SPACE EXPLORATION The age of space exploration opened in the sixth decade of the 20th century. In the few years since that time, robot probes and then human beings first ventured beyond the limits of the Earth's atmosphere and landed on another celestial object, the Moon. Probes have since gone on to explore the far realms of the solar system and beyond. Although the space age is still young, it is based on a long history of theoretical and practical developments. Long before the enabling technology for entering space was developed, a theoretical basis had been laid by science (see ASTRONAUTICS). The key to space exploration, however, lay in the production of the rocket engine (see ROCKETS AND MISSILES), which made possible the lofting of objects beyond the Earth's atmosphere. Once that was achieved, supporting technologies combined to yield the broad range of activities now being pursued in the realm of space. Such technologies include the development of scientific instruments to sense the conditions and processes found in outer space and to observe the objects encountered there, as well as the development of the transportation and communications hardware to support these activities. In order for human beings to survive in space, the effects of the vacuum, microgravity, and radiation conditions of that environment had to be studied as well (see SPACE MEDICINE), and the appropriate LIFE SUPPORT systems developed to meet those conditions. Space exploration today includes the investigation of celestial objects ranging in size from cosmic dust to the giant planets of the solar system and the Sun itself. The conditions encountered in outer space also alter familiar terrestrial processes, from simple chemical reactions to complex biological activities, and such effects are being explored to determine how they might prove useful, as in crystallization and drug purification processes. In addition, objects placed in orbit around the Earth provide platforms both for astronomical studies and for a wide range of scientific and practical activities relating to the Earth's surface, including surveys of resources, studies of weather processes, and the relaying of communications and television images between distant points. All of these subject areas lie within the field of space exploration, and this article also observes the social and military considerations that are necessarily involved. BASIC ROCKET TECHNOLOGY The basic principle of a rocket engine is that when fuel is burned in the engine, the reaction mass is expelled at high speed and pushes the engine in the opposite direction, in accordance with Isaac Newton's law of action and reaction. The energy to expel the reaction mass usually comes from some sort of exothermic (heat-producing) chemical reaction that causes the combustion products to expand violently and to stream out of a nozzle. In chemical reactions the actual reaction mass is usually the combustion products of the reaction. A number of other types of rocket engines are also possible, including ion engines in which electrically charged ions form the reaction mass. Thrust The thrust, or "push," of a rocket engine is measured either in units of weight (kilograms or pounds)--where one unit of thrust gives to the equivalent unit of weight an acceleration of one gravity, or "g" ((9.8 m/sec/sec, or 32.2 ft/sec/sec)--or, more properly, in newtons. A newton is a unit of force that gives one kilogram an acceleration of one meter per second. For any rocket, thrust in kilograms can be converted to force in newtons by multiplying thrust by g. Large values are measured in kilonewtons. Efficiency The efficiency of a rocket engine is a much more crucial indicator of its performance. Efficiency is measured by a quantity called specific impulse (Isp), which is equivalent to the propellant's exhaust velocity divided by g. The resulting unit of measure is seconds. An equivalent concept is that the Isp value is the duration of time for which one kilogram of propellant can produce one kilogram of thrust. The higher the exhaust velocity and specific impulse of a rocket engine are, the more efficient it is. Solid-fuel rocket engines tend to have Isp values of up to about 200. Simple liquid-rocket systems, such as those using kerosene and liquid oxygen, have Isp values that measure in the mid-200s. Hypergolic systems such as those using hydrazine and nitrogen tetroxide, where the components ignite upon contact, have Isp values exceeding 300. Cryogenic hydrogen fuel can deliver values in the mid-400s. A simple nuclear engine such as the NERVA project of the National Aeronautics and Space Administration (NASA) in 1970 can deliver 800 to 900 seconds, but with significant complications in safety. Staging Rocket staging is also required in order to create vehicles of sufficient power for spaceflight. When most of a rocket's fuel has been exhausted, the rocket is carrying a great deal of empty-casing weight and is using a rocket engine whose thrust has become too great for the remaining vehicle weight. Schemes were therefore developed to discard empty tanks and large engines during the course of a rocket's ascent. The simplest technique was to place an entire smaller rocket on top of a larger one; this is called tandem staging. Other approaches involve the use of side-mounted engines or even engine and tank assemblies that can be discarded in flight; this is known as parallel staging. MISSION