun by Pioneer 10.
The Pioneer spacecraft were designed to fit within the 3 m shroud of the Atlas-Centaur launch vehicle. Each spacecraft was stowed with its booms retracted and its antenna dish facing forward. The two spacecraft had to be extremely reliable and lightweight; their communications systems had to transmit information over extreme distances; and each had nonsolar heat sources to supply electrical power.
Each Pioneer spacecraft comprised several distinct subsystems: a general structure, an attitude control and propulsion system, a communications system, a thermal control system, an electrical power system, and a navigation system.
General Structure
Each spacecraft was 2.9 m long. The structure of each spacecraft centered around a 36 cm deep, flat equipment compartment, the top and bottom of which consisted of regular hexagons with sides 71 cm long. Attached to one side of this hexagon was a smaller hexagon compartment that carried most of the instruments for the scientific experiments.
A 2.74 m diameter, 46 cm deep, parabolic, dish-shaped, high-gain antenna of aluminum honeycomb sandwich material was attached to the front of the equipment compartment. Its feed was topped with a medium-gain antenna mounted on three struts which projected about 1.2 m forward. A low-gain, omnidirectional antenna extended about 0.76 m behind the equipment compartment mounted below the dish of the high-gain antenna. Two three-rod trusses, 120 degrees apart, projected from two sides of the equipment compartment. At there ends, radioisotope thermoelectric generators were held about 3 m from the center of the spacecraft. A third single-rod boom, 120 degrees from the two trusses, projected from the experiment compartment to position a magnometer sensor about 6.6 m from the center of the spacecraft. All three appendages were extended after launch.
Attitude Control and Propulsion
A starlight sensor on each spacecraft provided a reference on the bright southern star Canopus, and two sunlight sensors provided a reference to the Sun. Attitude was calculated from the reference directions to Earth and the Sun, with the known direction to Canopus provided as backup. Before Pioneer 11 was launched, the gain and threshold settings of its starlight sensor were modified to improve performance on the basis of experience gained during the first few months of Pioneer 10's flight.
Three pairs of rocket thrusters located near the rim of the antenna dish were used to direct the spin of each spacecraft, to keep it spinning at the desired rate of 4.8 rpm, and to change the spacecraft for in-flight maneuvers. The system's six thrusters could be commanded to fire steadily or in pulses. Each thruster developed its propulsive jet from the decomposition of liquid hydrazine by a catalyst in a small rocket thrust chamber to which the nozzles of the thruster were attached.
Attitude and velocity were changed by two thruster pairs mounted on opposite sides of the rim of the antenna dish. One thruster was fired forward, one aft, in brief pulses of thrust at a precisely timed position in the cycle of rotation of the spacecraft. Each thrust pulse, timed to the rotation, precessed the spin axis a few tenths of a degree until the desired attitude was reached.
To change velocity, the spin axis was first precessed until it pointed in the direction along which the correcting velocity had to be applied. Then two thrusters, one on each side of the antenna dish, were fired continuously, both in the same direction
To adjust the spin rate of the spacecraft , two more pairs of thrusters, also set along the rim of the antenna dish, were used. These thrusters were aligned tangential to the antenna rim, one pointing against the direction of spin and the other pointing with it. Thus, to reduce spin rate, two thrusters were fired against the direction of spin. To increase spin rate, they were fired with the direction of spin.
Communications
Each Pioneer spacecraft carried two identical receivers. The omnidirectional and medium-gain antennas operated together and were connected to one receiver, while the high-gain antenna was connected to the other. The receivers did not operate at the same time, but were interchanged by command or, if there was a period of inactivity, they were switched automatically. Thus, if a receiver had failed during the mission, the other would have automatically taken over.
Two radio transmitters, coupled to two traveling-wave-tube power amplifiers, each produced 8 W of transmitted power at S-band. The communications frequency uplink from Earth to the spacecraft was at 2110 MHz, the downlink to Earth, at 2292 MHz. The turnaround ratio, downlink to uplink, was precisely controlled to be compatible with the Deep Space Network.
The data system of each spacecraft converted scientific and engineering information into a specially coded stream of data bits for transmission by radio to Earth A convolutional encoder arranged the data in a form that allowed most errors to be detected and corrected by ground computer at the receiving site of Deep Space Network.
Thermal Control
Temperature was held between -23 degrees and 38 degrees Celsius inside the scientific instrument compartment, and at various other levels elsewhere so that the scientific equipment onboard the spacecraft operated properly.
The system of temperature control was designed to adapt to the gradual decrease in solar heating as the spacecraft moved away from the Sun, and to those frigid periods when the spacecraft passed through Jupiter's or Saturn's shadow during planetary encounters. The temperature control system also controlled the effects of heat from the third stage engine, atmospheric friction during launch, spacecraft thermoelectric power generators, and from other equipment.
Equipment compartments were insulated by multi-layered blankets of aluminized plastic. Temperature responsive louvers at the bottom of the equipment compartment, opened by temperature sensitive bimetallic springs, controlled the amount of excess heat allowed to escape. Other equipment was individually thermally insulated and was warmed as required by electric heaters and twelve 1 W radioisotope heaters fueled with plutonium-238.
Electrical Power
Nuclear fueled electric power for the Pioneer spacecraft was derived from SNAP-19 type radioisotope thermoelectric generators (RTGs), developed by the Atomic Energy Commission. These units converted heat from radioactive decay of plutonium-238 into electricity.
The RTGs were located on the opposite side of the spacecraft from the scientific instruments to reduce the effects of neutron radiation. Mounted in pairs on the end of each three-rod truss, these four RTGs developed about 155 W of electrical power for each spacecraft at launch. By the time each spacecraft reached Jupiter, the power output had decreased to about 140 W. The RTGs supplied adequate power for the mission because each spacecraft needed only 100 W to operate all its systems and experiments. Any excess power from the RTGs not required by the spacecraft was dissipated into space in the form of heat by a shunt resistor.
Navigation
Throughout the mission, the axis of the high-gain antenna was slightly offset from, but parallel to, the spin axis of each spacecraft within close tolerances. Except during a few course adjustments, the spin axis of each spacecraft always pointed toward Earth, within a tolerance of 1 degree, to provide best communication.
Analysts used the shift in frequency of the radio signals from the spacecraft together with angle tracking by the antennas of the Deep Space Network to calculate the speed, distance, and direction of the spacecraft from Earth. The motion of the spacecraft away from Earth caused the frequency of the spacecraft's signals to drop and their wavelengths to increase. This effect allowed the speed of the spacecraft to be calculated from measurements of the change in frequency of the signal received at Earth. As the spacecraft continued outward, angle tracking became less important. Pioneer's path was calculated by use of celestial mechanics, and the radio data provided information to keep the trajectory updated and to determine the masses of planetary bodies the Pioneers encountered.
The radio beam to Earth was offset 1 degree from the spin axis of the spacecraft. As a result, when the spin axis was not directed exactly toward Earth, uplink signals received by Pioneer from Earth varied in intensity synchronously with the rotation of the spacecraft. A system on the spacecraft, known as conical scan (CONSCAN) was originally intended to automatically change the attitude of the spacecraft in a direction that would reduce such variations in signal strength, thereby returning the axis to align with the direction of the Earth to within a threshold of 0.3 degrees.
References
Fimmel, R., Pioneer First To Jupiter, Saturn And Beyond, NASA- Government Printing Office, Washington D.C., 1980.
Gonzalez, L., Exploring Space, Derrydale Books, New York, 1979.
Jane's Spaceflight Directory, Jane's Publishing Inc., New York, 1986.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
