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Low Earth Orbit

Low Earth orbit

A low Earth orbit (LEO) is an orbit around Earth between the atmosphere and the Van Allen radiation belt, with a low angle of inclination. These boundaries are not firmly defined but are typically around 200 - 1200 km (124 - 726 miles) above the Earth's surface. This is generally below intermediate circular orbit (ICO) and far below geostationary orbit. Orbits lower than this are not stable and will decay rapidly because of atmospheric drag. Orbits higher than this are subject to early electronic failure because of intense radiation and charge accumulation. Orbits with a higher inclination angle are usually called polar orbits. Objects in low earth orbit encounter atmospheric gases in the thermosphere (approximately 80-500 km up) or exosphere (approximately 500 km and up), depending on orbit height. Most manned spaceflights have been in LEO, including all Space Shuttle and various space station missions; the only exceptions have been suborbital test flights such as the early Project Mercury missions and the flights of the X-15 rocket plane (which was not intended to reach LEO), and the Project Apollo missions to the Moon (which went beyond LEO). Most artificial satellites are placed in LEO, where they travel at about 27,400 km/h (8 km/s), making one revolution in about 90 minutes. The primary exception are communication satellites that require geostationary orbit. However, it requires less energy to place a satellite into a LEO and the satellite needs less powerful transmitters for data transfer, so LEO is still used for many communication applications. Because these orbits are not geostationary, a network of satellites is required to provide continuous coverage. Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun synchronous LEO orbits at an altitude of about 800km and near polar inclination. ENVISAT is one example of an earth observation satellite that makes use of this special type of LEO. The LEO environment is becoming congested, not least with space debris. The United States Space Command tracks more than 8,000 objects larger than 10cm in LEO. Although gravity in LEO is not much less than on the surface of the Earth (it reduces 1% every 30 km), people and objects in orbit experience weightlessness (see article). Atmospheric and gravity drag associated with launch typically add 1,500-2,000 m/s to the delta-V required to reach normal LEO orbital velocity of 7,800 m/s.

Alternatives

Airships have been proposed to hover above the Earth at an altitude of around 13 miles (20 kilometres) as communication stations, to provide cellular voice and data service. Solar-powered unpiloted aircraft (UAVs) are also proposed for this purpose.

Orbit

.]] :For other meanings of the term "orbit", see orbit (disambiguation) In physics, an orbit is the path that an object makes around another object while under the influence of a source of centripetal force, such as gravity.

History

Orbits were first analysed mathematically by Johannes Kepler who formulated his results in his laws of planetary motion. He found that the orbits of the planets in our solar system are elliptical, not circular (or epicyclic), as had previously been believed. Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation and that, in general, the orbits of bodies responding to the force of gravity were conic sections. Newton showed that a pair of bodies follow orbits of dimensions that are in inverse proportion to their masses about their common center of mass. Where one body is much more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body.

Planetary orbits

Within a planetary system, planets, asteroids, comets and space debris orbit the central star in elliptical orbits. Any comet in a parabolic or hyperbolic orbit about the central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. To date, no comet has been observed in our solar system with a distinctly hyperbolic orbit. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about that planet. Due to mutual gravitational perturbations, the eccentricities of the orbits of the planets in our solar system vary over time. Pluto and Mercury have the most eccentric orbits. At the present epoch, Mars has the next largest eccentricity while the smallest eccentricities are those of the orbits of Venus and Neptune. As an object orbits another, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest from each other. In the elliptical orbit, the centre of mass of the orbiting-orbited system will sit at one focus of both orbits, with nothing present at the other focus. As a planet approaches periapsis, the planet will increase in velocity. As a planet approaches apoapsis, the planet will decrease in velocity. See also: Kepler's laws of planetary motion

Understanding orbits

There are a few common ways of understanding orbits.
- As the object moves sideways, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
- A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
- As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center. As an illustration of the orbit around a planet (eg Earth), the much-used cannon model may prove useful (see image below). Imagine a cannon sitting on top of a (very) tall mountain, which fires a cannonball horizontally. The mountain needs to be very tall, so that the cannon will be above the Earth's atmosphere and we can ignore the effects of air friction on the cannon ball. 300px If the cannon fires its ball with a low initial velocity, the trajectory of the ball will curve downwards and hit the ground (A). As the firing velocity is increased, the cannonball will hit the ground further (B) and further (C) away from the cannon, because while the ball is still falling towards the ground, the ground is curving away from it (see first point, above). If the cannonball is fired with sufficient velocity, the ground will curve away from the ball at the same rate as the ball falls - it is now in orbit (D). The orbit may be circular like (D) or if the firing velocity is increased even more, the orbit may become more (E) and more (F) elliptical. At a certain even faster velocity (called the escape velocity) the motion changes from an elliptical orbit to a parabola.

Newton's laws of motion

For a system of only two bodies that are only influenced by their mutual gravity, their orbits can be exactly calculated by Newton's laws of motion and gravity. Briefly, the sum of the forces will equal the mass times its acceleration. Gravity is proportional to mass, and falls off proportionally to the square of distance. To calculate, it is convenient to describe the motion in a coordinate system that is centered on the heavier body, and we can say that the lighter body is in orbit around the heavier body. An unmoving body that's far from a large object has more energy than one that's close. This is because it can fall farther. This is called "potential energy" because it is not yet actual. With two bodies, an orbit is a flat curve. The orbit can be open (so the object never returns) or closed (returning), depending on the total kinetic + potential energy of the system. In the case of an open orbit, the speed at any position of the orbit is at least the escape velocity for that position, in the case of a closed orbit, always less. The path of a free-falling (orbiting) body is always a conic section. An open orbit has the shape of a hyperbola (or in the limiting case, a parabola); the bodies approach each other for a while, curve around each other around the time of their closest approach, and then separate again forever. This is often the case with comets that occasionally approach the Sun. A closed orbit has the shape of an ellipse (or in the limiting case, a circle). The point where the orbiting body is closest to Earth is the perigee, called periapsis (less properly, "perifocus" or "pericentron") when the orbit is around a body other than Earth. The point where the satellite is farthest from Earth is called apogee, apoapsis, or sometimes apifocus or apocentron. A line drawn from periapsis to apoapsis is the line-of-apsides. This is the major axis of the ellipse, the line through its longest part. Orbiting bodies in closed orbits repeat their path after a constant period of time. This motion is described by the empirical laws of Kepler, which can be mathematically derived from Newton's laws. These can be formulated as follows: # The orbit of a planet around the Sun is an ellipse, with the Sun in one of the focal points of the ellipse. Therefore the orbit lies in a plane, called the orbital plane. The point on the orbit closest to the attracting body is the periapsis. The point farthest from the attracting body is called the apoapsis. There are also specific terms for orbits around particular bodies; things orbiting the Sun have a perihelion and aphelion, things orbiting the Earth have a perigee and apogee, and things orbiting the Moon have a perilune and apolune (or, synonymously, periselene and aposelene). An orbit around any star, not just the Sun, has a periastron and an apastron # As the planet moves around its orbit during a fixed amount of time, the line from Sun to planet sweeps a constant area of the orbital plane, regardless of which part of its orbit the planet traces during that period of time. This means that the planet moves faster near its perihelion than near its aphelion, because at the smaller distance it needs to trace a greater arc to cover the same area. This law is usually stated as "equal areas in equal time." # For each planet, the ratio of the 3rd power of its semi-major axis to the 2nd power of its period is the same constant value for all planets. Except for special cases like Lagrangian points, no method is known to solve the equations of motion for a system with four or more bodies. The 2-body solutions were published by Newton in Principia in 1687. In 1912, K. F. Sundman developed a converging infinite series that solves the 3-body problem, however it converges too slowly to be of much use. Instead, orbits can be approximated with arbitrarily high accuracy. These approximations take two forms. One form takes the pure elliptic motion as a basis, and adds perturbation terms to account for the gravitational influence of multiple bodies. This is convenient for calculating the positions of astronomical bodies. The equations of motion of the moon, planets and other bodies are known with great accuracy, and are used to generate tables for celestial navigation. The differential equation form is used for scientific or mission-planning purposes. According to Newton's laws, the sum of all the forces will equal the mass times its acceleration (F = ma). Therefore accelerations can be expressed in terms of positions. The perturbation terms are much easier to describe in this form. Predicting subsequent positions and velocities from initial ones corresponds to solving an initial value problem. Numerical methods calculate the positions and velocities of the objects a tiny time in the future, then repeat this. However, tiny arithmetic errors from the limited accuracy of a computer's math accumulate, limiting the accuracy of this approach. Differential simulations with large numbers of objects perform the calculations in a hierarchical pairwise fashion between centers of mass. Using this scheme, galaxies, star clusters and other large objects have been simulated.

