. Cassini-Huygens Travel . Cassini's instruments  . Huygens's instruments . Communication's System . Spacecrafts' Classification . Extreme In-Space Environments . Saturn's fast facts

On Oct. 15, 1997, a seven-year journey to the ringed planet Saturn began with the liftoff of a Titan IVB/Centaur carrying the Cassini orbiter and its attached Huygens probe. Credit: NASASolarSystem





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On October 15, 1997, the Cassini–Huygens spacecraft was launched on an almost 7-year journey to the Saturn system. On its way, Cassini– Huygens passes Venus (twice), Earth, and Jupiter — arriving at the Saturn system in 2004.




Future Mission to Titan coming probably very soon!


After almost 20 years in space, the Cassini mission ending on September 15, 2017 at 5:07 a.m. PDT (8:07 a.m. EDT).

On April 22, 2017, Cassini leaped over the rings for its final series of daring dives between the planet and the inner edge of the rings. This is the Cassini "Grand Finale." After 22 of these orbits, each taking 6days to complete, the spacecraft, plunged into the upper atmosphere of the gas giant planet, where it burnt up like a meteor, ending the epic mission to the Saturn system.

Reference: saturn.jpl.nasa.gov/mission/grand-finale/overview/

Cassini-Huygens... mission 2017

Cassini-Huygens was one of the most ambitious missions ever launched into space. Loaded with an array of powerful instruments and cameras, the spacecraft took accurate measurements and detailed images in a variety of atmospheric conditions and light spectra.

The mission of Cassini’s Orbiter consisted of delivering the ESA Huygens probe to Titan, and remaining in orbit around Saturn for detailed geophysical studies of the planet, its rings, and satellites, particularly Titan. Read the fascinating suite of that Historic Exploration just below.



Launched in October 1997 by a Titan IVB booster rocket with an orbiter of 2,125 kg, the Huygens probe of 320 kg and 3,132 kg of propellants, the spacecraft weighed a total of 5,712 kg. That is one of the largest, heaviest and most complex interplanetary spacecraft ever built. After nearly 7 years of travel, it reached Saturn and its moons in July 2004.

The Cassini-Huygens spacecraft during vibration and thermal testing in 1996. Credit: NASA

With its total height of nearly 7 m and a 4-m high–gain antenna on the top, Cassini became the largest interplanetary spacecraft ever constructed by NASA. The magnetometer instrument is mounted on an 11-m (36 feet) boom that extends outward from the spacecraft. Three other 10-m (32 feet) rod-like booms that act as the antennas for the radio plasma wave subsystem extend outward from the spacecraft in a Y shape. The complexity of the spacecraft was necessary because of its flight path to Saturn and the ambitious program of scientific observations to be undertaken.

In total, Cassini-Huygens has a three-axis stabilizer equipped for 27 different science investigations. Of that, the Cassini orbiter has 12 instruments and the Huygens probe, 6, and many of these have multiple functions.

To summarize, this included the spectrometers, cosmic dust analyzers, magnetometers, radar, imaging technology and 3 Radioisotope Thermoelectric Generators (RTGs). The RTGs provide power for the spacecraft's instruments, computers, radio transmitters on-board, attitude thrusters and reaction wheel.

How to communicate with it? We can communicate with the spacecraft through 1 High-Gain and 2 Low-Gain Antennas. The latter were used in the event of a power failure or other emergency situation. (look to the right for details)

To navigate, Cassini-Huygens used 16 small rocket thrusters powered by hydrazine propellant to control the orientation. For the main engine, monomethyl hydrazine fuel was used combined with nitrogen tetroxide as oxidizer to provide thrust for trajectory changes.

Also, to reach the target, the Cassini spacecraft used the Attitude and Articulation Control Subsystems (AACS).

The path a rocket takes during powered flight is directly influenced by its attitude, such as its orientation in space. During the atmospheric flight, fins may deflect to steer a rocket. Outside the atmosphere, the articulation of exhaust nozzles permits to change the direction or the rocket's attitude flight path. Thus the term guidance and control has become associated with attitude control during the powered ascent phase of a spacecraft's mission. A few minutes after the launch, a spacecraft may face a mission of many years in free fall, during which its attitude has no relation to guidance except during short, infrequent propulsive maneuvers.

