Outer space

Outer Space by Rishabh Sharma on Scribd

The interface between the Earth's surface and outer space. The Kármán line at a height of 100 km (62 mi) is shown. The layers of the atmosphere are drawn to scale, whereas objects within them, such as the International Space Station, are not.

Outer space, deep space, or just space, is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium as well as electromagnetic radiation, magnetic fields, neutrinos, dust and cosmic rays. The baseline temperature, as set by the background radiation from the Big Bang, is 2.7 kelvins (K) (−270.45 °C; −454.81 °F). Plasma with a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies. In most galaxies, observations provide evidence that 90% of the mass is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces. Data indicates that the majority of the mass-energy in the observable universe is a poorly understood vacuum energy of space which astronomers label dark energy. Intergalactic space takes up most of the volume of the Universe, but even galaxies and star systems consist almost entirely of empty space.

There is no firm boundary where outer space begins. However the Kármán line, at an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping. The framework for international space law was established by the Outer Space Treaty, which was passed by the United Nations in 1967. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit.

Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights, followed by manned rocket launches. Earth orbit was first achieved by Yuri Gagarin of the Soviet Union in 1961 and unmanned spacecraft have since reached all of the known planets in the Solar System. Due to the high cost of getting into space, manned spaceflight has been limited to low Earth orbit and the Moon.

Outer space represents a challenging environment for human exploration because of the dual hazards of vacuum and radiation. Microgravity also has a negative effect on human physiology that causes both muscle atrophy and bone loss. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is high.

Discovery

In 350 BCE, Greek philosopher Aristotle suggested that nature abhors a vacuum, a principle that became known as the horror vacui. This concept built upon a 5th-century BCE ontological argument by the Greek philosopher Parmenides, who denied the possible existence of a void in space. Based on this idea that a vacuum could not exist, in the West it was widely held for many centuries that space could not be empty. As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled.

In ancient China, there were various schools of thought concerning the nature of the heavens, some of which bear a resemblance to the modern understanding. In the 2nd century, astronomer Zhang Heng became convinced that space must be infinite, extending well beyond the mechanism that supported the Sun and the stars. The surviving books of the Hsüan Yeh school said that the heavens were boundless, "empty and void of substance". Likewise, the "sun, moon, and the company of stars float in the empty space, moving or standing still".

The Italian scientist Galileo Galilei knew that air had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. However, it would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. The French mathematician Blaise Pascal reasoned that if the column of mercury was supported by air then the column ought to be shorter at higher altitude where the air pressure is lower. In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puy de Dôme mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually expand, then contract upon descent.

The original Magdeburg hemispheres (lower left) used to demonstrate Otto von Guericke's vacuum pump (right)

In 1650, German scientist Otto von Guericke constructed the first vacuum pump: a device that would further refute the principle of horror vacui. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon.

Back in the 15th century, German theologian Nicolaus Cusanus speculated that the Universe lacked a center and a circumference. He believed that the Universe, while not infinite, could not be held as finite as it lacked any bounds within which it could be contained. These ideas led to speculations as to the infinite dimension of space by the Italian philosopher Giordano Bruno in the 16th century. He extended the Copernican heliocentric cosmology to the concept of an infinite Universe filled with a substance he called aether, which did not cause resistance to the motions of heavenly bodies. English philosopher William Gilbert arrived at a similar conclusion, arguing that the stars are visible to us only because they are surrounded by a thin aether or a void. This concept of an aether originated with ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies moved.

The concept of a Universe filled with a luminiferous aether remained in vogue among some scientists until the early 20th century. This form of aether was viewed as the medium through which light could propagate. In 1887, the Michelson–Morley experiment tried to detect the Earth's motion through this medium by looking for changes in the speed of light depending on the direction of the planet's motion. However, the null result indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by Albert Einstein's theory of special relativity, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference.

