Space Probe
Aspace probe is any unmanned instrumented spacecraft designed to carry out physical studies of space environment. As distinguished from satellites orbiting Earth under the influence of gravitational attraction, a space probe is rocketed into space with sufficient speed to achieve escape velocity (the velocity needed to obtain parabolic or hyperbolic orbit) and to reach a trajectory aimed at a pre-selected target.
The first recorded mention of a possibility of an unmanned probe dates back to 1919, when American physicist R. H. Goddard (1882–1945) suggested a series of space based experiments. However, in large part to Goddard's advancements in rocketry, it took only 33 years for the concept of space experiment to reappear. In 1952, the term "space probe" was introduced by E. Burgess and C. A. Cross in a short paper presented to the British Interplanetary Society.
The space probe is used mostly for the acquisition of scientific data enriching general knowledge on properties of outer space and heavenly bodies. Each probe (sometimes a series of several identical craft) is constructed to meet specific goals of a particular mission, and thus, represents a unique and sophisticated creation of contemporary engineering. Nevertheless, whether it is an Earth satellite, a crewed flight, or an automated probe, there are some common problems underlying any space mission: how to get to the destination point, how to collect the information required, and, finally,
how to transfer the information back to Earth. Successful resolution of these principal issues is impossible without a developed net of high-tech Earth-based facilities used for assembling and testing the spacecraft-rocket system, for launching the probe into the desired trajectory, and for providing necessary control of probe-equipment operation, as well as for receiving data transmitted back to Earth.
As compared to crewed flights, automated space missions are far more economical and, of course, less risky to human life.
A probe's journey into far space can be divided into several stages. First, the probe has to overcome Earth's gravity. Escape velocities vary for different types of trajectories. During the second stage, the probe continues to move under the influence of its initial momentum and the combined gravitational influences of the Sun and bodies with substantial mass near its flight path. The third (approach) stage starts when the probe falls under the gravitational attraction of its destination target. The calculation of the entire trajectory from Earth to the point of destination is a complicated task. It must take into consideration numerous mutually conflicting demands: to maximize the payload but to minimize the cost, to shorten mission duration but to avoid such hazards as solar flares or meteoroid swarms, to remain within the range of the communication system but to avoid the unfavorable influence of large spatial bodies, etc.
Sometimes, strong gravitational fields of planets can be utilized to increase the probe's velocity and to change its direction considerably without firing the engines and using fuel. For instance, if used properly, Jupiter's massive gravitational pull can accelerate a probe enough to leave the solar system in any direction. The gravitational assistance or "swing-by" effect was successfully used, for example, in the American missions to Mercury via Venus, and in the voyage of the Galileo craft to Jupiter.
Projecting of payloads into designated trajectories is achieved by means of expendable launch vehicles (ELVs). A wide variety of ELVs possessed by the United States uses the same basic technology—two or more rocket-powered stages that are discarded when their engine burns are completed. Similar to the operation of a jet aircraft, the motion of a rocket is caused by a continuous ejection of a stream of hot gases in the opposite direction. The rocket's role as a prime mover makes it very important for the system's overall performance and cost. Out of 52 space-probe missions launched in the United States during the period from 1958 to 1988, 13 failed because of launch vehicle failures and only five because of probe equipment's malfunctions.
All supporting Earth-based facilities can be divided into three major categories: test grounds, where the spacecraft and its components are exposed to different extreme conditions to make sure that they are able to withstand tough stresses of outer space; 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 following the flight of each probe and in intercepting the data it transmits toward Earth. Already-developed facilities always have to be adopted in accordance with the specific spacecraft design. Today, the United States, Russia, and France (for unmanned flights only) possess major launch ranges, worldwide tracking networks, and dozens of publicly and privately owned test facilities. China is also actively developing space launch facilities and, in 1999, launched its first unmanned test of a program designed to enable China to launch a manned mission by 2003.
Any space probe is a self-contained piece of machinery designed to perform a variety of prescribed complex operations for a long time, sometimes for decades. There are ten major constituents of the spacecraft entity that are responsible for its vital functions: (1) power supply, (2) propulsion, (3) attitude control, (4) environmental control, (5) computer subsystem, (6) communications, (7) engineering, (8) scientific instrumentation, (9) guidance control, and (10) structural platform.
(1) The power supply provides well-regulated electrical power to keep the spacecraft active. Usually the solar-cell arrays transforming the Sun's illumination into electricity are used. Far from the Sun, where solar energy becomes too feeble, electricity may be generated by nuclear power devices. (2) The propulsion subsystem enables the spacecraft to maneuver when necessary, either in space or in a planet's atmosphere, and has a specific configuration depending upon the mission's goals. (3) The attitude-control subsystem allows orientation of the spacecraft for a specific purpose, such as to aim solar panels at the Sun, antennas at Earth, and sensors at scientific targets. It also aligns engines in the proper direction during the maneuver. (4) The environmental-control subsystem maintains the temperature, pressure, radiation and magnetic field inside the craft within the acceptable levels to secure proper functioning of equipment. (5) The computer subsystem performs data processing, coding, and storage along with routines for internal checking and maintenance. It times and initiates the pre-programmed actions independently of Earth. (6) The communication subsystem transmits data and receives commands from Earth. It also transmits identifying signals that allow ground crews to track the probe. (7) The engineering-instrumentation subsystem continuously monitors the "health" of the spacecraft's "organism" and submits status reports to Earth. (8) The scientific-instrumentation subsystem is designed to carry out the experiments selected for a particular mission, for example, to explore planetary geography, geology, atmospheric physics or electromagnetic environment. (9) The guidance-and-control subsystem is supposed to detect deviations from proper performance, determine corrections and to dispatch appropriate commands. In many respects, this subsystem resembles a human brain, since it makes active decisions, having analyzed all available information on the spacecraft's status. (10) The structural subsystem is a skeleton of the spacecraft; it supports, unites and protects all other subsystems.
Depending upon a mission's target, the probes may be classed as lunar, solar, planetary (Mercurian, Venusian, Martian, Jovian) or interplanetary probes. Another classification is based upon the mission type: flyby, orbiter, or soft-lander.
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