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Cassini-Huygens
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Cassini-Huygens is a joint NASA/ESA/ASI unmanned space mission intended to study Saturn and its moons. The spacecraft consists of two main elements: the Cassini orbiter and the Huygens probe. It was launched on October 15, 1997 and entered Saturn's orbit on July 1, 2004. On December 25 2004 the probe separated from the orbiter at approximately 02:00 UTC, with deployment confirmed by the Jet Propulsion Laboratory. The probe is scheduled to reach Saturn's moon Titan on 15 January 2005 , where it will make an atmospheric descent to the surface and relay scientific information. It is the first spacecraft to orbit Saturn and just the fourth spacecraft to visit Saturn.
Overview
Cassinis principal objectives are to:
- Determine the three-dimensional structure and dynamic behavior of the rings
- Determine the composition of the satellite surfaces and the geological history of each object
- Determine the nature and origin of the dark material on Iapetus's leading hemisphere
- Measure the three-dimensional structure and dynamic behavior of the magnetosphere
- Study the dynamic behavior of Saturn's atmosphere at cloud level
- Study the time variability of Titan's clouds and hazes
- Characterize Titan's surface on a regional scale
The Cassini-Huygens spacecraft was launched on October 15, 1997 from Cape Canaveral Air Force Station's Launch Complex 40 using a U.S. Air Force Titan IVB/Centaur launch vehicle. The launch vehicle was made up of a two-stage Titan IV booster rocket, two strap-on solid rocket motors, the Centaur upper stage, and a payload enclosure, or fairing. The complete Cassini flight system was composed of the launch vehicle and the spacecraft.
The spacecraft is composed of the Cassini orbiter and the Huygens probe. The Cassini orbiter is planned to orbit Saturn and its moons for four years, and the plan sees the Huygens probe dive into the atmosphere of Titan and land on its surface. Cassini-Huygens is an international collaboration between three space agencies. Seventeen nations contributed to building the spacecraft. The Cassini orbiter was built and managed by NASA/CalTech's Jet Propulsion Laboratory. The Huygens probe was built by the European Space Agency. The Italian Space Agency provided Cassini's high-gain communication antenna.
The total cost of the Cassini-Huygens mission is about US$3.26 billion, including $1.4 billion for pre-launch development, $704 million for mission operations, $54 million for tracking and $422 million for the launch vehicle. The U.S. contributed $2.6 billion, the European Space Agency $500 million and the Italian Space Agency $160 million.
A list of Cassini-Huygens abbreviations is available.
Spacecraft design
The spacecraft was originally planned to be the second three-axis stabilized, RTG-powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. Cassini was being developed together with the Comet Rendezvous Asteroid Flyby (CRAF) spacecraft, but various budget cuts and rescopings of the project forced NASA to terminate CRAF development in order to save Cassini. As a result, the Cassini spacecraft became a more specialized design, cancelling the implementation of the Mariner Mark II series.
The Cassini spacecraft, including the orbiter and the Huygens probe, is the largest, heaviest, and most complex interplanetary spacecraft built to date. The orbiter alone has a mass of 2150 kilograms. When the 350-kilogram Huygens probe, launch vehicle adapter, and 3132 kilograms of propellants were loaded at launch, the spacecraft had a mass of about 5600 kilograms. Only the two Phobos spacecraft sent to Mars by the Soviet Union were heavier. The Cassini spacecraft stood more than 6.8 metres (22.3 feet) high, and was more than 4 metres (13.1 feet) wide. The complexity of the spacecraft is necessitated both by its trajectory, or flight path, to Saturn, and by the ambitious program of scientific observations to be undertaken once the spacecraft reaches its destination. It functions with 1,630 interconnect circuits, 22,000 wire connections, and over 14 kilometres (8.7 miles) of cabling.
When Cassini is at Saturn, it will be between 8.2 and 10.2 astronomical units from Earth. Because of this, it will take 68 to 84 minutes for signals to travel from Earth to the spacecraft, and vice versa. In practical terms, this means that ground controllers will not be able to give "real-time" instructions to the spacecraft, either for day-to-day operations or in cases of unexpected in-flight events. Even if controllers respond immediately after becoming aware of a problem, nearly three hours will have passed before they receive a response from the spacecraft.
