# Running The Gravity Assist Mechanical Simulator

O K ,   I T ' S   A   M A G N E T I C   A S S I S T ,   B U T   W H A T ' S   T H E   D I F F E R E N C E ?

( E I T H E R   W A Y ,   I T ' S   T H E   P L A N E T ' S   M O M E N T U M   Y O U   N E E D . )

1. Clean the glass "ecliptic plane" and the BB "spacecraft" really well.

2. Turn off the motor and move Jupiter out of the way. Adjust everything so you can run the BB on a repeatable "default trajectory" in which the ball passes by Jupiter's "orbit" disk and ends up in the left side pocket every time.

3. Set Jupiter revolving. Adjust it to go just a little faster than the BB's natural speed along its default trajectory.

4. Run lots of BB launches. Make small adjustments in Jupiter's speed, and in the ball's launch-timing. Run several launches for each adjustment. Record the results:

• No assist achieved

• Collided with Jupiter (BB stuck to magnet)

• Assist achieved, but inward toward Sun

• Assist achieved, outward toward Saturn

• Assist achieved all the way to Saturn's orbit!

When an assist is observed, where does the energy come from to boost the BB higher up the inclined glass surface? The Jupiter magnet connects its revolving motion to the BB, as the BB rolls by. The BB picks up motion from Jupiter. If the BB rolls by a little faster than the Jupiter magnet's "escape velocity," then the boost is successful. If less, the BB "crashes" into Jupiter and sticks to the magnet. Of course the motor supplies the revolving energy for the Jupiter magnet, while the real Jupiter draws upon its huge primordial momentum as it coasts along in orbit.

Finer points: At the top of this page is the question, "...what's the difference?" Only a subtle difference. The simulator's Jupiter and spacecraft are of similar mass, and the magnet does all the pulling. As it does, the Jupiter magnet feels a backward tug as it supplies the energy to accelerate the BB. In interplanetary space, the scale is so much larger that gravitation makes the connection, Jupiter pulling on spacecraft and spacecraft pulling on Jupiter. Another fine point is that if you were to place an accelerometer on the BB, it would register acceleration as the boost takes place on the simulator. This would not be true on a spacecraft. A spacecraft feels nothing but continuous freefall as it picks up a large increment of speed during a gravity assist. This is because there is basically no gravity gradient across the spacecraft or its accelerometer.

The Video Webcast linked from the Gravity Assist Primer page fills in many of the quantities and formulae touched upon here.