So, to stabilize and control the spacecraft's attitude, its High-Gain Antenna must be accurately pointed to Earth for communications. It is important for on-board experiments to accomplish precise pointing for accurate collection and subsequent interpretation of data. So, when short propulsive maneuvers are executed in the right direction, the heating and cooling effects of sunlight and shadow may be used for thermal control and guidance.

But, the stabilization comes by setting the vehicle to spin, like the Pioneer 10 and 11 spacecrafts in the outer solar system, the Lunar Prospector, and the Galileo Jupiter orbiter spacecraft, and their atmospheric probe.

No matter what choices are made — spin or 3-axis stabilization, thrusters or reaction wheels, or any combinations of these — the task of attitude and articulation control falls to an Attitude and Articulation Control Subsystems (AACS) computer operating with a highly evolved software.

To navigate in space, a MAP is needed!

So, Cassini used a Stellar Reference Unit nearly continuously to navigate in the right direction. In the Celestial Reference, many different devices provide attitude reference by observing celestial bodies, such the star trackers, star scanners, solar trackers, sun sensors, and planetary limb sensors and trackers.

Many of today's Celestial Reference devices have a great capability, for example automated recognition of observed objects based on built-in star catalogs. It is the case of Voyager's AACS that takes input from a sun sensor for yaw and pitch reference. Its roll reference comes from a star tracker focused continuously on a single bright star (Canopus).

Also, Galileo took its references from a star scanner which rotated with the spinning part of the spacecraft, and a sun sensor was available for maneuvers. Another example is Magellan using a star scanner to take a fix on 2 bright stars during a special maneuver once every orbit or two. Its solar panels held a sun sensor each. Because its Celestial References were only used during specific scan maneuvers, Magellan utilized most of time the Gyroscopes. If necessary, the spacecraft can use its Celestial Reference as inertial devices to provide signals to AACS.

Hard conditions need Excellent Protections !

During its travel, Cassini crossed the Saturn ring plane and comet debris in the outer Solar System. That gives a higher risk of damage to the Spacecraft. To protect it, Multi Layer Insulation (MLI) blankets were applied on critical areas, such as the propellant and helium tanks. These blankets are composed of layers of Kapton polyamide or Mylar and gold foil on one side and silver on the other, providing very effective thermals insulation and radiation transfer.

Though it appears to be gold foil covering the spacecraft, the shiny gold coloring of Cassini's blankets is due to the combination of a transparent layer of amber-colored material on top of a reflective aluminized fabric. Credit: NASA

As a projectile shield, the blanket works to break up the projectile before it strikes an exterior wall, disperse and reduce the velocity of the fragments below of the original size. Like a tissue paper, a MLI layer density is sufficient to stop most strikes due to the very small mass of the typical micro-meteoroid.

On Cassini, MLI layers were used to shield certain components of the rocket nozzles, as shown on the picture on the right.

But, because many analysis have showed that the highest risk of catastrophic damage comes from the main engines, they have to be protected too. In fact, the thin disilicide refractory ceramic coating inside the engines is very vulnerable to micro-meteoroids damage. So, a retractable cover was mounted below the engines. It was important because it can lead to burn–through and engine loss. NASA accepted to keep a risk of engine nozzle loss to less than 3% for that mission.

The cover could be extended and retracted at least 25 times, a pyrotechnic ejection mechanism was in place to jettison it if a mechanical problem interfered with engine operation. When cruising, the main engines were not used, so the cover remained closed.

Blankets are built for long-term durability and high thermal requirements, as well keeping temperatures on-board at room temperature. This is good because, in space, temperatures on surfaces not protected range from about -364 to +482 degrees Fahrenheit (about -220 to +250 degrees Celsius).

Some of the blankets used on Cassini were sewn with layers of a canvas-like, carbon-coated fabric called beta cloth that is especially effective in protecting against micro-meteoroids.

Cassini-Huygens has done a Very Good Job... See!

A few results of many are shown in this section 

The Imaging Science Subsystem (ISS) have a wide-angle and a narrow-angle digital camera. These cameras detect visible wavelengths of light and some infrared and ultraviolet wavelengths and, with the help of several filters mounted on wheels, it can select the wavelengths to be sampled in each image.

Color image of Saturn backlit by the Sun

ISS served as Cassini’s main set of eyes for viewing the Saturn system and assisted the spacecraft for navigation. The narrow-angle camera used the known positions of Saturn’s moons along with a map of the stars to determine Cassini's position based on Saturn's images.

