Space probe: Definition from Answers.com

An automated, crewless vehicle, the payload of a rocket-launching system, designed for flight missions to other planets, to the Moon, and into interplanetary space, as distinguished from Earth-orbiting satellites (see table).

Important space probes
Name	Launch date	Comments
Luna 1	Jan. 2, 1959	Lunar probe; now in solar orbit; passed within 3278 mi (5275 km) of the Moon
Pioneer 4	Mar. 3, 1959	Cosmic rays; passed 37,300 mi (60,000 km) from Moon
Luna 2	Sept. 2, 1959	Impacted Moon
Luna 3	Oct. 4, 1959	Photographed far side of Moon
Pioneer 5	Mar. 11, 1960	First deep-space probe; magnetic fields and cosmic rays
Mariner 2	Aug. 26, 1962	First planetary flyby; Venus probe
Ranger 7	July 28, 1964	Lunar impact and approach photography
Mariner 4	Nov. 28, 1964	Mars encounter; photography, magnetic fields, cosmic rays
Ranger 8	Feb. 17, 1965	Lunar impact and approach photographs
Ranger 9	Mar. 21, 1965	Lunar impact and Alphonsus; approach photography
Zond 3	July 18, 1965	Photographs from lunar encounter
Pioneer 6	Dec. 16, 1965	Solar orbit
Luna 9	Jan. 31, 1966	First photographs from lunar surface
Luna 10	Mar. 31, 1966	Lunar and interplanetary data
Surveyor 1	May 30, 1966	Soft landing on Moon: environmental data and photography
Lunar Orbiter 1	Aug. 10, 1966	Lunar photographs
Pioneer 7	Aug. 17, 1966	Solar orbit
Luna 11	Aug. 24, 1966	Lunar data
Luna 12	Oct. 22, 1966	Lunar orbital photography and other data
Lunar Orbiter 2	Nov. 6, 1966	Lunar orbital photography
Luna 13	Dec. 21, 1966	Lunar surface photography and soil information
Lunar Orbiter 3	Feb. 5, 1967	Lunar orbital photography
Surveyor 3	Apr. 17, 1967	Lunar surface photography and surface properties
Lunar Orbiter 4	May 4, 1967	Lunar orbital photography
Venera 4	June 12, 1967	Analysis of Venus atmosphere; first instrumented landing on another planet
Mariner 5	June 14, 1967	Venus probe; atmospheric and magnetospheric data
Lunar Orbiter 5	Aug. 1, 1967	Lunar orbital photography
Surveyor 5	Sept. 8, 1967	Lunar surface photography and surface properties, including elemental analysis of surface
Surveyor 6	Nov. 7, 1967	Same as Surveyor 5; landing in Sinus Medii
Pioneer 8	Dec. 13, 1967	Solar orbit
Surveyor 7	Jan. 7, 1968	Same as Surveyor 5
Zond 5	Sept. 14, 1968	Circled Moon; recovered Sept. 21, 1968
Pioneer 9	Nov. 8, 1968	Solar orbit
Zond 6	Nov. 10, 1968	Circled Moon; recovered Nov. 17, 1968
Venera 5	Jan. 5, 1969	Same as Venera 4
Venera 6	Jan. 10, 1969	Same as Venera 4
Mariner 6	Feb. 25, 1969	Photography and analysis of surface and atmosphere of Mars
Mariner 7	Mar. 27, 1969	Same as Mariner 6
Luna 15	July 14, 1969	Lunar reconnaissance (crashed during attempted lunar landing)
Zond 7	Aug. 8, 1969	Reentered Aug. 14, 1969; third uncrewed circumlunar flight; recovered in the Soviet Union
Venera 7	Aug. 17, 1970	Lander capsule transmitted 23 min from surface of Venus, Dec. 15, 1970
Luna 16	Sept. 12, 1970	Reentered Sept. 24, 1970; uncrewed Moon lander touched down on Sea of Fertility Sept. 20, 1970; returned lunar soil samples
Zond 8	Oct. 20, 1970	Circled Moon; recovered Oct. 27, 1970
Luna 17	Nov. 10, 1970	Landed on Moon Nov. 17, 1970; uncrewed Moon rover
Mars 2	May 19, 1971	First Soviet Mars landing
Mars 3	May 28, 1971	Mars probe
Mariner 9	May 30, 1971	Mars probe
Luna 18	Sept. 2, 1971	Impacted Moon Sept. 11, 1971
Luna 19	Sept. 28, 1971	Lunar photography mission
Luna 20	Feb. 14, 1972	Recovered Feb. 25, 1972; returned lunar sample
Pioneer 10	Mar. 2, 1972	Jupiter encounter; transjovian interplanetary probe
Venera 8	Mar. 27, 1972	Venus landing July 22, 1972
Luna 21	Jan. 8, 1972	Moon landing Jan. 16, 1972, with Lunikhod rover
Pioneer 11	Apr. 5, 1973	Jupiter encounter and transjovian interplanetary probe; also Saturn encounter
Mars 4	July 21, 1973	Mars orbiter
Mars 5	July 25, 1973	Mars orbiter
Mars 6	Aug. 5, 1973	Mars lander
