The age of space exploration opened in the sixth decade of the 20th century. In its first two decades, robot probes and then human beings first ventured beyond the limits of Earth's atmosphere. Eventually, humans landed on the Moon. Probes have since gone on to explore the far realms of the solar system and beyond.
The space age is still young. But it is based on a long history of theoretical and practical developments. A theoretical basis had been laid by science well before the technology for entering space was developed. However, the key to space exploration lay in the production of the rocket engine that 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. Space technologies also include the development of transportation and communications hardware. In order for human beings to survive in space, conditions of that environment had to be studied as well. Scientists researched the effects of microgravity and radiation. Appropriate life-support systems were required to meet those conditions.
Space exploration today includes the investigation of celestial objects. These range 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. 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. Activities include 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. 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. This is 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. The reaction 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 using electrically charged ions.
The thrust, or "push," of a rocket engine is measured in one of two units. When measured in units of weight (kilograms or pounds), 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). However, thrust is more properly measured in newtons. A newton is a unit of force that gives one kilogram an acceleration of one meter per second 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.
The efficiency of a rocket engine is a much more crucial indicator of its performance. Efficiency is measured by a quantity called specific impulse. Specific impulse is equivalent to the propellant's exhaust velocity divided by g. The resulting unit of measure is seconds. An equivalent concept is that the specific-impulse 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 specific-impulse values of up to about 200. Simple liquid-rocket systems have specific-impuse values that measure in the mid-200s. These simple systems typically use kerosene and liquid oxygen. Hypergolic systems use hydrazine and nitrogen tetroxide. The components ignite upon contact and have specific impulse values exceeding 300. Cryogenic hydrogen fuel can deliver values in the mid-400s. A simple nuclear engine can deliver 800 to 900 seconds. An example is the NERVA project of the National Aeronautics and Space Administration (NASA) in 1970. But nuclear technology carries significant complications in safety.
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. The thrust of the rocket engine has become much 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 than can be discarded in flight; this is known as parallel staging.
A typical space-exploration mission initially requires that a vehicle be launched from the Earth's surface into outer space. The vehicle must be able to survive and operate in space. The vehicle sometimes returns to Earth. Each of these mission phases has special challenges that must be met by space scientists and engineers.
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. It achieves orbital altitude and velocity when it is more than 160 km (100 mi) above the Earth's surface and moving horizontally at about 7,600 m/sec (25,000 ft/sec). By then the booster must have expended about 9,100 m/sec (30,000 ft/sec) of velocity gain. This generally takes approximately eight to nine minutes, for an average acceleration of 2 gs.
Satellites commonly enter orbit close to Earth. 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 (GEO). This orbit is commonly used by communications and weather satellites because it keeps 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. This is the velocity needed for an object to effectively escape the Earth's gravitational influence. Escape velocity from Earth is roughly 11,200 m/sec (36,700 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. 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. The excess velocity is added to or subtracted from the Earth's own orbital velocity around the Sun, depending on the direction of aim. The velocity produces a new interplanetary orbit that may intersect the orbit of the intended target. With proper timing of the launch — the launch "window" — a target planet will be at the point where such an interception can occur.
The inflight operations of a spacecraft involve guidance, navigation, and control. These terms have specific meanings in space usage. Guidance refers to the determination of which way a probe should go to achieve a desired end position. The goal could be 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 later be 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 with a desired condition. The difference is used to determine the changes needed in current motion that would reduce the final differences. The future position is computed by extending the vehicle's "state vector" (current position and velocity) forward in time. All gravitational influences must be taken into account. In addition, smaller disturbances due to atmospheric drag, solar wind, and spacecraft venting must be considered. Significant course deviations can also be introduced by imprecise performance of the launch vehicle. A certain amount of state-vector dispersion must be expected due to imperfect executions of maneuvers.