Analysis of orbital motion

(see also orbit equation and Kepler's first law) To analyse the motion of a body moving under the influence of a force which is always directed towards a fixed point, it is convenient to use polar coordinates with the origin coinciding with the centre of force. In such coordinates the radial and transverse components of the acceleration are, respectively: :

\frac - r\left( \frac \right)^2

and :

\frac\frac\left( r^2\frac \right)

. Since the force is always radial, the transverse acceleration is zero, and it follows that: :

\frac = hu^2

, where h is a constant of integration and we have introduced the auxiliary variable u defined as 1/r. If magnitude of the radial force is f(r) per unit mass of the orbiting body, then the elimination of the time variable from the radial component of the equation of motion yields: :

\frac + u = \frac

. In the case of an inverse square force law the right hand side of the equation becomes a constant and the equation is seen to be the harmonic equation (up to a shift of origin of the dependent variable). The equation of the orbit described by the particle is thus: :

r = \frac = \frac

, where φ and e are constants of integration and L is the Semi-latus rectum. This can be recognised as the equation of a conic section in polar coordinates.

Orbital parameters

See: Orbital elements For a general elliptic orbit, the relations between the axis, eccentricity, and least and largest distance are: :Semimajor axis = (periapsis + apoapsis)/2 = mean of the extreme radii :Periapsis = semimajor axis × (1 - eccentricity) = least distance :Apoapsis = semimajor axis × (1 + eccentricity) = largest distance Note that there are alternative definitions for a "mean radius" or "average distance": if you average the radius over time for one orbit (mean anomaly), or over the orbital angle as observed by the primary (true anomaly), then you get a different result. See here for details.

Orbital period

See: orbital period

Orbital decay

If some part of a body's orbit enters an atmosphere, its orbit can decay because of drag. At each periapsis, the object scrapes the air, losing energy. Each time, the orbit grows less eccentric (more circular) because the object loses kinetic energy precisely when that energy is at its maximum. Eventually, the orbit circularises and then the object spirals into the atmosphere. The bounds of an atmosphere vary wildly. During solar maxima, the Earth's atmosphere causes drag up to a hundred kilometres higher than during solar minimums. Some satellites with long conductive tethers can also decay because of electromagnetic drag from the Earth's magnetic field. Basically, the wire cuts the magnetic field, and acts as a generator. The wire moves electrons from the near vacuum on one end to the near-vacuum on the other end. The orbital energy is converted to heat in the wire. Another method of artificially influencing an orbit is through the use of solar sails or magnetic sails. These forms of propulsion require no propellant or energy input, and so can be used indefinitely. See statite for one such proposed use. Orbital decay can also occur due to tidal forces for objects below the synchronous orbit for the body they're orbiting. The gravity of the orbiting object raises tidal bulges in the primary, and since below the synchronous orbit the orbiting object is moving faster than the body's surface the bulges lag a short angle behind it. The gravity of the bulges is slightly off of the primary-satellite axis and thus has a component along the satellite's motion. The near bulge slows the object more than the far bulge speeds it up, and as a result the orbit decays. Conversely, the gravity of the satellite on the bulges applies torque on the primary and speeds up its rotation. Artificial satellites are too small to have an appreciable tidal effect on the planets they orbit, but several moons in the solar system are undergoing orbital decay by this mechanism. Mars' innermost moon Phobos is a prime example, and is expected to either impact Mars' surface or break up into a ring within 50 million years. Finally, orbits can decay via the emission of gravitational waves. This mechanism is extremely weak for most stellar objects, only becoming significant in cases where there is a combination of extreme mass and extreme acceleration, such as with black holes or neutron stars that are orbiting each other closely.

Earth orbits

See Earth orbit for more details.
- Low Earth orbit
- High Earth Orbit
- Intermediate circular orbit
- Geostationary orbit
- Geosynchronous orbit
- Geostationary transfer orbit
- Molniya orbit
- Polar orbit
- Polar Sun Synchronous Orbit (this is not a complete list).

Scaling in gravity

The gravitational constant G is defined to be:
- 6.6742 × 10⁻¹¹ N·m²/kg²
- 6.6742 × 10⁻¹¹ m³/(kg·s²)
- 6.6742 × 10⁻¹¹(kg/m³)^-1s^-2. Thus the constant has dimension density^-1 time^-2. This corresponds to the following properties. Scaling of distances (including sizes of bodies, while keeping the densities the same) gives similar orbits without scaling the time: if for example distances are halved, masses are divided by 8, gravitational forces by 16 and gravitational accelerations by 2. Hence orbital periods remain the same. Similarly, when an object is dropped from a tower, the time it takes to fall to the ground remains the same with a scale model of the tower on a scale model of the earth. When all densities are multiplied by four, orbits are the same, but with orbital velocities doubled. When all densities are multiplied by four, and all sizes are halved, orbits are similar, with the same orbital velocities. These properties are illustrated in the formula :

GT^2 \sigma = 3\pi \left( \frac \right)^3,

for an elliptical orbit with semi-major axis a, of a small body around a spherical body with radius r and average density σ, where T is the orbital period.

Role in the evolution of atomic theory

When atomic structure was first probed experimentally early in the twentieth century, an early picture of the atom portrayed it as a miniature solar system bound by the coulomb force rather than by gravity. This was inconsistent with electrodynamics and the model was progressively refined as quantum theory evolved, but there is a legacy of the picture in the term orbital for the wave function of an energetically bound electron state.

External links

- An on-line orbit plotter: http://www.bridgewater.edu/departments/physics/ISAW/PlanetOrbit.html
- [http://www.braeunig.us/space/orbmech.htm Orbital Mechanics] (Rocket and Space Technology) Category:Celestial mechanics Category:Solar System als:Umlaufbahn ja:軌道 (力学) simple:Orbit th:วงโคจร

Van Allen radiation belt

The Van Allen radiation belt is a torus of energetic charged particles (ie. a plasma) around Earth, trapped by Earth's magnetic field. When the belts "overload", particles strike the upper atmosphere and fluoresce, causing the polar aurora. The presence of a radiation belt had been theorized prior to the Space Age and the belt's presence was confirmed by the Explorer I on January 31, 1958 and Explorer III missions, under Doctor James Van Allen. The trapped radiation was first mapped out by Explorer IV and Pioneer III. Qualitatively, it is very useful to view this belt as consisting of two belts around Earth, the inner radiation belt and the outer radiation belt. The particles are distributed such that the inner belt consists mostly of protons while the outer belt consists mostly of electrons. Within these belts are particles capable of penetrating about 1 g/cm² ₍₂₎ of shielding (e.g., 1 millimetre of lead). The term Van Allen Belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The atmosphere limits the belts' particles to regions above 200-1000 km ₍₁₎, while the belts do not extend past 7 Earth radii R_E ₍₁₎. The belts are confined to an area which extends about 65° ₍₁₎ from the celestial equator.

The Outer Van Allen Belt

celestial equator in his terrella, a magnetised anode globe in an evacutated chamber.]] The big outer radiation belt extends from an altitude of about 10,000–65,000 km and has its greatest intensity between 14,500–19,000 km. The outer belt is thought to consist of plasma trapped by the Earth's magnetosphere. The USSR's Luna 1 reported that there were very few particles of high energy within the outer belt. The electrons here have a high flux and along the outer edge and electrons with kinetic energy E > 40 keV can drop to normal interplanetary levels within about 100 km (a decrease by a factor of 1000). This drop-off is a result of the solar wind. The particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source. The outer belt is larger and more diffuse than the inner, surrounded by a low-intensity region known as the ring current. Unlike the inner belt, the outer belt's particle population fluctuates widely and is generally weaker in intensity (less than 1 MeV), rising when magnetic storms inject fresh particles from the tail of the magnetosphere, and then falling off again. There is debate as to whether the outer belt was discovered by the US Explorer IV or the USSR Sputnik II/III.

