Cassini Fun Facts Archive

April 12, 2005
This amazing ring photograph is actually a mosaic made of six separate images extending for a distance of about 62,000 kilometers along the ring plane. Staring from the center of the planet (off to the left in the photograph above) the rings go from a radius of 74,565 kilometers to 136,780 kilometers (46,333 to 84,991 miles).

The images were taken using red, green and blue spectral filters that were then combined to create this natural color mosaic. Like so much of the really exciting optical imagery from Cassini, these images were acquired using the spacecraft’s narrow angle camera. The NAC is shown in red on the spacecraft image below left. A more detailed photograph of the exterior of the NAC is shown below right.

 

The cutaway drawing to the left shows the interior components of the NAC. Almost in the center of the drawing there is a circular disk that shows the filter wheel aperture. This is the device that allows the camera to select specific wavelengths that can either be studied separately or combined in order to enhance selected features.

In the case of this week’s release the separate images were combined to create a single natural-color image for each of the six separate pictures that make up the entire photograph. For more information on Saturn’s amazing rings see the Cassini website.

March 14, 2004
The images in the animation movie were taken with the narrow angle camera using a filter to capture selected frequencies of light. The NAC is shown in red on the spacecraft image below left. A more detailed photograph of the exterior of the NAC is shown below right.

 

The cutaway drawing to the left shows the interior components of the NAC. Almost in the center of the drawing there is a circular disk that shows the filter wheel aperture. This is the device that allows the camera to select specific wavelengths that can either be studied separately or combined in order to enhance selected features.

For more information on the various moons of Saturn or the instruments aboard the Cassini spacecraft and Huygens probe please see the Cassini website.

February 28, 2005
Except for the photograph of Titan, the images above were taken by the Cassini spacecraft’s Narrow Angle Camera (NAC). The NAC is shown in red on the spacecraft image below left. A more detailed photograph of the exterior of the NAC is shown below right.

 

The photograph of Titan’s river tributaries was taken by the Descent Imager/Spectral Radiometer aboard the Huygen’s probe as it plummeted through Titan’s thick atmosphere to its landing spot. The DI/SR is shown in the photograph to the left. The Huygen’s probe (prior to departing from Cassini) is shown opposite the NAC in the photo above left.

For more information on the various moons of Saturn or the instruments aboard the Cassini spacecraft and Huygens probe please see the Cassini website.

February 11, 2005

The temperature maps of Saturn's moons Iapetus and Phoebe were constructed from observations of infrared heat radiation taken with the Cassini composite infrared spectrometer (CIRS) instrument. (CIRS) is a remote sensing instrument that measures the infrared light coming from an object (such as an atmosphere or moon surface) to help scientists learn more about its temperature and what it's made of. The photo shows the location the CIRS instrument on the Cassini spacecraft.

For more technical details on the design and operation of the CIRS see the NASA website.

January 21, 2005

The pictures that were taken by the Huygens probe during its decent to Titan’s surface came from an instrument called the Descent Imager/Spectral Radiometer. It’s one of six scientific instruments on the probe and one of two built by NASA. The figure below depicts the decent of the Huygens probe after it entered Titan’s thick atmosphere. Scientists report that the little spacecraft experienced a very rough ride down to the surface of the moon. It began slowing rapidly when its heat shield first encountered Titan’s atmosphere. After slowing down from its initial entry speed of 6 thousand meters per second it deployed parachutes to further reduce its speed. The decent took a total of 2.5 hours.

For more technical details on the design and operation of the Huygens probe instruments see the NASA website.

December 31, 2004

The images of Dione were taken by the Cassini spacecraft’s narrow angle camera in visible light. The NAC is one of two cameras that make up the Cassini imaging subsystem (ISS) and it photographs small areas in great detail. The other camera is a wide-angle camera (WAC) that is used to take in broad views at lower resolution. The NAC, which is the higher-resolution camera, can see a penny, 1.5 centimeters (0.5 inches) across, from a distance of nearly 4 kilometers (2.5 miles). The images to the right provide an idea of the components that make up the NAC. The image on the top shows the outer camera assembly with the camera aperture facing to the left. The image on the bottom shows an exploded view of the interior components of the instrument. For lots more technical details on the design and operation of the ISS and NAC please see the NASA website.