The first professional astronomer to support the concept of an infinite Universe was the Englishman Thomas Digges in 1576. But the scale of the Universe remained unknown until the first successful measurement of the distance to a nearby star in 1838 by the German astronomer Friedrich Bessel. He showed that the star 61 Cygni had a parallax of just 0.31 arcseconds (compared to the modern value of 0.287″). This corresponds to a distance of over 10 light years. The distance to the Andromeda Galaxy was determined in 1923 by American astronomer Edwin Hubble by measuring the brightness of cepheid variables in that galaxy, a new technique discovered by Henrietta Leavitt. This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way.

The earliest known estimate of the temperature of outer space was by the Swiss physicist Charles É. Guillaume in 1896. Using the estimated radiation of the background stars, he concluded that space must be heated to a temperature of 5–6 K. British physicist Arthur Eddington made a similar calculation to derive a temperature of 3.18 K in 1926. 1933 German physicist Erich Regener used the total measured energy of cosmic rays to estimate an intergalactic temperature of 2.8 K.

The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître. This theory holds that the observable universe originated from a very compact form that has since undergone continuous expansion. The background energy released during the initial expansion has steadily decreased in density, leading to a 1948 prediction by American physicts Ralph Alpher and Robert Herman of a temperature of 5 K for the temperature of space.

The term outer space was used in 1842 by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow". The expression outer space was used as an astronomical term by Alexander von Humboldt in 1845. It was later popularized in the writings of H. G. Wells in 1901. The shorter term space is actually older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.

Formation and state

This is an artist's concept of the metric expansion of space, where a volume of the Universe is represented at each time interval by the circular sections. At left is depicted the rapid inflation from the initial state, followed thereafter by steady expansion to the present day, shown at right.

According to the Big Bang theory, the Universe originated in an extremely hot and dense state about 13.8 billion years ago and began expanding rapidly. About 380,000 years later the Universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen—the so-called recombination epoch. When this happened, matter and energy became decoupled, allowing photons to travel freely through space. The matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep vacuum that forms what is now called outer space. As light has a finite velocity, this theory also constrains the size of the directly observable universe. This leaves open the question as to whether the Universe is finite or infinite.

The present day shape of the Universe has been determined from measurements of the cosmic microwave background using satellites like the Wilkinson Microwave Anisotropy Probe. These observations indicate that the observable universe is "flat", meaning that photons on parallel paths at one point will remain parallel as they travel through space to the limit of the observable universe, except for local gravity. The flat Universe, combined with the measured mass density of the Universe and the accelerating expansion of the Universe, indicates that space has a non-zero vacuum energy, which is called dark energy.

Estimates put the average energy density of the Universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and baryonic matter (ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters. The density of the Universe, however, is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and black holes—to conditions in vast voids that have much lower density, at least in terms of visible matter. Unlike the matter and dark matter, the dark energy seems not to be concentrated in galaxies: although dark energy may account for a majority of the mass-energy in the Universe, dark energy's influence is 5 orders of magnitude smaller than the influence of gravity from matter and dark matter within the Milky Way.

Environment

Part of the Hubble Ultra-Deep Field image showing a typical section of space containing galaxies interspersed by deep vacuum. Given the finite speed of light, this view covers the last 13 billion years of the history of outer space.

Outer space is the closest known approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along their ideal orbits. However, even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter. By comparison, the air we breathe contains about 10²⁵ molecules per cubic meter. The sparse density of matter in outer space means that electromagnetic radiation can travel great distances without being scattered: the mean free path of a photon in intergalactic space is about 10²³ km, or 10 billion light years. In spite of this, extinction, which is the absorption and scattering of photons by dust and gas, is an important factor in galactic and intergalactic astronomy.

Stars, planets and moons retain their atmospheres by gravitational attraction. Atmospheres have no clearly delineated boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from the surrounding environment. The Earth's atmospheric pressure drops to about 0.032 Pa at 100 kilometres (62 miles) of altitude, compared to 100,000 Pa for the International Union of Pure and Applied Chemistry (IUPAC) definition of standard pressure. Beyond this altitude, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar wind. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather.

