Engineering
Introduction
The Cassini-Huygens spacecraft is one of the largest, heaviest and most complex interplanetary spacecraft ever built. The main body of the orbiter is a nearly cylindrical stack consisting of a lower equipment module, a propulsion module and an upper equipment module, and is topped by the fixed, four-meter diameter high-gain antenna. Attached about halfway up the stack are a remote sensing pallet, which carries cameras and other remote sensing instruments, and a fields and particles pallet, which carries instruments that study magnetic fields and charged particles. The two pallets carry most of the Cassini orbiter's science instruments. In general, the entire spacecraft must be turned to orient the instruments in the correct observing direction, although three of the instruments possess their own single-axis articulation capability.
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Cassini Characteristics |
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| Dry mass (orbiter only) | 2125 kg | |
| Launch mass (orbiter, Huygens descent probe, launch vehicle adapter, fuel) | 5712 kg | |
| Height |
6.7 m
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| Width |
4 m
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| Power (beginning of mission) |
885 W
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| Power (end of nominal mission) |
633 W
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Mechanical
Structure
The Structure Subsystem provides mechanical support and alignment for all flight equipment, including the Huygens probe. In addition to its skeletal function, it provides thermal conductivity, serves as an equipotential surface and is an electrical grounding reference. It is also used as shielding from radio frequency interference and protects other spacecraft equipment from radiation and micrometeoroids. Before launch, it provided attachment points for ground handling.
Mechanical Devices
The Mechanical Device Subsystems supply equipment to the spacecraft that provides non-feedback controlled motion. These subsystems supply a number of mechanisms for separating Cassini from the Centaur launch vehicle, as well as all of the required pyrotechnic devices and initiators. They also provide the deployable Magnetometer Science Boom Assembly (MAG); an articulated platform for the redundant reaction wheel; Thermal Louvre Assemblies (TLAs) for passive heat transfer; and Variable Radioisotope Heater Units (VRHUs).
Thermal
As its name implies, the Temperature Control Subsystem is responsible for maintaining the temperature of the spacecraft within an acceptable range. Cassini's circuitous route to Saturn will expose it to extreme variations in its thermal environment. When flying by Venus, the solar heating will be nearly three times greater than it is at the Earth's distance from the sun. At the other extreme, when Cassini is at Saturn, solar irradiance will be nearly 100 times less than at 1 AU and extreme cold becomes a concern.
Temperature on the Cassini-Huygens spacecraft is maintained through a combination of special hardware and special handling procedures. For example, during the cruise to Saturn, the high-gain antenna is oriented toward the sun when the spacecraft to Sun range is less than 2.7 AU to shield most of the other spacecraft components. Special temperature control hardware includes thermal blankets, shades, thermal shields, louvers and heaters. Thermal blankets provide insulation. Thermal shields shade components from the Sun. Louvres dissipate heat from electronics bays. Each instrument has an electrical heater, but they are used sparingly, to bring equipment up to operating temperature. However, because of clever design techniques, few other electrical heaters are needed, as waste heat from the Radioisotope Thermoelectric Generators (RTGs) is used to heat electronic equipment.
Electric Power
The Power and Pyrotechnics Subsystem provides regulated 30 Volts DC electrical power to the spacecraft. The power is derived from three Radioisotope Thermoelectric Generators (RTGs), each of which use heat from the radioactive decay of 10.9 kg of plutonium dioxide to generate 300 Watts of electrical power at launch, reducing to around 210 Watts at the end of the nominal, eleven-year mission.
The power from the RTGs is conditioned and distributed to the powered spacecraft components. This subsystem also initiates electro-explosive, or pyrotechnic, devices. These devices are used throughout the spacecraft to initiate one-time events such as separating the spacecraft from the Centaur launch vehicle.
Cabling
The Cabling Subsystem provides system wiring for all of the other subsystems. Interconnections are required for power, instrumentation, command, data, signal and pyrotechnic device actuations. The Cassini Cabling System is a passive system - it contains no active electronic components, generated no signals of its own, and requires no power. Its sole function is to transfer electrical signals from one subsystem to another.
Propulsion
The Propulsion Module Subsystem provides thrust for spacecraft trajectory and orbit changes, and for attitude control. The main engine is used for spacecraft velocity and trajectory correction changes. There are two identical main engines, one in use and the other available as a backup. The engines use nitrogen tetroxide and monomethyl-hydrazine as the oxidiser and fuel, respectively. Each engine is capable of producing a thrust of 445 N.