Instrumentation
Cassinis instrumentation consists of: a RADAR mapper, a CCD imaging system, a visible/infrared mapping spectrometer, a composite infrared spectrometer, a cosmic dust analyzer, a radio and plasma wave experiment, a plasma spectrometer, an ultraviolet imaging spectrograph, a magnetospheric imaging instrument, a magnetometer, an ion/neutral mass spectrometer. Telemetry from the communications antenna as well as other special transmitters (an S-band transmitter and a dual-frequency Ka-band system) will also be used to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.
Cassini Plasma Spectrometer (CAPS)
The Cassini Plasma Spectrometer (CAPS) is a direct sensing instrument that measures the energy and electrical charge of particles such as electrons and protons that the instrument encounters. CAPS will measure the molecules originating from Saturn's ionosphere and also determine the configuration of Saturn's magnetic field. CAPS will also investigate plasma in these areas as well as the solar wind within Saturn's magnetosphere.[1] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-caps.cfm
Cosmic Dust Analyzer (CDA)
The Cosmic Dust Analyzer (CDA) is a direct sensing instrument that measures the size, speed, and direction of tiny dust grains near Saturn. Some of these particles are orbiting Saturn, while others may come from other solar systems. The Cosmic Dust Analyzer onboard the Cassini orbiter is ultimately designed to help discover more about these mysterious particles, and significantly add to the knowledge of the materials in other celestial bodies and potentially more about the origins of the universe.[2] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-cda.cfm
Composite Infrared Spectrometer (CIRS)
The Composite Infrared Spectrometer (CIRS) is a remote sensing instrument that measures the infrared light coming from an object (such as an atmosphere or moon surface) to learn more about its temperature and what it's made of. Throughout the Cassini-Huygens mission, CIRS will measure infrared emissions from atmospheres, rings and surfaces in the vast Saturn system to determine their composition, temperatures and thermal properties. It will map the atmosphere of Saturn in three dimensions to determine temperature and pressure profiles with altitude, gas composition, and the distribution of aerosols and clouds. This instrument will also measure thermal characteristics and the composition of satellite surfaces and rings.[3] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-cirs.cfm
Ion and Neutral Mass Spectrometer (INMS)
The Ion and Neutral Mass Spectrometer (INMS) is a direct sensing instrument that analyzes charged particles (like protons and heavier ions) and neutral particles (like atoms) near Titan and Saturn to learn more about their atmospheres. INMS is intended also to measure the positive ion and neutral environments of Saturn's icy satellites and rings.[4] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-inms.cfm
Imaging Science Subsystem (ISS)
The Imaging Science Subsystem (ISS) is a remote sensing instrument that captures images in visible light, and some in infrared and ultraviolet light. The ISS has a camera that can take a broad, wide-angle picture and a camera that can record small areas in fine detail. Scientists anticipate that Cassini scientists will be able to use ISS to return hundreds of thousands of images of Saturn and its rings and moons. ISS includes two cameras; a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). Each uses a sensitive charge-coupled device (CCD) as its detector. Each CCD consists of a 1,024 square array of pixels, 12 μm on a side. The camera's system allows for many data collection modes, including on-chip data compression. Both cameras are fitted with spectral filters that rotate on a wheel—to view different bands within the electromagnetic spectrum ranging from 0.2 to 1.1 μm.[5] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-iss.cfm
Dual Technique Magnetometer (MAG)
The Dual Technique Magnetometer (MAG) is a direct sensing instrument that measures the strength and direction of the magnetic field around Saturn. The magnetic fields are generated partly by the intensely hot molten core at Saturn's center. 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. MAG's goals are to develop a three-dimensional model of Saturn's magnetosphere, as well as determine the magnetic state of Titan and its atmosphere, and the icy satellites and their role in the magnetosphere of Saturn.[6] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-mag.cfm
Magnetospheric Imaging Instrument (MIMI)
The Magnetospheric Imaging Instrument (MIMI) is both a direct and remote sensing instrument that produces images and other data about the particles trapped in Saturn's huge magnetic field, or magnetosphere. This information will be used to study the overall configuration and dynamics of the magnetosphere and its interactions with the solar wind, Saturn's atmosphere, Titan, rings, and icy satellites.[7] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-mimi.cfm
