My question relates to travel times of the Galileo and Cassini space probes to the planets. My question is why did Galileo take so long (6 years) to reach Jupiter? It was launched in 1989 and reached Jupiter in 1995. Cassini was launched in 1997 and reached Saturn in 2004. That's 7 years travel time to Saturn -- why so long? Pioneer 10/11 only took a couple of years to reach Saturn as did Voyager 1/2- launched in 1977 reaching Saturn in 1980/81. So why did it take Cassini twice as long to get there? Was the booster that launched it less powerful than Voyager's? Or is it because the planets were aligned in 1977 and the distances were smaller?
An excellent question with an interesting answer!
The reason is that the Pioneers and Voyagers followed direct trajectories, while Galileo and Cassini followed more circuitous routes.
The Pioneer and Voyager spacecraft launched from Earth and took a direct orbit to Jupiter. This was originally the plan for Galileo and Cassini, as well, but to do this requires that the spacecraft be boosted by a large rocket. After the Challenger disaster, NASA would not allow the large rockets of Galileo and Cassini to be carried in the Shuttle Cargo Bay, ruling out a direct trajectory approach.
Galileo launched with a smaller rocket aboard the Space Shuttle Atlantis. Cassini was launched using a Titan IVB launch vehicle, also carrying a smaller rocket than originally planned.
Interestingly enough, it was lessons learned from Voyager that allowed Galileo and Cassini to reach the outer Solar System using less fuel. The Voyager missions perfected the technique of using a close encounter with a planet to boost the spacecraft's speed.
The spacecraft travels a path just behind the planet in its orbit, and the planet's gravity pulls the spacecraft ahead so that it can "slingshot" away, carrying some of the planet's orbital energy. (The planet, being much larger than the spacecraft, has enough orbital energy that losing a tiny amount won't affect it much at all.)
Galileo followed a VEEGA trajectory: Venus Earth Earth Gravity Assist. A close encounter with Venus and two close encounters with Earth gave it enough energy to reach Jupiter--though its journey required six years instead of two or three. The spacecraft's instruments were mostly dormant during its cruise, though, so it didn't significantly impact the spacecraft's mission lifetime.
Cassini's VVEJGA trajectory relied on gravity assists from Venus, Earth, and Jupiter. It arrived at Saturn in 2004.
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