We have arrived! Cassini now orbits Saturn, with our sensor onboard:

The Cassini RPWS Langmuir Probe

After seven years in space, Cassini went into orbit around Saturn on July 1, 2004. The first data obtained at Saturn and Titan by the Langmuir probe, designed and built at the Swedish Institute of Space Physics in Uppsala, look very nice and exciting, particularly in the context given by the Huygens probe at its succesful mission in January 14, 2005.

NEWS

Saturn's magnetopshere does not corotate. (Sep 13, 2005) Not all the way in, that is: far out, it does corotate with the planet, as it should because of the magnetic field. However, further in among the rings, our data shows that the plasma moves rather more like the rings themselves, indicating a strong dust-plasma coupling. See IRF press release (in Swedish) and paper in Geophysical Research Letters.

Latest news on Titans ionosphere! (May 13, 2005) The journal Science today published our results from the first Titan flybys. One of the findings is that about a kilogram of Titan's atmosphere is lost every second. See also IRF press release (in Swedish).

RPWS/LP data access (Password required)

The Cassini/Huygens project

Cassini/Huygens, a joint NASA/ESA mission launched in October 15, around 10:43 Swedish time, 1997, is the most ambitious human planetary expedition ever to the outer giant planets. With a size as a bus, a weight of 5.8 tons, and equipped with more than 20 scientific instruments it aims to investigate in detail the planet Saturn and its many icy moons in the years 2004-2008. In particular, the large moon Titan and its thick atmosphere is the main target of the Cassini/Huygens mission, where a small spacecraft (Huygens) will be detached from Cassini and descend through Titan's atmosphere in January 2005. In the left picture above the main spacecraft Cassini is depicted passing over the Saturnian moon Titan, dropping the probe Huygens (to the left and below the Cassini spacecraft body) into the atmosphere of Titan.

Cassini has now arrived at Saturn, and went into orbit around the ringed planet on July 1, 2004. The spacecraft has used the planets Venus (twice), Earth and Jupiter to gain speed and thereby shorten the trip to Saturn to merely seven years. The project planning started in 1982, and we began the planning for our instrument in 1989. This illustrates the necessary time span for planning of these type of missions.

Follow the links above to read more about Cassini/Huygens.

The Cassini Langmuir probe

So what have we done for Cassini, and what do we want to do in the coming years?

We have built a kind of "cosmic weather station" called a Langmuir probe, to study the very tenuous gas of charged particles that we normally find in space. Hey, what, isn't space a vacuum? Well, with Earthly standards, it is, but the vacuum is not perfect: essentially everywhere, there is a tenuous gas of charged particles. Such a gas of free ions and electrons is called a plasma. The basic principle for measuring this plasma with a Langmuir probe is simple: put a sphere (the probe) on some sort of boom on your spacecraft, apply a positive potential to the probe, and measure the current that flows from the spacecraft to the probe. As we put the probe at a positive potential, the current is due to collection of free electrons from the plasma. The higher the current, the more free electrons there must be around. Voila, we have measured the density! By some more intricate means, we can get the temperature as well, and sometimes also the plasma flow speed, i.e. the "wind speed", and the mean ion mass. Hence, we have a weather station in space, observing about the same things that Earthbound weather observatories do: temperature, wind speed, and pressure (which essentially is density times temperature).

Simple as this principle may sound, there are of course a lot of difficulties. Constructing electronics for low-noise measurments of currents down below the nanoamp level is a challenge in itself, and in addition there are a lot of factors complicating the elegant principle above. We have have long experience of building this kind of instruments for many spacecraft (see our group home page). Still, the design and construction of the Langmuir probe instrument spanned some eight years from first conception to final launch! Above the photo at right shows our Langmuir probe on Cassini, goldish because its surface layer of titanium nitride.

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What we want to do

OK, but what do we want to do, now that we have spent so much time on building this instrument?

Our main scientific interests are the tenuous plasmas found around Saturn and in the upper atmosphere of the moon Titan (see above for what a plasma is).

Saturn's magnetosphere is a gigantic object, due to the strong magnetic field from the planet and the low pressure from the solar wind. It spans all of the ring system and most of the moons. This magnetosphere is in some ways similar to the magnetosphere of the Earth, but in other respects different. To learn exactly where they differ and where they look the same is important: this can tell us something about what we can expect around other magnetized objects in space. For us, the distribution of plasma in the Saturnian magnetosphere is one of the most interseting things. We already have very nice data from the first ring plane crossing: the interaction of the ring material with the plasma is of course one thing we want to dwell further on.

Titan's ionosphere, i.e. its uppermost, ionized, atmospheric layers, is a fascinating place. Titan's atmosphere has a complex organic chemistry, with the possibility for complicated reactions, perhaps creating large organic molecules, in the plasma layers at top of the atmosphere. Studying this plasma during the many Cassini flybys of Titan is perhaps what we most look forward to do. From the first flybys, we have learnt that Titan's ionosphere is eroding at a rate of around a kilogram per second (paper in Science, May 13, 2005), and there are lots of other fun stuff to dig out from the data.

Learn more about Cassini's investigations of Saturn's magnetosphere and Titan at the JPL Cassini web site.

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The Radio and Plasma Wave Science investigation

We participate on Cassini as a co-investigator (CoI) team within the Radio and Plasma Wave Science (RPWS) investigation, led by the Department of Physics and Astronomy at the University of Iowa.. RPWS includes of three different kinds of sensors: the three main radio antennas, a search coil magnetometer, and our Langmuir probe. RPWS have already provided very nice data: for example, you can listen to the sounds heard by RPWS during the ring plane crossing in this QuickTime animation at the JPL Cassini site.

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To Space Plasma Physics research programme
To Swedish Institute of Space Physics, Uppsala

Last modified Tuesday, 21-Apr-2009 11:14:14 CEST by Anders.Eriksson@irfu.se