- Grades: 6–8, 9–12
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 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. The key to space exploration, however, lay in the production of the rocket engine, 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, and the appropriate life support systems developed to meet those conditions.
Space exploration today includes the investigation of celestial objects ranging in size from atmospheric 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 using electrically charged ions.
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 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, 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 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, such as those using kerosene and liquid oxygen, have specific impuse values that measure in the mid-200s. Hypergolic systems such as those using hydrazine and nitrogen tetroxide, where the components ignite upon contact, have specific impulse 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 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 requires, first of all, that a vehicle be launched from the Earth's surface into outer space. The vehicle must 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 mi) 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 effectively escape the Earth's gravitational influence. 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. 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 orbital 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 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 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 the 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 the actual position of a space vehicle is a complex task. Powerful radar stations on Earth can track satellites out to several thousand kilometers by bouncing radar beams off the satellites. Some satellites have transponders that automatically echo such a transmitted radar pulse, which greatly facilitates tracking. Satellites in GEO, however, usually can be detected from Earth only by their own radio transmissions, by optically tracking the satellites through powerful telescope cameras, or by using very powerful radar pulses. For vehicles in deep space, tracking is accomplished through the analysis of returned radio signals, 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 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 direction, 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.
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 Tracking Station 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, the network of three satellites and three spares 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 worldwide tracking sites were shut down.
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," or 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 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 - thermoelectric systems in which the heat from radioactive decay is converted by thermocouples into electricity. The U.S. Cassini 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, in which 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 that uses onboard propulsion 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 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 an efficient insulator such as the material 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. 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.