December 24, 2004
After separation, the Huygens probe will continue to drift toward its destination, arriving at the upper atmosphere of Titan on January 14, 2005. A timer then wakes up the probe so that it can collect data as it plummets to Titan’s surface for about 2.5 hours.

The artist’s rendering shows the leading edge of the Huygens probe heating up from friction with the molecules in Titan’s upper atmosphere. The probe will decelerate from 22,000 km/hr to 1,400 km/hr in just under two minutes. ESA scientists predict that the temperature of the gas in the shock wave in front of the heat shield may reach 12,000° C, with the shield itself reaching 1,800°. That’s hot. And it promises to be a pretty wild ride for the little probe.

Additional information about the Huygens probe can be obtained from the ESA website. After reaching the link, wait for a moment for the animation to load. It will draw the path of the Cassini spacecraft and allow you to interact with the simulation in order to find out the exact location of the Huygens probe after its release.

December 1, 2004

The ring image above was created from data acquired by Cassini’s ultraviolet imaging spectrograph (UVIS), which actually consists of four separate instruments designed to provide measurements of both reflected and emitted ultraviolet light.

The UVIS takes measurements in wavelengths ranging from 5.8 to 190 nanometers. By contrast, our eyes see in the wavelengths that range from 400 to 700 nanometers, which are much longer than for the UV part of the spectrum. For a very clear and thorough explanation of the electromagnetic spectrum (and the different measurements that one can use to describe the various portions of it) see the NASA Goddard web page.

November 12, 2004

The key to determining Saturn’s temperatures lies in the ability to measure a part of the electromagnetic spectrum that we can’t see. The Composite Infrared Spectrometer (CIRS) measures the infrared light coming from Saturn’s atmosphere.

Throughout the course of the mission it will provide data in three dimensions. Scientists can then determine temperature and pressure profiles by altitude at every place on the planet. The velocity of the particles in various parts of Saturn’s atmosphere can then be determined through the use of the infrared data -- just another terrific example of how an understanding of fundamental relationships allows scientists to get data that cannot be measured directly.

October 29, 2004

[Click image for larger version]

On a complex mission like the Cassini-Huygens tour of Saturn nothing is left to chance. Scientists carefully pre-plan the spacecraft’s flight path and determine in advance those areas they hope to study and at what level of detail. The image to the right shows the areas studied when Cassini made its closest approach to Titan on October 26th.

The horizontal axis shows longitude and the vertical shows latitude. The spacecraft used its sensors to collect as much data as possible about the mysterious moon. Look closely and you will see a small yellow “X” just inside the red boundary line in the photo. This is the point at which the detachable Huygens probe is supposed to land after entering Titan’s thick atmosphere in January 2005. In the mean time the orbiting Cassini spacecraft took its highest resolution photographs within the area bounded in red, capturing features 1 to 2 kilometers (0.6 to 1.2 mile) across. The larger area in green reveals features on the order of 5 kilometers (3.0 miles) across. The largest area (blue) resolves features on the scale of 20 kilometers (12 miles) across.

October 15, 2004

The instrument that provided the data for the top part of the graph is called the dual technique magnetometer and is easily visible as the long boom extending from the Cassini spacecraft.

The data for the bottom part of the graph was collected by a separate instrument on the spacecraft called the Cassini plasma spectrometer (CAPS), which measures the energy and electric charge of particles such as protons and electrons. The CAPS is shown in position essentially opposite to the location of the magnetometer boom. A photograph of the actual instrument is also shown.