Mars 7	Aug. 9, 1973	Mars lander
Mariner 10	Nov. 3, 1973	Venus and Mercury encounter
Luna 22	May 29, 1974	Lunar probe
Helios 1	Dec. 10, 1974	Inner solar system, solar wind exploration
Venera 9	June 8, 1975	Venus probe
Venera 10	June 14, 1975	Venus probe
Viking 1	Aug. 20, 1975	Mars lander and orbiter
Viking 2	Sept. 9, 1975	Mars lander and orbiter
Helios 2	Jan. 15, 1976	Interplanetary; similar objectives to those of Helios 1
Luna 24	Aug. 9, 1976	Recovered Aug. 25, 1976; returned lunar sample
Voyager 2	Aug. 20, 1977	Jupiter, Saturn, Uranus, and Neptune encounters; also satellites and ring systems
Voyager 1	Sept. 5, 1977	Same objectives as Voyager 2 with some orbital differences giving differing encounter trajectories
Pioneer Venus Orbiter	May 20, 1978	Returning atmospheric, surface, and particle and field information
Pioneer Venus Multi-Probe Bus	Aug. 8, 1978	Penetration of Venus atmosphere by four probes; returned atmospheric data
Venera 11	Sept. 8, 1978	Venus lander; returned information on surface properties; detection of lightning and thunderlike sounds
Venera 12	Sept. 14, 1978	Similar mission to Venera 11
Venera 13	Oct. 30, 1981	Venus lander
Venera 14	Nov. 4, 1981	Venus lander
Venera 15	June 2, 1983	Venus lander; surface topography
Venera 16	June 7, 1983	Similar mission to Venera 15
International	—	Originally International Sun-Earth Explorer 3 (ISEE 3) Earth satellite, redirected using a lunar swingby on Dec. 22, 1983, to encounter with Comet Giacobini-Zinner; plasma and magnetic field
Cometary
Explorer (ICE)
Vega 1	Dec. 15, 1984	Venus probe-Halley intercept
Vega 2	Dec. 21, 1984	Venus probe-Halley intercept
Sokigake	Jan. 8, 1985	Halley intercept; precursor to Suisei, upgraded to full mission
Giotto	July 2, 1985	Halley intercept
Suisei	Aug. 19, 1985	Halley intercept; plasma and magnetic field measurements
Phobos 1	July 7, 1988	Mars/Phobos probe, lost by command error
Phobos 2	July 12, 1988	Mars/Phobos probe; some data but communications lost
Magellan	May 4, 1989	Venus radar mapper
Galileo	Oct. 18, 1989	Jupiter orbiter and atmospheric probe
Muses	Jan. 24, 1990	Moon orbiter and relay probe; orbiter transmitter malfunctioned
Ulysses	Oct. 6, 1990	Solar polar orbiter
Mars Observer	Sept. 25, 1992	Contact lost 3 days before Mars arrival
Clementine	Jan. 25, 1994	Orbited Moon; thruster malfunction prevented asteroid flyby
Solar and Heliospheric Observatory (SOHO)	Dec. 2, 1995	Orbits L1 libration point to study the Sun
NEAR-Shoemaker	Feb. 17, 1996	Asteroid orbiter
Mars Global Surveyor	Nov. 7, 1996	Mars orbiter
Mars 96	Nov. 16, 1996	Mars orbiter and landers; launch vehicles failed
Mars Pathfinder	Dec. 4, 1996	Mars lander and rover
Advanced Composition Explorer	Aug. 25, 1997	Orbits L1 libration point to study charged particles
Cassini	Oct. 15, 1997	Saturn orbiter/Titan descent probe
Lunar Prospector	Jan. 6, 1998	Lunar orbiter
Nozomi (Planet-B)	July 4, 1998	Mars orbiter; orbit insertion failed
Deep Space 1	Oct. 24, 1998	Test of ion engine and 11 other advanced technologies; asteroid flyby
Mars Climate Orbiter	Dec. 11, 1998	Lost during Mars arrival
Mars Polar Lander	Jan. 3, 1999	Lost during Mars arrival
Stardust	Feb. 7, 1999	Comet flyby, dust sample return
Mars Odyssey	April 7, 2001	Mars orbiter
Wilkinson Microwave Anisotropy Probe	June 30, 2001	Orbits L2 libration point to study cosmic background radiation
Genesis	August 8, 2001	Orbits L1 libration point to collect solar wind samples and return them
Hyabusa (Muses-C)	May 9, 2003	Asteroid sample return mission
Mars Express	June 2, 2003	Mars orbiter and lander; orbiter was successful but Beagle 2 lander failed
MER-A/Spirit	June 10, 2003	Mars rover
MER-B/Opportunity	July 7, 2003	Mars rover
Spitzer Space Telescope (Space Infrared Telescope Facility)	August 25, 2003	Infrared observatory in Earth-trailing orbit
MESSENGER	August 3, 2004	Mercury orbiter

