Space Probe

Probe flight and supporting facilities

Design and classification

Space probe families

Recent and future space probes

Resources

A space probe is any uncrewed (unmanned), instrument-carrying spacecraft designed to travel to an extraterrestrial environment beyond the orbit of Earth. 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 human 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, is 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.

For over four decades the United States and the Soviet Union (now, Russia) were the only powers technologically and economically capable of sustaining major space-exploration programs. Beginning in the late 1950s, when the Soviet Union’s Sputnik became the first manufactured object to orbit 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 July 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., NASA’s 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 (via the European Space Agency), India, China, Japan, and a few other countries) 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; have landed on the asteroid 433 Eros; have orbited or flown by every major body, and many minor bodies, in the solar system; and have traveled so that, as of October 2006, are nearly outside the solar system and on their way to interstellar space (through the efforts of Voyager 1 and 2 and Pioneer 10 and 11 spacecraft).

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 Earth temporarily before proceeding to deep space. To leave Earth on a non-returning orbit, a probe must achieve a velocity of approximately 25,000 mph (40,000 km/h) relative to Earth, the speed termed escape velocity (Figure 1).

Having escaped Earth’s 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 pay-load (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 United States Mariner missions to Mercury (boost from Venus); the Voyager missions to the far planets of the solar system (boosts from Jupiter, Saturn, and Neptune); the Galileo probe to Jupiter (boosts from Venus and Earth); and the current operational mission of the 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. China, Japan, India, Russia, Ukraine, the countries of the European Space Agency (Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Norway, Portugal,

Spain, Sweden, Switzerland, and the United Kingdom), and the United States manufacture ELVs today. 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 five because equipment on the probe itself malfunctioned. ELVs have, however, tended to become more reliable with time.

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’s environment; 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 worldwide 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 generally used for missions to 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 programmers on Earth.
6. The communications subsystem transmits data and receives commands from the 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 detects deviations from proper course and performance, determines corrections, and dispatches 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 almost left the solar system and continue to transmit data from the edge of the solar system. (In May 2005, NASA stated that Voyager 1 was in the heliosheath, and that it should reach the heliopause [the generally accepted edge of the solar system] in 2015. As of August 2005, it was about 100 AU from the sun, the furthest human made object from Earth. According to NASA, Voyager 2 left the heliosheath in December 2004 and is about 80.5 AU away from Earth, as of September 2006. On March 4, 2006, NASA lost contact with Pioneer 10. Three months earlier, it was about 89.7AUawayfromEarth.NASAlost contact with Pioneer 11 in November 1995.) Another scheme of classification is based upon the mission type: flyby, orbiter, or soft-lander.

Space probe families

The scores of 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 encountered 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. However, since the twin disasters of 1999 and 2000, NASA has successfully completed numerous missions.

The Stardust mission was launched in February 1999 in order to investigate the comet Wild 2. It completed its mission when it returned a capsule to the Earth in January 2006 that contained samples of the comet. The Stardust mission was the first mission to return comet dust to scientists on the Earth.

NASA launched the Mars Odyssey in 2001 and orbited it around Mars in 2002. It uses its instruments to search for water and volcanic activity on Mars. Then, the lander mission to Mars, Mars Exploration Rover, was launched in 2003. The mission features duplicate probes much like the successful (and expensive) Viking landers of the 1970s. Each Mars Exploration Rover probe deployed one sophisticated rover (Spirit and Opportunity) onto the surface of Mars in 2004, exploiting technologies tested during the Pathfinder landing of 1997. As of October 2006, both probes are continuing their explorations of Mars with the help of the orbiting Mars Odyssey.

NASA’s Mars Reconnaissance Orbiter (MRO) was launched in August 2005. It reached its orbit about Mars in March 2006 and is expected to proceed with its mission to investigate the planet beginning in November 2006.

MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) was launched by NASA in August 2004 in order to study the planet Mercury while in orbit about the planet. With flybys of Earth and Venus, the probe is expected to begin orbiting Mercury in March 2011 for a one-year mission to investigate Mercury’s surface composition and geology, magnetic field, interior core, poles, and atmosphere.

KEY TERMS

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.

NASA launched Deep Impact in January 2005 to study the composition of comet Tempel 1. It traveled about 429 million kilometers in about six months before it was directed to make a controlled impact of the comet’s nucleus. The resulting collision produced an impact crater, which showed a dustier and less icy interior than expected.

