CASSINI MISSION and RESULTS
Group Four
Group Leader: Frederick Bailey
Group Members: Frederick Bailey and Paul Rabstejnek
BACKGROUND:
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.
SATURN ORBITER INSTRUMENTS
Cassini Plasma Spectrometer – Explores plasma within and
near Saturn’s magnetic field
Composite Infrared Spectrometer – Studies temperatures and
compositions of Saturn and its moons and rings
Cosmic Dust Analyzer – Studies ice and dust
grains in an near the Saturn system
Dual Technique Magnetometer – Studies Saturn’s magnetic
field and its interactions with the rings, the moons, and the solar wind
Imaging Science Subsystem – Takes pictures in visible,
near ultraviolet and near infrared light
Ion and Neutral Mass Spectrometer – Studies extended
atmospheres and ionospheres of Saturn, Titan, and the icy satellites
Magnetospheric Imaging Instrument – Takes pictures of the
distribution of charged particles in and near Saturn’s magnetic field
Titan Radar – Maps the surface of Titan and measures the
height of its surface features
Radio and Plasma Wave Science – Investigates plasma waves,
natural emissions of radio energy, and dust
Radio Science Subsystem – Searches for gravitational
waves; measures masses and structures of atmospheres
Ultraviolet Imaging Spectrograph – Studies structure,
chemistry, and composition of atmospheres and rings
Visual and Infrared Mapping Spectrometer – Identifies chemical
composition of surfaces, atmospheres, and rings
HUYGENS PROBE INSTRUMENTS
Aerosol Collector and Pyrolyser – Collects Titan’s aerosols
for chemical composition analysis
Descent Imager/Spectral Radiometer – Makes spectral
measurements and takes pictures of Titan’s surface and atmospheric hazes
Doppler Wind Experiment – Uses radio signals to
deduce wind speeds on Titan
Gas Chromatograph/Mass Spectrometer – Identifies and quantifies
various atmospheric constituents on Titan
Huygens Atmospheric Structure Instrument – Measures the physical and
electrical properties of the Titan atmosphere
Surface Science Package – Determines the physical
properties of Titan’s surface
References:
http://www.jpl.nasa.gov/cassini/mission/tour.html
http://observe.ivv.nasa.gov/nasa/space/Cassini/Cassini1.html
http://www-b.jpl.nasa.gov/cassini/rtg/orbiter.htm
http://www.jpl.nasa.gov/cassini/today/
PRESENT
HAPPENINGS:
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.
References:
http://cassini-radar.jpl.nasa.gov
http://southport.jpl.nasa.gov/radar/bigpicture/index.html
http://www.jpl.nasa.gov/cassini/images/poster/planposter.html
http://www.jpl.nasa.gov/releases/99/csearthflyby.html
FUTURE PROSPECTS:
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.
QUESTIONS CASSINI WILL TRY TO ANSWER
Saturn – What is the
atmosphere like below the cloud tops?
How do storms get started and die out?
How are the polar regions different from areas near the equator? What is Saturn’s interior like?
Titan – What is the surface
like? Are there lakes, oceans, or
rivers? Does it rain? In what direction do winds blow? How much sunlight reaches the surface? Could life ever exist there?
Ring System – What besides
particles of ice, makes up the rings?
What are the sizes of the particles?
How do the rings form and change?
Are there more moons hidden in the rings?
Icy Satellites – What is the
past history of these moons? Is Phoebe
a captured asteroid? Why is Iapetus
surface half dark and half bright? Why
do some moons have the same orbits?
Magnetosphere – What kinds of particles are trapped there? Do they change with time? How do they affect the rings and moons? What does the magnetosphere tell us about the inside of Saturn?
SPECIFIC EXPERIMENTS
Saturn Icy Satellites
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
Titan Magnetosphere
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.
EXTENDED
MISSION
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.
http://www.jpl.nasa.gov/Cassini/mission/ext.html
http://www.jpl.nasa.gov/cassini/msnstatus/missionadj.html
http://www-b.jpl.nasa.gov/cassini/rtg/swingby.html
http://www.jpl.nasa.gov/cassini/msnstatus/2000.html
http://www.jpl.nasa.gov/cassini/msnstatus/2001.html
http://www.jpl.nasa.gov/cassini/MoreInfo/pressrel/status000414.html
http://observe.ivv.nasa.gov/nasa/space/Cassini/Cassini5.html
http://observe.ivv.nasa.gov/nasa/space/Cassini/Cassini6a.html
http://observe.ivv.nasa.gov/nasa/space/Cassini/Cassini6c.html
http://southport.jpl.nasa.gov/radar/index.html
