Saturn

When the revolutionary Voyager-1 and Voyager-2 space missions launched in 1977 bound for Jupiter and Saturn, their planned trajectories took them outward toward their target planets. Cassini was not able to follow this path.

Cassini-Huygens is a massive spacecraft. It is carefully designed to brake into Saturn's orbit, as well as being loaded with an array of powerful instruments, cameras and sensors that will optimize the exploration of Saturn's vast, distant system.

No existing launch vehicle could have sent the 6000-kg craft directly to Saturn.

The mission designers found that a technique called "gravity assist" was the answer. Gravity assist works because of the mutual gravitational pull between a moving planet and a spacecraft. The planet, of course, pulls on the spacecraft. But the spacecraft's own mass also pulls on the planet. This permits an exchange of energy. For a detailed explanation of how the gravity assist technique works, you can read more about it in the Gravity Assist Primer.

Cassini Trajectory Movie

Cassini-Huygens has now looped around the Sun twice. On the first loop it flew close behind Venus in its solar orbit, where it "stole" some of the planet's orbital momentum on April 26, 1998. The next loop provided a second flyby of Venus, on June 24, 1999, and one of Earth, on August 18, 1999. Given these three "gravity assist" boosts, Cassini-Huygens finally had enough orbital momentum to reach the outer solar system. One last gravity assist from Jupiter on December 30, 2000 gave Cassini-Huygens the final thrust of energy it needed to project itself all the way to Saturn.

Gravity assist has often been called the "slingshot effect," but in reality, it is a different example altogether. Before letting go, a person wielding a slingshot whirls a projectile around and around, adding strength and refining aim each time, before letting go. The power comes from the thrower's muscles.

On the other hand, a spacecraft can obtain a gravity assist boost because the planet and the spacecraft tug on one another while orbiting the Sun. The spacecraft makes the planet lose some of its orbital momentum. From a planet's point of view, the spacecraft departs with no net energy gain, but from the Sun's point of view, the energy exchange makes a very small change in the planet's solar orbit. That amount of energy, though, benefits the tiny spacecraft substantially. The power comes from the planet's motion about the Sun.

Cassini Interplanetary Trajectory

Once Cassini-Huygens reached Saturn in July of 2004, the spacecraft began to fall toward the massive gas giant. At just the right moment, Cassini fired its main rocket engine for about 95 minutes to slow down. Instead of simply whizzing by, the spacecraft became forever trapped in orbit like one of Saturn's moons. The Huygens probe separated from the Cassini orbiter and descended into Titan's murky atmosphere, while the orbiter examines Saturn's fascinating realm as it repeatedly loops around the planet.