Radio Detection and Ranging Instrument (RADAR)
The Radio Detection and Ranging Instrument (RADAR) is a remote active and remote passive sensing instrument that will produce maps of Titan's surface and measures the height of surface objects (like mountains and canyons) by bouncing radio signals off of Titan's surface and timing their return. Radio waves can penetrate the thick veil of haze surrounding Titan. In addition to bouncing radio waves, the RADAR instrument will listen for radio waves that Saturn or its moons may be producing.[8] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-radar.cfm
Radio and Plasma Wave Science instrument (RPWS)
The Radio and Plasma Wave Science instrument (RPWS) is a direct and remote sensing instrument that 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 major functions of the RPWS are to measure the electric and magnetic wave fields in the interplanetary medium and planetary magnetospheres. The instrument will also determine the electron density and temperature near Titan and in some regions of Saturn's magnetosphere. RPWS studies the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR), as well as monitoring and mapping Saturn's ionosphere, plasma, and lightning from Saturn's atmosphere.[9] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-rpws.cfm
Radio Science Subsystem (RSS)
The Radio Science Subsystem (RSS) is a remote sensing instrument that uses radio antennas on Earth to observe the way radio signals from the spacecraft change as they are sent through objects, such as Titan's atmosphere or Saturn's rings, or even behind the sun. The RSS also studies the compositions, pressures and temperatures of atmospheres and ionospheres, radial structure and particle size distribution within rings, body and system masses and gravitational waves. The instrument uses the spacecraft X-band communication link as well as S-band downlink and Ka-band uplink and downlink.[10] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-rss.cfm
Ultraviolet Imaging Spectrograph (UVIS)
The Ultraviolet Imaging Spectrograph (UVIS) is a remote sensing instrument that captures images of the ultraviolet light reflected off an object, such as the clouds of Saturn and/or its rings, to learn more about their structure and composition. Designed to measure ultraviolet light over wavelengths from 55.8 to 190 nm, this instrument is also a valuable tool to help determine the composition, distribution, aerosol particle content and temperatures of their atmospheres. This sensitive instrument is different from other types of spectrometers because it can take both spectral and spatial readings. It is particularly adept at determining the composition of gases. Spatial observations take a wide-by-narrow view, only one pixel tall and 60 pixels across. The spectral dimension is 1,024 pixels per spatial pixel. Additionally, it is capable of taking so many images that it can create movies to show the ways in which this material is moved around by other forces.[11] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-uvis.cfm
Visible and Infrared Mapping Spectrometer (VIMS)
The Visible and Infrared Mapping Spectrometer (VIMS) is a remote sensing instrument that is actually made up of two cameras in one: one is used to measure visible wavelengths, the other infrared. VIMS captures images using visible and infrared light to learn more about the composition of moon surfaces, the rings, and the atmospheres of Saturn and Titan. VIMS also observes the sunlight and starlight that passes through the rings to learn more about ring structure. VIMS is designed to measure reflected and emitted radiation from atmospheres, rings and surfaces over wavelengths from 0.35 to 5.1 mm. It will also help determine the compositions, temperatures and structures of these objects. With VIMS, scientists also plan to perform long-term studies of cloud movement and morphology in the Saturn system, to determine the planet's weather patterns.[12] http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-vims.cfm
Plutonium power source and controversy
Because of Saturn's distance from the Sun, solar arrays were not feasible power sources for the spacecraft. To generate enough power, such arrays would have been too large and heavy. Thus, the Cassini orbiter gets its power from three radioisotope thermoelectric generators or RTGs, which use heat from the natural decay of plutonium (in the form of plutonium dioxide) to generate direct current electricity. These RTGs are of the same design as those which flew on the Galileo and Ulysses spacecraft and are designed to have a long operational lifetime. At the end of the 11-year Cassini mission, they will still be capable of producing at least 628 watts of power.