NASA is planning to launch Mars Science Laboratory (MSL) in December 2009 for a landing on the planet in October 2010. It will carry three times as many instruments as the MER rovers Spirit and Opportunity for its investigation of whether Mars supported life in the past or is supporting life at the present.

NASA’s New Horizon’s spacecraft, launched in January 2006, will cross the orbits of all of the planets past Earth on its way past Pluto and Charon in 2015. It will be the first spacecraft to visit the Pluto-Charon system. The spacecraft is expected to fly within 6,200 mi (10,000 km) of Pluto with a relative velocity of 8.6 mi/sec (13.78 km/sec) at closest approach. When it flies by Charon it will come as close as 16,800 mi (27,000 km). After passing by Pluto and Charon, New Horizons will travel further into the Kuiper Belt. Over the next four to five years, the spacecraft is expected to examine several KBOs (Kuiper Belt Objects).

NASA’s Small Explorer program is expected to launch IBEX (Interstellar Boundary Explorer) in June 2006 for a mission to map the diffuse boundary between the solar system and interstellar space.

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 in the near future, but other nations are beginning to do so. Japan, China, the countries of the European Space Agency, Australia, India, Korea, Russia, Ukraine, and others are all launching civilian and military probes into space for their own particular purposes. For instance, SELENE (Selenological and Engineering Explorer) is a Japanese spacecraft that is expected to orbit the moon, with an expected launch year of 2007. In addition, some missions are international in nature, as several countries contribute specific instruments and materials to the spacecraft.

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.

Matloff, Gregory L. Deep Space Probes: To the Outer Solar System and Beyond. Berlin, Germany: Springer, 2005.

Russell, Christopher T. Deep Impact Mission: Looking Beneath the Surface of a Cometary Nucleus. Dordrecht, Netherlands and Norwell, MA: Springer, 2005.

OTHER

National Aeronautics and Space Administration. “Solar

System Exploration.” <http://www.solarsystem.nasa.gov/index.cfm> (accessed October 27, 2006).

Elena V. Ryzhov

Larry Gilman

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

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.

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.

See also Astronomy; History of manned space exploration; Space and planetary geology; Spacecraft, manned

Space probe

A space probe is any unmanned spacecraft designed to carry out physical studies of the Moon, other planets, or outer space. Space probes take pictures, measure atmospheric conditions, and collect soil samples then bring or report the data back to Earth.

More than 30 space probes have been launched since the former Soviet Union first fired Luna 1 toward the Moon in 1959. Probes have now visited every planet in the solar system except for Pluto. Two have even left the solar system and headed into the interstellar medium.

Moon probes

The earliest probes traveled to the Moon. The Soviets launched a series of Luna probes that took the first pictures of the far side of the Moon. In 1966, Luna 9 made the first successful landing on the Moon and sent back television footage from the Moon's surface.

The National Aeronautics and Space Administration (NASA) landed Surveyor on the Moon four months after Luna 9. The Surveyor had more sophisticated landing capability and sent back more than 11,000 pictures.

Planetary probes

In the meantime, NASA launched the first series of planetary probes, called Mariner. Mariner 2 first reached Venus in 1962. Later Mariner spacecraft flew by Mars in 1964 and 1969, providing detailed images of that planet. In 1971, Mariner 9 became the first spacecraft to orbit Mars. During its year in orbit, Mariner 9 transmitted footage of an intense Martian dust storm as well as images of 90 percent of the planet's surface and the two Martian moons.

The Soviets also put probes in orbit around Mars in 1971. Mars 2 and Mars 3 carried landing vehicles that successfully dropped to the planet's surface, but in each case radio contact was lost after about 20 seconds.

In 1976, the U.S. probes Viking 1 and Viking 2 had more direct encounters with Mars. Viking 1 made the first successful soft landing on Mars on July 20, 1976. Soon after, Viking 2 landed on the opposite side of the planet. The Viking probes reported on the Martian weather and photographed almost the entire surface of the planet. Twenty years after the Voyager probes were released, NASA launched the Mars Global Surveyor and the Mars Pathfinder to revisit Mars. The Mars Global Surveyor completed its mapping mission of Mars in early 2001 after having sent back tens of thousands of images of the planet. Its main mission accomplished, NASA engineers hope to use Surveyor to relay commands to twin rovers slated to land on the planet in early 2004. The Mars Pathfinder landed on the planet's surface on July 4, 1997, and released the Sojourner rover, which sent back to Earth images and analyses of the Martian terrain, including chemical analyses of rocks and the soil.