September 29, 2004

August 20, 2004
The Cassini spacecraft’s cameras took a series of 55 exposures to capture the data that allowed Drs. Charnoz and Brahic to discover the new moons. For S/2004 S2, the camera took 28 frames in just over nine hours. It covered about 10 degrees of the moon's orbit on the left side of Saturn followed by a gap of approximately 80 degrees. The tiny moon can then be seen moving through another 30-degree swath on the right side of the planet. The S1 movie spans about 90 degrees of that moon's orbit around the left side of the planet and consists of 27 frames taken over a period of six hours. The overexposed object seen orbiting Saturn is another moon - Mimas. Although the spacecraft provided enough data to identify these two new satellites, more imagery is required in order to establish accurate orbits for these two tiny moons. Scientists hope to photograph the satellites again during the four-year Cassini tour of Saturn.

(click image for a larger version)

July 9, 2004
Here on Earth we often think of entering orbit as a simple two-dimensional maneuver. For example, we see the Space Shuttle clear the launch tower, ascend for several hundred feet, roll to its intended inclination and fly along a single plane through the atmosphere and into orbit around the Earth. Getting Cassini into orbit around Saturn was not so simple. The maneuver was both critical and complex—requiring movement in three dimensions simultaneously.

The images below break Cassini’s motion into separate components and help us to understand its actual flight path during orbit insertion. Recall from earlier updates how the spacecraft approached from the southern hemisphere of the planet. The rings were tilted upward and sunlight shone on the lower surface of the rings. One earlier image (see archive from May 3rd) showed how the light slipped through the Cassini gap to cast a silvery sliver of light on Saturn’s upper atmosphere.

Cassini's path over the rings of Saturn (click image for a larger version)

The adjacent image (an artist’s illustration) shows Saturn with its rings nearly edge-on. Notice how they almost disappear. The curved line (purple and speckled) shows how Cassini actually crossed the rings (through the gap between rings F and G), rising to its maximum point above the rings, then diving downward again to make the second ring crossing and completing orbit capture. However, this is only part of the story.

Cassini's path looking down on Saturn (click image for larger version)

The second figure shows the path of the spacecraft from the top, looking directly down on Saturn. Now the flight path can be seen as a large arc bending to the left as Cassini moves from the top to the bottom of the illustration. The gravitational pull of the planet causes the curvature in Cassini’s path. The red part of the flight path shows the portion during which the main engine burned to achieve orbit. That lasted for a total of 96 minutes. The sun is at the top of the figure (also the direction of Earth), and the tiny green circles show the places where Cassini ascended through the ring plane (Ascent Ring Plane Crossing) and descended back through it again (DRPC). This was an amazing bit of navigation on the part of the NASA-JPL team—kind of like standing in Los Angeles and threading a needle in New York. Having completed its initial double ring-plane crossing for orbit insertion, no future orbits will bring Cassini inside the rings.

June 29, 2004
To make sure that the Saturn orbit insertion (SOI) maneuver goes as smoothly as possible, NASA engineers have tried to anticipate potential difficulties and design appropriate solutions for the mission. The 96-minute main engine firing is essential for orbit capture; therefore, to help assure that this maneuver goes smoothly the spacecraft carries an extra main engine. The transition through the ring plane was also given careful consideration with the use of the most powerful telescopes (Earth and space-based) to search for potential hazards. However, knowing that particles too small to be seen from the Earth could prove fatal to the spacecraft engineers will turn the spacecraft to that its high gain antenna can be used as a shield against small particles. Therefore, NASA’s engineers will use both the redundancy in hardware design and the operation of the Cassini spacecraft as tools to assure a successful mission.

June 16, 2004
Scientists used several instruments to study Phoebe during Cassini’s close approach. One of these, called the Visual and Infrared Mapping Spectrometer (VIMS), determines chemical composition by measuring colors of visible light and infrared energy given off by Phoebe’s surface. Color might not seem like a good way to determine the chemical composition of an object, but it’s a perfect example of what happens when known scientific principles meet good data. Matter affects light, absorbing some colors (or wavelengths) more than others. For example, when the sun is low on the horizon, its light traverses long distances through our atmosphere which transmits more red light than other frequencies. Each chemical substance affects light in its own way, transmitting some frequencies while absorbing others, and scientists have a solid understanding about the relationship between chemical substance and color.