As said before, the System had a wide-angle camera to provide context and a narrow-angle camera to give higher resolution of specific targets (craters or fractures).

The wide-angle camera captured broad scenes from more than 1.5 million km away. It used lenses to focus light on a detector called a Charge-Coupled Device (CCD) that converted light into a digital image made of pixels.

The narrow-angle camera used mirrors to focus light on a detector that also converted light into a digital image made of pixels, instead of an image with close to 1 megapixel.

Yes, 1 megapixel! Cassini’s cameras were far more carefully calibrated than the one on Earth and were built to survive more than 10 years in the harsh conditions of outer space. And, they continue to capture fantastic images nearly 20 years after the spacecraft left Earth!

Rhea in front of Titan

Cassini’s cameras were sensitive to all wavelengths of light between the near-ultraviolet and the near-infrared, a range greater than typical visible-light cameras (like on Earth). Each camera included a variety of color filters mounted on wheels that select the range of wavelengths (colors) to record with each image.

For example, using a filter that blocked all but ultraviolet light, researchers were able to study high hazes in the Saturn and Titan's atmospheres. Also, when the filter blocked certain infrared wavelengths, researchers were able to peer through the obscuring smog in the atmosphere of Titan to reveal its surface.

Cassini’s cameras studied the structure and the motion of Saturn’s and Titan’s atmospheres, such as the size, thickness, composition and other characteristics of Saturn’s rings, as well as the interaction between its moons and rings with the gravity. By taking lots of pictures, scientists created maps of Rhea's and Titan's surfaces, and Saturn and its rings. PHOTO

Specifically, these cameras captured views in color by taking 3 images, each with a different colors filter, which was then combined back on Earth.

Finally, the Spacecraft used the narrow-angle camera tonavigate in outer space. To do so, it used the known positions of Saturn’s moons along with a map of the stars, to determine its position based on its images. For its orbit, that was a crucial point. 

The International Team

. Hundreds of scientists and engineers from 16 European countries and 33 states of the United States make up the team responsible for designing, building, flying and collecting data from the Cassini orbiter and Huygens probe.

. Cassini's mission was managed by NASA’s Jet Propulsion Laboratory in California, where the orbiter was designed and assembled.

. The prime contractor for the Huygens Probe was Alcatel in France, but it was managed by the European Space Technology and Research Center.

. The Cassini orbiter and its instruments represented a $1.422 billion investment by NASA. The agency budgeted a total of $710 million to support the cruise and orbital operations phase of the mission. Other contributions included $54 million in NASA tracking costs and about $144 million from the U.S. Department of Energy to support the Radioisotope Thermoelectric Generators and Radioisotope Heater Units. The launch vehicle, provided by the U.S. Air Force, cost $422 million.

. The European Space Agency’s contribution to the Cassini program totalled approximately $500 million for the Huygens probe, its instruments and probe science and engineering operations. The Italian space agency, Agenzia Spaziale Italiana,

contributed the Cassini orbiter’s dish-shaped high-gain antenna as well as significant portions of 3 science instruments; its contribution was $160 million.

.Communications with Cassini during the mission were carried out through stations of NASA’s Deep Space Network in California, Spain and Australia. Data from the Huygens probe were received and relayed by the network and sent to the European Space Agency operations complex in Darmstadt, Germany.

FAST FACTS - Name of Saturn: Roman God of agriculture

Mean Distance from the Sun: 1,426.666 million km (886,489 million mi)
Orbit Period: 29.4 Earth years (10,755.7 Earth days)
Orbit Eccentricity: (Circular Orbit = 0) 0.05386179
Orbit Inclination to Ecliptic: 2.486 deg
Inclination of Equator to Orbit: 26.73 deg
Rotation Period: 10.656 hours
Equatorial Radius: 60,268 km (37,449 mi)
Mass: 95.16 of Earth’s
Density: 0.70 g/cm3
Gravity: 7.207 m/sec2 (23.64 ft/sec2)
Atmosphere Primary Components: hydrogen, helium
Effective Temperature: –178 deg C (–288 deg F)
Known Moons: 53 (Plus 9 awaiting official confirmation, total 62, as ofJuly 2013)
Rings: 7 main rings (C, B, A, D, F, G, E)

What are Extreme In-space Environments? 

