MISSION PHASES A typical space exploration mission requires, first of all, that a vehicle be launched from the Earth's surface
into outer space. The vehicle must be able to survive and operate in space, after which it is sometimes returned to Earth. Each of these mission phases has special challenges that must be met by space scientists and engineers. Launch The initial phase of launch must use engines with high thrust and compact fuel. In practice, the launch engine involves either kerosene/liquid oxygen or solid-fuel boosters. As a booster pushes against air resistance and the tug of gravity, it loses much of its energy during the ascent. By the time it has achieved orbital altitude and
velocity, more than 160 km (100 mi) above the Earth's surface and moving horizontally at about 7,600 m/sec (25,000 ft/sec), the booster must have expended about 9,100 m/sec (30,000 ft/sec) of velocity gain. This generally takes about eight or nine minutes, for an average acceleration of 2 gs.
Satellites commonly enter orbit close to Earth, and this region of space is referred to as low Earth orbit, or LEO. Propulsive stages can carry the payload higher, or into the 24-hour geosynchronous orbit, or GEO, commonly used by communications and weather satellites because they keep the satellites in position above a selected point on the Earth's surface. Alternatively, an upper stage on a rocket can fire to push a payload to escape velocity, or the velocity needed if an object is to effectively escape the Earth's gravitational influence. Escape velocity from Earth is about 10,800 m/sec (35,000 ft/sec). Vehicles departing from Earth are slowed by the tug of gravity, but as they attain greater and greater distances from the Earth, the gravitational pull decreases by the inverse square law (see gravitation). Ultimately, a probe launched with the exact velocity for escape from Earth would reach an infinite distance with no speed left. In practice, vehicles are effectively out of Earth's influence at a distance of about 1,600,000 km (1,000,000 mi). At this distance they would drift in orbit about the Sun near Earth's own orbit. In order to reach another planet, a vehicle must have a velocity that exceeds escape velocity. Added to or subtracted from the Earth's own orbital velocity around the Sun, depending on the direction of aim, this excess velocity produces a new interplanetary orbit that may intersect the orbit of the intended target. With proper timing of the launchÑthe so-called launch "window"Ña target planet will be at the point where such an interception can occur.