PHASES A typical space exploration mission requires, first of all, that a vehicle be launched from the Earth's surface into outer space. The vehicle must then be able to survive and operate in space, after which it is sometimes returned to Earth. Each of these mission phases has special challenges that must be met by space scientists and engineers. Launch The initial phase of launch must use engines with high thrust and compact fuel. In practice, the launch engine involves either kerosene/liquid oxygen or solid-fuel boosters. As a booster pushes against air resistance and the tug of gravity, it loses much of its energy during the ascent. By the time it has achieved orbital altitude and velocity, more than 160 km (100 miles) above the Earth's surface and moving horizontally at about 7,600 m/sec (25,000 ft/sec), the booster must have expended about 9,100 m/sec (30,000 ft/sec) of velocity gain. This generally takes about eight or nine minutes, for an average acceleration of 2 gs. Satellites commonly enter orbit close to Earth, and this region of space is referred to as low Earth orbit, or LEO. Propulsive stages can carry the payload higher, or into the 24-hour geosynchronous orbit, or GEO, commonly used by communications and weather satellites because they keep the satellites in position above a selected point on the Earth's surface. Alternatively, an upper stage on a rocket can fire to push a payload to ESCAPE VELOCITY, or the velocity needed if an object is to totally escape the Earth's gravitational influence. (This is speaking in practical terms since the gravitational influence of any object in the universe is actually pervasive.) Escape velocity from Earth is about 10,800 m/sec (35,000 ft/sec). Vehicles departing from earth are slowed by the tug of gravity, but as they attain greater and greater distances from the Earth, the gravitational pull decreases by the inverse square law (see GRAVITATION). Ultimately, a probe launched with the exact velocity for escape from Earth would reach an infinite distance with no speed left. In practice, vehicles are effectively out of Earth's influence at a distance of about 1,600,000 km (1,000,000 mi). At this distance they would drift in orbit about the Sun near earth's own orbit. In order to reach another planet, a vehicle must have a velocity that exceeds escape velocity. Added to or subtracted from the Earth's own velocity around the Sun, depending on the direction of aim, this excess velocity produces a new interplanetary orbit that may intersect the orbit of the intended target. With proper timing of the launch--the so-called launch "window"--a target planet will be at the point where such an interception can occur. Inflight Operations The inflight operations of a spacecraft involve guidance, navigation, and control. In space usage, these terms have specific meanings. Guidance refers to the determination of which way a probe should go to achieve a desired end position, such as a planetary intercept or a rendezvous with another vehicle. Navigation refers to the process of determining exactly where a probe is at any given time, and where it will be later along that same course. Control refers to means of altering the flight path of a probe, usually by means of small rockets. Guidance is accomplished by computing a space vehicle's end position compared to a desired condition, and then using the differences to determine what changes in current motion would result in smaller final differences. The future position is computed by propagating a vehicle's "state vector" (current position and velocity) forward in time, taking all gravitational influences into account as well as smaller perturbations due to atmospheric drag, solar wind, spacecraft venting, and similar disturbances. Significant course deviations can also be introduced by imprecise launch-vehicle performance. A certain amount of state-vector dispersion must be expected due to imperfect executions of maneuvers. Measuring a space vehicle's actual position is a complex task. Powerful radar stations of Earth can track satellites out to several thousand kilometers by bouncing radar beams off them. Some satellites have transponders that automatically echo such a transmitted pulse, which greatly facilitates trackings. Satellites in GEO can be detected from Earth only by their own radio transmissions, however, or by optical tracking through powerful telescope cameras. For deep-space vehicles, tracking is accomplished by analysis of returned radio signals, in terms both of line-of-sight to the probe and of Doppler shifts (see DOPPLER EFFECT) of the signal from the probe as it passes through changing gravitational fields. These measurements are compared to computer models of where the probe would be traveling if a certain initial position were assumed. From this comparison, a best estimate of position can then be determined. A space vehicle's own attitude, or pointing directions is determined onboard. Precise angles can be measured relative to an outside inertial frame of reference by means of periodic star sightings, and GYROSCOPES are used to measure any variations that occur subsequently. A precise knowledge of attitude is required to perform proper course changes. Once required velocity changes have been computed, the vehicle must rotate itself in space in order to point its propulsion system in the proper direction, and then perform the firing at the precise