Spectrometry - Plasma, Ion and Neutral Mass
Spectrometers work to study and measure the light that the human eye cannot see. The Cassini Plasma Spectrometer (CAPS) measures the energy and electrical charge of particles such as electrons and protons that the instrument encounters. The instrument is used to study the composition, density, flow, velocity, and temperature of ions and electrons in Saturn's magnetosphere. Shaped much like a teardrop, Saturn's magnetosphere is huge. It spreads out almost a million miles, engulfing the orbits of Titan and most of the ringed planet's icy moons, as well as the famous rings. The Ion and Neutral Mass Spectrometer (INMS) is collecting data to determine the composition and structure of positive ions and neutral particles in the upper atmosphere of Titan and the magnetosphere of Saturn. It is also measuring the positive ion and neutral environments of Saturn's rings and icy moons.
The instrument consists of three sensors: an electron spectrometer, an ion beam spectrometer, and an ion mass spectrometer. A motor-driven actuator rotates the sensor package to provide 208-degree scanning in the azimuth of the spacecraft. The electron spectrometer makes measurements of the energy of incoming electrons; its energy range is 0.7 to 30,000 electron volts. The ion beam spectrometer determines the energy to charge ratio of an ion; its energy range is 1 electron volt to 50 kilo-electron volts. The ion mass spectrometer's energy range is 1 electron volt to 50 kilo-electron volts.
Radar is an acronym that stands for "radio detection and ranging." The Cassini Radar (RADAR) instrument consists of a 4-meter-diameter (13-foot) high-gain antenna and a smaller low-gain antenna. Radar takes pictures like a camera but it "sees" using microwaves instead of light. It measures how objects reflect microwaves, which tells scientists how rough they are, or how they would conduct electricity. This information helps scientists to deduce what a celestial body's landscape looks like and figure out its composition. Radar can also pierce through an atmosphere, even one as thick and murky as the one engulfing Titan. By bouncing radio signals off Titan's surface and timing their return, the RADAR produces maps of Titan's surface and measures the height of surface objects such as mountains and canyons. The radar will take four types of observations: imaging, altimetry, backscatter and radiometry. In imaging mode, pulses of microwave energy are bounced off the surface of Titan. Radar then records the time it takes the pulses to return to the spacecraft. To obtain the precise altitudes of surface features, Cassini uses the altimetry mode, which also bounces microwave pulses off the surface of a body. Backscatter mode collects data about the composition and roughness of the surface. In radiometry mode, the instrument simply records heat emanating from the surface of Titan.
Imaging Science Subsystem
The "eyes" of Cassini, the Imaging Science Subsystem (ISS) consists of a wide-angle camera and a narrow-angle camera. The narrow-angle camera provides high-resolution images of targets of interest, while the wide-angle camera allows more extended spatial coverage at lower resolution.
At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 microns on a side. The data system allows many options for data collection, including choices for on-chip summing and data compression. The narrow-angle camera packs plenty of power too, and could see a quarter -- 2.4 centimeters (0.9 inches) across -- from a distance of nearly 4 kilometers (2.5 miles).
The narrow-angle camera also provides high-resolution images of targets of interest, while the wide-angle camera allows more extended spatial coverage at lower resolution.
To increase the images' scientific value, each camera on Cassini has two filter wheels designed to take images at specific wavelengths of light. The narrow-angle camera has 12 filters in each wheel for a total of 24 filters; the wide-angle has 9 in each wheel for a total of 18. Some filters only allow light of a certain color to reach the sensor. Combining three such images can produce a color image. The most scientifically interesting images are calibrated in order to turn the electrical signals that emerge from the CCDs into an absolute measure of brightness.
Dual Technique Magnetometer
The Dual Technique Magnetometer (MAG) is a Direct Sensing Instrument (think smell or taste) that measures the strength and direction of the magnetic field around Saturn (MAG shown in light blue). Measuring the magnetic field is one of the ways to probe the core, even though it is far too hot and deep to actually visit. The goal is to develop a three-dimensional model of Saturn's magnetosphere, as well as determine the magnetic state of Titan and its atmosphere, and investigate the icy satellites and their role in the magnetosphere of Saturn. The MAG "sees" through celestial bodies in order to measure the strength and direction of Saturn's magnetic field near the spacecraft. To protect it from the normal electromagnetic radiation coming from the body of the Cassini spacecraft, the sensitive MAG is mounted at the end of a 13-meter-long (42-foot) boom that extends outward from the spacecraft. To detect and measure the strength of magnetic fields in the vicinity of the spacecraft, the MAG is composed of two instruments; the Vector/Scalar Helium Magnetometer (V/SHM), and the Fluxgate Magnetometer. Together, the instruments measure the magnitude and direction of magnetic fields. Since magnetometers are sensitive to electric currents and ferrous components, they are generally placed on an extended boom, as far from the spacecraft as possible.
