Space probe

A space probe is any uncrewed, instrument-carrying spacecraft designed to travel to an extraterrestrial environment beyond Earth orbit. The first recorded mention of a possibility of a true space probe dates to 1919, when United States physicist and rocketry pioneer Robert Goddard (1882–1945) suggested that a flash from an explosion produced by a rocket on the Moon's surface could be observed from Earth with a telescope . The slight scientific value of such an experiment, along with the absence of the technology for its realization, made Goddard's idea premature. However, in coming years it was to reappear in more mature forms, first on paper and then in reality. In 1952, the term "interplanetary probe" was introduced in a paper by two members of the British Interplanetary Society (a private group of space enthusiasts); by the end of that decade, the dream had at last begun to materialize in hardware projects.

Space probes are used to enrich our scientific knowledge of conditions and bodies in space (asteroids, comets , stars, planets, the solar wind , etc.). Every probe is constructed to fulfill the goals of a particular mission, and thus represents a unique and sophisticated creation of engineering art. Nevertheless, there are some common basic problems underlying any space mission, whether Earth satellite , crewed flight, or automated probe: how to get to the destination, how to collect information, and how to return information to Earth. Successful resolution of these issues is impossible without a highly developed network of Earth-based facilities for assembling and testing the spacecraft-and-launcher system, for launching the spacecraft onto a desired trajectory, for remote control of devices in flight, and for receiving information transmitted back to Earth.

In general, a space probe may thus be considered a combination of interacting systems: on-ground facilities, the launch vehicle, and the spacecraft itself, all communicating with each other through numerous mechanical, electronic, and human interfaces. Each system, in turn, can be split into a set of subsystems interacting through interfaces of their own. A successful space probe therefore requires the fusion of cutting-edge knowledge from many fields. Celestial mechanics , rocketry, precision instrumentation, and telecommunications are only a few of the fields involved.

Automated space missions are, in general, far less costly than crewed missions; a camera or radiation detector, unlike an astronaut, does not require a massive life-support system. Uncrewed spacecraft are still, however, expensive. Today, for example, a probe to Mars is considered relatively inexpensive exploration.

Crewed or not, space missions are an expensive business. For over three decades the United States and the Soviet Union were the only powers technologically and economically capable of sustaining major space-exploration programs. Beginning in the late 1950s, when the Soviet Union's simple Sputnik became the first manufactured object to orbit the Earth, both superpowers spent many billions of dollars on space flight—especially crewed space flight, with its obvious power to impress the world. The prestige motive largely faded, however, after the U.S. won the space race, as it was popularly called, by landing on the Moon in 1969. After the collapse of the Soviet Union in 1991, Russia inherited its space program and continued it in a much reduced form, with emphasis on the Mir space station rather than on solar-system exploration via instrumented probes. In the U.S., the space shuttle and the International Space Station have continued to absorb most of the space budget. However, some funding has always remained available in the U.S. (and to a much smaller extent in Europe and Japan) for small, uncrewed, remote-controlled space vehicles capable of exploring the solar system .

The scientific returns from these missions have been of incalculable value. Scientific probes have been landed on the Moon, Venus , and Mars, and have orbited or flown by every major body in the solar system.

Probe flight and supporting facilities

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, 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.

Design and classification

A space probe is a largely self-contained mechanical system designed to perform a variety of prescribed operations for a long time, sometimes decades. There are ten major constituents of the spacecraft that are responsible for its vital functions: (1) power supply, (2) propulsion, (3) altitude control, (4) environmental control, (5) computers, (6) communications, (7) engineering-instrumentation, (8) scientific instrumentation, (9) guidance control, and (10) structural platform.

(1) The power supply provides regulated electrical power to keep the spacecraft active. Solar-cell arrays that transform sunlight into electricity are used for missions to the inner solar system; thermoelectric generators run by plutonium are used for missions the outer solar system, where sunlight is dim, and have also been used for some planetary-lander probes (e.g., Viking I and II on Mars). (2) The propulsion subsystem enables the spacecraft to maneuver during either space travel or landing (if any), and must be specifically configuration depending upon the mission's goals. (3) The altitude-control subsystem allows the spacecraft to orient itself in space. Solar panels must be aimed at the Sun, antennas at Earth, and sensors at scientific targets. This subsystem also aligns rockets in the proper direction during course-change maneuvers. (4) The environmental-control subsystem maintains the temperature and other aspects of the craft's internal environment within the acceptable levels to secure proper functioning of equipment. (5) The computer subsystem controls all the other subsystems. It performs processing and storage of scientific data, executes routines for internal checking and maintenance, instructs onboard instruments to perform scientific studies, aids in the diagnosis of equipment faults, and initiates pre-programmed actions independently of Earth. (6) The communications 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 other systems and submits status reports to Earth via the computer and communications subsystems. (8) The scientific-instrumentation subsystem carries out the experiments selected for a particular mission, as, for example, to explore a planet's geography, geology , atmospheric physics , and electromagnetic environment. (9) The guidance-and-control subsystem is supposed to detect deviations from proper course and performance, determine corrections, and to dispatch appropriate corrective commands. (10) The structural subsystem is the mechanical skeleton of the spacecraft; it supports, unites, and protects all other subsystems.

