Category Archives: Cassini

Saturn’s C Ring

Cassini keeps on giving even after it became part of Saturn and the end of the epic mission.

Original caption from NASA:

Saturn’s C ring is home to a surprisingly rich array of structures and textures (see also PIA21618). Much of the structure seen in the outer portions of Saturn’s rings is the result of gravitational perturbations on ring particles by moons of Saturn.

Such interactions are called resonances. However, scientists are not clear as to the origin of the structures seen in this image which has captured an inner ring region sparsely populated with particles, making interactions between ring particles rare, and with few satellite resonances.

In this image, a bright and narrow ringlet located toward the outer edge of the C ring is flanked by two broader features called plateaus, each about 100 miles (160 kilometers) wide.

Plateaus are unique to the C ring. Cassini data indicates that the plateaus do not necessarily contain more ring material than the C ring at large, but the ring particles in the plateaus may be smaller, enhancing their brightness. This view looks toward the sunlit side of the rings from about 53 degrees above the ring plane. The image was taken in green light with the Cassini spacecraft narrow-angle camera on Aug. 14, 2017.

The view was acquired at a distance of approximately 117,000 miles (189,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 74 degrees. Image scale is 3,000 feet (1 kilometer) per pixel.

The Cassini spacecraft ended its mission on Sept. 15, 2017.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

Image: NASA/JPL-Caltech/Space Science Institute

Saturn Moon Dione

One of the last images of the Saturn moon Dione from the Cassini spacecraft shows nice detail on the moon surface. Scientists say that the subtle variations in brightness hint at differences in composition, as well as the size and shape of grains in Dione’s surface material, or regolith.

The view was obtained at a distance of approximately 224,000 miles (360,000 kilometers) from Dione which isn’t too much different than our Earth-moon distance. Image scale is 1.4 mile (2.2 kilometers) per pixel

The Cassini spacecraft entered the Saturn atmosphere right on schedule on 15 September 2017.

Image: NASA/JPL-Caltech/Space Science Institute

The Last Moments of Cassini

The Cassini team released a description of Cassini’s last moments. The spacecraft was a fighter to the very end especially when one considers that it was traveling around 4.5 times faster than the ISS is around Earth.

Here’s a excerpt from the NASA description:

Data show that as Cassini began its final approach, in the hour before atmospheric entry it was subtly rocking back and forth by fractions of a degree, gently pulsing its thrusters every few minutes to keep its antenna pointed at Earth. The only perturbing force at that time was a slight tug from Saturn’s gravity that tried to rotate the spacecraft.

“To keep the antenna pointed at Earth, we used what’s called ‘bang-bang control,'” said Julie Webster, Cassini’s spacecraft operations chief at NASA’s Jet Propulsion Laboratory, Pasadena, California. “We give the spacecraft a narrow range over which it can rotate, and when it bangs up against that limit in one direction, it fires a thruster to tip back the other way.” (This range was indeed small: just two milliradians, which equals 0.1 degree. The reconstructed data show Cassini was subtly correcting its orientation in this way until about three minutes before loss of signal.)

At this point, about 1,200 miles (1,900 kilometers) above the cloud tops, the spacecraft began to encounter Saturn’s atmosphere. Cassini approached Saturn with its 36-foot-long (11-meter) magnetometer boom pointing out from the spacecraft’s side. The tenuous gas began to push against the boom like a lever, forcing it to rotate slightly toward the aft (or backward) direction. In response, the thrusters fired corrective gas jets to stop the boom from rotating any farther. Over the next couple of minutes, as engineers had predicted, the thrusters began firing longer, more frequent pulses. The battle with Saturn had begun.

With its thrusters firing almost continuously, the spacecraft held its own for 91 seconds against Saturn’s atmosphere — the thrusters reaching 100 percent of their capacity during the last 20 seconds or so before the signal was lost. The final eight seconds of data show that Cassini started to slowly tip over backward. As this happened, the antenna’s narrowly focused radio signal began to point away from Earth, and 83 minutes later (the travel time for a signal from Saturn), Cassini’s voice disappeared from monitors in JPL mission control. First, the actual telemetry data disappeared, leaving only a radio carrier signal. Then, 24 seconds after the loss of telemetry, silence.