Cassinis use of plutonium — 32.8 kg, at the time the most ever launched into space — attracted significant protest from environmental groups, physicists, and some former NASA staff. NASA made several statements about the safety of the mission, all of which intended to mean the mission was acceptably safe: the chances of radioactive release during the first 3 1/2 minutes after launch were 1 in 1,400; the chances of a release later in the rocket's climb into orbit were 1 in 476; the chances of the craft falling to earth later into the mission were less than 1 in a million; a worst-case scenario would mean 120 humans could die from Cassini-caused cancer over 50 years. These figures were derided as wild guesses by commentators that included the theoretical physicist Professor Michio Kaku, who suggested 200,000 would die if the plutonium canisters survived reentry and crashed in a heavily populated area.
To gain momentum for the voyage to Saturn, Cassinis trajectory included several gravitational slingshot maneuvers: two passes of Venus, one past the Earth, then one past Jupiter. The Earth fly-by, which occurred successfully on August 18, 1999, was the final point at which Cassini posed any danger to humans. Had it suffered a malfunction that caused it to impact, it was estimated by NASA's Cassini final environmental impact study [13] http://saturn.jpl.nasa.gov/spacecraft/safety-eis.cfm that a significant fraction of the plutonium contents of the RTGs would have been dispersed into Earth's atmosphere. A small number of activists continued to protest after the maneuver. Counter-demonstrators from the National Space Society carried signs reading CASSINI IS GO.
The Huygens probe
The Huygens probe, supplied by the European Space Agency (ESA) and named after the Dutch 17th century astronomer Christiaan Huygens, will scrutinize the clouds, atmosphere, and surface of Saturn's moon Titan. It is designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface. The Huygens probe system consists of the probe itself, which will descend to Titan, and the probe support equipment (PSE), which will remain attached to the orbiting spacecraft. The PSE includes the electronics necessary to track the probe, to recover the data gathered during its descent, and to process and deliver the data to the orbiter, from which it will be transmitted or "downlinked" to the ground.
The probe remained dormant throughout the 6.7-year interplanetary cruise, except for bi-annual health checks. These checkouts followed preprogrammed descent scenario sequences as closely as possible, and the results were relayed to Earth for examination by system and payload experts.
Prior to the probe's separation from the orbiter on December 25 2004, a final health check was performed. The "coast" timer was loaded with the precise time necessary to turn on the probe systems (15 minutes before its encounter with Titan's atmosphere), then the probe detached from the orbiter and will coast to Titan for 22 days with no systems active except for its wake-up timer.
The main mission phase will be the parachute descent through Titan's atmosphere. The batteries and all other resources are sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half hour or more) on Titan's surface. The probe's radio link will be activated early in the descent phase, and the orbiter will "listen" to the probe for the next 3 hours, which includes the descent plus 30 minutes after impact. Not long after the end of this three-hour communication window, Cassini's high-gain antenna (HGA) will be turned away from Titan and toward Earth. Very large radio telescopes on Earth will also be listening to Huygens's 10-watt transmission using the technique of very long baseline interferometry and aperture synthesis mode on the following telescopes: the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia and eight of the ten telescopes of the continent-wide VLBA, located at Pie Town and Los Alamos, NM, Fort Davis, TX, North Liberty, IA, Kitt Peak, AZ, Brewster, WA, Owens Valley, CA, and Mauna Kea, HI. The signal strength received at Earth from Huygens will be comparable to that from the Galileo probe (the atmospheric descent probe) as recieved by the VLA, and will thus be too weak to detect in real time because of the signal modulation by the (then) unknown telemetry. Instead, wide-band recordings of the probe signal will be made throughout the three-hour descent. After the Probe telemetry is relayed from Cassini to Earth, the recorded signal is processed against a telemetry template, enabling signal integration over several seconds for determining the probe frequency. It is expected that through analysis of the Doppler shifting of Huygens's signal as it descends through the atmosphere of Titan, wind speed and direction will be able to be determined with some degree of accuracy. Through interferometry, it is also expected that the radio telescopes will allow determination of Huygenss landing site on Titan with exquisite precision, measuring its position to within 1 km (about two-thirds of a mile) at a distance from Earth of about 1.2 Tm (750 million miles). This represents an angular resolution of approximately 170 microarcseconds. A similar technique was used to determine the landing site of the Mars exploration rovers by listening to their telemetry alone.
