Why does Galileos speed change? Why doesn it stay constant?
On June 29, 1995, Galileo was travelling at roughly 25,900 kilometers per hour relative to the Sun, but this speed isn’t constant — the spacecraft is actually slowing down. Since the spacecraft is essentially in an elliptical orbit about the Sun, its speed naturally drops as it moves away from the Sun (if not for Jupiter’s influence, it would speed up as it started moving back towards the Sun). You notice the same behavior when throwing a long, high fly ball — the ball slows down and “hangs” at the top of its path. In addition, as Galileo draws very near to Jupiter, its speed is changed quite dramatically by the gravity of the giant planet. However, this effect won’t be significant until a few days before arrival at Jupiter (on December 7, 1995, and subsequently as the spacecraft draws near each encounter).