As an optical instrument, the human eye (through the cones in its retina) is actually sensitive to only three frequencies that we interpret as blue, yellow and red. All of the other colors result from the brain’s ability to make sense of the relative amounts of the basic three colors. By contrast, the VIMS instrument actually takes pictures in 352 discrete colors at the same time. In other words, it provides a full spectrum of color for each point on the surface of an object at which the spectrometer is pointed, giving it a much greater spectral resolution than the human eye. Furthermore, the VIMS color range encompasses the entire range of human vision and beyond, allowing it to see into the infrared as well. This allows the VIMS instrument to be able to very accurately quantify the light it detects. Very accurate measurements combined with well-known principles of science yield a very powerful tool for the scientists on the Cassini mission.

June 10, 2004
Most of the imagery released by NASA during the 15-week period leading up to the encounter with Phoebe relied upon the Narrow Angle Camera that uses filters in various combinations to enhance features of special interest. This instrument is great for taking photographs of the entire planet and provided scientists with new insights into the composition of Saturn’s complex atmosphere.

For the Phoebe encounter on June 11th scientists will pull out all the stops by using several instruments carried by the Imaging Science Subsystem (ISS). These include the Visual and Infrared Mapping Spectrometer (VIMS) that will help to derive detailed maps allowing scientists to understand what materials lay on Phoebe’s surface. The Ultraviolet Imaging Spectrograph (UVIS) will provide the first ultraviolet map of this moon. Radar will penetrate the surface to assess the relative ice cleanliness in the upper layer of rock and soil. Cassini’s Radio and Plasma Wave Science Instrument (RPWS) will look for evidence of any interaction between Phoebe and the solar wind, and the Cosmic Dust Analyzer (CDA) will collect data about the composition of the moon's surface as well as about what processes generate the dust. In contrast to the restricted use of the Narrow Angle Camera for the full-Saturn photographs, this represents an all-out data assault on the tiny moon. Scientists must be expecting some very interesting findings.

June 1, 2004
The terms “scientist” and “engineer” are often used together when referring to space exploration. Indeed, both skills are essential for the Cassini-Huygens mission to Saturn. But what’s the difference between these two teams of professionals, and how do they divide up their responsibilities on a project like Cassini? The team at NASA’s Jet Propulsion Laboratory offered a humorous insight into the distinction by claiming that the engineers are like valet parking attendants. They navigate the spacecraft to those points of interest called out by the scientists. The engineering teams on the Cassini mission are responsible for the design, operation and maintenance of 15 separate subsystems on the spacecraft. These include temperature controls, radio frequencies, power and pyrotechnics, propulsion, etc.

Once the spacecraft has arrived at the proper location the scientists take over and operate their instruments, each of which is designed to address a certain question (or set of questions). So, while the engineers take care of the spacecraft’s operation and navigation, the scientists focus on the collection of scientific data that reflects their field of study. As noted in earlier updates, there are 18 scientific instruments on the spacecraft - six of which are on the detachable Huygens probe.

May 25, 2004
The detachable Hugyens probe carries six instruments designed especially to study this moon of Saturn. It will break away from the Cassini spacecraft on December 25th and then coast for a period of 22 days until it reaches Titan on January 14, 2005. As soon as the spacecraft senses the outer edge of Titan’s atmosphere (which is rich in nitrogen) the scientific instruments will turn on, allowing the probe to begin data collection. The Huygens probe’s mission life of 153 minutes (including a 2.5 hour trip through the atmosphere) includes 30 minutes of data collection on the surface of Titan. The instruments include a camera, programmed to capture about 1,100 images, along with five other instruments designed to sample Titan’s atmosphere.