Instrumentation
The Huygens probe has six complex instruments aboard that will take in a wide range of scientific data after the probe descends into Titan's atmosphere. The six instruments are:
Huygens Atmospheric Structure Instrument (HASI)
This instrument contains a suite of sensors that will measure the physical and electrical properties of Titan's atmosphere. Accelerometers will measure forces in all three axes as the probe descends through the atmosphere. With the aerodynamic properties of the probe already known, it will be possible to determine the density of Titan's atmosphere and to detect wind gusts. In the event of a landing on a liquid surface, the probe motion due to waves will also be measurable. Temperature and pressure sensors will also measure the thermal properties of the atmosphere. The Permittivity and Electromagnetic Wave Analyzer component will measure the electron and ion (i.e., positively charged particle) conductivities of the atmosphere and search for electromagnetic wave activity. On the surface of Titan, the conductivity and permittivity (i.e., the ratio of electric flux density produced to the strength of the electric field producing the flux) of the surface material will be measured. The HASI subsystem also contains a microphone, which will be used to record any acoustic events during probe descent and landing. If the Huygens mission succeeds, it will be only the second time in history (a Venera-13 recording being the first) that audible sounds from another planetary body have been recorded.
Doppler Wind Experiment (DWE)
This experiment will use an ultra-stable oscillator to improve communication with the probe by giving it a very stable carrier frequency. The probe drift caused by winds in Titan's atmosphere will induce a measurable Doppler shift in the carrier signal. The swinging motion of the probe beneath its parachute due to atmospheric properties may also be detected.
Descent Imager/Spectral Radiometer (DISR)
This instrument will make a range of imaging and spectral observations using several sensors and fields of view. By measuring the upward and downward flow of radiation, the radiation balance (or imbalance) of the thick Titan atmosphere will be measured. Solar sensors will measure the light intensity around the Sun due to scattering by aerosols in the atmosphere. This will permit the calculation of the size and number density of the suspended particles. Two imagers (one visible, one infrared) will observe the surface during the latter stages of the descent and, as the probe slowly spins, build up a mosaic of pictures around the landing site. There will also be a side-view visible imager to get a horizontal view of the horizon and the underside of the cloud deck. For spectral measurements of the surface, a lamp that will switch on shortly before landing will augment the weak sunlight.
Gas Chromatograph Mass Spectrometer (GCMS)
This instrument will be a versatile gas chemical analyzer designed to identify and measure chemicals in Titan's atmosphere. It will be equipped with samplers that will be filled at high altitude for analysis. The mass spectrometer will build a model of the molecular masses of each gas, and a more powerful separation of molecular and isotopic species will be accomplished by the gas chromatograph. During descent, the GCMS will also analyze pyrolysis products (i.e., samples altered by heating) passed to it from the Aerosol Collector Pyrolyser. Finally, the GCMS will measure the composition of Titan's surface in the event of a safe landing. This investigation will be made possible by heating the GCMS instrument just prior to impact in order to vaporize the surface material upon contact.
Aerosol Collector and Pyrolyser (ACP)
This experiment will draw in aerosol particles from the atmosphere through filters, then heat the trapped samples in ovens (the process of pyrolysis) to vaporize volatiles and decompose the complex organic materials. The products will then be flushed along a pipe to the GCMS instrument for analysis. Two filters will be provided to collect samples at different altitudes.
Surface-Science Package (SSP)
The SSP contains a number of sensors designed to determine the physical properties of Titan's surface at the point of impact, whether the surface is solid or liquid. An acoustic sounder, activated during the last 100 meters of the descent, will continuously determine the distance to the surface, measuring the rate of descent and the surface roughness (e.g., due to waves). If the surface is liquid, the sounder will measure the speed of sound in the "ocean" and possibly also the subsurface structure (depth). During descent, measurements of the speed of sound will give information on atmospheric composition and temperature, and an accelerometer will accurately record the deceleration profile at impact, indicating the hardness and structure of the surface. A tilt sensor will measure any pendulum motion during the descent and will indicate the probe attitude after landing and show any motion due to waves. If the surface is, indeed, liquid, other sensors will measure its density, temperature and light reflecting properties, thermal conductivity, heat capacity, and electrical permittivity.
