Pluto via Jupiter
The New Horizons spacecraft is on its way to Pluto and has begun a leisurely encounter with the planet Jupiter that will alter the trajectory of the spacecraft to put it on course for its Pluto encounter in July 2015. The New Horizons team will have an opportunity to exercise their instruments as well as their procedures while taking some useful scientific data on the Jovian system. The reason for the flyby, however, is not scientific, but energetic. In order to get to Pluto, whose orbit is tilted 17 degrees to the orbit of the Earth which means that with the exception of the two times each Pluto orbit that it is crossing the Earth’s orbit, Pluto is either North or South of the Earth’s orbit. The Earth’s orbit matters because that’s where all spacecraft leaving Earth start out. Relative to the Sun, New Horizons, like all other interplanetary spacecraft, starts off moving with the Earth’s velocity around the Sun, plus the extra little bit that the rocket provides it. It takes a lot of energy to things moving in a different orbital plane, and this is where Jupiter can be so helpful.
Jupiter’s orbit is tilted less than 2 degrees relative to the Earth’s, so it’s fairly easy to get there. All you need is enough energy to get 5.2 AU from the Sun, where the Earth, by definition, is on average 1 AU from the Sun. New Horizons will fly by Jupiter in such a way that Jupiter’s gravity will deflect it toward Pluto. No extra energy is required because New Horizons will steal some of Jupiter’s energy. Jupiter can spare it, because while New Horizons will get a big change in its velocity, Jupiter’s velocity change will be smaller by the ratio of the mass of the New Horizons spacecraft to the mass of Jupiter, and that means about 4 million billion billion times less (or 4*10^24, or 4,000,000,000,000,000,000,000,000).
Information on New Horizons’ flyby of Jupiter can be found here. Closest approach is at the end of February, and observations of the giant planet continue from now until the end of June.