Space Probe

A probe's journey into space can be divided into several stages. First, the probe has to leave the Earth's surface. Once in space, most probes orbit the Earth temporarily before proceeding to deep space. To leave the Earth on a nonreturning orbit, a probe must achieve a velocity of approximately 25,000 MPH (40,000 km/h) relative to the Earth, the speed termed "escape velocity."

Having escaped Earth orbit, a probe moves primarily under the gravitational influence of the Sun during its journey across the solar system. Some probes take up orbit around the Sun itself; others are targeted at other bodies in the solar system. The final (i.e., approach) stage of a probe's trip starts when the probe experiences significant gravitational attraction from the target body. The calculation of the entire trajectory from Earth to the point of destination is a complex task because it must take into consideration several mutually conflicting demands: maximize payload (i.e., mass of instrumentation delivered to the destination) but minimize cost; shorten mission duration but avoid hazards (e.g., solar flares or meteor swarms); and so forth.

Sometimes, the gravitational fields of planets can be utilized to increase a probe's velocity and to change its direction and velocity without using rocket fuel. For instance, Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. Jupiter's massive gravitational pull can accelerate a probe enough to leave the solar system, or to proceed at greatly increased velocity toward more distant planets. This "gravity assist" effect was successfully used in the U.S.'s Mariner missions to Mercury (boost from Venus), its Voyager missions to the far planets of the solar system (boosts from Jupiter, Saturn, and Neptune), its still-functioning Galileo probe to Jupiter (boosts from Venus and Earth), and its en-route Cassini probe to Saturn (boost from Jupiter). Figure 2 illustrates how a planet's gravity can accelerate a probe.

Projecting of payloads into designated trajectories is achieved by means of expendable launch vehicles (ELVs), that is, non-reusable booster rockets. ELVs are manufactured today by mainland China, France, Japan, Russia, and the United States. Most ELVs use the same basic concept: two or more stacked rocket stages are burned in succession, with each lowermost stage being discarded when its burn is completed. The motion of a rocket is caused by a continuous ejection of hot gases in the opposite direction. Momentum gained by the gases ejected behind the rocket is balanced by forward momentum gained by the rocket itself. No other device can produce rapid acceleration in a vacuum, making rockets essential to space flight. (Solar sails, which exploit the faint pressure exerted by the Sun's light, may have application in the future for missions where very slow acceleration is acceptable.) The rocket's role as a prime mover makes it key to any mission's overall performance and cost. Out of 52 space-probe missions launched in the United States from 1958 to 1988, 13 failed because of ELV failures and only 5 because equipment on the probe itself malfunctioned. (ELVs have, however, tended to become more reliable with time, and several dramatic failures or near-failures of probes since 1990 have been due to)

Earth-based support facilities can be divided into three major categories: test grounds, where the spacecraft and its components are exposed to extreme conditions to make sure that they are able to withstand the stresses of launch, space travel, and the destination environement; check-out and launch ranges, where the lift-off procedure is preceded by a thorough examination of all spacecraft-rocket interfaces; and post-launch facilities, which are used to track, communicate with, and process the data received from the probe.

Hundreds of people and billions of dollars' worth of facilities are involved in tracking a probe and intercepting the data it transmits toward Earth. Preexisting facilities must be modified in accordance with the design of each specific spacecraft. Today, the United States uses two major launch ranges, several world-wide tracking networks, and dozens of publicly and privately owned test facilities.