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Frequently Asked Questions - Trajectory

Frequently Asked Questions - Trajectory

How long would it take for a spacecraft to travel to Saturn using a Hohmann transfer orbit? Why was this not used for the Cassini mission to Saturn?

A Hohmann transfer orbit is like the orbit of a planet with perihelion at Earth's orbit and aphelion at Saturn's orbit. Therefore, the transfer time would be half a year of a (fictitious) planet with a semimajor axis close to the average for Earth and Saturn. That average is 5.13 AU, so the transfer time would be close to half a Jupiter year, or just under six Earth years. We didn't use this method because no existing launch rocket could give Cassini-Huygens enough energy to fly to the distance at which Saturn orbits the Sun. Instead, we had to use a gravity-assist trajectory. To learn more, visit "Basics of Space Flight" at

Why did Cassini-Huygens take such a roundabout path to Saturn? Why didn't it just fly straight there?

Cassini-Huygens is quite massive. Even the most powerful launch rockets we have today could not give the spacecraft enough speed to enable it to counter the Sun's pull and reach Saturn. So Cassini-Huygens took a "gravity-assist" route, in which it passed by Venus (twice), Earth, and Jupiter in such a way that the gravity of those planets gave the spacecraft an extra boost, increasing its speed enough to travel to Saturn. A little bit of the momentum that the planets had in their orbit around the Sun was actually transferred to the spacecraft -- not enough to make a difference to the planets, but enough to propel Cassini-Huygens to Saturn! For further explanation, visit or