Color image of Saturn backlit by the sun.

On July 19, 2013, in an event celebrated the world over, NASA's Cassini spacecraft slipped into Saturn's shadow and turned to image the planet, seven of its moons, its inner rings -- and, in the background, our home planet, Earth.

How It Works

The Imaging Science Subsystem consists of a wide-angle and a narrow-angle digital camera. The cameras are sensitive to visible wavelengths of light and to some infrared and ultraviolet wavelengths. Each camera has several filters mounted on wheels to select the wavelengths to be sampled in each image.

How We Use It

The Imaging Science Subsystem serves as Cassini’s main set of eyes for viewing the Saturn system. It studies features as small as boulders and as large as Saturn’s rings. The instrument also assists in navigation and captures the beautiful images for which Cassini is so well known.

Not to be confused with the International Space Station, which goes by the same acronym, Cassini’s Imaging Science Subsystem, or ISS, is a powerful camera system that sends back a steady stream of beautiful, science-packed images from Saturn.

Cassini's imaging system has two parts: a wide-angle camera (to provide context) and a narrow-angle camera (for higher resolution). The wide-angle camera captures broad scenes, such as the entirety of Saturn and its rings from more than a million miles (1.5 million km) away, while the narrow-angle camera provides higher-resolution images of specific targets, such as craters or fractures on the surfaces of Saturn’s moons.

The two cameras are similar in some ways to digital cameras on Earth. The wide-angle camera uses lenses, while the narrow-angle camera uses mirrors, to focus light on a detector called a Charge-Coupled Device (CCD) that converts light into a digital image made of pixels. But while today’s cameras on Earth have detectors with dozens of megapixels, Cassini’s cameras -- built in the early 1990s -- are closer to 1 megapixel.

Cornell University planetary scientist Peter Thomas, a member of the imaging team who studies Saturn’s moons with Cassini’s cameras, said the cameras are far more carefully calibrated than Earth cameras and were built to survive more than 10 years in the harsh conditions of outer space. The cameras were built so well, in fact, that they continue capturing mind-blowing images to this day, nearly 20 years after the spacecraft left Earth.

Rhea and Titan
Rhea in front of Titan.

Cassini’s cameras provide far more information than consumer-level digital cameras because they’re sensitive to all wavelengths of light between the near-ultraviolet and the near-infrared, a range greater than typical visible-light cameras. Each camera has a variety of color filters mounted on wheels that select the range of wavelengths, or color, for the camera to record with each image.

Using the different filters helps researchers investigate specific aspects of the Saturn system. For example, by using a filter that blocks all but ultraviolet light, researchers can study high hazes in the atmospheres of Saturn and Titan. Or, using a filter that blocks all but certain infrared wavelengths, researchers can peer through the obscuring smog in the atmosphere of Saturn's moon Titan to reveal its surface. Thomas said the imaging team was pleased that some of Titan’s surface was visible in the near-infrared. “We weren’t totally confident beforehand that we’d be able to do that,” he says. “But the filters were well-chosen.”

Cassini’s cameras help researchers study the structure and motion of Saturn’s and Titan’s atmospheres. They also study the size, thickness, composition and other characteristics of Saturn’s rings, and how Saturn’s moons and rings interact through gravity.

By taking lots of pictures, the cameras create maps of the moons’ surfaces, Saturn’s rings and the planet itself. Researchers also use the images to understand the moons’ topography (the heights and shapes of features on the surfaces), composition (what materials those features are made of), and to measure the orbits, shapes and sizes of Saturn’s moons with exquisite precision.

The cameras were included on the Cassini spacecraft for scientific purposes, but the bonus is that Earthlings get to see Saturn’s exotic beauty closer, clearer and sharper than we ever could from Earth. Cassini’s cameras capture views in color by taking three images, each with a different color filter, which are then combined back on Earth. The resulting images show us Saturn as our eyes would see it were we actually there, silently orbiting Saturn alongside Cassini.

In addition to scientific observations and beautiful spacescapes, Cassini also uses the narrow-angle camera for navigation (the wide-angle camera is also capable of doing the job if needed). A spacecraft navigating in outer space is similar to a backpacker navigating in the wilderness, but instead of landmarks, a map and a compass, the spacecraft uses the known positions of Saturn’s moons along with a map of the stars to determine its position based on its images. “It’s a crucial item for tweaking the orbit,” Thomas said.

Illustration of spacecraft with ISS called out.

At a Glance

  • Wide Angle Camera [WAC](20 cm f/3.5 refractor; 380-1100 nm; 18 filters; 3.5ox3.5o)
  • Narrow Angle Camera [NAC](2 m f/10.5 reflector; 200-1100 nm; 24 filters; 0.35ox0.35o)
  • Mass (current best estimate) = 57.83 kg
  • Peak Operating Power (current best estimate) = 55.90 W
  • Peak Data Rate (current best estimate) = 365.568 kilobits/sec
  • Dimensions (approximate) = 95x40x33 cm (NAC); 55x35x33 cm (WAC)
  • Mass (current best estimate) = 57.83 kg
  • Peak Operating Power (current best estimate) = 55.90 W
  • Peak Data Rate (current best estimate) = 365.568 kilobits/sec
  • Dimensions (approximate) = 95x40x33 cm (NAC); 55x35x33 cm (WAC)

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