Not all probe sent to Mars were as productive as the Mars Global Surveyor and the Mars Pathfinder. In 1999, NASA lost two probes, the Mars Climate Orbiter and the Mars Polar Lander. As its name implies the Mars Climate Orbiter was to have explored the Martian atmosphere, while the Mars Polar Lander was to have explored the planet's landscape in search of water. Neither was able to land successfully due to an error in converting English and metric measurements (for the Mars Climate Orbiter ) and a software glitch (for the Mars Polar Lander ).

From 1970 to 1983, the Soviets concentrated mostly on exploring Venus. They sent out a series of Venera and Vega probes that landed on Venus, analyzed its oil, took detailed photographs, studied the atmosphere, and mapped the planet using radar.

Mercury was visited by a probe in 1974 when Mariner 10 came within 470 miles (756 kilometers) of the planet and photographed about 40 percent of its surface. The probe then went into orbit around the Sun and flew past Mercury twice more in the next year before running out of fuel.

Space probes to the outer planets

NASA sent Pioneer probes to explore the outer planets. Pioneer 10 reached Jupiter in 1973 and took the first close-up photos of the giant planet. It then kept traveling, crossing the orbit of Pluto and leaving the solar system in 1983. Pioneer 11 traveled to Saturn, where it collected valuable information about the planet's rings.

NASA next introduced the Voyager 1 and 2 probes, more sophisticated versions of the Pioneers. Launched in 1977, they flew by Jupiter two years later and took pictures of the planet's swirling colors, volcanic moons, and its previously undiscovered ring.

The Voyager space probes then headed for Saturn. In 1980 and 1981, they sent back detailed photos of Saturn's spectacular rings and its vast collection of moons. Voyager 2 then traveled to Neptune, which it reached in 1989, while Voyager 1 continued on a path to the edge of the solar system and beyond.

After many delays, the U.S probe Galileo was launched from the space shuttle Atlantis in 1989. It reached Jupiter in December 1995, and dropped a barbecue-grill-sized mini-probe down to the planet's surface. That mini-probe spent 58 minutes taking extremely detailed pictures of the gaseous planet before being incinerated near the surface. As of the

beginning of 2001, Galileo was still sending valuable scientific information about Jupiter and its moons back to Earth.

In February 1996, NASA launched NEAR (Near Earth Asteroid Rendezvous) Shoemaker, an unmanned spacecraft that was to become the first to orbit an asteroid. In April 2000, it began a circular orbit around the asteroid Eros. During its one-year mission around Eros, the spacecraft took measurements to determine the mass, density, chemical composition, and other geological characteristics of the asteroid. It also beamed some 160,000 images of Eros back to Earth. In February 2001, NEAR Shoemaker used the last of its fuel in a successful attempt to land on the surface of the asteroid. Once on the surface, it continued to collect invaluable data about the oddly shaped Eros before it was finally shut down by NASA.

Future space probe missions

NASA has plans underway for many more space probes. The Cassini orbiter, which was launched in October 1997, will study Saturn and its moon. It is scheduled to reach the planet in 2004. Cassini will drop a mini-probe, called Huygens, onto the surface of Titan, Saturn's largest moon, for a detailed look. Cassini will then go into orbit around Saturn.

With a desire to return to Mars, NASA launched the Mars Odyssey in April 2001. Once in orbit around the planet by the fall of that year, the spacecraft will examine the composition of the planet's surface and try to detect water and shallow buried ice. In a mission planned by the European Space Agency and the Italian space agency, NASA will launch the Mars Express in mid-2003. It will search for subsurface water from orbit and deliver a lander, Beagle 2, to the Martian surface. NASA will also launch two powerful rovers to Mars in 2003, each to a different region of the planet to look for water. And in 2005, NASA plans to launch the Mars Reconnaissance Orbiter, a powerful scientific orbiter that will map the Martian surface with an high-definition camera.

NASA had hoped to explore Pluto sometime in the twenty-first century by sending the Pluto-Kuiper Express. This probe was to have consisted of two spacecraft, each taking about eight years to reach the farthest planet in our solar system. Originally scheduled to launch in 2004 and arrive at Pluto in 2012, the Pluto-Kuiper Express was put on hold by NASA in the fall of 2000 because of high costs. It was then scraped in the spring of 2001 after President George W. Bush unveiled his 2002 budget, which provided no money for the project.

[See also Jupiter; Mars; Mercury; Moon; Neptune; Pluto; Satellite; Saturn; Spacecraft, manned; Venus ]

space probe • n. see probe.