The temperature of the vacuum is measured in terms of the kinetic activity of the gas, as it is on Earth. However, the radiation that fills the vacuum has a different temperature than the kinetic temperature of the gas, meaning that the gas and radiation are not in thermodynamic equilibrium. All of the observable universe is filled with photons that were created during the Big Bang, which is known as the cosmic microwave background radiation (CMB). (There is quite likely a correspondingly large number of neutrinos called the cosmic neutrino background.) The current black body temperature of the background radiation is about 3 K (−270 °C; −454 °F). The gas temperatures in outer space are always at least the temperature of the CMB but can be much higher. For example, the corona of the Sun has temperatures which range over 1.2–2.6 million K.

Outside a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic subatomic particles known as cosmic rays. These particles have energies ranging from about 10⁶ eV up to an extreme 10²⁰ eV of ultra-high-energy cosmic rays. The peak flux of cosmic rays occurs at energies of about 10⁹ eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of electrons is only about 1% of that of protons. Cosmic rays can damage electronic components and pose a health threat to space travelers. According to astronauts, like Don Pettit, space has a burned/metallic odor that clings to their suits and equipment, similar to the scent of an arc welding torch.

Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of lichen carried on the ESA BIOPAN facility survived exposure for ten days in 2007. Seeds of Arabidopsis thaliana and Nicotiana tabacum germinated after being exposed to space for 1.5 years. A strain of bacillus subtilis has survived 559 days when exposed to low-Earth orbit or a simulated martian environment. The lithopanspermia hypothesis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms to another habitable world. A conjecture is that just such a scenario occurred early in the history of the Solar System, with potentially microorganism-bearing rocks being exchanged between Venus, Earth, and Mars.

Effect on human bodies

Because of the hazards of a vacuum, astronauts must wear a pressurized space suit while off-Earth and outside their spacecraft.

Sudden exposure to very low pressure, such as during a rapid decompression, can cause pulmonary barotrauma—a rupture of the lungs, due to the large pressure differential between inside and outside the chest. Even if the victim's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture. Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to hypoxia.

As a consequence of rapid decompression, any oxygen dissolved in the blood will empty into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrives at the brain, humans and animals will lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids boil when the pressure drops below 6.3 kPa, and this condition is called ebullism. The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. Swelling and ebullism can be reduced by containment in a flight suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for Shuttle astronauts, prevents ebullism at pressures as low as 2 kPa. Space suits are needed at 8 km (5.0 mi) to provide enough oxygen for breathing and to prevent water loss, while above 20 km (12 mi) they are essential to prevent ebullism. Most space suits use around 30–39 kPa of pure oxygen, about the same as on the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause decompression sickness and gas embolisms if not managed.

Humans evolved for life in Earth gravity, and exposure to weightlessness has been shown to have deleterious effects on the health of the human body. Initially, more than 50% of astronauts experience space motion sickness. This can cause nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. The duration of space sickness varies, but it typically lasts for 1–3 days, after which the body adjusts to the new environment. Longer term exposure to weightlessness results in muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise. Other effects include fluid redistribution, slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face.

For long duration space travel, radiation can pose an acute health hazard. Exposure to radiation sources such as high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the eyes, nervous system, lungs and the gastrointestinal tract. On a round-trip Mars mission lasting three years, nearly the entire body would be traversed by high energy nuclei, each of which can cause ionization damage to cells. Fortunately, most such particles are significantly attenuated by the shielding provided by the aluminium walls of a spacecraft, and can be further diminished by water containers and other barriers. However, the impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research will be needed to assess the radiation hazards and determine suitable countermeasures.

Boundary

SpaceShipOne completed the first manned private spaceflight in 2004, reaching an altitude of 100.12 km (62.21 mi).

There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely:

• The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself.
• The United States designates people who travel above an altitude of 50 miles (80 km) as astronauts.
• NASA's Space Shuttle used 400,000 feet (76 mi, 122 km) as its re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable, thus beginning the process of switching from steering with thrusters to maneuvering with air surfaces.

In 2009, scientists at the University of Calgary reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at 118 km (73 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 268 m/s (600 mph).