The Van Allen Belt's impact on space travel

Solar cells, integrated circuits, and sensors can be damaged by radiation. In 1962, the Van Allen belts were temporarily amplified by a high-altitude nuclear explosion (the Starfish Prime test) and several satellites ceased operation. Magnetic storms occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics and logic circuits have made satellites more vulnerable to radiation, as incoming ions may be as large as the circuit's charge. Electronics on satellites must be hardened against radiation to operate reliably. The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation. An object satellite shielded by 3 mm of aluminum will receive about 2500 rem ₍₃₎ (25 Sv) per year. Proponents of the Apollo Moon Landing Hoax have argued that space travel to the moon is impossible because the Van Allen radiation would kill or incapacitate an astronaut who made the trip. In practice, Apollo astronauts who travelled to the moon spent very little time in the belts and received a harmless dose. [http://spider.ipac.caltech.edu/staff/waw/mad/mad19.html]. Nevertheless NASA deliberately timed Apollo launches, and used lunar transfer orbits that only skirted the edge of the belt over the equator to minimise the radiation. Astronauts who visited the moon probably have a slightly higher risk of cancer during their lifetimes, but still remain unlikely to become ill because of it.

The Van Allen Belts and why they exist

It is generally understood that the Van Allen belts are a result of the collision of Earth's magnetic field with the solar wind. Radiation from the solar wind then becomes trapped within the magnetosphere. The trapped particles are repelled from regions of stronger magnetic field, where field lines converge. This causes the particle to bounce back and forth between the earth's poles, where the magnetic field increases. The gap between the inner and outer Van Allen belts is caused by low-frequency radio waves that eject any particles that would otherwise accumulate there. Solar outbursts can pump particles into the gap but they drain again in a matter of days. The radio waves were originally thought to be generated by turbulence in the radiation belts, but recent work by James Green of the NASA Goddard Space Flight Center comparing maps of lightning activity collected by the Micro Lab 1 spacecraft with data on radio waves in the radiation-belt gap from the IMAGE spacecraft suggests that they're actually generated by lightning within Earth's atmosphere. The radio waves they generate only strike the ionosphere at the right angle to pass through it only at high latitudes, where the lower ends of the gap approach the upper atmosphere. The Soviets once accused the U.S. of creating the inner belt as a result of nuclear testing in Nevada. The U.S. has, likewise, accused the USSR of creating the outer belt through nuclear testing. It is uncertain how particles from such testing could escape the atmosphere and reach the altitudes of the radiation belts. Likewise, it is unclear why, if this is the case, the belts have not weakened since atmospheric testing was banned by treaty. Tom Gold has argued that the outer belt is left over from the aurora while Alex Dessler has argued that the belt is a result of volcanic activity.

Removing the belts

The belts are a hazard for artificial satellites and moderately dangerous for human beings and difficult and expensive to shield against. There is a proposal by the late Robert L. Forward called HiVolt which may be a way to drain at least the inner belt to 1% of its natural level within a year. The proposal involves deploying highly electrically charged tethers in orbit. The idea is that the electrons would be deflected by the large electrostatic fields and intersect the atmosphere and harmlessly dissipate. Some scientists, however, theorize that the Van Allen belts carry some additional protection against solar wind, which means that a weakening of the belts could harm electronics and organisms.

References

- 1 : Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)
- 2 : The Radiation Belt and Magnetosphere by Wilmot Hess (1968)
- 3 : NASA http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970228a.html

External links

- [http://www.phy6.org/Education/Iradbelt.html An explanation of the belts] Category:Earth Category:Space plasmas ja:ヴァン・アレン帯

1 E5 m²

To help compare sizes of different geographic regions, we list here areas between 10 hectares (100,000 m²) and 100 hectares (1,000,000 m²). See also areas of other orders of magnitude.
- Areas smaller than 0.1 km²
- 0.1 km² is equal to:
- 10 hectares
- 10 hectares -- 1.08 million square feet
- 19 hectares -- Irish National Botanic Gardens
- 35 hectares -- Palace of the Parliament, Bucharest, Romania
- 44 hectares -- Vatican City
- Areas larger than 1 km² See also: Orders of magnitude

External link

[http://www.ex.ac.uk/trol/scol/ccarea.htm Conversion Calculator for Units of AREA] Category:Orders of magnitude (area)

1 E6 m

To help compare different orders of magnitude this page lists lengths starting at 10⁶ m (1 Mm or 1,000 km). Distances shorter than 10⁶ m

Conversions

1,000 km is equal to:
- approximately 621.37 miles.
- Side of square with area 1,000,000 km²

Human-built structures

- 2,451 km — Length of the Alaska Highway
- 3,069 km — Length of Interstate 95 (from Houlton, Maine to Miami, Florida)
- 3,846 km — Length of U.S. Highway 1 (from Fort Kent, Maine to Key West, Florida)
- 5,007 km — Estimated length of Interstate 90 (Seattle, Washington to Boston, Massachusetts)
- 6,400 km — Length of the Great Wall of China
- 7,821 km — Length of the Trans-Canada Highway, the world's longest national highway (from Victoria, British Columbia to St. John's, Newfoundland)
- 9,289 km — Length of the Trans-Siberian railway

Nature

- 2,000 km — Distance from Beijing to Hong Kong as the crow flies
- 2,800 km — Narrowest width of Atlantic Ocean (Brazil-West Africa)
- 2,850 km — Length of the Danube river
- 2,205 km — Length of Sweden's total land boundaries
- 2,515 km — Length of Norway's total land bounaries
- 3,690 km — Length of the Volga river, longest in Europe
- 4,350 km — Length of the Huang He
- 4,715 km — Length of the Nile
- 4,800 km — Widest width of Atlantic Ocean (U.S.-Northern Africa)
- 5,650 km — Coastline of New Zealand
- 5,100 km — Distance from Dublin to New York as the crow flies
- 6,270 km — Length of the Mississippi-Missouri River system
- 6,380 km — Length of the Yangtze River
- 6,762 km — Length of the Amazon system, longest on Earth
- 8,200 km — Distance from Dublin to San Francisco as the crow flies

Astronomical

- 1,186 km — diameter of Charon, Pluto's moon
- 1,280 km — diameter of the Trans-Neptunian object 50000 Quaoar
- 1,436 km — diameter of Iapetus, one of Saturn's major moons
- 1,578 km — diameter of Titania, the largest of Uranus' moons
- 2,320 km — Diameter of Pluto
- 2,707 km — Diameter of Triton, largest moon of Neptune
- 3,475 km — Diameter of Earth's Moon
- 3,643 km — Diameter of Io (moon of Jupiter)
- 4,879 km — Diameter of Mercury
- 6,366 km — Earth radius
- 6,792 km — Diameter of Mars Distances longer than 10⁷ m

Geostationary orbit

A geostationary orbit (abbreviated GEO) is a circular orbit directly above the Earth's equator (0º latitude). Any point on the equator plane revolves about the Earth in the same direction and with the same period (speed) as the Earth's rotation. It is a special case of the geosynchronous orbit (abbreviated GSO), and the one which is of most interest to operators of artificial satellites (including communication and television satellites). Satellite locations may differ by longitude only (remember in Geostationary orbit latitude is zero). The idea of a geosynchronous satellite for communication purposes was first published in 1928 by Herman Potocnik. Geosynchronous and geostationary orbits were first popularised by science fiction author Arthur C. Clarke in 1945 as useful orbits for communications satellites. As a result they are sometimes referred to as Clarke orbits. Similarly, the "Clarke Belt" is the part of space approximately 35,786 km above mean sea level in the plane of the equator where near-geostationary orbits may be achieved. Geostationary orbits are useful because they cause a satellite to appear stationary with respect to a fixed point on the rotating Earth. As a result, an antenna can point in a fixed direction and maintain a link with the satellite. The satellite orbits in the direction of the Earth's rotation, at an altitude of approximately 35,786 km (22,240 statute miles) above ground. This altitude is significant because it produces an orbital period equal to the Earth's period of rotation, known as the sidereal day.

