Space and Astrology
Pluto Facts And Measurements Including Pluto's Gravity
By nasa
3/13/04

From Pluto and Charon, edited by Alan Stern and Dave Tholen, University of Arizona Press (chapter by Dave Tholen and Marc Buie).

Pluto's Orbit. Here is a diagram showing Pluto's orbit with the orbits of all the other planets in the solar system. (The figure comes from an excellent introductory astronomy textbook - Cosmic Perspectives). You may also be wondering why Pluto does not collide with Neptune, since their orbits are close. This is because their orbits are in resonance. Pluto orbits the Sun twice for every 3 orbits of Neptune. As a result, Pluto and Neptune are never very close to each other. Here is a diagram which illustrates the resonance of their orbits. The upper diagram is the usual map of the solar system looking down from above the Sun. The lower diagram has been made by keeping Neptune and the Sun fixed - the planets are then allowed to follow their orbits over thousands of years. You will see Pluto's eccentric orbit maps out a strange, curly path - but never comes close to Neptune. (From the chapter by Malhotra and Williams in the book Pluto and Charon, edited by Alan Stern and Dave Tholen).

Pluto's gravity. Since the mass of Pluto is 1/455 that of Earth, you might first guess that the gravity would be 455 times less. But you also need to take into account the fact that the size of Pluto is 1/5.5 times less than the Earth so that when you stand on Pluto you are 5.5 times closer to the center of mass. Since the force of gravity decreases with the square of the distance from the center of mass, the gravity on Pluto will be changed by a total of (5.5 x 5.5) / 455 = 0.07 or 1/15. This means that a person on Pluto would weigh 1/15 what they weigh on Earth (nice way to lose weight - but a chilly way to do it!). For comparison, the astronauts on the Moon weighed 1/6 what they weighed on Earth.