• Gravity Assist Landing Screen:

Welcome to the Gravity Assist Simulator. Here you will explore gravity assist maneuvers by first learning about collisions, a related concept that is a little bit closer to home! Of course spacecraft don't actually collide with planets in order to change their speed and trajectory; but they do interact through gravity, and the idea of a collision helps us to understand the effects of that interaction. Explore the simulated encounters of the New Horizons spacecraft with Jupiter and the MESSENGER spacecraft with Venus. How does the distance of closest approach affect the speed and trajectory of such encounters? Check it out...


Navigate through this simulator using the "main menu" link in the upper left corner, or use the navigation panel along the middle of each Part. Don't forget to explore the "learn more" sections, accessible via a link in the upper right corner of each Part.

• Part 1 - Elastic Collisions - Introduction:

Part 1 explores elastic collisions using four different scenarios. In each scenario, note which is moving faster before and after the collision: the ball or the wall? Don't forget to "learn more" in the upper right corner!

Scenario 1: a ping pong ball collides with a stationary wall. The speed of the ball before the collision is the same as the speed after the collision, however the direction of travel has changed; the ball is traveling in the opposite direction after the collision. The wall does not move since it is so much larger than the ball.
Scenario 2: the ping pong ball and the wall are moving in opposite directions toward one another. The speed of the ball increases with the collision and the direction also changes. The wall does not change speed or direction since it is so much larger than the ball.
Scenario 3: the ping pong ball and the wall are moving in the same direction and the ball is moving faster than the wall so it overtakes the wall. The speed of the ball decreases with the collision and the direction also changes. The wall does not change speed or direction since it is so much larger than the ball.
Scenario 4. the ping pong ball and the wall are moving in the same direction, but in this scenario the wall is on the opposite side of the ball and moving faster. When they collide the speed of the ball is increased and it does NOT change direction. Meanwhile the speed and direction of travel of the wall remains unchanged since it is much larger than the ball.

• Part 2 - Stationary Planet Flyby Basics:

If you were standing on Jupiter we would say you were in the reference frame of the planet, and it would appear to be stationary- just as the Earth appears to be stationary when we are within its reference frame. In Part 2 you will explore how the trajectory and speed of a spacecraft would change if observed from the planet's reference frame. You can select one flyby distance at a time, all three at once, or any combination thereof!

In this interactive, a planet remains stationary while a spacecraft passes by at three different distances. The force of gravity between the planet and the spacecraft changes both the speed and direction of the spacecraft. This is a "collision" similar to those observed in Part 1, even though the spacecraft and planet do not actually touch. The planet remains unchanged by the collision because it is much, much larger than the spacecraft. There are three different distances of closest approach in this part of the interactive. The speed and direction of the spacecraft are changed most during the flyby that is closest to the planet.

• Part 3 - New Horizons Jupiter Gravity Assist:

Indeed, spacecraft really use the planet's gravitational attraction to change their speed and direction. The New Horizons spacecraft, on a long journey to Pluto, used the gravity of Jupiter for just such a maneuver. See a simplified version in Part 3. Again, you can select one distance of closest approach, all three at once, or any combination thereof.

This part of the interactive is similar to the last (Part 2), except now the planet is moving as the spacecraft flies past. The planet Jupiter is moving much faster that the spacecraft relative to the Sun, so it pulls the spacecraft, causing it to speed up as it flies past. Again you will notice that the closer the spacecraft is to the planet as it passes the greater the boost in speed it receives.

• Part 4 - MESSENGER Venus Gravity Assist:

The MESSENGER spacecraft used the gravity of Venus to change its speed and trajectory on its journey to Mercury. Observe how each of the distances of closest approach changes the speed and trajectory of the spacecraft. Don't forget to visit the "learn more" link in the upper right corner.

Venus is moving in this part of the interactive as the spacecraft flies past. Only now the flyby causes the spacecraft to slow down instead of speed up. And just as in the previous parts of the interactive, the smaller the distance of closest approach, the greater the change in the speed of the spacecraft.

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