Slingshot continued...

Excerpt from the above pages: Starting out from a low Earth orbit, a spacecraft needs to increase its speed by 9 kilometers per second (19,440 mph) in order to reach Jupiter. Navigators refer to a needed speed change as "delta V," where "delta" indicates "change" and "V" stands for velocity.

Keep in mind, though, that Jupiter's orbit about the Sun doesn't lie in the same plane as the Earth's, so a spacecraft going to Jupiter would have to move out of the plane of the ecliptic. This is known as a "broken-plane" maneuver. Couldn't the spacecraft go "directly" to Jupiter without having to make the broken-plane maneuver? Yes, but that usually means that the spacecraft needs to be going even faster to begin with - around 11 km/sec.

By comparison, Galileo's Venus-Earth-Earth Gravity Assist (VEEGA) trajectory required that the spacecraft provide a delta-V of only 4.094 km/s to reach Jupiter. Of this total, 4 km/s was provided by the IUS booster; the other .094 km/s of delta-V came from Galileo's thrusters (the spacecraft also produced an additional 100 meters/sec of delta-V that was used to for science purposes on the way to Jupiter, e.g. for asteroid flybys). The additional delta-V needed to get to Jupiter was provided by the planetary flybys (2.0 km/sec (4,320 mph) from Venus, 5.2 km/sec (11,600 mph) from the first Earth flyby, 3.7 km/ sec (7,992 mph) from the second Earth flyby). Note that this doesn't add up to 9 km/sec total delta-V; that's because we're actually giving changes in velocity (which involves direction), not just speed, and velocity changes add as vectors.

As a bonus, Galileo didn't have to perform a broken-plane maneuver - that was thrown in "for free" by the flybys.