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Cassini probes outer edge of Saturn's atmosphere

Morgan Rehnberg, Correspondent
May 8, 2015, 23:55 UTC

Sen—This week the Cassini spacecraft, in orbit about Saturn since 2004, has been observing the planet's troposphere, the highest, thinnest part of the planet's atmosphere.

Although this part of the atmosphere might not be visible to the naked eye, it could play a vital role when the Cassini mission comes to an end in 2017.

Here on Earth, our troposphere is so high that it extends hundreds of kilometers above the orbit of the International Space Station.

How could such a thin atmosphere affect the bus-sized Cassini? Even a little gas exerts drag on objects passing through it and the faster that object is travelling, the more drag it experiences. Spacecraft travel incredibly fast (Cassini’s current speed is nearly 23,000 kph) and therefore feel significant drag from even the thinnest of atmospheres. The drag felt by the ISS, for example, is enough to require monthly orbital boosts to avoid eventually spiralling down to Earth.

When Cassini approaches close to the planet at the end of its mission in 2017, the drag it experiences will hasten its demise. Observing the troposphere now will allow scientists to better understand its density and thus predict the drag it will cause.

But the thermosphere is not just thin, it is also the hottest region around Saturn. Energetic radiation from the Sun is absorbed by these lofty atoms and molecules which, once warmed up, emit the light observed this week by Cassini. Stick your hand outside, though, and it would not burn, but freeze! That is because while each particle has a lot of energy—and thus a high temperature—there are not many of them to strike you and transfer their heat.

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.