Use in artificial satellites

Geostationary orbits can only be achieved very close to the ring 35,786 km directly above the equator. All other circular non-active geosynchronous orbits will cross the geostationary orbit and possibly collide with satellites there. In practice this means that all geostationary satellites have to exist on this ring, which poses problems for satellites needing to be decommissioned at the end of their service life (for example when they run out of thruster fuel). Such satelites are typically raised to a Disposal Orbit. A geostationary transfer orbit is used to move a satellite from Low Earth orbit (LEO) into a geostationary orbit. A worldwide network of operational geostationary satellites are used by meteorological satellites to provide visible, as well as infrared images of Earth's surface and atmosphere. These satellite systems include:
- the US GOES
- METEOSAT, launched by the European Space Agency and operated by the European Weather Satellite Organization, EUMETSAT
- the Japanese GMS Most commercial communications satellites (and all television satellites) operate in geostationary orbits. A statite, a hypothetical satellite that uses a solar sail to modify its orbit, can theoretically hold itself in a geostationary orbit with different altitude and/or inclination from the "traditional" equatorial geostationary orbit.

Derivation of geostationary altitude

In geostationary orbit, a satellite is neither plunging towards the earth nor flying away from it. Therefore, the inward and outward forces on the satellite must equal each another (by Newton's first law of motion). To calculate the geostationary orbit altitude, one equates the two forces:

F_ = F_

By Newton's second law of motion, we can replace the forces

F

with the mass of the object multiplied by the acceleration felt by the object due to that force:

m_ \cdot a_ = m_ \cdot a_

We note that the mass of the satellite,

m_

, appears on both sides -- geostationary orbit is independent of the mass of the satellite! So, calculating the altitude simplifies into calculating the point where the magnitudes of the centrifugal acceleration derived from orbital motion and the centripetal acceleration provided by Earth's gravity are equal. The centrifugal acceleration's magnitude is:

|a_c| = \omega^2 \cdot r

...where

\omega

is the angular velocity in radians per second, and

r

is the orbital radius in metres as measured from the Earth's centre of mass. The magnitude of the gravitational attraction is:

|a_g| = \frac

...where

M_e

is the mass of Earth in kilograms, and

G

is the gravitational constant. Equating the two accelerations gives:

r^3 = \frac

r = \sqrt[3]

We can express this in a slightly different form by replacing

M_e \cdot G

\mu

, the geocentric gravitational constant:

r = \sqrt[3]

The angular velocity

\omega

is found by dividing the angle travelled in one revolution (

360^\circ = 2 \cdot \pi\ rad

) by the orbital period (the time it takes to make one full revolution: one sidereal day, or 86,164 seconds). This gives:

\omega = \frac = 7.29 \cdot 10^\ \mathrm \cdot \mathrm^

The resulting orbital radius is 42,164 km. Subtracting the Earth's equatorial radius, 6,378 km, gives the altitude of 35,786 km. Orbital velocity (how fast the satellite is flying through space) is calculated by multiplying the angular velocity by the orbital radius:

v = \omega \cdot r = 3.07\ \mathrm \cdot \mathrm^ = 11,052\ \mathrm

References

- Federal Standard 1037C
- MIL-STD-188

External links

- [http://www.braeunig.us/space/orbmech.htm ORBITAL MECHANICS] (Rocket and Space Technology)
- [http://www.satsig.net/sslist.htm List of satellites in geostationary orbit]. Category:Astrodynamics Category:Celestial mechanics Category:Earth orbits Category:Satellites ja:静止軌道

Thermosphere

The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. (see also: ionosphere) The thermosphere, named from the Greek θερμός (thermos) for heat, begins about 80 km above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures are highly dependent on solar activity, and can rise to 2,000°C. Radiation causes the air particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000km above the Earth's surface, the atmosphere blends into space. The few particles of gas here can reach 2,500°C (4500°F) during the day.

Manned spaceflight

Human spaceflight is space exploration with a human crew and possibly passengers, which is in contrast to robotic space probes or remotely-controlled unmanned space missions. On occasion, passengers of other species have ridden aboard spacecraft, although not all survived the return to earth. Dogs, not humans, were the first large mammals launched from Earth. The first human spaceflight was Vostok 1 on April 12, 1961; Soviet cosmonaut Yuri Gagarin made one orbit around the earth. Perhaps the highest of Earth orbits was Gemini 11 in 1966, which reached a height of 1374 km. The Space Shuttle on the missions to launch and service the Hubble Space Telescope has also reached high earth orbit at an altitude of around 600 km. The destination of human spaceflight missions beyond Earth orbit has only been the Moon. On the first such mission, Apollo 8, the crew orbited the Moon. Apollo 10 was the next mission, and it tested the lunar landing craft in lunar orbit without actually landing. The six missions that landed were Apollo 11-17, excluding Apollo 13. On each mission, two of the three astronauts involved landed on the moon; thus, in the late 1960s and early 1970s NASA's Apollo program landed twelve men on the Moon--returning them all to Earth. As of 2005 piloted space missions have been carried out by Russia, the People's Republic of China, and the United States. Missions carried out by the United States are both governmental (NASA) and civilian (Scaled Composites, a California-based company). Canada, Europe, India, and Japan also have active space programs. The Indian Parliament recently sanctioned funds to the Indian Space Research Organization for a human spaceflight by 2008 (although the programme has now been scaled down to start with an unmanned orbiting satellite for surveying--see Chandrayan). Japan has announced a program to place a person on the moon by 2025. Currently the following spacecrafts and spaceports are used:
- International Space Station (includes Soyuz TMA as an emergency lander; normal crew transport with the following two spacecraft)
- Soyuz TMA with Soyuz launch vehicle - Baikonur Cosmodrome
- Space Shuttle - John F. Kennedy Space Center
- Shenzhou spacecraft with Long March rocket - Jiuquan Satellite Launch Center
- Scaled Composites SpaceShipOne with Scaled Composites White Knight (the latter does not enter space itself) - Mojave Spaceport In an attempt to win the $10 million X-Prize, numerous private companies attempted to build their own manned spacecraft capable of repeated sub-orbital flights. The first private spaceflight took place on June 21 2004, when SpaceShipOne conducted a sub-orbital flight. With its second flight within one week, SpaceShipOne captured the prize on October 4, 2004. NASA uses the term "human spaceflight" to refer to its programme of launching people into space. Traditionally, these endeavours have been referred to as "manned space missions". The term "manned" is accurate in terms of gender when speaking of all U.S. spaceflight programs before the Space Shuttle program and Soviet spaceflights before Vostok 6. Although it only denotes gender in one of several definitions of the word, the term "manned" is considered sexist by some, and they may prefer to use the term "crewed"' or "piloted space missions."

External links

- [http://spaceflight.nasa.gov/ NASA Human Space Flight]
- [http://www.thespacereview.com/article/352/1 The top three reasons for humans in space]
- [http://www.chrisvalentines.com/sts107/videoessay.html 20 Minute Video Essay on Human Space Exploration] Category:Human spaceflight