moment for which it was computed. The velocity change actually executed can be observed by accelerometers on the probe. Following such correction maneuvers, additional periods of tracking may occur so that more navigation can refine the knowledge of the probe's trajectory and end point, and additional course corrections can then be performed as needed (see NAVIGATION). Radio communications are required to command a probe and to receive information about its status and about the findings of its instruments. Information received over a radio link is called TELEMETRY. Control is exercised by sending coded instructions that are received by the spacecraft, interpreted by a circuit called the command decoder, and then executed as necessary by the probe's computer autopilot. For deep-space probes, the round-trip time of radio signals can become excessive. Round-trip time to the Moon, for example, is only a matter of seconds, but it can reach tens of minutes for Mars probes and many hours for probes in the outer solar system. Direct real-time commanding therefore cannot always be accomplished, and a great deal of flexibility and anticipatory programs are involved in preparing for such distant probes. The use of ground TRACKING STATIONS is very different for LEO satellites and for deep-space vehicles. The latter move through the Earth's sky very slowly and can remain in sight of a single tracking site for up to 12 hours. Because of the low altitude and relatively great speed of LEO satellites, however, they quickly cross the sky of any ground site, moving from horizon to horizon in five or six minutes. This explains why satellites near Earth spend most of their time out of radio contact even though 10 to 20 tracking sites are available for use, whereas vehicles millions of kilometers out in space can be continuously monitored by only a handful of sites strategically spread around the globe. NASA's deep-space network has three main sites, at Goldstone in California, Madrid in Spain, and near Canberra in Australia. In order to overcome this geographical restriction and to reduce the expenses associated with maintaining worldwide radio stations, both NASA and the Soviet space program have been developing geosynchronous relay satellites for use by other satellites. Known in the United States as the Tracking and Data Relay Satellite System (TDRSS), the network of two satellites and one spare is supposed to relay near-continuous data from manned and some unmanned satellite missions. The equivalent Soviet system is called Luch ("ray"). Once operational, such relay satellites could eliminate the need for the string of worldwide tracking sites that are now devoted to LEO payloads. Electrical power is another feature common to all space vehicles. Probes on missions lasting only a few days may use batteries or high-efficiency fuel cells that convert cryogenic oxygen and hydrogen reactions into electricity, with water as a waste product. Solar cells, arranged either in flat panels, or "wings," wrapped around the outer surface of a probe, are the most common power source. They must be supplemented by batteries, however, for those times when spacecraft pass through planetary shadows. For missions to Jupiter and beyond, where sunlight is only a few percent as great as near Earth, solar energy is too weak. In such cases small nuclear devices are used--thermoelectric systems in which the heat from radioactive decay is converted by thermocouples into electricity. Full-fledged nuclear reactors have also been used in LEO, where large power output is required from compact units. The most notable example is the Soviet radar ocean surveillance program, in which two satellites and their reactors have fallen to Earth, one in 1978 and the other in 1983. Return to Earth For spacecraft that are to be returned to Earth, a controlled descent is required. This is initiated by a "deorbit" maneuver that uses onboard propulsion to slow the vehicle's speed by about 1 percent. This slight amount is sufficient to lower the orbital path into the upper atmosphere, where drag will slow it further. Entry into the atmosphere poses special craft-survival problems. Tremendous heat builds up, not on the skin of the vehicle (friction is not involved) but just ahead of it, where a shock wave creates severe air compression. The resulting PLASMA can reach temperatures as high as on the surface of the Sun. This heat will soak into the vehicle unless it is shielded, either by an ablative covering that carries heat away as it boils off, by a very efficient insulator such as the material used in the SPACE SHUTTLE tiles, or by an active cooling system. Atmospheric resistance slows the vehicle sharply, creating deceleration forces of up to ten times the force of gravity. Superheated air becomes ionized and surrounds the vehicle with a sheath that blocks all radio communications. These effects require a precisely guided descent profile to enable a safe return.