A critical design flaw
Long after launch, a few persistent engineers discovered that the communication equipment on Cassini had a fatal design flaw, which would have caused the loss of all data transmitted by the Huygens probe.
As Huygens is too small to transmit directly to Earth, it is designed to radio the telemetry data obtained while descending to Titan to Cassini, which would relay it to Earth using its large 4-meter diameter main antenna. Some engineers, most notably ESA Darmstadt employee Boris Smeds, felt uneasy about the fact that, in their opinion, this feature had not been tested before launch under sufficiently realistic conditions. Smeds managed with quite some difficulty to convince superiors to perform additional tests while Cassini was in flight. In early 2000, he sent simulated telemetry data at varying power and Doppler shift levels from Earth to Cassini. It turned out that Cassini was unable to relay the data correctly.
The reason: When Huygens descends to Titan, it will accelerate relative to Cassini, causing its signal to be Doppler shifted. Consequently, the hardware of Cassinis receiver was designed to be able to receive over a range of shifted frequencies. However, the firmware was not: The Doppler shift changes not only the carrier frequency but also the timing of the payload bits, coded by phase-shift keying at 8192 bits per second, and this, the programming of the module fails to take into account.
Reprogramming the firmware was impossible, and as a solution the trajectory had to be changed: Huygens will detach a month later (December 2004 instead of November) and approach Titan in such a way that its transmissions travel perpendicularly to its direction of motion relative to Cassini, greatly reducing the Doppler shift. (See IEEE Spectrum article http://www.spectrum.ieee.org/WEBONLY/publicfeature/oct04/1004titan.html for the full story.)
Important events and discoveries
Timeline
A chronology of the mission can be found under Cassini-Huygens timeline. Following is a discussion of the more notable events and discoveries.
Jupiter flyby
Cassini made its closest approach to Jupiter on December 30, 2000, and performed many scientific measurements. About 26 thousand images were taken of Jupiter during the course of the months-long flyby. The most detailed global color portrait of Jupiter ever was produced (see image at right), in which the smallest visible features are approximately 60 km (37 miles) across.
A major finding of the Jupiter flyby, announced[14] http://saturn.jpl.nasa.gov/news/press-releases-03/20030306-pr-a.cfm on March 6, 2003, was of the nature of Jupiter's atmospheric circulation. Dark "belts" alternate with light "zones" in the atmosphere. Scientists had long considered the zones, with their pale clouds, to be areas of upwelling air, partly because many clouds on Earth form where air is rising. Analysis of Cassini imagery, however, told a new story. Individual storm cells of upwelling bright-white clouds, too small to see from Earth, pop up almost without exception in the dark belts. According to Anthony Del Genio of NASA's Goddard Institute for Space Studies, "We have a clear picture emerging that the belts must be the areas of net-rising atmospheric motion on Jupiter, with the implication that the net motion in the zones has to be sinking."
Other atmospheric observations made included a swirling dark oval of high-atmosphere haze, about the size of the Great Red Spot, near Jupiter's north pole. Infrared imagery revealed aspects of circulation near the poles, with bands of globe-encircling winds, with adjacent bands moving in opposite directions.
The same announcement also discussed the nature of Jupiter's rings. Light scattering by particle in the rings revealed the particles were irregularly shaped (as opposed to being spherical) and likely originate as ejecta from micrometeorite impacts on Jupiter's moons, probably Metis and Adrastea.
Test of Einstein's theory of general relativity
On October 10, 2003, the Cassini science team announced the results of a test of Einstein's theory of general relativity, using radio signals from the Cassini probe. The researchers observed a frequency shift in the radio waves to and from the spacecraft, as those signals traveled close to the Sun. According to the theory of general relativity, a massive object like the Sun causes space-time to curve, and a beam of radio waves (or light) that passes by the Sun has to travel further because of the curvature. The extra distance that the radio waves travel from Cassini past the Sun to the Earth delays their arrival; the amount of the delay provides a sensitive test of the predictions of Einstein's theory. Although deviations from general relativity are expected in some cosmological models, none were found in this experiment. Past tests were in agreement with the theoretical predictions with an accuracy of one part in one thousand. The Cassini experiment improved this to about 20 parts in a million, with the data still supporting Einstein's theory.