Space Shuttle

). For the first two missions only, the external fuel tank spray-on foam insulation (SOFI) was painted white. Subsequent missions have featured an unpainted tank thus exposing the orange/rust colored foam insulation. This resulted in a weight saving of over 1,000 lb (450 kg), a savings that translated directly to added payload capacity to orbit.]] : This article is about the NASA Space Shuttle. For information on the Soviet Space Shuttles, see the articles Shuttle Buran, Ptichka, Shuttle 2.01, Shuttle 2.02 and Shuttle 2.03. NASA's Space Shuttle, officially called Space Transportation System (STS), is the United States government's sole manned launch vehicle currently in service. The Space Shuttle orbiter was manufactured by North American Rockwell, now part of the Boeing Company. Martin Marietta (now part of Lockheed Martin) designed the external fuel tank and Morton Thiokol (now part of Alliant Techsystems (ATK)) designed the solid rocket boosters. The Shuttle is the first orbital spacecraft designed for partial reusability. It carries large payloads to various orbits, provides crew rotation for the International Space Station (ISS), and performs servicing missions. While the vehicle was designed with the capacity to recover satellites and other payloads from orbit and return them to Earth, this capacity has not been used often; it is, however, an important use of the Space Shuttle in the context of the ISS program, as only very small amounts of experimental material, hardware needing to be repaired, and trash can be returned by Soyuz. Each Shuttle was designed for a projected lifespan of 100 launches or 10-years operational life. The program started in the late 1960s and has dominated NASA's manned operations since the mid-1970s. According to the Vision for Space Exploration, use of the Space Shuttle will be focused on completing assembly of the ISS in 2010, after which it will be replaced by the yet-to-be-developed Crew Exploration Vehicle (CEV). However, following the STS-114 return-to-flight mission in August 2005, the Shuttle program is currently grounded pending repairs and the solution of outstanding safety issues. Further aggravating the shuttle's return to space, also in August 2005, the Space Shuttle external tank construction site, Michoud Assembly Facility located in New Orleans, Louisiana was damaged by Hurricane Katrina, with all work shifts cancelled up to September 26, 2005. This could potentially set back further Shuttle flights by more than two months. The NASA Chief Administrator Michael Griffin has recently suggested the decision to develop the Space Shuttle and International Space Station was a mistake by saying, "It is now commonly accepted that was not the right path. We are now trying to change the path while doing as little damage as we can." [http://www.usatoday.com/printedition/news/20050928/1a_bottomstrip28.art.htm]

History

The Shuttle decision

NASA had conducted a series of paper projects throughout the 1960s on the topic of reusable spacecraft to replace their expedient "one-off" systems like Mercury, Gemini, and Apollo. Meanwhile, the U.S. Air Force had a continuing interest in smaller systems with more rapid turn-around times, and were involved in their own spaceplane project, the X-20 Dyna-Soar. In several instances groups from both worked together to investigate the state of the art. With the major Apollo development effort winding down in the second half of the 1960s, NASA started looking to the future of the space program. They envisioned an ambitious program consisting of a large space station being launched on huge boosters, served by a reusable logistics "space shuttle", both providing services for a permanently manned Lunar colony and eventual manned missions to Mars. However, in reality, NASA found itself with a rapidly plunging budget. Rather than trying to adapt their long-term future to their dire financial situation, they attempted to save as many of the individual projects as possible. The mission to Mars was rapidly dismissed, but the Space Station and Shuttle conserved. Eventually only one of them could be saved, so it stood to reason that a low-cost Shuttle system would be the better option, because without it a large station would never be affordable. A number of designs were proposed, but many of them were complex and varied widely in their systems. An attempt to re-simplify was made in the form of the "DC-3" by one of the few people left in NASA with the political importance to accomplish it, Maxime Faget, who had designed the Mercury capsule, among other vehicles. The DC-3 was a small craft with a 20,000-pound (9 tonne) (or less) payload, a four-man capacity, and limited maneuverability. At a minimum, the DC-3 provided a baseline "workable" (but not significantly advanced) system by which other systems could be compared for price/performance compromises. Mercury The defining moment for NASA was when they, in desperation to see their only remaining project saved, went to the Air Force for its blessing. NASA asked that the USAF place all of their future launches on the Shuttle instead of their current expendable launchers (like the Titan II), in return for which they would no longer have to continue spending money upgrading those designs — the Shuttle would provide more than enough capability. The Air Force reluctantly agreed, but only after demanding a large increase in capability to allow for launching their projected spy satellites (mirrors are heavy). These were quite large, weighing an estimated 40,000 pounds (18 tonnes), and needed to be put into polar orbits, which require higher energies than lower inclination orbits; and since the Air Force also wanted to be able to abort after a single orbit (as did NASA), and in addition land at the launch site (unlike NASA), the spacecraft would also require the ability to maneuver significantly to either side of its orbital track to adjust for launch-point rotational drift while in polar orbit — for example, in a 90-minute orbit, Vandenberg AFB would drift over 1,000 miles (1,600 km), whereas in more equatorially aligned orbits, the required cross-range would be less than 250 mi/400 km. This large "cross-range" capability for polar orbits meant the craft had to have a greater lift-to-drag ratio than originally planned, requiring the addition of bigger, heavier wings. The result was that the simple "DC-3" was clearly irrelevant because it had neither the cargo capacity nor the cross-range the Air Force demanded. In fact, all existing designs were far too small, as a 40,000-pound (18 tonnes) delivery to polar orbit equates to a 65,000-pound (29 tonne) delivery to an eastwardly launched orbit with typical 28-degree inclination. Additionally, any design using simple straight or foldout wings was not going to meet the cross-range requirements, so any future design would require a more complex, heavier delta wing. Of further concern, any increase in the weight of the upper portion of a launch vehicle, which had just occurred, required an even bigger increase in the capability of the lower stage used to launch it. Suddenly, the two-stage system had grown in size to something larger than the Saturn V, and the complexity and costs to develop it soared. While all of this was going on, others were suggesting a completely different approach to the future. They stated that NASA would fare better using the existing Saturn to launch their space station, supplied and manned using modified Gemini capsules on top of the Air Force's newer Titan II-M. The cost of development for this looked to be considerably less than the Shuttle alone, and would have a large space station in orbit earlier. In reply, advocates of the Shuttle answered that given enough launches, a reusable system would more than pay for the cost of development when compared with the launch costs of disposable rockets. Another factor in the cost-benefit analysis was inflation, and in the 1970s this was high enough that the payback from the development had to happen very quickly to see a positive return. Hence, a high launch rate was needed to make the system economically feasible. But it was infeasible that a space station or Air Force payloads could demand such rates (roughly one or two a week), so they insisted and suggested that all future U.S. launches would take place on the Shuttle, once built. In order to do this, the cost of launching the Shuttle would have to be lower than any other system, with the exception of very small rockets, ignored for practical reasons, and very large boosters, which were rare and excessively expensive in any case. With a baseline project now gelling, NASA started to work through the process of obtaining stable funding for the five years the project would take to develop. Once again, they found themselves in an increasingly deplorable situation. With the budgets being pressed by inflation in the U.S. and the Vietnam War abroad, Congress and the Administration were generally uninterested in long-term projects such as space exploration. Some members were therefore looking to further cut NASA's budget; but with a single long-term project confirmed, they could do little in terms of cutting whole projects — the Shuttle was the single one left, and its cancellation would mean that there would be no U.S. manned space program by 1980. Instead, they looked to reduce the year-to-year costs of development to a stable figure. That is, they wished to see the development budgets spread out over several more years. This was somewhat impractical and in conflict with the planned funding and development. The result was another intense series of redesigns in which the reusable booster was eventually abandoned due to its high price. Unsurprisingly, some designs for reusable boosters amounted to vehicles the size of the then-new Boeing 747, which would have to fly faster than the record-holding — and considerably smaller — X-15 rocket plane. Instead, a series of simpler rockets would launch the system and then drop away for recovery. Another change was that the fuel for the Shuttle itself was placed in an external tank instead of internal tanks as in the previous designs. This allowed a larger payload bay in an otherwise much smaller craft, although it also meant throwing away the tankage after each launch. The last remaining debate was over the nature of the boosters. NASA had been looking at no less than four solutions to this problem: one a development of the existing Saturn lower stage, another using simple pressure-fed liquid-fuel engines of a new design, and finally either a large, single solid rocket, or two (or more) smaller ones. The decision was eventually made on the smaller solids due to their lower development costs (a decision that had been echoed throughout the whole Shuttle program). While the liquid-fueled systems provided better performance and enhanced safety, delivery capability to orbit is much more a function of the upper-stage performance and weight than the lower; the money was hence spent elsewhere.