[37;40;1m THE SPACE ENVIRONMENT One of the first discoveries made by early Earth satellites was that an unexpectedly high number of charged particles were trapped in the Earth's magnetic field. Soviet instruments had earlier detected hints of this, but it was the U.S. Explorer 1 satellite that helped determine that the Earth is encircled by what are now known as the VAN ALLEN RADIATION BELTS--named for the scientist who designed the instruments aboard Explorer 1 and properly interpreted the readings. Other characteristics of the space environment, some anticipated and some unexpected, where also experienced by these early probes. Such characteristics include a very hard (that is, relatively very pure) vacuum, so-called zero gravity, high solar illumination levels, radiation, and micrometeorite hazards. The vacuum conditions encountered in space required the encapsulation of apparatuses and passengers in a space vehicle, or else the special and expensive design of equipment that could work without an air environment. The cooling of electronics systems became a problem, and moving parts required special lubricating systems because they otherwise tended to stick together when operating in space. The free fall of satellites in low Earth orbits created the condition commonly called zero gravity. Technically, this term is a misnomer. The force of gravity in low Earth orbits is scarcely diminished from that experienced at the Earth's surface; it is the motion of the satellite that results in the effect of weightlessness. The term "zero-G" has passed into common usage, however, and is going to remain in writings about space. Because slight accelerations actually do occur even on a satellite--due mainly to air drag and satellite motion--the more recent term microgravity has generally been adopted as well. Unfiltered solar radiation can cause illuminated portions of a spacecraft to rise to high temperatures. Meanwhile, shaded portions of the craft will radiate their warmth into space and cool below the freezing point of common fluids such as water and storable rocket fuels. All such fluid containers and lines are commonly equipped with electrical heaters, while overall temperatures are moderated by rotating the spacecraft along an axis perpendicular to the spacecraft-Sun line. This is known as passive thermal control, or, more colorfully, "barbecue mode." Unmanned spacecraft to the inner planets must be equipped with parasols to reflect away unwanted solar heat. Those sent to the outer solar system--or to the the Moon's surface, with its two-week long nights--often use radioisotope heaters (see SNAP). Radiation effects on spaceflights also took some time to appreciate. Satellites in LEO are protected by the magnetosphere from solar charged particles and from a large percentage of the cosmic rays arriving from outer space. Vehicles operating at GEO or on interplanetary missions, however, received the full force of these radiations. Cosmic rays have been known to penetrate integrated circuits in spacecraft autopilots and to alter data and commands. A space version of static electricity has been built up on other space vehicles during solar storms, resulting in electrical sparks that caused severe problems in onboard electronics. Experienced design of such systems has reduced the effects of these influences. The danger from micrometeorites, on the other hand, has proved to be slight. Although numerous impacts have been recorded--and, on at least one occasion, actually heard by an orbiting crew--no spacecraft is known to have been seriously damaged by such particles. Debris from other artificial satellites appears to be increasing as a significant danger, however, and one Soviet satellites may actually have been destroyed by such a collision with "space junk." ORBITAL SPACE EXPLORATION The practical uses of satellites in orbit around the Earth are described in many separate articles and in more general entries such as COMMUNICATIONS SATELLITES. The scientific exploration of space and the Earth has been advanced by many other orbital systems. NASA, for example, has successfully launched eight Orbiting Solar Observatories (see OSO) and the Solar Maximum Mission for studying the Sun. To study regions beyond the solar system, NASA launched three Small Astronomical Satellites (see UHURU), two Orbiting Astronomical Observatories (see OAO), and three HIGH ENERGY ASTRONOMICAL OBSERVATORIES. With the European Space Agency (ESA) and the United Kingdom it has launched the International Ultraviolet Explorer (see ULTRAVIOLET ASTRONOMY), and, with the Netherlands, the Infrared Astronomy Satellite (see INFRARED ASTRONOMY). ESA, for its part, has launched the Exosat astronomy satellite for exploring the X-ray region of the electromagnetic spectrum (see X-RAY ASTRONOMY). Earth-observing satellites have included NASA's six Orbiting Geophysical Observatories (see OGO) and many of its EXPLORER series. One of the most ambitious of all scientific programs, NASA's SPACE TELESCOPE, is to be launched by the Space Shuttle. Geodetic surveys and accurate navigational data may be obtained from precise tracking of the positions of the U.S. Navy's TRANSIT satellites and the more recent NAVSTAR series. The most accurate geodetic data can be obtained from pulsed laser beams. The Laser Geodynamic Satellite, or LAGEOS, placed in orbit in 1976, permits measurements of the movements of the Earth's crust to an accuracy of within 2 cm (0.8 in), thereby providing information vital to earthquake-prediction research. Geosynchronous orbits provide ideal observational positions for many applications satellites, but another highly useful location for Earth observation is a Sun-synchronous orbit, which is slightly tilted off true polar orbit so that the orbital plane shifts to keep pace with the Sun's annual motion through the ecliptic. INTERPLANETARY PROBES The exploration of other