Educator Features

Escape Velocity: Fun and Games
Delta IV rocket launch at night
Did you ever watch a group of children playing "Red Rover?" Arms linked up for strength, they chant, "Red Rover, Red Rover, let Sally come over," and Sally's challenge is to break through that chain of linked arms. If she does it, Sally wins.

Image to left: A Delta IV rocket launches at night. Credit: NASA

If Sally breaks through the chain of arms, she's also demonstrated several key aspects to the space concept of escape velocity. Escape velocity (or a rousing game of Red Rover) requires an object to propel itself with enough speed and thrust to break through a barrier. Sally's reward is the cheers of her teammates. A spacecraft's reward is a journey into space or orbit.

Escape velocity is the speed at which an object must travel to break free of a planet or moon's gravitational force and enter orbit. A spacecraft leaving the surface of Earth, for example, needs to be going about 11 kilometers (7 miles) per second, or over 40,000 kilometers per hour (25,000 miles per hour), to enter orbit.

An Endless Cycle

Saturn V rocket being prepared for launch
Achieving escape velocity is one of the biggest challenges facing space travel. The vehicle requires an enormous amount of fuel to break through Earth's gravitational pull. All that fuel adds significant weight to the spacecraft, and when an object is heavier, it takes more thrust to lift it. To create more thrust, you need more fuel. It's a cycle that scientists are hoping to resolve by creating lighter vehicles, more efficient fuels and new methods of propulsion that don't require the same ingredients to attain great speeds.

Image to right: A Saturn V rocket is prepared for launch. Credit: NASA

That cycle of speed, fuel and weight was a primary reason the Saturn V rocket that took the first astronauts to the Moon was so large. It required such enormous quantities of fuel to break free of the Earth's gravitational pull that a vehicle of this size was the only workable solution. The Space Shuttle in use now is much smaller, but it doesn't have nearly as far to travel or nearly as much gravitational force to overcome. Future space propulsion projects, such as magnetic levitation, could reduce size requirements because speed and propulsion will be created in a manner that doesn't require large fuel tanks.

Illustration of orbit and gravitational pull
Image to right: This illustration shows possible orbital paths. Credit: NASA

In astronomy, the term orbit refers to the path of an object whose motion through space is controlled by the gravitational pull of another object. The Moon orbits the Earth, and the Earth, in turn, orbits the Sun. Spacecraft can also orbit the Earth. If an object gains enough speed to attain escape velocity, its orbit becomes an open curve called a parabola. If it continues moving faster than escape velocity, its orbit is a flattened curve called a hyperbola. A spacecraft that leaves its orbit around the Earth on a journey toward another planet travels in a hyperbolic orbit.

Illustration of parabola and hyperbola
Image to right: This illustration shows a parabola and a hyperbola. Credit: NASA

Using Sally's "Red Rover" game as an example, think how much more easily she could break through the chain if she approached the line on turbo-charged roller skates or if she had a spear-shaped battering ram in front of her. Alternate methods of propulsion and maximized aerodynamics are two factors scientists are researching as they explore the possibilities for achieving escape velocity with less difficulty.