Development

Vietnam War The Shuttle program was launched on January 5, 1972, when President Richard M. Nixon announced that NASA would proceed with the development of a reusable, low-cost Space Shuttle system. The project was already to take longer than originally anticipated due to the year-to-year funding caps. Nevertheless, work started quickly and several test articles were available within a few years. Most notable among these was the first complete Orbiter, originally to be known as Constitution. However, a massive write-in campaign from fans of the Star Trek television series convinced the White House to change the name to Enterprise. Amid great fanfare, the Enterprise was rolled out on September 17, 1976, and later conducted a successful series of glide-approach and landing tests that were the first real validation of the design. The first fully functional Shuttle Orbiter, built in Palmdale, California, was the Columbia, which was delivered to Kennedy Space Center on March 25, 1979, and was first launched on April 12, 1981—the 20th anniversary of Yuri Gagarin's space flight—with a crew of two. Challenger was delivered to KSC in July 1982, Discovery was delivered in November 1983, and Atlantis was delivered in April 1985. The Shuttle was meant to visit Space Station Freedom, announced in 1984, an ambitious and much-delayed project later downsized and merged into the International Space Station program. Challenger was destroyed in an explosion during launch on January 28, 1986, with the loss of all seven astronauts on board. Endeavour was built to replace it (using spare parts originally intended for the other Orbiters) and delivered in May 1991. Columbia was lost, with all seven crew members, during reentry on February 1, 2003, and has not been replaced.

Description

2003 The Shuttle has a large 60 by 15 ft (18 by 4.6 m) payload bay, filling most of the fuselage. The payload bay doors have heat radiators mounted on their inner surfaces, and so are kept open for thermal control while the Shuttle is in orbit. Thermal control is also maintained by adjusting the orientation of the Shuttle relative to Earth and Sun. Inside the payload bay is the Remote Manipulator System, also known as the Canadarm, a robot arm used to retrieve and deploy payloads. Until the loss of Columbia, the Canadarm had been used only on those missions where it was needed. Since the arm is a crucial part of the Thermal Protection Inspection procedures now required for Shuttle flights, it will probably be included on all future flights. The Space Shuttle system has undergone numerous improvements over the years. The Orbiter has changed its thermal protection system several times in order to save weight and ease workload. The original silica-based ceramic tiles need to be removed for inspection for damage after every flight, and they also soak up water and thus need to be protected from the rain. The latter problem was initially fixed by spraying the tiles with Scotchgard, but a custom solution was adopted. Later, many of the tiles on the cooler portions of the Shuttle were replaced by large blankets of insulating feltlike material, which means huge areas (notably the cargo bay area) no longer have to be inspected as often. Internally the Shuttle remains largely similar to the original design, with the exception that the avionics continue to be improved. The original systems were "hardened" IBM 360 computers connected to analog displays in the cockpit similar to contemporary airliners like the DC-10. Today the cockpits have been replaced with "all glass" systems and the computers themselves are many times faster. The computers use the HAL/S programming language. In the Apollo-Soyuz Test Project tradition, programmable calculators are carried as well (originally the HP-41C). In addition to the "glass cockpit," several improvements have been made for safety reasons after the Challenger explosion, including a crew escape system for use in a narrow range of situations that require the Orbiter to "ditch." With the coming of the Space Station, the Orbiter's internal airlocks are being replaced with external docking systems to allow for a greater amount of cargo to be stored on the Shuttle's mid-deck during Station resupply missions. HP-41C The Space Shuttle Main Engines have had several improvements to enhance reliability and power. This is why during launch you may hear curious phrases such as "Go to throttle-up at 106%." This does not mean the engines are being run over limit. The 100% figure is the power level for the original main engines. The actual engine contract requirement was for 109%. The original flight engines could handle 102%. The 109% number was finally reached in flight hardware with the Block II engines in 2001. For STS-1 and STS-2 the external tank was painted white to protect the insulation that covers much of the tank, but improvements and testing showed that it was not required. This saved considerable weight, and thereby increases the payload the Orbiter can carry into orbit. Additional weight was saved by removing some of the internal "stringers" in the hydrogen tank that proved unnecessary. The resulting "light-weight external tank" has been used on the vast majority of Shuttle missions. STS-91 saw the first flight of the "super light-weight external tank". This version of the tank is made of the 2195 Aluminum-Lithium alloy. It weighs 7,500 lb (3.4 t) less than the last run of lightweight tanks. As the Shuttle cannot fly unmanned, each of these improvements has been "tested" on operational flights. And, of course, the SRBs (Solid Rocket Boosters) have undergone improvements as well. Notable is the adding of a third O-ring seal to the joints between the segments, which occurred after the Challenger accident. A number of other SRB improvements were planned in order to improve performance and safety, but never came to be. These culminated in the considerably simpler, lower cost, probably safer and better performing Advanced Solid Rocket Booster which was to have entered production in the early to mid-1990s to support the Space Station, but was later cancelled to save money after the expenditure of $2.2 billion. The loss of the ASRB program forced the development of the SLWT, which provides some of the increased payload capability, while not providing any of the safety improvements. In addition the Air Force developed their own much lighter single-piece design using a filament-wound system, but this too was cancelled. A cargo-only, unmanned variant of the Shuttle has been variously proposed and rejected since the 1980s. It is called the Shuttle-C and would trade re-usability for cargo capability with large potential savings from reusing technology developed for the Space Shuttle.

Components

The Space Shuttle consists of three main components: the reusable Orbiter itself, a large, brown, expendable external fuel tank, and a pair of white, reusable solid-fuel booster rockets. The fuel tank and booster rockets are jettisoned during ascent, so only the Orbiter goes into orbit.
- The reusable Orbiter Vehicle (OV), with a large payload bay and three main engines (fed from the external tank) and an orbital maneuvering system with two smaller engines (used after jettisoning the external tank). There are currently three orbiters, rotated between missions.
- A large expendable external fuel tank (ET) containing liquid oxygen and liquid hydrogen (at the forward and aft ends, respectively) for the three main engines of the Orbiter; it is discarded 8.5 minutes after launch at an altitude of 60 nautical miles (111 km) and breaks up in the atmosphere upon reentry. The pieces fall in the ocean and are not recovered.
- A pair of reusable solid-fuel rocket boosters (SRB); the propellant consists mainly of ammonium perchlorate (oxidizer, 70% by weight) and aluminum (fuel, 16 %); they are separated two minutes after launch at a height of 36 nautical miles (67 km) and are recovered after landing in the ocean, their fall slowed by parachutes. Initial plans for the so-called Space Transportation System included space tugs and extra fuel tanks for the orbital-maneuvering-system engines, among many other concepts. None of this hardware has actually ever been built.

Technical data

parachute Shuttle Carrier Aircraft (SCA), 1998 (NASA)]]
- System stack height: 184.2 ft (56.14 m)
- Orbiter length: 122.17 ft (37.236 m)
- Wingspan: 78.06 ft (23.79 m)
- Gross liftoff: 4.5 million lb (2,040,000 kg)
- ET: 1.7 million lb (751,000 kg)
- SRBs: 1.3 million lb (590,000 kg) each (x 2)
- Orbiter: 240,000 lb (109,000 kg)
- Total liftoff thrust: 7.82 million lbf (34.8 MN)
- SSMEs: 400,000 lbf (1.8 MN) each (x 3) = 1.2 million lbf (5.3 MN)
- SRBs: 3.30 million lbf (14.7 MN) each (x 2) = 6.61 million lbf (29.4 MN)
- Maximum landing: 230,000 lb (104,000 kg)
- Maximum launch payload: 63,500 lb (28,800 kg)
- Operational altitude: 100 to 520 nmi (185 to 1000 km)
- Speed: 25,404 ft/s (7743 m/s, 27 875 km/h, 17 321 mi/h)
- Passenger capacity: 10 Astronauts (crews other than 5 to 7 are uncommon)

