Ballistic Flight

The first manned Mercury spaceflight, launched by the United States in 1961, was ballistic. A rocket carried the space capsule into space. Forty-two seconds after liftoff, the rocket shut down, and the capsule separated from it. The cap­sule’s momentum carried it on upward. The capsule did not go into orbit around Earth. Gravity slowed its upward flight until, at a height of 118 miles (190 kilometers), it stopped climbing and began falling. Parachutes opened to slow the capsule’s fall before splashdown in the Atlantic Ocean.

THE BALLISTA

The word ballistic comes from a Greek word meaning "to throw." A weapon called the ballista was invented in ancient Greece in about 400 b. c.e. It was a throwing machine, like a huge crossbow. It hurled heavy stone balls or spears.

A spacecraft coming back to Earth can follow a ballistic trajectory, which simply means that it falls through the atmosphere. A ballistic reentry is uncomfortable for astronauts, however. The strong forces produced by drag slow the spacecraft down suddenly, like a car braking hard. Also, once the spacecraft begins its return to Earth on a ballistic reentry, it cannot be steered toward a particular landing area.

Another way to return to Earth is to use the spacecraft’s shape to create lift instead of letting it fall back to Earth. A spacecraft produces lift simply by tilt­ing, like a kite. As it plunges through the atmosphere, its angle to the oncoming air lifts it up.

A spacecraft returning from space in this way slows down more gently than in a ballistic reentry. By changing the amount and direction of its tilt, the
spacecraft can then be steered through the atmosphere toward a chosen landing site. The Space Shuttle comes back to Earth in this way.

SpaceShipOne, the first private spacecraft, uses a ballistic reentry. When its rocket engine shuts down after launch, at a height of 150,000 feet (45,720 meters), the spacecraft’s momen­tum carries it up another 150,000 feet (45,720 meters). From that point, SpaceShipOne falls back through the atmosphere. The pilot then retakes control and flies the spaceship like a glider to a landing on a runway.

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SEE ALSO:

• Gravity • Lift and Drag • Space­flight • Space Shuttle • Takeoff and Landing