Normally, we need some criterias to reach: - Heat flux at atmospheric entry: Heat flux exceeding 1 kW/ cm2 Hypervel
- Hypervolacity impact: Higher than 20 km/sec
- Low temperature: Lower than −55◦C
- High temperature: Exceeding +125◦C
- Thermal cycling: Cycling between temperature extremes outside of the military standard range of −55◦C to +125◦C
- High pressures: Exceeding 20 bars
- High radiation: Total ionizing dose (TID) exceeding 300 krad (Si)
Additional extremes include deceleration (g–loading) exceeding 100g, acidic environments, and dusty environments.

Source: Extreme Environments Technologies for Future Space Science Mission, September 19, 2007, NASA

Cassini-Huygens Spacecraft use 3 General Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) Systems of 56 kg each.

Credit: NASA/JPL- Installation of RTG3 in Cassini-Huygens Spacecraft

After the Multi-Hundred Watt (MHW)-RTG, the Department Of Energy (DOE) developed the GPHS module to provide a standard, modular plutonium dioxide-based heat source. It was the first of these standardized designs, using 18 GPHS modules and Si-Ge (silicon-germanium) thermoelectrics to generate nearly 300 We at the beginning of life. That System has been used very successfully on 4planetary missions: Galileo, Ulysses, Cassini and New Horizons. The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is the newest addition to the family, using updated Step 2 GPHS modules. The latter is currently powering the Mars Science Laboratory (MSL) rover Curiosity, which was launched in 2011 and has been exploring the Red Planet since August 2012.

Unlike the MMRTG, the GPHS-RTG is the first standardized RTG design, using GPHS modules to encase the fuel and developed to operate only in vacuum.

Reference: "Radioisotope Power Systems Reference Book for Mission Designers and Planners", Radioisotope Power System Program Office, by Young Lee & Brian Bairstow, Jet Propulsion Laboratory.


The spacecraft communicates its discoveries to Earth using a High-Gain Antenna (HGA) and 2 Low-Gain Antennas (LGH1 & LGA2).

Click on image to see it bigger

Dish-shaped HGAs are principally used for long-range communications with Earth. The amount of gain achieved (High, Low, or Medium) is relative to the amount of incoming radio power collected and focused into the spacecraft's receiving subsystems, and to the outbound this latter can transmit. In the frequency ranges used by spacecraft, this means that HGAs incorporate large paraboloidal reflectors steerable or fixed to its bus.

A HGA on an interplanetary spacecraft must be pointed to Earth within a fraction of a degree for communications to be feasible. Once this is achieved, communications take place over the highly-focused radio signal.

A HGA can shade from the sun or protect the structure from thousands of micro-meteoroids impacts when a spacecraft (ex. Cassini) points at the sun.

The Low-gain antennas (LGAs) provide wide-angle coverage at the expense of gain. Coverage is nearly omnidirectional, except for areas shadowed by the spacecraft's body. Like the name says, LGAs are used for low data rates and, are operational only when the spacecraft is relatively close, for example several Astronautic Units. The transmission will be established if the Deep Space Network transmitter is powerful enough.

A Travelling Wave Tube Amplifier (TWTA) is a vacuum tube using the interaction between the field of a wave propagated along a waveguide, and a beam of electrons travelling along the wave. The electrons tend to travel slightly faster than the wave, and on the average, are slowed slightly by the wave. The effect amplifies the wave's total energy. TWTAs require a regulated source of high voltage. 

The output of the power amplifier is passed through waveguides and can be switched to the antenna receiver of choice, HGA, MGA, or LGA. The receiver is an electronic device sensitive to a narrow band of radio frequencies, generally plus and minus a few kHz of a single frequency selected during mission design. Once an uplink is detected within its bandwidth, the receiver's phase-lock-loop circuitry (PLL) will follow any changes in its frequency. JPL invented the PLL technology in the early 1960s, and it has since become standard throughout the telecommunications industry.

The receiver's circuitry can provide the transmitter with a frequency reference coherent with the received uplink. This means the downlink signal's phase bears a constant relation to that of the uplink signal. Once detected, the received uplink locks onto the uplink signal and decreases its frequency.

Usually, transmitters and receivers are combined into a single electronic device on a spacecraft, which is called a transponder.

