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Spaceflight is used for placing in Earth's orbit communications satellites, reconnaissance satellites, Earth observation satellites, but also for space exploration such as space observatories and space probes, or even for space tourism.
- Overview
- The space environment
- Kinds of spacecraft
- Gravity
- Staging
- Acceleration rates
- Flight trajectories
- Sounding rocket
- Earth orbit
- Earth escape
spaceflight, flight beyond Earth’s atmosphere. This article deals with the basic concepts associated with the launch and return of unmanned and manned spacecraft and their travel, navigation, and rendezvous and docking in space. For the development of space travel and discussions of spacecraft and space programs and their contributions to scientifi...
Space, as considered here, is defined as all the reaches of the universe beyond Earth’s atmosphere. There is no definitive boundary above Earth at which space begins, but, in terms of the limiting altitude for vehicles designed for atmospheric flight, it may be considered to be as low as 45 km (28 miles). The lowest practical orbit for an artificial satellite around Earth is about 160 km (100 miles). By comparison, Earth’s natural satellite, the Moon, orbits the planet at a mean distance about 2,400 times greater—at 384,400 km (239,000 miles). Even this distance, however, is small compared with the size of the solar system, where spacecraft must traverse interplanetary distances measured in the hundreds of millions to billions of kilometres, and it is infinitesimal compared with the size of the universe. Earth’s nearest neighbouring stars lie more than 40 trillion km (25 trillion miles) away.
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Space Odyssey
The space that separates cosmic objects is not entirely empty. Throughout this void, matter—mostly hydrogen—is scattered at extremely low densities. Nevertheless, space constitutes a much greater vacuum than has been achieved on Earth. Additionally, space is permeated by gravitational and magnetic fields, a wide spectrum of electromagnetic radiation, and high-energy cosmic ray particles. Until the end of World War II, all deductions about space had been made from observations through the distorting atmosphere of Earth. With the advent of sounding rockets in the late 1940s and then of instrumented satellites, space observatories, probes, and manned spacecraft, it became possible to directly explore the complexities of space phenomena.
Spacecraft is a general term for objects launched into space—e.g., Earth-orbiting satellites and space probes, experiment capsules, the orbiting modules of some launch vehicles (e.g., the U.S. space shuttle or the Russian Soyuz), and space stations. Spacecraft are considered separately from the rocket-powered vehicles that launch them vertically into space or into orbit or boost them away from Earth’s vicinity (see sounding rocket and launch vehicle). A space probe is an unmanned spacecraft that is given a velocity great enough to allow it to escape Earth’s gravitational attraction. A deep-space probe is a probe sent beyond the Earth-Moon system; if sent to explore other planets, it is also called a planetary probe. An experiment capsule is a small unmanned laboratory that is often recovered after its flight. A space station is an artificial structure placed in orbit and equipped to support human habitation for extended periods.
Spacecraft differ greatly in size, shape, complexity, and purpose. Those that share similarities in design, function, or both are often grouped into program families—e.g., Gorizont, Meteor, Molniya, Resurs, Soyuz, and Uragan in Russia; Explorer, Galaxy, Iridium, Milstar, Navstar, Nimbus, Orbview, Telstar, and Voyager in the United States; Astra, Europestar, Envisat, Hotbird, Meteosat, and SPOT in Europe; Anik and Radarsat in Canada; Dong Fang Hong, Fengyun, and Shenzhou in China; Insat in India; and Ofeq in Israel. Lightness of weight and functional reliability are primary features of spacecraft design. Depending on their mission, spacecraft may spend minutes, days, months, or years in the environment of space. Mission functions must be performed while exposed to high vacuum, microgravity, extreme variations in temperature, and strong radiation.
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A general differentiation of spacecraft is by function—scientific or applications. A scientific satellite or probe carries instruments to obtain data on magnetic fields, space radiation, Earth and its atmosphere, the Sun or other stars, planets and their moons, and other astronomical objects and phenomena. Applications spacecraft have utilitarian tasks, such as telecommunications, Earth observation, military reconnaissance, navigation and position-location, power transmission, and space manufacturing.
Although the designs of the various spacecraft families and special-purpose spacecraft vary widely, there are nine general categories of subsystems found on most spacecraft. They are (1) the power supply, (2) onboard propulsion, (3) communications, (4) attitude control (i.e., maintaining a spacecraft’s orientation toward a specific direction and pointing its instruments precisely at selected targets), (5) environmental control (mainly regulation of the spacecraft components’ temperatures), (6) guidance, navigation, and flight control, (7) computer and data processing, (8) structure (the skeleton framework of the spacecraft that physically supports all other subsystems), and (9) a "health-monitoring" system that monitors the status of the spacecraft and its payload.
Earth’s gravitational attraction was one of the major obstacles to spaceflight. Because of the observations and calculations of earlier scientists, rocket pioneers understood Newton’s laws of motion and other principles of spaceflight, but the application of those principles had to await the development of rocket power to launch a spacecraft to the altitude and velocity required for its mission.