Missing spokes
A new, high-resolution picture of Saturn taken by Cassini on February 9, 2004 was publicly released a few weeks later. Mission scientists were puzzled by the fact that no "spokes" in Saturn's ring were visible. These dark structures in the "B" section of the ring had been discovered in pictures taken by the Voyager probe in 1981. (See JPL Press Release Image http://photojournal.jpl.nasa.gov/catalog/PIA05380 )
New moons of Saturn
Using images taken by Cassini, two new moons of Saturn were discovered in June, 2004. They are both very small and were given the provisional names S/2004 S 1 and S/2004 S 2 before being named Methone and Pallene at the end of 2004.
Phoebe flyby
On June 11, 2004, Cassini flew by the moon Phoebe. This was the first opportunity for close-up studies of this moon since the Voyager 2 flyby. It also was Cassinis only possible flyby for Phoebe due to the mechanics of the available orbits around Saturn.
First close up images were received on June 12, and mission scientists immediately realized that the surface of Phoebe looks different from other asteroids visited by spacecraft. Parts of the heavily cratered surfaces look very bright in those pictures, and it is currently believed that a large amount of water ice exists under its immediate surface.
Saturn rotation
In an announcement on June 28, Cassini scientists described the measurement of the rotational period of Saturn. Since there are no fixed features on the surface that can be used to obtain this period, the repetition of radio emissions was used. This new data agree with the latest values measured from Earth, and constitute a puzzle to the scientists. It turns out that the radio rotational period has changed since it was first measured in 1980 by Voyager, and that it is now 6 minutes longer. This doesn't indicate a change in the overall spin of the planet, but is thought to be due to movement of the source of the radio emissions to a different latitude, at which the rotation rate is different.
Orbiting Saturn
On July 1, 2004, the spacecraft flew through a gap in the thin outermost area of Saturn's rings and achieved orbit, after a seven year voyage. It is the first spacecraft to ever orbit Saturn. The Saturn Orbital Insertion (SOI) maneuver performed by Cassini was notably complex, requiring the craft to orient its High-Gain Antenna away from Earth and along its flight path, in order to shield its instruments from particles in Saturn's rings. Once the craft crossed the ring plane, it then had to rotate again so that its engine was pointed along its flight path, and then the engine fired to decelerate the craft and allow Saturn to capture it. Cassini was captured by Saturn's gravity at around 8:54 PM Pacific Daylight Time on June 30. During the maneuver Cassini passed within 20,000 km (13,000 miles) of Saturn's cloud tops.
Titan flybys
Cassini had its first distant flyby of Saturn's largest moon, Titan, on July 2, 2004, only a day after orbit insertion, when it approached to within 339,000 kilometers (211,000 miles) of Titan and provided the best look at the moon's surface to date. Images taken through special filters (able to see through the moon's global haze) showed south polar clouds thought to be composed of methane and surface features with widely differing brightness. On October 27, 2004 the spacecraft executed the first of the 45 planned close flybys of Titan when it flew a mere 1,200 kilometers above the moon. Almost four gigabits of data were collected and transmitted to Earth, including the first radar images of the moon's haze-enshrouded surface. Radar imagery observed no conclusive evidence of lakes of liquid hydrocarbons, though it did not dismiss the possibility such lakes could exist. It also revealed the surface of Titan (at least the area covered by radar) to be relatively flat, with topography reaching no more than about 50 meters in altitude. The flyby provided a remarkable increase in imaging resolution over previous coverage. Images with up to 100 times higher resolution were taken and are typical of resolutions planned for subsequent Titan flybys.
Huygens encounter with Titan
Cassini released the Huygens probe on 25 December 2004 by means of a spring. It will enter the atmosphere of Titan on 14 January 2005. On that day it is supposed to touch down on the Xanadu Regio site. It is not yet certain whether it is a mountain range, a flat plain, an ocean, or something else, and it is hoped that analysis of data from Cassini will help to answer these questions. Based on recent pictures taken by Cassini about 745 miles (1200 kilometers) away from Titan, the landing site appears to be "for lack of a better word, shoreline," although the probe will likely touch down on a solid, rather than a liquid, surface1.