bodies in the solar system began within a few years of the first satellites. Both U.S. and Soviet space engineers set their sights on the Moon. Early Soviet launches in 1958 all failed and were never announced. Several U.S. launches also failed, although two of them (Pioneers 1 and 3) reached nearly 100,000 km into space before falling back to Earth. The first probe to escape Earth's gravity was LUNA 1, launched on Jan. 2, 1959, which passed the Moon and continued into interplanetary space. The U.S. probe Pioneer 4, launched two months later, followed the same path. Later Soviet probes either hit the Moon or passed it and took photographs of the hidden far side, relaying them back to Earth. In the mid-1960s three NASA programs pursued the lunar objective. RANGER probes crashed into the Moon's surface but succeeded in sending high-resolution photographs prior to impact. SURVEYOR probes soft-landed on the Moon and analyzed its surface, while LUNAR ORBITER probes circled the Moon and sent back pictures both of potential landing sites for astronauts and of areas of general scientific interest. Soviet efforts proceeded along similar lines and achieved limited successes shortly before their U.S. equivalents (Luna 9 made the first successful soft landing on the moon in 1966, and Luna 10 became the first probe to enter lunar orbit a few months later). The Soviets also operated a series of heavy automatic probes that retrieved small amounts of lunar soil, deployed wheeled rovers called LUNOKHODS, and made lunar-orbit surveys. The USSR also aimed three Soviet rockets towards MARS in 1960, but all failed; the last caused loss of life when it exploded during a launchpad checkout. Other shots in the Soviet series also failed, including later heavy-landing attempts. The first successful Mars probe was the U.S. MARINER 4, in 1964. Two more fly-by missions and an orbital photographic flight followed before the sophisticated landings of VIKING spacecraft in 1976. The USSR sent two probes to reach the Martian moon Phobos in 1989, but one went out of control shortly after launch, and contact was lost with the other soon after it began returning data form Mars orbit. Soviet probes toward VENUS also failed in early attempts, and the first spacecraft to reach the planet successfully was NASA's Mariner 2, in 1962. Later Soviet VENERA atmospheric probes eventually returned some basic data, and Mariner 5 provided sophisticated measurements during a fly-by. A Soviet probe survived briefly on the surface of Venus in 1971, with more advanced landers following in 1975. Two NASA pioneer missions in 1978 provided additional atmospheric and topographical data, while a pair of Soviet probes conducted a radar-mapping mission of the planet's northern hemisphere in 1983. Two Soviet spacecraft dropped instrumented balloons into the upper atmosphere in 1985. The only mission to Mercury was the triple fly-by performed by Mariner 10 in 1974-75. The craft returned detailed photographs and environmental measurements of the planet. Among the giant outer planets, the first missions to Jupiter were NASA's Pioneer 10 and Pioneer 11 probes, launched by Atlas-Centaur boosters in 1972-73. They each took more than two years to reach their destination before continuing on toward the outer solar system. Their measurements of the environment of Jupiter and particularly of its severe radiation belts paved the way for the VOYAGER probes launched in 1977. Voyager 1 and Voyager 2 discovered new features of the Jupiter system, such as new moonlets, another ring, and active volcanism on Jupiter's satellite IO. Saturn was first visited by the Pioneer 11 spacecraft in 1979, and in the following year both Voyager probes passed the planet, measuring its environment and observing its atmosphere, satellites, and impressive system of rings. Voyager 2 went on to become the first probe to reach the planets URANUS, in 1986, Neptune, in 1989. This left Pluto as the only planet unvisited by investigatory probes. The first probe to a comet was the International Sun-Earth Explorer launched by NASA in 1978 into a high lunar orbit and then diverted five years later for an interception of Comet Giacobini-Zinner. In 1986, four probes from Earth--two Soviet Vega missions, ESA's Giotto, and a Japanese payload--passed near HALLEY'S COMET. Many early Pioneer probes were launched to observe the Sun from different angles. Two German spacecraft, Helios 1 and Helios 2, were sent well within the orbit of Mercury to measure the Sun's magnetic field and other environmental features. A later European probe, named Ulysses, is intended to explore the polar regions of the Sun. Two further planetary probes got under way in 1989 after being launched from the Space Shuttles. One, Magellan, is to study Venus, while GALILEO was sent toward Jupiter. MANNED SPACE MISSIONS The most challenging and exciting aspect of space exploration has been manned space flight. Soon after the first satellites were launched, both Soviet and American design teams began work on manned space vehicles. The Soviet team was able to make use of a launch vehicle three times as powerful as the ones available to NASA, so the USSR was able to choose familiar systems known to be reliable. NASA's need to venture into new technological disciplines, on the other hand, later proved crucial to the success of manned lunar flight--a challenge that the Soviet design teams were unable to meet. After