There are also sixteen 0.5 N thrusters arranged in four groups of four, which are used for attitude control and also for small (< 0.5 m s-1) velocity-change manoeuvres. The thrusters use hydrazine fuel which undergoes catalytic decomposition to generate thrust.
Attitude Control
The Attitude and Articulation Control Subsystem is responsible for three functions, with varying abilities. Its first responsibility is to maintain attitude control of the spacecraft, which means its position along three axes. The second, to a much lesser degree, is articulation, and the third function is pointing control of the main propulsion engines relative to the spacecraft.
In order to alter its position, the spacecraft must first know its orientation and current location. This is called attitude determination and is achieved in the Attitude and Articulation Control Subsystem by three Inertial Reference Units (IRUs) and a Stellar Reference Unit (SRU), or star tracker. The inertial reference units use solid-state gyroscopes. The stellar reference unit navigates by detecting the stars in its field of view and comparing them with its onboard catalog of 5,000 stars. Finally, Reaction Wheel Assemblies (RWAs) are one of the two systems used to provide pointing control of the spacecraft in flight (with the thrusters of the Propulsion System as the other). The reaction wheel assemblies contain electrically powered wheels. They are mounted along three orthogonal axes aboard the spacecraft. Varying the rotation speed of the reaction wheels causes a torque on the spacecraft about the axis of the wheel.
Communications
Radio Frequency Subsystem
The Radio Frequency Subsystem, together with the antenna subsystem, provides communication functions for the spacecraft to and from Earth. Part of the radio frequency subsystem is also used by the Radio Science Instrument. For telecommunications, the radio frequency subsystem produces an X-band carrier at 8.4 Ghz, modulates it with data received from the Command and Data System, amplifies the X-band carrier band-carrier power to produce 20 Watts from the Traveling Wave Tube Amplifiers (TWTA), and delivers it to the antenna subsystem.
The parts of this subsystem used for the radio science instruments are: The High-gain Antenna (ANT), the Ultra Stable Oscillator (USO), the Deep Space Transponders (DSTs), the X-band Traveling Wave Tube Amplifiers (X-TWTAs), and the X-band Traveling Wave Tube Amplifier.
Antennas
The Antenna Subsystem consists of the High-Gain Antenna (HGA) and two Low-Gain Antennas
To prevent the harmful rays of the sun from reaching the spacecraft's instruments during most of the early portion of the long journey to Saturn, the high-gain antenna was pointed toward the sun, functioning as an umbrella. With its most powerful antenna not pointed toward Earth, the spacecraft used the low-gain antennas to exchange information with ground controllers. Low-gain antennas provide omni directional coverage allowing relaxed spacecraft pointing requirements, as opposed to the high-gain antenna, which must be accurately pointed Once Cassini-Huygens was far enough from the Sun, it finally began using the high-gain antenna for communicating with Earth, thus achieving much faster transmission rates.
Distances and Data Rates
When Cassini is in orbit around Saturn, its distance from Earth will vary from 8.6 to 10.6 AU (1.3 to 1.6 ×109 km). Radio signals will take from 68 to 84 minutes to travel between the spacecraft and the ground station.
Depending on the mission phase, the data transmission rate will vary between 5 bits per second and 249 kilobits per second.
Data Handling
Command and Data Subsystem
The Command and Data Subsystem stores and processes data from all of the subsystems, sensors and science instruments. It also provides commands to all of the subsystems and instruments. The orbiter requires extensive on-board computing capability because most of the Cassini mission is performed while the spacecraft is not in direct communication with ground controllers. The heart of this subsystem is the Engineering Flight Computer, which connects to all the spacecraft components through a bus interface system.
Solid State Recorder
The Solid State Recorder Subsystem records science data and information on the spacecraft's health and status. The recorder has no moving parts, and Cassini-Huygens is the first deep space mission to use this technology. Older missions had to rely on flight tape recorders to store the data collected. In addition to its recording and playback functions, the recorder is used to store critical flight programs. Science data is periodically sent to Earth and then erased from the recorder in order to free space for new data. The Solid State Recorder has a capacity of four gigabits.
Electronic Packaging
The Electronic Packaging Subsystem contains almost all of the electronic equipment for the orbiter. This subsystem consists of a circular electronics bus made up of twelve standardized bays containing the electronics modules. The packaging of all electronic assemblies was designed with attention to functional, cabling, temperature control, radiation, magnetic and centre-of-gravity considerations. In addition, the electronics assemblies are shielded from electromagnetic interference and electric cross-coupling.