Normal ascent

Initially the main engines are lit and checked out; if successful, the SRBs are lit and the vehicle is then committed to takeoff. At takeoff the vast majority (~90%) of the thrust is provided by the SRBs. Shortly after clearing the tower the Shuttle rotates so that the vehicle is below the main tank and SRBs. The vehicle climbs in a progressively flattening arc, accelerating as the weight of the SRBs and main tank reduces. To reach orbit, the vehicle needs to reach high altitude, but more importantly it needs to achieve mach 25 sideways, and so must spend as much time as possible accelerating horizontally. Around a point called "max-q", where the aerodynamic forces are at their maximum, the main engines are temporarily throttled back to avoid overspeeding and hence overstressing the Shuttle (particularly vulnerable parts such as the wings). 126 seconds after launch, the SRBs thrust reduces and then explosive bolts release them from the vehicle and they fall back to the ocean to be reused. The Shuttle then begins accelerating to orbit on the Space Shuttle Main Engines. The vehicle at that point in the flight has a thrust to weight ratio of less than one — the main engines actually have insufficient thrust to exceed the force of gravity, and the vertical speed given to it by the SRBs temporarily decreases. However, as the burn continues, the weight of the propellant reduces, the ever-lighter vehicle produces more and more acceleration until the thrust to weight ratio exceeds 1 again and the vehicle can hold itself up. The vehicle continues to climb and takes on a somewhat nose-up angle to the horizon — it uses the main engines to gain and then maintain altitude whilst it accelerates horizontally towards orbit. Finally, in the last tens of seconds towards the end of the main engine burn, the mass of the vehicle is low enough, that the acceleration is throttled back to keep the vehicle accelerating at no more than 3g, largely for astronaut health and comfort. Before complete depletion of propellant (running dry would destroy the engines) the main engines are shutdown, and the empty main tank is released by firing explosive bolts. The tank then falls to largely burn up in the atmosphere, with some fragments falling into the Indian Ocean. At this point the Shuttle is still slightly suborbital, since the trajectory intersects the atmosphere. The Shuttle then fires the OMS engines to circularize the orbit and avoid reentry.

Ascent abort modes

There are five abort modes available during ascent, classified as intact aborts and contingency aborts [http://www.shuttlepresskit.com/STS-93/REF86.htm]. The choice of abort mode depends on estimates of what the orbiter's situation would be at the time of main engine cutoff (TMECO). The abort modes cover a wide range of potential problems, but the most common expected problem is that the orbiter failing to achieve orbital speed by TMECO, technically known as "MECO underspeed".

Intact abort modes

There are four intact abort modes. Intact aborts are designed to provide a safe return of the orbiter to a planned landing site.
- Return To Launch Site (RTLS) — has never been tried, but would involve turning the Shuttle around while continuing to burn the SSMEs, jettisoning the ET, and gliding to a landing at Kennedy Space Center.
- East Coast Abort Landing (ECAL) — involves landing at predetermined locations on the east coast of North America in the U.S. and Canada. This has never occurred.
- Transoceanic Abort Landing (TAL) — involves landing at predetermined locations in Africa and western Europe. This has never occurred.
- Abort to Orbit (ATO) — occurs when the intended orbit cannot be reached but a stable alternate orbit is possible. This occurred on STS-51-F mission; required mission replanning, but the mission was nevertheless declared a success.
- Abort Once Around (AOA) — occurs when entering a stable orbit is not possible. This has never occurred.

Contingency abort mode

Contingency aborts are designed to permit flight crew survival following more severe failures when an intact abort is not possible. A contingency abort would generally result in a ditch operation. To the extent that the hydrogen and oxygen are not needed, they are used up deliberately to allow the ET to be discarded safely. The designated sites for ECAL are Bangor, Maine, Wilmington, North Carolina; MCAS Cherry Point, North Carolina; NAS Oceana; Wallops Flight Facility; Dover Air Force Base; Atlantic City, New Jersey; Gabreski, New York; Otis ANGB; Pease International Airport (all USA); Halifax; Stephenville; St John's; Gander; and Goose Bay (all Canada). A TAL would be declared between roughly T+2:30 minutes (liftoff plus 2 minutes, 30 seconds) and Main Engine Cutoff (MECO), about T+8:30 minutes. The Shuttle would then land at a predesignated friendly airstrip in Africa or Europe. Potential sites include Istres Air Base in France; Banjul International Airport in The Gambia; and Zaragoza Air Base and Morón Air Base in Spain. Prior to a Shuttle launch, two of them are selected depending on the flight plan, and staffed with standby personnel in case they are used. The list of TAL sites has changed over time; most recently Ben Guerir Air Base in Morocco was eliminated due to terrorism concerns. Past TAL sites have included Kano, Nigeria; Easter Island (for Vandenberg launches); Rota, Spain; Casablanca, Morocco; and Dakar, Senegal. Emergency landing sites for the Orbiter include Lajes, Beja, (both Portugal), Keflavik (Iceland), Shannon International Airport (Ireland), RAF Fairford (UK), Köln Bonn Airport (Germany), Ankara (Turkey), Riyadh (Saudi Arabia), Diego Garcia (British Indian Ocean Territory). Were the Orbiter unable to reach a runway, it could ditch in water, or could land on terrain other than a landing site. It would be unlikely for the flight crew still on board to survive. However, if the Orbiter ascent were aborted in a narrow set of circumstances in which controlled gliding flight could be achieved, the In-flight Crew Escape System would allow the crew to escape with parachutes. A special Escape Pole would take each crewmember on a trajectory beneath the Orbiter's left wing. In the two disasters, things went wrong so fast that little could be done. In the case of Challenger, the SRBs were still burning as they tore free from the rest of the stack. The orbiter disintegrated almost instantly because of aerodynamic stresses as the stack broke up. The Columbia disaster occurred high in the atmosphere during reentry. Even if the crew had been able to bail out, they would have been killed by the heat generated by the friction of the air.

Shuttles

Columbia disaster, Discovery, Atlantis and Endeavour. Not illustrated: Enterprise and Pathfinder.]] Individual Orbiters are both named, in a manner similar to ships, and numbered using the NASA Orbiter Vehicle Designation system. Whilst all three are externally very similar, they have minor internal differences; new equipment is fitted on a rotating basis as they are maintained, and the newer Orbiters tend to be structurally lighter.
- Handling test article designed with no spaceflight capability whatsoever:
- Pathfinder (Orbiter Simulator, no series number)
- Main propulsion test article, with no spaceflight capability whatsoever:
- MPTA-ET (External Tank) which is now attached to Pathfinder
- MPTA-098 suffered major damage due to engine failure.
- Structural test article, with no spaceflight capability:
- STA-099 which became Challenger
- Test vehicle suitable only for glide/landing tests, with no spaceflight capability without major refit:
- Enterprise (OV-101)
- Lost in accidents (see below):
- Challenger (OV-099, ex-STA-099) - destroyed after liftoff - January 28, 1986
- Columbia (OV-102) - destroyed during reentry February 1, 2003
- In use:
- Atlantis (OV-104)
- Discovery (OV-103)
- Endeavour (OV-105)

Applications

- Crew rotation of the ISS
- Manned servicing missions, such as to the Hubble Space Telescope (HST)
- Manned experiments in LEO
- Carry to LEO:
- Large satellites — these have included the HST
- Components for the construction of the ISS
- Supplies
- Carry satellites with a booster, the Payload Assist Module (PAM-D) or the Inertial Upper Stage (IUS), to the point where the booster sends the satellite to:
- A higher Earth orbit; these have included:
- Chandra X-ray Observatory
- Many TDRS satellites
- Two DSCS-III (Defense Satellite Communications System) communications satellites in one mission
- A Defense Support Program satellite
- An interplanetary orbit; these have included:
- Magellan probe
- Galileo spacecraft
- Ulysses probe

Flight statistics (as of August 25, 2005)

† Satellites deployed

- This was flight STS-80, during November 1996.

Accidents

Two Shuttles have been destroyed in 114 missions, both with the loss of the entire crew of seven:
- Challenger — lost 73 seconds after liftoff, January 28, 1986; see STS-51-L.
- Columbia — lost during reentry, February 1, 2003; see Space Shuttle Columbia disaster. This gives a 2% death rate per astronaut per flight. While the technical details of the accidents are quite different, the organisational problems show remarkable similarities. In both cases events happened which were not planned for or anticipated. In both cases, instead of dealing with the issue as unexpected and in need of complete explanation, at significant cost in time and money, the lazy attitude was to allow the unexpected events to happen as the damage done was not deemed to endanger the shuttle, although this was not actually known to be the case. In the case of Challenger, an O-ring which should not have eroded at all did, in fact, erode on earlier shuttle launches. Instead of finding out why, it was noted that it had not eroded by more than 30%, and the assumption was made was that this was not a hazard as there was a safety margin of a factor of 3. Despite repeated pleas by NASA's engineers to cancel or reschedule the launch, the managers allowed the shuttle to launch. Challenger's O-ring eroded right through, fatally, shortly after its last launch. Columbia failed as a consequence of damage caused by being struck by a piece of foam which broke off from the bipod during ascent. The foam had not been designed or expected to break off, but had been observed in the past to do so without incident.