May 14, 2004
As Cassini-Huygens approaches Saturn, engineers at NASA/JPL will send a command to slow the spacecraft in order to allow it to be inserted into orbit around the planet. They’ll do this by pointing the main rocket engine of the spacecraft in the direction of flight to start a 96-minute burn that will slow the spacecraft enough to allow capture by Saturn’s huge gravitational field. The spacecraft has two rocket engines, one which is carried as a backup. A hemispherical cover conceals the engines to protect them from impacts until needed. The photograph shows the full-scale model of the Cassini-Huygens spacecraft on display in the Air and Space Gallery. The engine cover is shown in the closed position, which reflects the condition of the spacecraft during the overwhelming majority of its mission.

May 7, 2004
The Cassini-Huygens spacecraft is now in its final cruise phase toward Saturn and has enough velocity to arrive there on the target date of July 1st. Using the distances reported by NASA for each week’s photograph release, and the number of days between observations, you can approximate Cassini’s current speed. It’s slowed down to less than half the speed that it reached during its last gravity assist from Jupiter in December 2000. For a fun challenge, see if you can calculate the distance of Cassini-Huygens from Saturn for each week between now and the July 1st arrival. All the data you need to calculate the speed is archived in the Image of the Week section. Then, multiply that speed by the number of elapsed days (remember to convert to the proper units). NASA will publish the actual distance each Thursday or Friday so you can check your calculations. Check out NASA's official Cassini site to see how well you did.

A map of Cassini's trip through the solar system. Click above for a larger version of the image in PDF format. Courtesy of NASA/JPL.

May 3, 2004
Did you know that ever since its launch in October 1997 Cassini has waged a gravity battle with the Sun? It’s true. In order to reach its distant solar system destination (i.e. the beautiful ringed planet) the spacecraft must have sufficient velocity to prevail against the Sun’s ever-present gravitational tug. The rocket that launched Cassini simply could not provide the energy needed to send the spacecraft directly to Saturn. So, to achieve the necessary speed, engineers at NASA made Cassini fly twice by Venus, once by Earth and once by Jupiter in order to pick up additional momentum.

April 26, 2004
When studying planets in our own solar system, nothing beats getting a camera close enough to take a good look for lots of high-resolution pictures. With this week’s release the Cassini-Huygens spacecraft crossed the point at which it got close enough to Saturn to send back images that have greater resolution than those provided by the Hubble Space Telescope. Cassini’s higher-resolution camera, called the Narrow Angle Camera, is one of the instruments in the optical remote sensing package. It has 24 filters and has been the source of all NASA weekly releases up to this point. The NAC can see a penny, 1.5 cm (0.5 in) across, from a distance of nearly 4 km (2.5 mi). The clarity of these images, along with the ability to photograph the ringed giant in different wavelengths, is already allowing scientists to uncover new mysteries about Saturn. With a planned four-year mission through the Saturnian system, and 17 other scientific instruments, scientists stand to collect a staggering amount of data.

April 21, 2004
Most of the structure of the Cassini-Huygens spacecraft is covered by a loose-fitting gold-colored blanketing that required hand-stitching by a special team of spacecraft technicians. The blanketing is composed of a shiny amber-colored material on top of a reflective aluminized fabric. In some places there are up to 24 layers of fabric which include aluminized Kapton, mylar, Dacron and other materials. The blanketing protects the spacecraft from thermal extremes and can also absorb the energy of impacting particles of dust. On the full-scale engineering model of the Cassini-Huygens spacecraft that is on display in the Air and Space Gallery, the blanketing has been pulled back in several places in order to expose the underlying structure.

April 9, 2004
Aside from its large 4-meter high-gain antenna (and of course the detachable Huygens probe), perhaps the most distinguishable feature of the Cassini-Huygens spacecraft is the 11-meter long magnetometer boom that is attached to the spacecraft. It's one of six instruments designed to collect data on particles and fields. The magnetometer boom actually carries two magnetometers which will, among other things, help distinguish the magnetic field of Saturn from that produced by the spacecraft itself. The magnetometer boom was folded into a small canister during launch and deployed after separation from the launch vehicle and before the Earth flyby, which was in August 1999.

