Group Members: Frederick Bailey and Paul Rabstejnek
In 1655, Dutch scientist Christiaan Huygens developed a new technique for grinding and polishing lenses. In 1659, with his improved optics, he observed Saturn’s rings, which he described as a “smooth ribbon”, and which Galileo earlier had called “strange arms”. While studying the rings, Huygens was the first person to observe and document the Saturnian moon Titan. In 1675, the Italian-French astronomer Giovanni Domenico Cassini discovered four additional Saturnian moons – Iapetus, Rhea, Dione, and Tethys – as well as a gap in the planet’s ring system that is now known as the Cassini Division. Since Saturn was visible through the division, astronomers saw this as evidence that the rings were not solid. By the mid-nineteenth century, observers had seen and determined that there were three consecutive rings containing several divisions but still struggled to explain what they were.
Saturn was last visited by the twin Voyager missions in 1980 and in 1981. Voyager 1 and Voyager 2 returned spectacular images and data that added to the understanding of the Saturnian system, but they were flybys lasting only a few hours and raised more questions than were answered. In an effort to answer some of the continuing questions about Saturn, the National Aeronautical and Space Administration (NASA) joined an international venture with the European Space Agency (ESA), the Italian Space Agency (ASI), as well as other European partners. On October 15, 1997, a United States Air Force Titan IV/Centaur rocket blasted off carrying a six-ton spacecraft named Cassini on a two billion-mile journey to Saturn. Piggybacking on the Cassini spacecraft was Huygens, a probe designed to study the atmosphere and, if only briefly, the surface of Titan, Saturn’s cloud-shrouded moon. The launch of Cassini began a new era in the exploration of Saturn, its rings, and several of its moons, even though prior to its launch, there had been a controversy, as activists protested NASA’s decision to use a plutonium isotope in the Cassini power supply. Cassini with its more sophisticated instruments will go into orbit around Saturn where it will return images and data for at least four years
The Cassini Mission is split into two major segments: the Cruise, which covers all the activities before the spacecraft nears Saturn and the Tour, which includes everything after the destination is achieved. The tour is the focus of Cassini’s science goals, which require many different observations of Saturn and its system from a wide variety of perspectives. Cassini is the best equipped spacecraft that has ever been sent to another planet and carries instruments to “see” in three types of light; a system to create radar images; instruments for measuring dust-particles, radio waves, and plasma waves; equipment for measuring the magnetic fields in space and particularly around Saturn; and instruments to measure particles near the spacecraft. There will be over three hundred thousand color images taken of Saturn and its rings.
The Huygens probe, which was developed by the ESA, carries instruments to study the atmosphere and surface of Titan. One instrument will take more than a thousand images of Titan’s surface and clouds. The probe will be released by Cassini in November of 2004 and will drop into Titan’s atmosphere in approximately three weeks. As the probe enters the atmosphere it will begin taking measurements in the upper stratosphere. As it descends – first on a main parachute and later on a drogue chute for stability – various instruments will measure the temperature, pressure, density, and energy balance in the atmosphere. Another instrument will use radio signals to measure Titan’s winds, and three sensors will analyze the moon’s atmosphere.
After the probe is released, the orbiter will perform a propulsive maneuver to target for the flyby and delay its arrival to Titan so that it can have the proper geometry to view the probe descent region. During the probe mission, the orbiter will fly above Titan and listen with its High Gain Antenna (HGA) that was developed by the ASI for data transmitted by the probe. This data will be first stored on the orbiter’s Solid State Recorder before it is downlinked to Earth. The project has a number of strategies, including downlinking the probe data multiple times, to ensure that the probe data gets to Earth with no problems.
As the Huygens probe breaks through the cloud deck, a camera will capture pictures of the Titan panorama. Other instruments will directly measure the organic chemistry in Titan’s atmosphere – providing what some scientists believe may be the equivalent of a time machine to examine the chemistry of the early Earth. Instruments will also be used to study properties of the moon’s surface remotely, and perhaps even directly after landing on the surface. After the probe mission is completed, Cassini will turn its HGA to Earth and begin transmitting the recorded probe data. The data will be transmitted twice and be verified on the ground before it can be overwritten on the data recorders. Once the data is verified, the probe mission is considered to be complete. Many scientists theorize that Titan may have lakes and oceans of methane or ethane. This remains a mystery – the laws of thermodynamics say such a possibility should exist, but radar studies conducted from Earth have turned up no direct evidence of them. The resolution of this puzzle is up to Cassini and Huygens.
Since Cassini and Huygens will not reach their destinations until July and November of 2004 respectfully, all of the current happenings pertain to the Cruise aspect of the mission.
After liftoff in October of 1997, Cassini headed in the opposite direction of its Saturn destination – toward Venus. Using Venus' gravitational pull – a sort of slingshot effect called gravity-assist flyby – Cassini swung around the planet in April 1998 and again in June 1999. It flew by Earth in August 1999, for another slingshot assist and will fly by Jupiter in December 2000, picking up enough additional speed to carry it on to Saturn, where it is to arrive in July 2004. This is called the VVEJGA Interplanetary Trajectory (Venus-Venus-Earth-Jupiter Gravitational Assist) and named for the flybys and the gravity assists. Once near Saturn, Cassini will use Titan's gravitational field for gravity-assist flybys to change the spacecraft's orbital direction and speed. When Titan is unavailable, Cassini will use its two main engines to make orbital changes. Cassini will reorient itself on a daily basis and point toward Earth to transmit data back home.