Assuming the landing site is non-solid, the Huygens probe is designed to survive the impact and splash-down with Titan's liquid surface for several minutes and send back data on the conditions there. If that occurred it would be the first time a human probe would land in an extraterrestrial (i.e. non-Earth) ocean. The spacecraft has no more than three hours of battery life, a majority of which will be taken up by the descent. Engineers only expect to get at best 30 minutes of data from the surface.
The probe will communicate with the Cassini mothership via radio.
Detailed Huygens activity timeline:
- Huygens probe separated from Cassini orbiter at 02:00 UTC on December 25, 2004 in SCET.
- Huygens probe is scheduled to enter Titan's atomsphere at 09:06 UTC on January 14, 2005 in SCET.
- The probe will land on the surface of the moon at ~162.351 degrees east and ~10.604 degrees south around 11:24 UTC in SCET.
There will be a transit of the Earth/Moon across the Sun as seen from Saturn/Titan just hours before the landing. For the transit of the Earth as seen from the landing site, 1st contact will happen at 18:45:47 UTC on 2005-Jan-13, 2nd contact will happen at 18:53:00 UTC on 2005-Jan-13, 3rd contact will happen at 06:15:22 on 2005-Jan-14, 4th contact will happen at 06:22:35, the greatest will happen around 00:34:20 on 2005-Jan-14. Huygens probe will enter the upper layer of Titan's atomsphere 2.7 hours after the end of the transit of the Earth, or only one or two minutes after the end of the transit of the Moon. However, the transit will not interfere with Cassini orbiter or Huygens probe. First, although they cannot receive any signal from us because we are in front of the Sun, we can still listen to them. Second, Huygens does not send any readable data to the Earth; it transmits data to Cassini orbiter, which relays the data received to the Earth later. For details about transits of the Earth as seen from Saturn, see also Transit of Earth from Saturn.
See also Detailed timeline of Huygens mission.
Trajectory
Notes
- Kevin Grazier, science planning engineer at NASA's Jet Propulsion Laboratory, on CNN http://www.cnn.com/2004/TECH/space/12/24/cassini.titan/index.html
External links
- Cassini Mission Homepage http://saturn.jpl.nasa.gov/ by the Jet Propulsion Laboratory
- Cassini Imaging Homepage http://ciclops.lpl.arizona.edu/
- Cassini information http://nssdc.gsfc.nasa.gov/planetary/cassini.html by the National Space Science Data Center (NSSDC).
- ESA Huygens Homepage http://sci.esa.int/home/huygens/index.cfm about the probe that will study Titan's atmosphere and possibly the surface
- ESA - Where is Cassini-Huygens now? http://www.esa.int/SPECIALS/Cassini-Huygens/SEMD6E2VQUD_0.html Interactiv Flash-Animation of Cassini orbits through 2008
- Cassini's Tour de Saturn part A http://www.spacedaily.com/news/cassini-01a.html , B http://www.spacedaily.com/news/cassini-01b.html , C http://www.spacedaily.com/news/cassini-01c.html , D http://www.spacedaily.com/news/cassini-01d1.html , E http://www.spacedaily.com/news/cassini-01e.html , F http://www.spacedaily.com/news/cassini-01f1.html - descriptions of the 4-year tour of Saturn by Bruce Moomaw
- SpaceflightNow news coverage of the mission http://www.spaceflightnow.com/cassini/
- Countdown to Cancel the Cassini Space Probe http://baltimorechronicle.com/cassini_probe_2.html by the Baltimore Peace Network in 1997 due to concerns over the use of plutonium
Books
- David Harland, "Mission to Saturn: Cassini and the Huygens Probe", 2002 ISBN 1852336560
- Ralph Lorenz & Jacqueline Mitton, "Lifting Titan's Veil: Exploring the Giant Moon of Saturn", 2002 ISBN 0521793483
| Saturn |
|---|
| Janus' group | Mimas | Enceladus | Tethys | Dione | Rhea |
| Titan | Hyperion | Iapetus | Siarnaq's group | Phoebe's group |
| (For other moons, see: Saturn's natural satellites) |
| see also: Cassini-Huygens |