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Cassini spends week observing Saturn's magnetic personality

Morgan Rehnberg, Correspondent
Jun 26, 2015, 15:57 UTC

Sen—There is a whole world of material out in space hidden from the naked eye and this week NASA’s Cassini spacecraft labored to make it visible. Its tool: one of the most unique instruments in all of planetary science.

Plasma—gas so energized it has lost some of its electrons—is everywhere in space. Because each bit of plasma carries an electric charge, its motion is controlled by Saturn’s magnetic field. Positively- and negatively-charged particles spiral oppositely as they are dragged along by the field’s rotation with the planet in a region known as the magnetosphere.

From the Ion and Neutral Mass Spectrometer to its magnetometer, Cassini carries a range of instruments designed to measure the plasma and field directly around the spacecraft. As the spacecraft orbits Saturn, it can sample the plasma at different locations. 

But what if astronomers want to study the plasma far from Cassini? For that they need the Magnetospheric Imaging Instrument (MIMI), a device that, when it launched aboard Cassini in 1997, was the first of its kind.

One of MIMI’s many functions is to make images of the otherwise-invisible plasma trapped within the magnetosphere. To do this, it relies on a phenomenon known as energetic neutral atoms, which arise when a fast-moving plasma encounters a slow cloud of uncharged gas. When bits of energetic plasma strike atoms of this gas, they can steal an electron and become neutral themselves. Instantly, the now-neutral particles are no longer contained by Saturn’s magnetic field and shoot off into space along straight lines.

By capturing some of these particles and measuring their trajectory and velocity, MIMI can trace their origin and construct an image of where the plasma must be.

Why is Cassini making these remote observations? MIMI allows scientists to observe the interaction of plasma and gas in regions that might otherwise be too dangerous for the spacecraft to go, as well as observe how the large-scale distribution of plasma in the magnetic field changes with time.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.