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Escape Velocity: Fun and Games

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

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.

Image to right: This illustration shows possible orbital paths. Credit: NASA
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