several unmanned test flights in 1960 and early 1961, the USSR launched the world's first manned spacecraft, VOSTOK, on Apr. 12, 1961. The pilot was a 26-year-old Russian named Yuri GAGARIN. The basic spacecraft consisted of a three-ton sphere and a two-ton service module. The sphere was later rebuilt to hold three crew members and was launched with the name VOSKHOD in 1964. In 1961, President John F. Kennedy had declared that the goal of the U.S. manned space program was to land a man on the Moon before the end of the decade. The story of that endeavor is recounted in the entries on the MERCURY PROGRAM, GEMINI PROGRAM, and APOLLO PROGRAM. The landing did take place on schedule, on July 20, 1969. Since those first years of flight, the Soviet and U.S. manned programs have taken divergent paths (see SPACE PROGRAMS, NATIONAL). POLICY ISSUES Several major policy questions are involved in planning for the future in space exploration. Such questions are concerned with the matter of international cooperation as opposed to international competition, the desirability of manned versus unmanned activities in space, the issue of long-range planning, the employment of reusable versus expendable systems, and problems relating to space industrialization and the orbiting of military systems. Because most nations who fund space exploration do so in order to accrue national benefits--whether technological military, or simply in national prestige--the development of international cooperation has long been balanced against concerns for such national benefits. It should be observed that many international space projects, such as Spacelab, Ariane, Vega, and Kvant, have proved particularly successful because each nation involved has contributed in its own areas of specialization. Such alliances have yielded large amounts of knowledge that could then be shared satisfactorily--as could the prestige involved, as well. The costliness of manned space activities has fueled the ongoing dispute over the desirability of involving astronauts and cosmonauts in operations that can also be conducted automatically by means of remote control. Many tasks of space exploration are in fact best performed by unmanned systems, but the most complex and unpredictable operations often exceed the capacity of computers and telemetric systems and instead require the presence of flexible, perceptive, imaginative humans. Experience on Earth has demonstrated that the most valuable discoveries in exploration and research are often those which were completely unexpected, and hence those which unmanned systems might overlook. Long-range planning for space exploration is bedeviled with down-to-earth problems of political and budgetary support. Despite the general desire for consistent levels of space funding, nations customarily do not plan budgets for decades in advance. National interest in space activities also waxes and wanes with dramatic short-term variations, based on opportunities that arise for the achievement of relatively short-term goals. Major space projects in the United States, at least, have tended to take place in sprints, with long pauses in between, and realistically speaking, they will probably continue to do so. The high cost of space exploration has led to major efforts to economize, and this in turn has created the conflict between reusable and expendable spacecraft systems, primarily in the area of launch vehicles. The issue is complicated by the fact that the largest costs of space operations usually are not for the hardware but instead for mission checkout, preparation, and management, and these costs may actually be increased if the space vehicle being prepared has already been used several times. In addition, reusable space systems must sacrifice a significant portion of their performance in order to include recovery aids, such as wings or parachutes. Thus, "throwaway" systems may actually be cheaper to operate, pending the development of advanced space vehicles that can be recycled economically. The promise of space industrialization remains thus far unrealized, because of the continuing high cost of space operations--primarily in the area of transportation--and because ground industrial technologies also continue to advance. The high start-up costs for the space industry, combined with the excessively long lead times between investment and payoff, have been major factors in discouraging the development of such projects by private Western corporations. Clearly, however, a number of high-value and low-volume products such as crystals, drugs, and glass could be manufactured in space at an advantage, and the commercial sale of such products has already begun. Few space-exploration issues are more emotional or have more diplomatic and ideological connotations than the conflict over space militarization versus treaty restrictions. Space is already a transit region for military missiles, and it has served as a vantage point for military reconnaissance since soon after the first days of Sputnik. Several limited antisatellite (ASAT) systems have also been developed, and the Soviet Union has an orbital "killer" satellite. Nevertheless, efforts to develop space-based weapons systems such as the U.S. STRATEGIC DEFENSE INITIATIVE program are considered by many to be excessive and inflammatory. Technical issues are poorly defined in this dispute, which is grounded mainly in political doctrines.