Illustration of lightning at Saturn

This artist concept shows how Cassini is able to detect radio signals from lightning on Saturn. Lightning strokes emit electromagnetic energy across a broad range of wavelengths, including the visual wavelengths we see and long radio wavelengths that cause static on an AM radio during a thunderstorm. Some of the radio waves propagate upwards and can be detected at long distances by the radio and plasma wave science instrument on Cassini.

How It Works

The Radio and Plasma Wave Science, or RPWS, instrument detects radio and plasma waves, as well as the plasma medium through which Cassini passes, using a suite of antennas and sensors. 

How We Use It

The instrument uses three types of sensors to study the intensity of waves across a broad frequency range, and the density and temperature of plasma in the magnetic bubble surrounding Saturn -- its magnetosphere. The instrument can even "hear" dust hitting Cassini and lightning in Saturn’s atmosphere. By studying radio and plasma waves around Saturn, scientists can better understand Saturn’s relationship with its moons and rings, as well as how the planet interacts with the solar wind. For example, Saturn's auroras emit radio waves in approximately the same frequency range as AM radio stations on Earth.

Saturn is a favorite target for amateur and professional astronomers alike, for its beauty and because on a clear night, the planet and its rings are visible to anyone with a small optical telescope.

But with radio telescopes, Saturn is difficult to see from Earth. “It’s invisible,” said Bill Kurth, a University of Iowa physicist and member of Cassini’s Radio and Plasma Wave Science team. “We can’t listen to Saturn radiation from Earth because our ionosphere blocks it.”

The ionosphere is what allows amateur radio operators on Earth to chat across different continents when the curvature of Earth should block communication — their radio signals bounce off Earth’s ionosphere like a racquetball off a concrete wall. And just as radio waves from Earth bounce back down, waves above Earth’s ionosphere bounce back out. The radio and plasma science instrument was included on the Cassini mission because it provides an up-close look at Saturn’s invisible environment that is impossible to see from Earth.  Saturn’s radio emissions provide an excellent way to know when Saturn’s auroras are bright without needing to take images of the auroras.

Data from the radio and plasma wave instrument show that the variation in radio waves controlled by the planet's rotation is different in the northern and southern hemispheres. Moreover, the northern and southern rotational variations also appear to change with the Saturnian seasons and the hemispheres have actually swapped rates. These two radio waves, converted to the human audio range, can be heard and seen in the new video.

The instrument’s electrical field sensor is one of the most prominent pieces of hardware on the Cassini spacecraft. Its three antennas are each 33 feet (10 meters) long and stick out like an insect’s feelers to detect radio waves rippling through the Saturnian environment.

The instrument also detects plasma, a “soup” of super-hot ions (atoms that have lost one or more electrons) and free electrons that have been stripped from atoms. Plasma races from the sun in the solar wind but is also captured by magnetic fields. So by studying plasma around Saturn and Titan — where it’s dense or thin, and where it’s hot or not — researchers can sense Saturn’s magnetosphere and watch how it moves.

The spacecraft also determines where and how much dust is orbiting between Saturn’s moons and rings, and it does so by touch. “We’re talking  dust particles the size of those in cigarette smoke, but there’s enough kinetic energy when that dust hits the spacecraft that it destroys the dust particle and a little part of the spacecraft.” The potent but miniscule collisions produce tiny puffs of plasma, which the instrument detects. When the dust impacts are converted to sound, the spacecraft’s passage through a dust cloud sounds like being in an automobile in a hail storm.

Radio and Plasma Wave Science

At a Glance

  • Mass: 6.80 kg
  • Average Operating Power: 7.00 W
  • Average Data Rate: 0.90 kilobits/s

For more information, see the RPWS Engineering and Technical Write-up.

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