When Cassini passed Earth at 03:28 Universal Time on August 16, 1999, the flyby gave the spacecraft a 5.5 kilometer-per-second (approximately 12,000 mile-per-hour) boost in speed sending the craft on towards Jupiter and the ringed planet still about one billion miles away. During the pass of Earth, nine of Cassini’s twelve instruments were activated to make observations of the Earth/Moon system. The spacecraft will pass Jupiter on December 30, 2000 and the giant planet’s gravitational pull will bend the Cassini flight path to put it on target with Saturn
In mid-April, Cassini successfully completed its passage through the solar system’s asteroid belt between Mars and Jupiter. This makes Cassini the seventh spacecraft ever to fly through the asteroid belt and before NASA’s Pioneer 10 spacecraft successfully passed through the region in 1972, it was not known whether a spacecraft could survive the trip. The spacecraft entered the belt in mid-December and while it was in the area, Cassini’s camera imaged the asteroid 2685 Masursky. Data gathered provided scientists with the first size estimates on the asteroid and preliminary evidence that it may have different material properties than previously believed. The belt contains a significant concentration of asteroids but now the area is not considered a hazard to spacecraft and engineers reoriented the Cosmic Dust Analyzer (CDA) to better study the environment. A cover over Cassini’s main engines has been in place since launch except when the main engines were fired for trajectory maneuvers and this cover protects the engines from any possible impacts.
A joint Cassini-Galileo Conjunction experiment was also performed. This conjunction, like the one in February, provided the opportunity to observe Jovian radio emissions in a stereoscopic sense. Both the Cassini and Galileo spacecraft were very nearly aligned relative to Jupiter and the viewing angle gave some understanding to the characteristics of Jovian radio emissions.
The most recent Cassini spacecraft telemetry data was acquired from the Madrid tracking station on Wednesday, June 21, 2000. NASA reported that the Cassini spacecraft is in an excellent state of health and is operating normally. Engineers continue to monitor and checkout its instrument flight software and to collect operating performance data. The speed and the current trajectory of the spacecraft can be viewed at http://www.jpl.nasa.gov/cassini/today/, “Where is Cassini Now” web page. Maintenance of the Visual and Infrared Mapping Spectrometer (VIMS) instrument was accomplished by turning on the Infrared Optics decontamination heaters for a twenty-four hour period following last weeks Trajectory Correction Maneuver-14. Updated flight software and Instrument Expanded Blocks (IEBs) for the Cassini Plasma Spectrometer (CAPS) and VIMS instruments were loaded on board. CAPS actuator and flight software checkouts were performed. VIMS dark frame collection, internal calibration, and flight software checkout were begun. The Attitude and Articulation Control Subsystem (AACS) completed a reaction wheel unload. A weekly ‘Significant Event” report as well as a quarterly status report on Cassini, can be obtained by subscribing at Cassini@jpl.nasa.gov.
Cassini’s nominal mission is to study Saturn, its moons, elaborate ring system, and its magnetic and radiation environment for a four-year period. From all evidence provided by NASA and the Jet Propulsion Laboratory (JPL), the spacecraft is proceeding without problems and its instrumentation is functioning correctly as of June 21, 2000.
1. Cloud properties/atmospheric composition 1. Mechanisms of surface modification
2. Winds and temperatures 2. Surface composition and distribution
3. Internal structure and rotation 3. Bulk composition and structure
4. Saturnian ionosphere 4. Surface characteristics
5. Origin and evolution of the planet 5. Magnetosphere interaction
1. Atmospheric constituent abundance 1. Configuration
2. Distribution of trace gases and aerosols 2. Particle composition and sources
3. Winds and temperatures 3. Magneto dynamics
4. Surface state and composition 4. Solar wind interaction
5. Upper atmosphere 5. Titan’s interaction
Major mission events still remaining for 2000 are Trajectory Correction Maneuvers 15 and 16, periodic instrument maintenance, and calibration of the Stellar Reference Units and the Radio and Plasma Wave Science Unit. The major mission events scheduled for the early part of 2001 include the Trajectory Correction Maneuver-17, testing for Gravitational Wave Experiments, and several calibrations of the HGA.
NASA scientists feel there are many exciting options available for an extended mission. One approach for an extended program is to repeat many of the observations taken during the original mission with different geometries and instruments and investigating a strange magnetic field anomaly. They believe Cassini (like the Voyager spacecraft which are still operating and collecting data on the outer solar system and the solar wind twenty years after their launch and Pioneer 11 launched in April 1973 and passed within 21,000 kilometers (13,000 miles) of Saturn in 1979 and is only now running out of operating power) will have the same success in an extended program. In fact, if attitude control were the only concern, Cassini should be able to survive up to two hundred years before losing its stable attitude.
