This summer, NASA’s New Horizons craft caused quite a stir when it zipped past Pluto in the middle of July this year, performing the first ever scientific study of the planet from a close range. For at least a week, the mainstream media was full of talk about Pluto, it’s heart-shaped field of icy plains and the vast amount of imagery returned by the spacecraft probe. However, New Horizons only spent about one Earth day looking at Pluto before taking off to study the wider Kuiper Belt. Not much attention is being paid to the NASA craft that has provided us with a close view of our solar system’s second-largest planet every day for more than a decade.
October 15th, 1997, saw the launch of the Cassini orbiter from Cape Canaveral, FL. By 2004, the craft had reached the Saturn system and completed its primary mission of studying that region of our solar system by 2008. Recently, the craft sent pictures of Dione, one of Saturn’s moons, back to researchers on Earth on its last flyby of that orbiting body before Cassini plunges into Saturn’s atmosphere sometime in 2017.
Cassini is named after Italian astronomer Giovanni Cassini who, in the late 17th century, discovered four satellite moons of Saturn as well as the existence of a division in Saturn’s rings, now named the Cassini Division. When launched, Cassini was partnered with the Huygens probe, a craft developed by the European Space Agency (ESA) and named 17th century Dutch astronomer Christiaan Huygens who discovered Saturn’s moon Titan in 1665, just six years before Cassini started finding the other moons. The homage to Titan’s discoverer makes a lot of sense because Huygens’ main mission involves descending to Titan to study its atmosphere and surface, which the probe did back in 2004.
Powering Cassini towards the sixth planet of our solar system, and helping it to perform the braking techniques required to pull the craft into orbit around Saturn, is a propulsion module constructed by Lockheed Martin Corporation (NYSE:LMT). This module, the largest U.S. planetary spacecraft propulsion system ever constructed, was fired up 16 times while Cassini traveled to Saturn and will be used more than 100 times over the entire course of the already extended mission. Lockheed also built the Titan IV/Centaur rocket that launched Cassini-Huygens into space back in 1997.
Other than the propulsion module, there are 11 other subsystems of the Cassini spacecraft that work to support the orbiter’s scientific mission. A command and data subsystem processes data from all of the other subsystems as well as the scientific instruments, relying heavily upon an Engineering Flight Computer developed and fabricated by IBM Corp. (NYSE:IBM). Data is stored in a solid state recorder, a type of digital recording medium using no moving parts; Cassini was the first NASA project to take advantage of solid state data storage. The propulsion module is aided by Cassini’s attitude and articulation control subsystem, which points the main propulsion engines and maintains Cassini’s position as it flies through space, and a power and pyrotechnic subsystem which provides 30 volts of DC electric energy to the craft and initiates electro-explosive devices that enable Cassini to separate from the Centaur launch rocket, for example.
Making sure that the components of the Cassini craft remains at a temperature within an acceptable operating range is the temperature control subsystem. This series of thermal blankets, shields and other hardware enabled Cassini to survive its flyby of Venus, where the sun’s warmth is three times the amount of heat the sun radiates towards Earth, as well as the extreme cold of the Saturn system, where temperatures are about 100 times lower than at Earth. The skeleton of Cassini is composed of an outer structure subsystem which protects components from radiation and micrometeoroids, the mechanical device subsystem which completes mechanisms for initiating pyrotechnic devices and separating Cassini from its launch rocket as well as the electronic packaging system, a circular electronics bus of 12 standardized bays containing all of the craft’s electronic modules.
Communication with scientists on Earth is achieved thanks to two subsystems: the antenna subsystem, consisting of one high-gain antenna and two low-gain antennas and doubling as an umbrella to block out the sun’s heat for certain parts of the mission; and the radio frequency subsystem which supplies the antennas with an amplified data signal to be sent to Earth. Rounding out the engineering design aspects of Cassini is the cabling system which provides wiring for all of the other subsystems. This passive system transmits electrical signals between every other subsystem found on the NASA craft.
