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Space exploration
Getting into space and back

flying in the space

Overcoming gravity is the biggest problem for a space mission. A spacecraft must be launched at a particular velocity (speed and direction).

Gravity gives everything on Earth its weight and accelerates free-falling objects downward. At the surface of Earth, acceleration due to gravity, called g, is about 32 feet (10 meters) per second each second.

A powerful rocket called a launch vehicle or booster helps a spacecraft overcome gravity. All launch vehicles have two or more rocket sections known as stages. The first stage must provide enough thrust (pushing force) to leave Earth's surface. To do so, this stage's thrust must exceed the weight of the entire launch vehicle and the spacecraft. The booster generates thrust by burning fuel and then expelling gases. Rocket engines run on a special mixture called propellant. Propellant consists of solid or liquid fuel and an oxidizer, a substance that supplies the oxygen needed to make the fuel burn in the airlessness of outer space. Lox, or liquid oxygen, is a frequently used oxidizer.

The minimum velocity required to overcome gravity and stay in orbit is called orbital velocity. At a rate of acceleration of 3 g's, or three times the acceleration due to gravity, a vehicle reaches orbital velocity in about nine minutes. At an altitude of 120 miles (190 kilometers), the speed needed for a spacecraft to maintain orbital velocity and thus stay in orbit is about 5 miles (8 kilometers) per second.

In many rocket launches, a truck or tractor moves the rocket and its payload (cargo) to the launch pad. At the launch pad, the rocket is moved into position over a flame pit, and workers load propellants into the rocket through special pipes.

At launch time, the rocket's first-stage engines ignite until their combined thrust exceeds the rocket's weight. The thrust causes the vehicle to lift off the launch pad. If the rocket is a multistage model, the first stage falls away a few minutes later, after its propellant has been used up. The second stage then begins to fire. A few minutes later, it, too, runs out of propellant and falls away. If needed, a small upper stage rocket then fires until orbital velocity is achieved.

The launch of a space shuttle is slightly different. The shuttle has solid-propellant boosters in addition to its main rocket engines, which burn liquid propellant. The boosters combined with the main engines provide the thrust to lift the vehicle off the launch pad. After slightly more than two minutes of flight, the boosters separate from the shuttle and return to Earth by parachute. The main engines continue to fire until the shuttle has almost reached orbital velocity. Small engines on the shuttle push it the remainder of the way to orbital velocity.

To reach a higher altitude, a spacecraft must make another rocket firing to increase its speed. When the spacecraft reaches a speed about 40 percent faster than orbital velocity, it achieves escape velocity, the speed necessary to break free of Earth's gravity.

Contributor: James Oberg, M.S., Spaceflight Engineer; author, UFOs and Outer Space Mysteries.
Source : World Book 2005

Space exploration
What is space?
Getting into space and back
Living in space
Meeting basic needs in space
Communicating with Earth
The dawn of the space age
Space probes
Probes to Venus
Probes to Jupiter and beyond
Probes to comets
Human beings enter space
Apollo: Mission to the moon
Exploring the moon
Returning to Earth
The International Space Station
Space shuttles
Types of shuttle missions