Using radio waves, Cassini's Radio Science instruments study the solar wind interaction with Saturn and the interior structure of the planet and its many natural satellites. The Radio and Plasma Wave Science (RPWS) instrument receives and measures the radio signals coming from Saturn, including the radio waves given off by the interaction of the solar wind with Saturn and Titan.
The instrument studies the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR), and also monitors and maps Saturn's ionosphere and plasma, and lightning from Saturn's atmosphere.
RPWS is also adept at determining the dust and meteoroid distributions throughout the Saturn system and between the icy satellites, the rings, and Titan.
Cassini's Radio Science Subsystem (RSS) is designed to take measurements using radio waves beamed to Earth that explore similar concepts at a distance of more than a billion kilometers (more than 621 million miles).
Spectrometry - Ultraviolet and Infrared
Cassini uses infrared and ultraviolet light to study the composition of Saturn and its moons. The Ultraviolet Imaging Spectrograph (UVIS) is a box of four telescopes that can see ultraviolet light. Ultraviolet (UV) light, known as the cause of sunburn on Earth, is invisible to the human eye. The instrument measures the views in ultraviolet light, and scientists use these measurements to produce pictures we can see. Since there is no table that maps ultraviolet "colors" to the colors that humans see, the team exercises creative freedom when it makes representative images from the collected data. With its capacity to see light that's redder than the red we see, the Composite Infrared Spectrometer (CIRS) searches for heat and is capable of discerning an object's composition. The Visual and Infrared Mapping Spectrometer (VIMS) is made up of two cameras in one: one is used to measure visible wavelengths, the other infrared. Combined, the two cameras gather a lot of information on the composition of moon surfaces, the rings, and the atmospheres of Saturn and Titan.
Capable of detecting the impact of tiny particles -- 1/1,000 of a millimeter wide, Cassini's Cosmic Dust Analyzer (CDA) measures the chemical composition, speed, size, and trajectory of dust particles to determine their origin. The CDA takes measurements when the dust particles impact the collection surface inside the instrument and become vaporized. As a result, a puff of plasma is created, which is extensively measured and analyzed. To understand their true size and consistency, this cosmic dust can best be visually compared to icy cigar smoke particles. Under certain conditions, the Cosmic Dust Analyzer (CDA) can even detect smaller dust grains called nano-dust. The instrument can also detect the impact of very tiny particles in the Saturnian system. Saturn's broad, diffuse E ring -- within which several of Saturn's major moons travel in their orbits -- is composed primarily of dust particles that are one-thousandth of a millimeter in size. The Cassini's Cosmic Dust Analyzer has the ability to directly measure the chemical composition of the dust particles impacting the Saturn system. In addition to detecting their speed, size and chemical composition, the instrument can also determine trajectories (orbits). This allows scientists to determine where the dust originated. The CDA requires very little power and is a passive instrument that allows particles to impact it rather than actively searching for them.
Magnetospheric Imaging Instrument
The Magnetospheric Imaging Instrument (MIMI) studies all the possible sources of energy in and around Saturn, which is crucial to scientists' understanding of the dynamics of Saturn's magnetic field and atmosphere. MIMI measures the composition, charge state, and energy distribution of energetic ions and electrons; detects fast neutral particles; and conducts remote imaging of Saturn's magnetosphere. The information gathered is used to study the overall configuration and dynamics of the magnetosphere and its interactions with the solar wind, Saturn's atmosphere, rings, and icy moons, and Titan. It is the first instrument ever designed to produce an image of a planetary magnetosphere. The MIMI instrumentation consists of three sensors: the Low Energy Magnetospheric Measurement System (LEMMS), the Charged Energy Mass Spectrometer (CHEMS), and the Ion and Neutral Camera (INCA). The low-energy magnetospheric measurements system (LEMMS) detector measures low- and high-energy proton, ion, and electron angular distributions. The ion and neutral camera (INCA) will take two different kinds of measurements. Highly sensitive, the INCA will collect the three-dimensional distribution, velocities, and rough composition of magnetospheric and interplanetary ions in the regions in which the energetic ion fluxes are very low. INCA will also obtain remote images of the global distribution of the energetic neutral emission of hot plasmas in the Saturnian magnetosphere. The Charge Energy Mass Spectrometer (CHEMS) measures the charge state, composition, and energy of ions.
Radioisotope Thermoelectric Generator
Saturn is more than nine times as far from our Sun as the Earth. At that great distance, the Cassini spacecraft is too far from the Sun (1.43 billion km (890 million miles)) to use solar panels to generate enough power to operate. Instead, the 2,125 kg (4,685 pounds) orbiter relies on three onboard Radioisotope Thermoelectric Generators (RTGs) to provide 885 watts of power by converting heat into electrical energy. The natural radioactive decay of (ceramic-form) plutonium-238 produces heat. (RTGs are not reactors, and the radioactive material is neither fissionable nor fusionable.) Thermocouples convert the heat into electricity, which is then distributed by the Power and Pyrotechnics Subsystem (PPS) to the spacecraft, its instruments, computers, radio transmitters, attitude thrusters, and reaction wheels.