Measuring the actual position of a space vehicle is a complex task. Powerful radar stations on Earth can track satellites out to several thousand kilometers. They do so by bouncing radar beams off the satellites. Some satellites have transponders that automatically echo such a transmitted radar pulse. This greatly facilitates tracking. However, satellites in GEO usually can be detected from Earth only by certain methods: by their own radio transmissions; by optical tracking of the satellites through powerful telescope cameras; or through the use of very powerful radar pulses. For vehicles in deep space, tracking is accomplished through the analysis of returned radio signals. These are analyzed in terms both of line-of-sight to the probe and of Doppler shifts of the signal from the probe as it passes through changing gravitational fields. These measurements are compared with 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 direction, is determined onboard. Precise angles can be measured relative to an outside inertial frame of reference by means of periodic star sightings. Gyroscopes are used to measure any subsequent variations that occur. A precise knowledge of attitude is required to perform proper course changes. Once required velocity changes have been computed, the vehicle must first rotate itself in space. This points the propulsion system in the proper direction. The vehicle must then fire at the precise moment that was computed. Any velocity change actually executed can be observed by accelerometers on the probe. Following such correction maneuvers, the vehicle is tracked. The probe's altered trajectory and end point are viewed. Additional course corrections can then be performed as needed.
Radio communications are required to command a probe. Such communications also provide information about the craft's status and the findings of its instruments. Information received over a radio link is called telemetry. Control is exercised by sending coded instructions to the spacecraft. These are interpreted by a circuit called the command decoder. The probe's computer autopilot executes the directions as necessary. For deep-space probes the round-trip time of radio signals can become excessive. For example, round-trip time to the Moon is only a matter of seconds. But it can take tens of minutes to reach Mars probes. For probes in the outer solar system the round-trip can take hours. Direct real-time commanding therefore cannot always be accomplished. A great deal of flexibility and anticipatory programs are involved in preparing for distant probes.
The use of ground tracking stations is very different for LEO satellites and for deep-space vehicles. Deep-space vehicles move through the Earth's sky very slowly. They can remain in sight of a single tracking site for up to 12 hours. However, LEO satellites travel at low altitude and at relatively great speeds. They quickly cross the sky of any ground site. Traveling from horizon to horizon may take only 5 to 6 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. But 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 expense of maintaining worldwide radio stations, both NASA and the Russian space program have developed geosynchronous relay satellites. Known in the United States as the Tracking and Data Relay Satellite system (TDRS), the network of nine satellites now routinely relays data from manned and some unmanned satellite missions. The equivalent Russian system is called Luch ("ray"). After the American system became fully operational in 1995, most of NASA's ground tracking sites were shut down. A set of second-generation tracking satellites are currently in development. The first was placed in orbit in 2000; others will follow through 2013.
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. These convert cryogenic oxygen and hydrogen reactions into electricity, with water as a waste product. The most common power source is solar cells. The cells are either arranged in flat panels or wrapped around the outer surface of a probe. However, they must be supplemented by batteries for those times when the spacecraft passes through planetary shadows. For missions to planets more distant from the Sun than the Earth is, solar cells would provide less energy. Therefore energy is sometimes provided instead by small nuclear devices. These compact systems of thermoelectricity use thermocouples to convert the heat from radioactive decay into electricity. The Cassini-Huygens spacecraft launched toward the planet Saturn in 1997 carries three such plutonium devices. Full-fledged nuclear reactors have been used in LEO, where large power output is required from compact units. The most notable example was the radar ocean surveillance program of the former Soviet Union: two satellites and their reactors fell 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. Onboard propulsion is used to slow the vehicle's speed by about 1%. 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 those on the surface of the Sun. This heat will soak into the vehicle unless it is shielded. Shields consist of either an ablative covering that carries heat away as it boils off; an efficient insulator such as the material in the Space Shuttle tiles; or an active cooling system. Atmospheric resistance slows the vehicle sharply, creating deceleration forces of up to ten times the force of gravity. Ionized, superheated air surrounds the vehicle with a sheath that blocks all radio communications. These effects require a precisely guided descent profile to enable a safe return.
James E. Oberg
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