The space probe is used primarily for scientific purposes, which are stated as the mission objectives. Missions generally fall into three categories according to destination: those to the rocky bodies in the inner solar system, including the Earth-like planets, asteroids, and comets; those to the giant gaseous planets in the outer solar system; and those designed to study solar physics and the properties of interplanetary space. Most spacecraft launched to a planet or other body also study the environment of charged particles and electromagnetic fields in interplanetary space during their cruise phase en route to the destination. See also Asteroid; Solar system.

Missions may also be categorized by complexity. The simplest are flyby spacecraft, which study their target body during a relatively brief encounter period from a distance of hundreds to thousands of miles as they continue past. Next are orbiters, which circle a planet or other body for extended study; some may carry atmospheric descent probes. Even more complex are lander missions, which touch down on a planet or other body for the collection of on-site data; some may bear exploration rovers designed to range beyond the immediate landing site. Finally, the most complex space probes envisaged are sample-return missions, which would collect specimen material from a target body and return it to Earth for detailed study.

Spacecraft subsystems

In the broadest terms, a space probe may be considered a vehicle that transports a payload of sensing instruments to the vicinity of a target body. Thus, the spacecraft must include a number of subsystems to provide power, to communicate with Earth, to maintain and modify attitude and perform maneuvers, to maintain acceptable on-board temperature, and to manage the spacecraft overall. See also Space technology.