April 2, 2004
The quantity of scientific data sought by the Cassini-Huygens spacecraft staggers the imagination. Scientists want every conceivable learning opportunity given the time and effort required to carry out this exciting mission. To collect this data, the spacecraft carries 18 scientific instruments, of which the detachable Huygens probe (more about that in another update) will carry six to the surface of Saturn's largest moon, Titan.

The 12 instruments aboard the main spacecraft cluster into three broad categories. One group conducts optical remote sensing. These instruments create imagery by "staring" directly at radiation reflected or emitted by different objects of interest (e.g. Saturn's clouds, its rings, etc.). A second cluster of instruments sends out radio pulses to bounce off different target objects. By measuring the amount of return energy and its round-trip travel time, scientists can build 3D models of cloud-covered surfaces and estimate surface roughness or atmospheric turbulence at levels below the cloud tops. The third cluster of instruments looks at particles and fields. By measuring field strengths, the velocities of charged particles and the interaction between the two, scientists can better understand the complex radiation environment of the Saturnian system and further refine theories about the interior core of the planet and the source of its magnetic field.

March 26, 2004
Imagine having a stiff neck and not being able to turn your head from left to right. You would have to rotate your entire body to gaze around the room. Believe it or not, that's how Cassini operates. Its scientific instruments are mounted on the body of the spacecraft—which means that the whole spacecraft must be turned to point them at various objects of interest. With a four-year mission, including close flybys of six Saturnian moons, that's a lot of turning!

As a result, the Propulsion Module Subsystem (PMS) is the largest subsystem onboard. It's broken into two parts that include a bi-propellant system used to change Cassini's trajectory and orbit, and a hydrazine system for attitude control and fine maneuvering. At the time of launch, the PMS fuel (approx. 3,100 kilograms) made up more than half the total mass of the spacecraft!

March 19, 2004
One of the most important functions of any deep space probe is sending scientific findings back to Earth. However, any antenna can only radiate so much energy. Low-gain antennae (LGA) radiate in all directions, so their energy is spread out in a spherical shape, getting thinner and thinner and thinner as the distance from the antenna increases. You can receive info from LGA from any direction, but their signals can get very weak.

High-gain antennae (HGA) concentrate their energy into a narrow beam—most often by using a parabolic shape, like a satellite dish. (Check out the Creative World whisper dishes to learn about "parabola power.") HGAs must be aimed just right to communicate, but their signal is a lot stronger than an LGA's. Cassini has two LGAs and one HGA, which is a large, white 4-meter-wide dish located on top of the spacecraft. (Check it out in the Air and Space Gallery). During the first 30 months after launch, the HGA faced into the sun to provide shade for the other systems on the spacecraft. This meant that the HGA almost never pointed at Earth, so the two LGAs were used to communicate with the spacecraft during this time. Now, deep into its mission and very far from Earth, Cassini calls home almost exclusively with its HGA.

March 12, 2004
Did you know that Cassini is a nuclear-powered spacecraft? It's true. Saturn lies so far out in space that solar panels aren't practical, so Cassini uses nuclear power to run its scientific instruments. The power comes from a device called a radioisotope thermoelectric generator (or RTG), which uses the heat from the natural decay of plutonium to generate direct current. RTGs have no moving parts and have proven to be reliable power sources on other deep space probes. Cassini uses 3 RTGs.

March 5, 2004
With a total of 18 science instruments, the Cassini spacecraft is the largest robotic deep space probe ever built. It will probably never be duplicated again. Today's mission design strategy calls for smaller spacecraft with more specialized science objectives. Smaller craft are less expensive and easier to design (because of fewer conflicts in the mission objective). Smaller craft can also be reproduced relatively cheaply so that two or more can be used to increase the quantity of scientific data (e.g. the Mars Spirit and Opportunity spacecraft).