A spacecraft and its launch vehicle are projected upward by the unbalanced pressure inside the rocket engine. There is high pressure on the closed front end of the rocket’s thrust chamber but much lower pressure on the open back end, where the exhaust gases flow out the chamber’s nozzle. This unbalanced force is called the rocket’s thrust. If the total thrust of the engines were exactly equal to the weight of the entire spacecraft–launch-vehicle assembly at liftoff, the assembly would not move. But if, for example, the thrust were twice that weight, the assembly would rise at an initial acceleration equal to the standard gravitational acceleration of 9.8 metres (32.2 feet) per second per second. As propellant mass is consumed and ejected from the rocket engines, the vehicle continually lightens. Therefore, if the thrust is maintained constant, the vehicle’s acceleration increases as it rises.
Earth’s gravitational pull on the rising spacecraft subsides gradually. At an altitude of 160 km (100 miles) it is still 95 percent of that at Earth’s surface, and at 2,700 km (1,680 miles) it is 50 percent (4.9 metres per second per second). For the purpose of spaceflight, the gravitational pull of Earth becomes negligible only at distances of several million kilometres, except when a spacecraft approaches the Moon and lunar gravity (one-sixth that of Earth) becomes predominant.
Most spacecraft are launched vertically. But if the vehicle’s velocity remains perpendicular to Earth’s surface, it will not go into orbit but will eventually fall back to Earth (unless it can attain a velocity high enough to escape Earth’s gravitational influence). To achieve Earth orbit, the launch vehicle must be turned so that its velocity vector is parallel to Earth’s surface. When it reaches a speed high enough that the centrifugal acceleration of its curved path around Earth exactly balances Earth’s gravitational pull at that altitude, the spacecraft will be in orbit.
Because it is very difficult to achieve the high speed required to achieve orbit, launch vehicles need several stages to reach that speed. The technique of staging uses two or more rocket systems mounted in linear sequence. Initially, the rearmost, or first, stage is ignited, and it (sometimes assisted by attached booster rockets) lifts the vehicle at increasing velocity until its propellants are exhausted. At that point the stage drops off, lightening the vehicle, and the second stage is ignited. This stage, which is smaller and of lower thrust than the first, then accelerates the remaining launch vehicle farther. The use of additional stages generally follows the same pattern, until the payload (the spacecraft) has reached a velocity adequate to provide the centrifugal acceleration needed to balance Earth’s gravity and go into orbit.
For some missions the final stage is not employed during the initial climb into space but reserved for a later step of the flight. For example, a spacecraft carried on a three-stage vehicle may use the first two stages to achieve a low “parking orbit” around Earth. It is then boosted to a higher orbit or away from Earth by the third stage.
In general, the longer it takes a space vehicle to leave Earth’s atmosphere and achieve required velocity, the less economical the procedure becomes. At low accelerations the launch vehicle wastes much of its propellant because, in effect, it is investing nearly 10 metres per second of velocity each second of travel just to counter Earth’s gravitat...
There are four general types of trajectories: sounding rocket, Earth orbit, Earth escape, and planetary.
Sounding rockets provide the only means of making scientific measurements at altitudes of 45–160 km (28–100 miles), between the maximum altitude of balloons and the minimum altitude of orbiting satellites. They can be single-stage or multistage vehicles and are launched nearly vertically. After all the rocket stages have expended their fuel and hav...
Flight into Earth orbit usually is achieved by launching a rocket vertically from Earth’s surface and then tilting its trajectory so that its flight is parallel to the surface at the time that the spacefaring portion of the vehicle reaches orbital velocity at the desired altitude. Orbital velocity is the speed that provides the centrifugal acceleration, or pull, needed to balance exactly the pull of Earth’s gravity on the vehicle at that altitude. At this point the rocket engine is shut down. At an altitude of 200 km (125 miles), the velocity required to orbit Earth is about 29,000 km (18,000 miles) per hour. Because this altitude is above most of the atmosphere, aerodynamic drag is not great, and the spacecraft will continue to orbit for an extended time.
The time required for an orbiting spacecraft to make one complete revolution is called the orbital period. At 200 km this is about 90 minutes. The orbital period increases with altitude for two reasons. First, as the altitude increases, Earth’s gravity decreases, so the orbital velocity needed to balance it decreases. Second, the spacecraft has to travel farther to circle Earth. For example, at an altitude of 1,730 km (1,075 miles), the orbital velocity is 25,400 km (15,780 miles) per hour, and the period is two hours.
At about 35,800 km (22,250 miles), a spacecraft’s velocity is 11,100 km (6,900 miles) per hour, and its orbital period has a special value. It is equal to a sidereal day (see sidereal time), the rotational period of Earth measured against the fixed stars (about four minutes shorter than the conventional 24-hour solar day). A spacecraft in this orbit has properties desirable for certain applications. For example, if the orbit lies in the plane of Earth’s Equator, the spacecraft appears to an observer on Earth to be stationary in the sky. This particular orbit, called a geostationary orbit, is used for communications and meteorological satellites.