These data explain why those watching the signal — appearing as a tall green spike on a squiggly plot of Cassini’s radio frequency — in mission control and live on NASA TV — saw what appeared to be a short reprieve, almost as though the spacecraft was making a brief comeback. The spike of the signal first began to diminish over a few seconds, but then rose briefly again before disappearing with finality.

Credit: NASA/JPL-Caltech

Crescent of Enceladus

Cassini may be gone, but the legacy lives on.

NASA – The brightly lit limb of a crescent Enceladus looks ethereal against the blackness of space. The rest of the moon, lit by light reflected from Saturn, presents a ghostly appearance.

Enceladus (313 miles or 504 kilometers across) is back-lit in this image, as is apparent by the thin crescent. However, the Sun-Enceladus-spacecraft (or phase) angle, at 141 degrees, is too low to make the moon’s famous plumes easily visible.

This view looks toward the Saturn-facing hemisphere of Enceladus. North on Enceladus is up. The above image is a composite of images taken with the Cassini spacecraft narrow-angle camera on March 29, 2017 using filters that allow infrared, green, and ultraviolet light. The image filter centered on 930 nm (IR) was is red in this image, the image filter centered on the green is green, and the image filter centered on 338 nm (UV) is blue.

The view was obtained at a distance of approximately 110,000 miles (180,000 kilometers) from Enceladus. Image scale is 0.6 miles (1 kilometer) per pixel.

The Cassini spacecraft ended its mission on Sept. 15, 2017.

Credit: NASA/JPL-Caltech/Space Science Institute

Last Light for Cassini

NASA —  This monochrome view is the last image taken by the imaging cameras on NASA’s Cassini spacecraft. It looks toward the planet’s night side, lit by reflected light from the rings, and shows the location at which the spacecraft would enter the planet’s atmosphere hours later.

A natural color view, created using images taken with red, green and blue spectral filters, is also provided (Figure 1). The imaging cameras obtained this view at approximately the same time that Cassini’s visual and infrared mapping spectrometer made its own observations of the impact area in the thermal infrared.

This location — the site of Cassini’s atmospheric entry — was at this time on the night side of the planet, but would rotate into daylight by the time Cassini made its final dive into Saturn’s upper atmosphere, ending its remarkable 13-year exploration of Saturn.

The view was acquired on Sept. 14, 2017 at 19:59 UTC (spacecraft event time). The view was taken in visible light using the Cassini spacecraft wide-angle camera at a distance of 394,000 miles (634,000 kilometers) from Saturn. Image scale is about 11 miles (17 kilometers).

The Last Signals from Cassini

The end of mission activities for Cassini will be here with live commentary from JPL at 11:00 UT/7:00 ET.

Expected LOS is 12:00 UTC / 08:00 EDT

About the image (NASA) – ASA’s Cassini spacecraft gazed toward the northern hemisphere of Saturn to spy subtle, multi-hued bands in the clouds there.

This view looks toward the terminator — the dividing line between night and day — at lower left. The sun shines at low angles along this boundary, in places highlighting vertical structure in the clouds. Some vertical relief is apparent in this view, with higher clouds casting shadows over those at lower altitude.

Images taken with the Cassini spacecraft narrow-angle camera using red, green and blue spectral filters were combined to create this natural-color view. The images were acquired on Aug. 31, 2017, at a distance of approximately 700,000 miles (1.1 million kilometers) from Saturn. Image scale is about 4 miles (6 kilometers) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

Image: NASA/JPL-Caltech/Space Science Institute

Apoapsis

On 12 September 2017 at 05:27 UT / 01:27 EDT the Cassini spacecraft reached apoapsis or the farthest point from Saturn it will reach. For Cassini apoapse was 1.3 million km / 800,000 miles.

Cassini is now headed right toward Saturn for the last time. The spacecraft will plunge into the Saturian atmosphere in just a few days, on 15 September 2017.

The signal from Cassini has to travel 83 minutes before reaching Earth. By the time we lose the signal from the spacecraft about 11:55 UT / 07:55 EDT on the 15th, the spacecraft will have already encountered Saturn. The time could change a little because of atmospheric drag during the final orbits.

If I calculated this correctly the “at Saturn” event time would be 10:32 UT / 06:32 EDT based on that original estimate.

Definition of Apoapsis vs. Apogee.

The image above of the moon Titan is one of the very last we will see from the mission and was taken on 10 September 2017.