Scientific Instruments on Cassini's Orbiter

Ion and neutral mass spectrometer: Examines neutral and charged particles near Titan, Saturn and the icy satellites to learn more about their extended atmospheres and ionospheres. 

Visible and infrared mapping spectrometer: Identifies the chemical composition of the surfaces, atmospheres and rings of Saturn and its moons by measuring colors of visible light and infrared energy given off by them.

Composite infrared spectrometer: Measures infrared energy from the surfaces, atmospheres and rings of Saturn and its moons to study their temperature and composition. 

Cosmic dust analyzer: Studies ice and dust grains in and near the Saturn system.

Radio and plasma wave instrument: Investigates plasma waves (generated by ionized gases flowing out from the Sun or orbiting Saturn), natural emissions of radio energy and dust. 

Cassini plasma spectrometer: Explores plasma (highly ionized gas) within and near Saturn’s magnetic field.

Ultraviolet imaging spectrograph: Measures ultraviolet energy from atmospheres and rings to study their structure, chemistry and composition.

Magnetospheric imaging instrument: StudiesSaturn’s magnetosphere and measures interactions between the magnetosphere and the solar wind, a flow of ionized gases streaming out from the Sun.

Dual technique magnetometer: Studies Saturn’s magnetic field and its interactions with the solar wind, the rings and the moons of Saturn.  

Imaging science subsystem: Takes pictures in visible, near-ultraviolet and near-infrared light.

Cassini radar: Maps surface of Titan using radar imager to pierce veil of haze. Also used to measure heights of surface features.

Radio science subsystem: Searches for gravitational waves in the universe; studies the atmosphere, rings and gravity fields of Saturn and its moons by measuring telltale changes in radio waves sent from the spacecraft.

Scientific Instruments on Huygens Probe

Descent imager and spectral radiometer: Makes images and measures temperatures of particles in Titan’s atmosphere and on Titan’s surface.

Huygens atmospheric structure instrument: Explores the structure and physical properties of Titan’s atmosphere. 

Gas chromatograph and mass spectrometer: Measures the chemical composition of gases and suspended particles in Titan’s atmosphere.

Aerosol collector pyrolyzer: Examines clouds and suspended particles in Titan’s atmosphere.

Surface science package: Investigates the physical properties of Titan’s surface.

Doppler wind experiment: Studies Titan’s winds from their effect on the probe during its descent.

Watch as NASA's Cassini spacecraft swoops past Saturn's largest moon, and takes snapshots of its lakes, clouds, and atmospheric haze!

Credit: Ian Regan, January 8, 2014.

On unexplored worlds, the sound of science is a harmonious melody of chimes, clicks and mechanical whirrs. At least that is how one scientist interpreted the January 2005 descent and landing of the European Space Agency’s Huygens probe on Titan.

Credit: NASA

A very, very interesting video of Saturn's approach by Cassini-Huygens

This video was created from approximately 25,000 images captured by the Cassini-Huygens mission to Saturn. The probe passed the planet Jupiter for a gravitational speed boost and captured some shots during the process. The following text is from the main mission page - www.ciclops.org:

"Data collection began with the spacecraft 3.8 degrees (deg) above Jupiter's equator plane and approaching the planet from a phase (Sun-Jupiter-spacecraft) angle of 20 deg with a distance of 84.7 million km. From this viewing geometry, Jupiter looked only slightly different than it did from Earth. By the middle of November, the phase angle dropped to 18 deg, and the distance decreased to the point where 4 narrow angle camera images were required to cover the planet. All throughout this period we made repeated observations of the atmosphere, and searched for previously undiscovered satellites in the region around Jupiter containing the Galilean satellites.

By the middle of December, the phase angle dropped to zero, repeated monitoring of the atmosphere ceased, and we began our observations of the rings and satellites. On December 18, we made our closest approach to Himalia, a small outer satellite of Jupiter. As Cassini swept through a large range of phase angle during the rest of the encounter, we monitored the light scattering behavior of the rings and Galilean satellites in a suite of spectral and polarimetric filters. (For a brief time surrounding closest approach, Jupiter was large enough to require 9 images to cover the planet.) And we were on the lookout for time-variability - in the rings and in the expected diffuse glows from the tenuous atmospheres of Io, Europa and Ganymede as they passed into Jupiter's shadow. (The Galileo spacecraft first observed such glows, as well as high temperature hot spots, from volcanically active Io.)