Scientists feel there are many opportunities for Cassini after its nominal mission ends. Cassini could go into orbit around Titan, or even use its gravity assist to escape Saturn altogether. It may be possible for Cassini to go to another planet or to visit an asteroid (though it's likely that this would take many years to accomplish). However, many scientists feel that the Saturn system is interesting enough – and may even have some surprises left. First of all, the way Saturn and its rings change with time is very important scientifically. Studying Saturn for a longer period of time, in and of itself, could be of immense value.
The following are some possible specifics for a Cassini extended mission:
Escape Saturn's Gravity: It is possible to escape Saturn by using Titan's gravity to change the orbit of the spacecraft. This process would require at least several flybys of Titan, and would likely take at least six months – depending on where the spacecraft ends up at the completion of the nominal mission. On the last Titan flyby, the spacecraft is in orbit around Saturn before the flyby, and then attains an escape trajectory leaving Saturn after the flyby. Where the spacecraft would go next is uncertain; but it's unlikely that Cassini could move very far from Saturn's orbit without later flybys of Saturn or a surplus of leftover fuel.
Fly closer to Titan: During the nominal tour, there are many flybys of Titan during which scientists will be investigating Titan's unique atmosphere. The spacecraft cannot get too close, however, or it will encounter too much drag in the "wind" of Titan's atmosphere. Significantly lower Titan flyby altitudes might still be an option for an extended mission to examine the atmospheric characteristics at lower altitudes. Some engineers believe it may even be possible to go into orbit around Titan. "Aerobraking”, or using a body's atmosphere to slow a spacecraft down, has been tested with other spacecraft and is planned for at least one JPL’s mission in the future. Aerobraking and/or spacecraft maneuvers could be sufficient to place Cassini in orbit around Titan, allowing the spacecraft to study Titan very closely over a long period of time.
Fly Closer to Saturn: After the four-year tour, the spacecraft will most likely be in a near polar orbit that flies over latitudes approaching +/- 80 deg. Another extended mission option is to further reduce the closest approach distance to Saturn, allowing the spacecraft to pass inside the “G” ring while skipping over the regions known to contain a high density of potentially damaging ring particles. This option also has the added advantage of increasing the maximum orbit inclination by a small amount. Traversing plane crossings inside the “G” ring may increase the hazard of ring particle impacts but such increased risk may be more acceptable in an extended mission.
More Flybys: Next to more observation time in general, this is one of the most obvious advantages of an extended mission. Additional close flybys of Titan could improve the RADAR mapping coverage, add data on atmospheric dynamics and composition, and map Titan's gravity field more accurately. If interesting features on the moon were detected during the nominal mission, the Titan flyby ground tracks during the extended mission could be designed to cover these interesting areas. Additional close flybys of the icy satellites are an obvious choice for an extended mission regardless of other extended mission objectives. These flybys would be a great boon, since the spacecraft can only get close (under several thousand kilometers at closest approach) to a handful of the icy satellites during the tour. Additional flybys of Titan also offer gravity-assist resources that trajectory designers can use to move the orbit to different and interesting regions of the Saturnian system.
Investigate Saturn's Rings more Closely: During the nominal tour, Saturn's inner rings will be observed from afar, since many ring particles pose a threat to the safety of the spacecraft. After completing the nominal tour, however, the project might be inclined to take more risk to get some good firsthand information about ring composition. In fact, there are some ring related events – flybys of moons near or through part of the ring system, for example – that happen in the few years following the end of Cassini's nominal mission. Observing a small moon plow through a ring may be interesting enough in itself to merit continued operations.
Alter Orbit Inclination: There are several science objectives on reaching different inclinations, or orbital tilt. This is important to fully map Saturn's magnetic field in three dimensions, and is very useful for high-resolution observations of the polar regions. Altering the inclination, however, takes gravity assists from Titan that would have to split among many other science objectives. During an extended mission, there would be more time to get higher or lower in inclination than can be afforded during the nominal tour – since all science objectives must be accomplished during the nominal tour.
Rotate the Orbit: During the tour, different orbital geometries will offer a wide variety of "phase angle" geometry for Saturn and its satellites. The phase angle at which a body is viewed is a measure of what angle the Sun strikes the surface. A full moon, for example, has a phase angle of near zero degrees, which is best for color and surface composition analysis. When the moon is half full, its phase angle is near ninety degrees and shadows of highlands and lowlands are easily discernible for altitude mapping. A new moon would have a phase angle of near 180 degrees, and is good for Sun occultations, when the Sun passes behind the planet (typically used to provide information on the body's atmosphere). Rotating the orbit in an extended mission would give researchers the opportunity to observe the Saturnian system at phase angles not available, or briefly so, during the nominal tour.
Many of these scenarios would take many Titan flybys and a good deal of time to accomplish. Also, extended mission options which result in greater risk to the spacecraft may become more attractive in an extended mission in which the science objectives have already been achieved, especially if these options provide opportunities to enhance what Cassini has already discovered. The type of extended mission will depend greatly on the amount of propellant left onboard, the spacecraft’s operational health, and the available funding to support continued operations here on Earth.