All the above figures assume a circular orbit, which for a spacecraft is often ideal but difficult to achieve. Usually a spacecraft’s orbit is an ellipse with a perigee altitude (nearest distance to Earth) and an apogee altitude (farthest distance from Earth). If thrust is available, a spacecraft’s orbit may be made more nearly circular by reducing the velocity at perigee (which lowers the apogee) or by increasing the velocity at apogee (which raises the perigee). Thrust in such instances is applied against or in the direction of flight, respectively.
In launching a spacecraft into Earth orbit, the launch vehicle most commonly is tilted after liftoff in an easterly direction. Launching to the east is done to take advantage of the velocity imparted to the vehicle by Earth’s eastward rotation. This rotational surface velocity is greatest at the Equator, about 1,670 km (1,037 miles) per hour, and it is 1,470 km (913 miles) per hour at the latitude of Cape Canaveral, Fla. At the still higher latitude of Russia’s Baikonur launch site in Kazakhstan, the surface velocity is 1,170 km (727 miles) per hour. It is possible to launch a spacecraft into a westerly orbit, but additional velocity, and thus additional propellant expenditure, is required to achieve an orbit of the same altitude compared with an easterly orbit.
If the spacecraft is to be put into a polar orbit—an orbit that crosses over Earth’s poles—it is launched in a northerly or southerly direction. Although the benefit of an easterly launch is lost, a spacecraft in an orbit perpendicular to the Equator offers other advantages. As Earth turns on its axis, the spacecraft travels over all parts of the globe every few revolutions. Satellites that monitor Earth’s environment, such as remote sensing satellites and some weather satellites, use polar orbits, as do some military surveillance satellites.
In order to escape completely from Earth’s gravity, a spacecraft requires a launch velocity of about 40,000 km (25,000 miles) per hour. If it subsequently does not come under the gravitational influence of another celestial body, it will go into an orbit around the Sun like a tiny planetoid. With precise timing, a spacecraft can be sent on a trajectory that will carry it near the Moon. In the case of the Apollo lunar landing flights, the spacecraft was placed on a trajectory calculated to pass ahead of the Moon and, under the influence of lunar gravity, to swing around the far side. If no velocity-changing maneuver had been made, the spacecraft would have looped around the Moon and returned on a trajectory toward Earth. By reducing flight speed on the Moon’s far side, Apollo astronauts placed their craft in a lunar orbit held by lunar gravity. Similar maneuvers were used to orbit a number of spacecraft around Mars, the Magellan spacecraft around Venus, the Galileo spacecraft around Jupiter, the Near Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker) spacecraft around the asteroid Eros, and the Cassini spacecraft around Saturn.
The so-called three-body problem of celestial mechanics (in the case of the Apollo missions, the relative motions of Earth, the spacecraft, and the Moon under their mutual gravitational influence) is extremely complex and has no general solution. Although equations expressing the relative motions can be written for specific cases, no expedient approximate solutions were possible before the development of high-speed digital computers for calculating trajectories of long-range missiles. Computers integrate the complicated equations of motion numerically, show the spacecraft’s complete trajectory at successive positions through space, and compare the actual flight path to the planned path at any point in time.
Apr 22, 2022 · NASA works with dozens of space agencies and thousands of companies around the world to develop innovative technologies to advance human space travel. The technical leaps and bounds since the dawn of the space age have found far-reaching applications to improve life on Earth.
The "gravity assist" concept has proven fundamental to exploring our "back yard" — the solar system. The technique has even been employed at least once to rescue an Earth-orbiting communications satellite whose launch vehicle failed to place it in its intended geosynchronous orbit.
- SpaceX is reusing rockets and landing boosters. In a stunning bit of rocket tech now considered routine, SpaceX recovers and reuses the first stages of both Falcon 9 and Falcon Heavy rockets.
- SpaceX helped reduce launch costs. Reusable rocket and spacecraft technology is the backbone upon which SpaceX builds its cost estimates, which tend to be lower than those of its competitors.
- SpaceX returned crewed orbital spaceflight to U.S. When NASA retired its space shuttle fleet in 2011, the agency was in the midst of supporting several American space companies working to develop the vehicles' replacement, including SpaceX.
- SpaceX ramped up orbital space tourism. We've already spoken about Inspiration4's mandate, but other aspects of the orbital space tourism mission are worth mentioning.
May 6, 2024 · space exploration, investigation, by means of crewed and uncrewed spacecraft, of the reaches of the universe beyond Earth ’s atmosphere and the use of the information so gained to increase knowledge of the cosmos and benefit humanity.
Space exploration is the use of astronomy and space technology to explore outer space. [1] . While the exploration of space is currently carried out mainly by astronomers with telescopes, its physical exploration is conducted both by uncrewed robotic space probes and human spaceflight.
