GO IN DEPTH

# Gravity Assist Simulator

On Earth, a "collision" occurs when two objects hit each other. To physicists—particularly those working on the New Horizons and Messenger missions—a "collision" is when two objects in space get close enough to each other that each one's speed and direction are changed by the other's gravitational pull. How does each one's speed and direction change? It depends on each one's size and how they are moving relative to one another. The Gravity Assist Simulator takes you through a series of simulated collisions to show how the New Horizon and Messenger spacecraft will use planned collisions to extend their missions.

Part 1 of Gravity Assist simulates a familiar kind of collision—between a ping-pong ball and a wall. Observing the motion of the ball in four different situations—with a non-moving wall and with a wall moving in various directions—will help you understand the next three parts, which simulate collisions between spacecraft and planets.

Part 2 shows an idealized collision between a stationary planet and an approaching spacecraft. (It is "idealized" because there is no such thing as a stationary planet!) This gives you a visual introduction to the type of collision NASA is interested in.

Parts 3 and 4 simulate the planned collisions that will "slingshot" New Horizons deeper into space, and slow Messenger in preparation for a safe orbit around Mercury.

For Educators

Starting from the main menu, students can go directly to Part 1, 2, 3, or 4, or click BEGIN to see all four in order. Part 1 consists of four separate simulations. Students can select one, view the simulation, and then click SCENARIO MENU to go on to the next one. Clicking GO TO PART 2: STATIONARY PLANET FLYBY BASICS takes students to Part 2. In each simulation, students can click MAIN MENU in the upper-left corner of the simulation window, or LEARN MORE to see a text display with instructions and more information.

You can use this tool as part of a discussion of astronomy, or one about the physics of motion and kinetic energy. If possible, display the Gravity Assist Simulator on a large screen and walk through it with the class. The following questions can help you guide the discussion.

PART 1: This part shows how the speed and direction of a ping-pong ball change when it collides with a wall. The first simulation should be easily predictable. Ask students, "What happens when the wall is moving in the same direction as the ball or in the opposite direction, slow or fast?" Challenge students to predict how the ball's speed and direction will change before you start each simulation.

PART 2: This simulation shows what could happen to the speed and direction of a small object (like a spacecraft) when it passes by a large planet. The object does not make contact with the planet, but it is affected by the planet's gravity. The simulation shows three possible paths. Ask students, "Which path do you think will produce the greatest change in the object's speed and direction? Which of the simulations from Part 1 does this one most resemble?" (The first one, because the large mass is stationary.)

PART 3: This simulation shows how the New Horizons spacecraft will aim for a collision with Jupiter, and use that planet's gravity to slingshot on toward Pluto. Again: "Which of the Part 1 simulations does this one most resemble?" (The fourth one, because the small object speeds up but its direction is not radically changed.)

PART 4: This is a simulation of Messenger's planned braking maneuver. "Which Part 1 simulation is this one closest to?" (The third one—the only one in which the small object slows down after the collision.)

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