Scientific instruments

The scientific payload may be divided into remote-sensing instruments, such as cameras, and direct-sensing instruments, such as magnetometers or dust detectors. They may be classified as passive instruments, which detect radiance given off by a target body, or active ones, which emit energy such as radar pulses to characterize a target body. See also Remote sensing.

Power subsystem

Electrical power is required for all spacecraft functions. The total required power ranges from about 300 to 2500 W for current missions, depending on the complexity of the spacecraft. The power subsystem must generate, store, and distribute electrical power. All space probes launched so far have generated power either via solar panels or via radioisotope thermoelectric generators (RTGs). See also Nuclear battery; Solar cell; Space power systems.

Telecommunications subsystem

In order to accomplish its mission, the spacecraft must maintain communications with Earth, such as receiving commands sent from ground controllers, and transmitting scientific data and routine engineering “housekeeping” data. All of these transmissions are made in various segments of the microwave spectrum. The design of the telecommunications subsystem takes into account the volume of data to be transmitted and the distance from Earth at which the spacecraft will operate, dictating such considerations as the size of antennas and the power of the on-board transmitters. See also Microwave.

Advanced planetary probes have carried a dish-shaped high-gain antenna which is the chief antenna used to both transmit and receive. These antennas typically consist of a large parabolic reflector, with a subreflector mounted at the main reflector's focus in a Cassegrain-type configuration. In the interest of redundancy and in the event that Earth pointing is lost, spacecraft virtually always carry other on-board antennas. These may be low-gain antennas, which typically offer nearly omnidirectional coverage except for blind spots shadowed by the spacecraft body, or medium-gain antennas, which provide a beam width of perhaps 20–30°. See also Antenna (electromagnetism).

Attitude-control subsystem

It would be impossible to navigate the spacecraft successfully or point its scientific instruments or antennas without closely controlling its orientation in space, or attitude. Some spacecraft, particularly earlier ones, have been spin-stabilized; during or shortly after launch, the spacecraft is set spinning at a rate on the order of a few revolutions per minute. Much like a rotating toy top, the spacecraft's orientation is stabilized by the gyroscopic action of its spinning mass. Most planetary spacecraft, however, are three-axis-stabilized, meaning that their attitude is fixed in relation to space. The spacecraft's attitude is maintained and changed via onboard thruster jets or reaction wheels, or a combination of both.

Propulsion subsystem

Most spacecraft are outfitted with a series of thruster jets, each of which produces approximately 0.2–2 pounds-force (1–10 newtons) of thrust. Thrusters are usually fueled with a monopropellant, hydrazine, which decomposes explosively when it contacts an electrically heated metallic catalyst within the thruster. In addition to maintaining the spacecraft's attitude, on-board thrusters are used for trajectory-correction maneuvers. Spacecraft designed to orbit a planet or similar target body must carry a larger propulsion element capable of decelerating the spacecraft into orbit upon arrival. See also Spacecraft propulsion.

Thermal control subsystem

In order to minimize the impact of temperature variations on the electronics on board, spacecraft nearly always incorporate some form of thermal control. Mechanical louvers, controlled by bimetallic strips similar to those in terrestrial thermostats, are often used to selectively radiate heat from the interior of the spacecraft into space. Other thermal strategies include painting exterior surfaces. In some cases, spacecraft may also carry one or more active forms of heating to maintain temperature at required minimums.

Command and data subsystem

This designation is given to the main computer that oversees management of spacecraft functions and handling of collected data. Blocks of commands transmitted from Earth are stored in memory in the command and data subsystem and are executed at prescribed times. This subsystem also contains the spacecraft clock in order to accurately pace its activities, as well as all the activities of the spacecraft.

Structure subsystem

The spacecraft's physical structure is considered a subsystem itself for the purposes of planning and design. Usually the heart of this structure is a spacecraft bus, often consisting of a number of bays, which houses the spacecraft's main subsystems. See also Spacecraft structure.

Space probe

Contents

Interplanetary trajectories

Some notable probes

See also

References

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