Ring, satellite and occasional atmospheric observations continued through closest approach until January 15, at which point the spacecraft was looking back on a crescent Jupiter from a distance of 18 million km (11 million miles) and 3 degrees below the equator plane. At this time, we returned to repeated imaging of the planet as we departed. The last Jupiter images were taken on March 22, 2001.

The closest approach distance to the planet was not very close at 9.72 million kilometers (6.04 million miles). Thus, Cassini images did not have the exquisitely high resolution of either Voyager or Galileo images. But the slow pace of the flyby, the large data collection and downlinking capability of the spacecraft, and the wide spectral range and fine photometric precision of the Imaging Science System (ISS), made it possible to acquire high quality time-lapse CCD imagery of Jupiter's ever changing atmosphere extending over several months in a large suite of atmosphere-probing wavelengths, and to search for time-variability in other Jovian targets ... something no previous Jupiter-bound spacecraft has ever done before."
All images can be found at: http://pds-rings-tools.seti.org/opus/
Credit: Lightning Rod, July 13, 2015.

On October 15, 1997, NASA's Cassini orbiter embarked on an epic, seven-year voyage to the Saturnian system. Hitching a ride was ESA's Huygens probe, destined for Saturn's largest moon, Titan. The final chapter of the interplanetary trek for Huygens began on December 25, 2004 when it deployed from the orbiter for a 22-day solo cruise toward the haze-shrouded moon. Plunging into Titan’s atmosphere, on January 14, 2005, the probe survived the hazardous 2 hour 27 minute descent to touch down safely on Titan’s frozen surface.

This narrated movie, created with data collected by the Huygens Descent Imager/Spectral Radiometer (DISR), depicts the view from Huygens during the last few hours of this historic journey.

This new version of the movie used updated DISR data and was released on January 14, 2015 on the occasion of the 10th anniversary of Huygen's landing on Titan. 
Credit: ESA/NASA/JPL/University of Arizona  Video: Erich Karkoschka, DISR team, University of Arizona. Script: Chuck See, DISR team, University of Arizona. Narration: David Harrington. Music: Beethoven's Piano Concerto No. 5 by Debbie Hu (Yelm, Washington, USA).

More information about this video can be found at http://sci.esa.int/cassini-huygens/39...
Credit: ESA Science & Technology, January 6, 2017.

Mission Events

On December 30, 2000, Cassini-Huygens took a six-month swing by Jupiter to pick up speed for its journey to Saturn and collaborated with NASA's Galileo spacecraft to study the Jovian system. 

On June 30, 2004, Cassini arrived at Saturn. 

On December 13, 2004, Cassini-Huygens made its 2 flybys of the Saturnian moons, Titan and Dione.

For a full list of Cassini's flybys since 2004, visit:http://saturn.jpl.nasa.gov/mission/flybys/

On December 24, 2004, Cassini released the European Space Agency-built Huygens probe on Saturn's moon Titan. 

On January 14, 2005, the Huygens probe made its descent through Titan's atmosphere to sample the chemical composition and surface properties of the Saturnian moon. 

In June 2008, Cassini completed its primary mission to explore the Saturn system and began its mission extension (Equinox Mission). 

In September 2010, Cassini completed its extended mission (Equinox Mission) and began its second mission extension (Solstice Mission), ending in 2017. This mission was the first observations of a complete seasonal period for Saturn and its moons. 

Learn more at http://saturn.jpl.nasa.gov/index.cfm 

In December 2011, Cassini used its synthetic aperture radar to obtain the highest resolution images yet of Saturn's moon Enceladus. 

In December 2012, Cassini used its visual and infrared mapping spectrometer (VIMS) instrument to track the transit of Venus. The exercise was to test the feasibility of using Cassini's VIMS to observe planets outside our solar system.

In March 2013, Cassini made its last flyby of Saturn's moon Rhea, probing the internal structure of the moon by measuring the gravitational pull of Rhea against the spacecraft's steady radio link to NASA's Deep Space Network here on Earth. 

In July 2013, Cassini took pictures of a backlit of Saturn to examine the planet's rings in fine detail and captured a pixel-size Earth in the process. In a campaign to raise awareness about the photo shoot, NASA encouraged Earthlings to go outside and wave at Saturn.

Reference: http://saturn.jpl.nasa.gov/