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Frequently Asked Questions - Technical Information

Frequently Asked Questions - Technical Information

Where can I find information on Cassini's instruments?

For descriptions of the instruments aboard the Cassini Orbiter, visit:

For instruments aboard the Huygens Probe, see

You'll also find links to web pages maintained by some of the science teams responsible for the instruments, and to the European Space Agency's Huygens web site.

The Cassini spacecraft carries three radioisotope thermoelectric generators. Where can I find information about them?

You'll find what you are looking for at The quick fact sheet in the "Cassini Launch" press kit has information on the RTGs, and there is more information at the end of the document. Also check "Spacecraft Power for Cassini," which is listed under "Other Fact Sheets" on the same web page.

How long does it take for transmissions from Cassini to reach Earth?

Traveling at the speed of light, radio signals from Cassini will take one hour and 24 minutes to reach Earth when Cassini arrives at Saturn. As Saturn and Earth move in their orbits around the Sun during the course of Cassini's four-year tour, the distance between the two planets will vary and the "one way light time" will change accordingly. To read more about time's impact on communications between Cassini and Earth, visit

What is the power margin and remaining fuel as well as the fuel usage rate on the Cassini spacecraft?

As of January 2010, Cassini has about 680 watts of power available to operate both the engineering subsystems and science instruments. The radioisotope thermoelectric generators that provide the spacecraft’s electricity lose about 9 watts a year, so about 671 watts will be available in January 2011. At launch in October 1997, there were 878 watts available. The rate of power loss was greater in the first few years, due to the expected Silicon-Germanium terminal degradation.

There are two types of propellant on Cassini: mono-propellant and bi-propellant. The mono-propellant, hydrazine, is used for small maneuvers, dead-band control, and for reaction wheels momentum management. The bi-propellant, mono-methyl hydrazine and nitrogen tetroxide, is used for big maneuvers (those larger than 0.3 meters/second). Cassini started with 132 kilograms of hydrazine and has about 68 kilograms remaining. The spacecraft also started with 1,131 kilograms of mono-methyl hydrazine and 1,869 kilograms of nitrogen tetroxide. The spacecraft is now down to 76 kilograms of mono-methyl hydrazine and 126 kilograms nitrogen tetroxide.

The bulk of the bi-propellant was used to get Cassini to Saturn, into orbit around Saturn, and then to adjust the spacecraft’s closest distance from Saturn as Cassini settled into orbit. The propellant usage rate is totally dependent on how the trajectory is shaped, since the spacecraft’s flight path is mostly governed by the gravity of Saturn and its moons, especially that of the massive moon Titan. The flight team keeps propellant use to a minimum, using just a few hundred grams every month depending on the demands of a given orbit. Even with less than 10 percent of the bi-propellant remaining, Cassini is expected to be able to continue its mission for at least eight more years.