Retrospect

Space Shuttle Columbia disaster

Costs

While the Shuttle has been a reasonably successful launch vehicle, it has been unable to meet its goal of radically reducing flight launch costs, as the average launch expenditures during its operations up to 2005 accumulates to $1.3 billion [http://sciencepolicy.colorado.edu/prometheus/archives/space_policy/], a rather large figure compared to the initial projections of $10 to $20 million. The total cost of the program has been $145 billion as of early 2005 ($112 billion of which was incurred while the program was operational) and is estimated at $174 billion when the Shuttle retires in 2010. NASA's budget for 2005 allocates 30%, or $5 billion, to Space Shuttle operations. [http://www.space.com/news/shuttle_cost_050211.html] The original mission of the Shuttle was to operate at a high flight rate, at low cost, and with high reliability. It was intended to improve greatly on the previous generation of single-use manned and unmanned vehicles. Although it did operate as the world's first reusable crew-carrying spacecraft, it did not improve on those parameters in any meaningful way, and is considered by some to have failed in its original purpose. Although the design is radically different from the original concept, the project was still supposed to meet the upgraded USAF goals, and to be much cheaper to fly in general. One reason behind this apparent failure appears to be inflation. During the 1970s the U.S. suffered from severe inflation, driving up costs about 200% by 1980. In contrast, the rate between 1990 and 2000 was only 34% in total. This magnified the development costs of the Shuttle. The original process by which contractors bid for Shuttle work has also inflated overall project costs as there were political and industrial pressures to spread Shuttle work around. For instance, the need for a single-piece SRB design was dismissed as only one company, Aerojet, was located close enough to the launch site to make this viable. The company that secured the SRB contract, Morton Thiokol, is based in Utah, necessitating the modular design that contributed to the Challenger loss. Ironically, the U.S. aerospace mergers of the 1990s mean that the vast majority of the STS contracts are now held by a single company (Boeing). However, this does not explain the high costs of the continued operations of the Shuttle. Even accounting for inflation, the launch costs on the original estimates should be about $100 million today. The remaining $400 million arises from the operational details of maintaining and servicing the Shuttle fleet, which have turned out to be tremendously more expensive than anticipated. Some of this can be attributed to operating beyond the 10-year anticipated lifespan of each Shuttle. The main reasons for higher costs can be ascribed to:

the reentry tiles turned out to be very expensive (averaging about 1 person week to replace a tile, with hundreds damaged with each launch)

engines were highly complex and marginal necessitating removal and maintenance after each flight

launch rate is much lower than ideal (studies showed that launching 50 times per year would have dramatically cut costs- the current shuttle launches about 4 times per year- the written record shows that NASA never installed any infrastructure to launch more than 12 times per year)

original costs of $118/lb were marginal costs, not total costs

Shuttle operations

When originally conceived, the Shuttle was to operate similarly to an airliner. After landing, the Orbiter would be checked out and start "mating" to the rest of the system (the ET and SRBs), and be ready for launch in as little as two weeks. Instead, this turnaround process takes months. Decisions to cut short-term development costs have resulted in a continued high-cost maintenance schedule. The documentation requirements have become extremely thorough. Dramatically increasing the number of support personnel needed to launch also caused a significant increase in costs. This was exacerbated in the aftermath of the Challenger disaster. Even simple changes require significant amounts of documentation. This paperwork results from the fact that, unlike current expendable launch vehicles, the Space Shuttle is manned and has no escape systems mode for most of the flight regime, and therefore any accident which would result in the loss of a booster would also result in the loss of the crew. Because loss of crew is unacceptable, the primary focus of the Shuttle program is to return the crew to Earth safely, which can conflict with other goals, namely to launch payloads cheaply. Furthermore, because there are cases where there are no abort modes — no potential way to prevent failure from becoming critical — many pieces of hardware simply must function perfectly and so must be carefully inspected before each flight. The result is a massively inflated labor cost, with around 25,000 workers in Shuttle operations and labor costs of about $1 billon per year. Initially NASA hoped the Shuttle's manned capacity would be justified as a "space taxi" to a revived Skylab or a Saturn V-launched "Skylab 2". With the go-ahead for the large, modular "Freedom" Space Station proposal the Shuttle appeared to have a continued justification with the prospect of a 6- to 10-crew outpost only being serviceable by the Shuttle. The scale back of the Space Station concept in the 1990s ultimately made the utility of the Shuttle as a manned ferry obsolete. NASA's justification of the STS for its own unmanned science missions has also declined. Following the Challenger disaster, use of the powerful Centaur upper stages required for interplanetary probes was ruled out. The Shuttle's history of unexpected delays also makes it liable to miss the narrow launch windows. Advances in technology over the last decade have made probes smaller and lighter, and as a result it is possible to reach Mars using a relatively cheap and reliable Delta launcher. Another possible impediment to the Shuttle system was the politically required participation of the United States Air Force. To receive the funding required, Congress mandated that the Shuttle replace all other launch vehicles in the national inventory as a cost-cutting measure. This requirement dramatically altered the size and scope of the program as the Air Force needed significant capabilities to allow it to meet national defense objectives. Ironically, neither NASA nor the Air Force got the system they wanted or needed, and the Air Force eventually returned to their older launch systems and abandoned their Vandenberg shuttle launch plans; many of the Air Force-imposed capabilities that most seriously hobbled the Shuttle system have never been used. Opinions differ on the lessons of the Shuttle. In general, however, future designers look to systems with only one stage, automated checkout and, in some cases, overdesigned (more durable) low-tech systems. Another consideration for future manned space flight is to pursue the construction and operation of "space planes", which could fly up to the edge of the atmosphere and then rocket out into Earth orbit, thereby being more efficient and versatile than such vehicles as the space shuttle.

Terrestrial transportation vehicles

- The Crawler-Transporter moves the Space Shuttle from the Vehicle Assembly Building to Launch Complex 39
- The Shuttle Carrier Aircraft is a modified Boeing 747 that flies the Space Shuttle from alternative landing sites back to Cape Canaveral.

References

- [http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html Reference manual]
- [http://science.howstuffworks.com/space-shuttle.htm How The Space Shuttle Works]
- [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810022734_1981022734.pdf NASA Space Shuttle News Reference - 1981 (PDF document)]
- [http://science.ksc.nasa.gov/shuttle/resources/orbiters/orbiters.html Orbiter Vehicles]
- [http://www.house.gov/science/hot/columbia/rs21411.pdf Shuttle Program Funding 1992 - 2002]

External links

- [http://www.chrisvalentines.com/sts107/index.html Columbia Disaster Multi-Media]
- [http://spaceflight.nasa.gov/shuttle/ NASA Human Spaceflight - Shuttle] Current status of Shuttle missions
- [http://www.nasa.gov/ntv NASA TV] View live streaming of launch and mission coverage
- [http://www.globalsecurity.org/space/facility/sts-els.htm List of all Shuttle Landing Sites]
- [http://www.srh.noaa.gov/smg/lsitegif.htm Map of Landing Sites]
- [http://gmaps.tommangan.us/spacecraft_tracking.html Track the Shuttle] with Google Maps
- [http://www.idlewords.com/2005/08/a_rocket_to_nowhere.htm "A Rocket To Nowhere"] criticism of the Space Shuttle program
- [http://www.washingtonmonthly.com/features/2001/8004.easterbrook-fulltext.html "Beam Me Out Of This Death Trap, Scotty"] Critical article on the Space Shuttle program, from 1981
- [http://www.theatlantic.com/doc/200311/langewiesche "Columbia's Last Flight"] Article from the Atlantic Monthly on the Columbia disaster and the subsequent investigation
- [http://www.spacedaily.com/news/shuttle-03p1.html "Explaining 30 years of Fudge"] - how the shuttle program was miss-sold to congress and where the $118/lb supposed costs came from ja:スペースシャトル