Depending upon a mission's target, it may be classed as lunar, solar, planetary, or interplanetary (i.e., visiting more than one planet ). Interstellar missions are also possible in principle. None have been launched, but several U.S. probes to the outer planets have left the solar system and continue to transmit data from interstellar space. Another scheme of classification is based upon the mission type: flyby, orbiter, or soft-lander.

Space probe families

The scores probes launched since 1959 are grouped into families, which usually encompass craft similar by design, mission, or both. The United States National Aeronautics and Space Administration (NASA) has launched a series of interplanetary probes (Pioneer, Voyager, etc.), of lunar probes (Ranger, Surveyor, Lunar Orbiter), of planetary probes (Mariner, Viking, Pioneer Venus). The former Soviet Union's probe families were Luna (Russian for Moon), Mars, and Venera (Russian for Venus). Although all Soviet (and, later, Russian) efforts to land a space probe on Mars have failed, only Soviet spacecraft have landed on the surface of Venus.

Recent and future space probes

A new program recently initiated by NASA, the Discovery program, has its objective to find cheaper ways to explore the solar system. It was largely inspired by the dramatic failure of the $1-billion Mars Observer mission in 1993, which exploded on arrival at Mars and is supposed to supplant large, expensive, infrequent missions with relatively small, inexpensive, frequent. It was Discovery's original goal to increase mission frequency to one every 12 to 18 months and to provide for a more continuous accumulation of diverse scientific information on asteroids, planets, and the Sun. In the frame of the Discovery program, the Mars Pathfinder and the Near-Earth Asteroid Rendezvous (NEAR) missions were launched in 1996. The Pathfinder lander mission, which landed successfully on Mars in 1997, included a low-power, low-mass instruments, and a small six-wheeled rover (named Sojourner) to analyze rock composition on the Martian surface. NEAR journeyed through the asteroid belt, flying by the asteroid Iliya in 1996, and after a gravity boost from Earth, NEAR encountering near-Earth object 433 Eros in December 1998. NEAR was completely successful in its mission to study 433 Eros at close range, and even managed to make a soft touchdown on its surface (an add-on mission for which it had not been originally designed).

However, the pace of the Discovery series of missions slowed drastically after the failure of two consecutive Mars probes in 1999 (Mars Orbiter) and 2000 (Mars Polar Lander). Critics charged that NASA had allowed its new "better, cheaper, faster" philosophy to compromise its engineering standards, and NASA, in the face of two consecutive catastrophes, agreed. Its missions have subsequently become less cheap and less fast. Since the twin disasters of 1999 and 2000, NASA has successfully orbited one craft around Mars (Mars Odyssey 2001). Its next major lander mission to Mars, Mars Exploration Rover, will feature duplicate probes much like the successful (and expensive) Viking landers of the 1970s. Each Mars Exploration Rover probe will, if successful, deploy a sophisticated rover onto the surface of Mars in 2004, exploiting technologies tested during the Pathfinder landing of 1997. Another orbiter, Mars Express, is due to arrive at Mars in 2003. Mars Express will deploy a small lander as well.

Possible future missions include a Mercury orbiter, a Pluto mission, a Venus environmental satellite, comet life history investigation, and NEARS (Near-Earth Asteroid Returned Samples), which would be equipped with a special "shooter" for firing sample tubes into an asteroid's surface. After collecting up to 21 oz (600 g), of sample material, the probe would return it to Earth. The realities of Earthly politics, however, make it difficult to fund interplanetary missions; each must be fought for by the scientists who believe in its value, and a valuable mission may be definitively canceled (or launched) only after years of vacillation at the political level.

The United States launches the vast majority of interplanetary probes, and will probably continue to do so, but other nations are also beginning to do so. Japan launched its Planet B orbiter toward Mars in 1998; it is expected to assume an orbit around that planet in January 2004.

See also Spacecraft, manned.

Resources

books

Harland, David M. Mission to Saturn: Cassini and the Huygens Probe. (Springer-Praxis Books in Astronomy and Space Sciences) Springer Verlag, 2002.

Kraemer, Robert S., and Roger D. Launius. Beyond the Moon: Golden Age of Planetary Exploration 1971-1978 (Smithsonian History of Aviation and Spaceflight Series) Smithsonian Institution Press, 2000.

Griffin, M. D., and J. R. French. Space Vehicle Design. American Institute of Aeronautics and Astronautics, 1991.

other

National Aeronautics and Space Administration. "Solar System Exploration." December 27, 2002 [cited December 30, 2002]. <http://www.solarsystem.nasa.gov/index.cfm>.

Elena V. Ryzhov
Larry Gilman

KEY TERMS

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Gravitation

—The force whereby any two particles of matter attract each other throughout the universe.

Interface

—A common boundary between two parts of a system, whether material or nonmaterial.

Trajectory

—The path described by any body moving through space.