In addition to the 12 engineering subsystems of the craft, Cassini holds a scientific payload of 12 data collecting instruments gathering information to conduct 27 different science investigations being supported by the orbiter’s mission. Four instruments provide optical remote sensing data collection techniques for studying Saturn, its rings and its moons in the electromagnetic spectrum. The visualization of heat sources on Saturn is performed by the Composite Infrared Spectrometer (CIRS), which can also detect the chemical composition of those heat sources. CIRS was responsible for the discovery that the cracks on the southern pole of Enceladus, another one of Saturn’s moons, were actually emanating heat. Those cracks on Enceladus were discovered by Cassini’s Imaging Science Subsystem (ISS), a charge coupled device device that can capture images as small as a quarter from a distance of about 2.5 miles. Images captured by Cassini, including the recent photos of Dione, are collected on Earth by the Cassini Imaging Central Laboratory for Operations (CICLOPS) for distribution throughout the world. Ultraviolet images are captured by the Ultraviolet Imaging Spectrograph (UVIS), which is able to visualize gases on Saturn that are invisible to the human eye. Finally, the Visual and Infrared Mapping Spectrometer (VIMS) also takes photos invisible to the human eye which detects chemical wavelengths in order to find a spectral signature, unlocking for scientists important clues as to the chemical composition of Saturn’s atmosphere and surface.
Data about the dust and plasma surrounding Saturn, along with the detection of magnetic fields in the Saturn system, is achieved by the fields, particles and waves scientific instruments. The energy and electrical charge of ions and electrons floating within Saturn’s magnetosphere, a massive region spreading out for a million miles, is detected by the Cassini Plasma Spectrometer (CAPS). This unit includes an electron spectrometer, an ion beam spectrometer and an ion mass spectrometer that together can detect energy levels ranging from 0.7 electron volts up to 50 kilo-electron volts. The study of positive ions and neutral particles found in Saturn’s magnetosphere and Titan’s upper atmosphere is conducted by the Ion and Neutral Mass Spectrometer (INMS). INMS is designed to detect the chemical, elemental and isotopic composition of gaseous and volatile components. The Magnetospheric Imaging Instrument (MII) is the first instrument ever produced in order to take a picture of a planet’s magnetosphere. This instrument will give us a greater understanding of the plasma, or ionized gases, as well as storm clouds and Saturn’s kilometric radiation, an intense non-thermal radio emission coming from Saturn. More data about the magnetic fields surrounding Saturn has been coming from the Dual Technique Magnetometer (MAG). MAG can measure magnetic fields which immediately surround the craft and those found within Saturn, which can help to identify the size of Saturn’s core. Cosmic dust, which was again discovered by Giovanni Cassini in the 1600s, is investigated by the Cassini craft’s Cosmic Dust Analyzer (CDA). The CDA can detect cosmic dust at nano-dust sizes, which measure about one-millionth of a millimeter; this has been compared to being able to detect a single drop of rain falling into the Gulf of Mexico. Rounding out this area of Cassini’s scientific mission is the Radio and Plasma Wave Science (RPWS) instruments that measure Saturn’s radio signals as well as radio waves created by the interaction of solar winds with Saturn and Titan.
Another area of scientific study receiving a decent amount of attention from the Cassini mission is microwave remote sensing studies that can map atmospheres and determine the mass of Saturn’s moons. Cassini’s RADAR component takes images using microwaves, which can cut through thick atmospheres and capture detailed images of features on Saturn’s surface, like mountains and rivers. We even know more about the quality of sound that can be heard on and around Saturn thanks to the Radio Science Subsystem (RSS), the single largest instrument found on Cassini. One half of it actually resides on Earth and radio waves transmitted between the two component parts of RSS, uncovering secrets about material that comes between Cassini and Earth, including the composition of Saturn’s rings and atmosphere.
Cassini still has a fair number of flybys remaining in the current phase of its mission, the next one being a close encounter with Titan on September 28th of this year. On December 19th, Cassini will begin a flyby of Enceladus that will last into 2017, providing us with useful information on what has been driving Titan’s geysers. In 2017, the final phase of Cassini’s mission, known as the Cassini Grand Finale, will take place. In this phase, Cassini will span the distance between Saturn’s upper atmosphere and its innermost ring 22 times before taking a huge dive down to Saturn’s surface. This phase will provide more information on Saturn’s gravitational forces and magnetic fields as well as the material making up Saturn’s rings.