Image: NASA/JPL-Caltech/Space Science Institute

Cassini’s Legacy

As the Cassini mission draws to a close, NASA shares thoughts about the mission and what the future might bring.  I for one am all in for the Ice Giants Mission idea, robotic missions have proven to be a good value.

The image shown here (credit: NASA/JPL-Caltech/Space Science Institute) was taken on 08 Sept 2017 and returned to Earth on 09 Sept 2017.

As the Cassini spacecraft nears the end of a long journey rich with scientific and technical accomplishments, it is already having a powerful influence on future exploration. In revealing that Saturn’s moon Enceladus has many of the ingredients needed for life, the mission has inspired a pivot to the exploration of “ocean worlds” that has been sweeping planetary science over the past decade.

“Cassini has transformed our thinking in so many ways, but especially with regard to surprising places in the solar system where life could potentially gain a foothold,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate at Headquarters in Washington. “Congratulations to the entire Cassini team!”

Onward to Europa

Jupiter’s moon Europa has been a prime target for future exploration since NASA’s Galileo mission, in the late 1990s, found strong evidence for a salty global ocean of liquid water beneath its icy crust. But the more recent revelation that a much smaller moon like Enceladus could also have not only liquid water, but also chemical energy that could potentially power biology, was staggering.

Many lessons learned during Cassini’s mission are being applied to planning NASA’s Europa Clipper mission, planned for launch in the 2020s. Europa Clipper will fly by the icy ocean moon dozens of times to investigate its potential habitability, using an orbital tour design derived from the way Cassini has explored Saturn. The Europa Clipper mission will orbit the giant planet — Jupiter in this case — using gravitational assists from its large moons to maneuver the spacecraft into repeated close encounters with Europa. This is similar to the way Cassini’s tour designers used the gravity of Saturn’s moon Titan to continually shape their spacecraft’s course.

In addition, many engineers and scientists from Cassini are serving on Europa Clipper and helping to develop its science investigations. For example, several members of the Cassini Ion and Neutral Mass Spectrometer and Cosmic Dust Analyzer teams are developing extremely sensitive, next-generation versions of their instruments for flight on Europa Clipper. What Cassini has learned about flying through the plume of material spraying from Enceladus will help inform planning for Europa Clipper, should plume activity be confirmed on Europa.

Returning to Saturn

Cassini also performed 127 close flybys of Saturn’s haze-enshrouded moon Titan, showing it to be a remarkably complex factory for organic chemicals — a natural laboratory for prebiotic chemistry. The mission investigated the cycling of liquid methane between clouds in its skies and great seas on its surface. By pulling back the veil on Titan, Cassini has ushered in a new era of extraterrestrial oceanography ­– plumbing the depths of alien seas — and delivered a fascinating example of earthlike processes occurring with chemistry and at temperatures markedly different from our home planet.

In the decades following Cassini, scientists hope to return to the Saturn system to follow up on the mission’s many discoveries. Mission concepts under consideration include spacecraft to drift on the methane seas of Titan and fly through the Enceladus plume to collect and analyze samples for signs of biology.

Giant Planet Atmospheres

Atmospheric probes to all four of the outer planets have long been a priority for the science community, and the most recent Planetary Science Decadal Surveycontinues to support interest in sending such a mission to Saturn. By directly sampling Saturn’s upper atmosphere during its last orbits and final plunge, Cassini is laying the groundwork for an eventual Saturn atmosphere probe.

Farther out in the solar system, scientists have long had their eyes set on exploring Uranus and Neptune. So far, each of these worlds has been visited by only one brief spacecraft flyby (Voyager 2, in 1986 and 1989, respectively). Collectively, Uranus and Neptune are referred to as “ice giant” planets, because they contain large amounts of materials (like water, ammonia and methane) that form ices in the cold depths of the outer solar system. This makes them fundamentally different from the gas giant planets, Jupiter and Saturn, which are almost all hydrogen and helium, and the inner, rocky planets like Earth or Mars. It’s not clear exactly how and where the ice giants formed, why their magnetic fields are strangely oriented, and what drives geologic activity on some of their moons. These mysteries make them scientifically important, and this importance is enhanced by the discovery that many planets around other stars appear to be similar to our own ice giants.

A variety of potential mission concepts are discussed in a recently completed study, delivered to NASA in preparation for the next Decadal Survey — including orbiters, flybys and probes that would dive into Uranus’ atmosphere to study its composition. Future missions to the ice giants might explore those worlds using an approach similar to Cassini’s mission.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.