The altitude where the atmospheric pressure matches the vapor pressure of water at the temperature of the human body is called the Armstrong line, named after American physician Harry G. Armstrong. It is located at an altitude of around 19.14 km (11.89 mi). At or above the Armstrong line, fluids in the throat and lungs will boil away. More specifically, exposed bodily liquids such as saliva, tears, and the liquids wetting the alveoli within the lungs will boil away. Hence, at this altitude the human body requires a pressure suit, or a pressurized capsule, to survive.

Legal status

2008 launch of the SM-3 missile used to destroy American spy satellite USA-193

The Outer Space Treaty provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of outer space the Moon and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty. It also prohibits the deployment of nuclear weapons in outer space. The treaty was passed by the United Nations General Assembly in 1963 and signed in 1967 by the USSR, the United States of America and the United Kingdom. As of January 1, 2008 the treaty has been ratified by 98 states and signed by an additional 27 states.

Beginning in 1958, outer space has been the subject of multiple resolutions by the United Nations General Assembly. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space. Four additional space law treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and anti-satellite weapons have been successfully tested by the US, USSR and China. The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. However, this treaty has not been ratified by any nation that currently practices manned spaceflight.

In 1976, eight equatorial states (Ecuador, Colombia, Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. They made the "Declaration of the First Meeting of Equatorial Countries," also known as "the Bogotá Declaration", where they made a claim to control the segment of the geosynchronous orbital path corresponding to each country. These claims are not internationally accepted.

Earth orbit

A spacecraft enters orbit when it has enough horizontal velocity for its centripetal acceleration due to gravity to be less than or equal to the centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about 7,800 m/s (28,100 km/h; 17,400 mph); by contrast, the fastest manned airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15.
To achieve an orbit, a spacecraft must travel faster than a sub-orbital spaceflight. The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude. Spacecraft with a perigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere, which will cause the orbital altitude to decrease. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about 300 km (190 mi), decay becomes more rapid with lifetimes measured in days. Once a satellite descends to 180 km (110 mi), it will start to burn up in the atmosphere. The escape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11,200 m/s (40,300 km/h; 25,100 mph).

Earth's gravity reaches out far past the Van Allen radiation belt and keeps the Moon in orbit at an average distance of 384,403 km (238,857 mi). The region of space where the gravity of a planet tends to dominate the motion of objects in the presence of other perturbing bodies (such as another planet) is known as the Hill sphere. For Earth, this sphere has a radius of about 1,500,000 km (930,000 mi).

Regions

Space is a partial vacuum: its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the solar wind of interplanetary space. Interplanetary space extends to the heliopause, whereupon the solar wind gives way to the winds of the interstellar medium. Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void.

Geospace

Aurora australis observed from the Space Shuttle Discovery, on STS-39, May 1991 (orbital altitude: 260 km)

Geospace is the region of outer space near Earth. Geospace includes the upper region of the atmosphere and the magnetosphere. The Van Allen radiation belt lies within the geospace. The outer boundary of geospace is the magnetopause, which forms an interface between the planet's magnetosphere and the solar wind. The inner boundary is the ionosphere. As the physical properties and behavior of near Earth space is affected by the behavior of the Sun and space weather, the field of geospace is interlinked with heliophysics; the study of the Sun and its impact on the Solar System planets.

The volume of geospace defined by the magnetopause is compacted in the direction of the Sun by the pressure of the solar wind, giving it a typical subsolar distance of 10 Earth radii from the center of the planet. However, the tail can extend outward to more than 100–200 Earth radii. The Moon passes through the geospace tail during roughly four days each month, during which time the surface is shielded from the solar wind.

Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the Earth's magnetic field. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth’s upper atmosphere. During geomagnetic storms two regions of geospace, the radiation belts and the ionosphere, can become strongly disturbed. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, disrupting telecommunications and GPS technologies, and can also be a hazard to astronauts, even in low Earth orbit. They also create aurorae seen near the magnetic poles.

Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites. This region contains material left over from previous manned and unmanned launches that are a potential hazard to spacecraft. Some of this debris re-enters Earth's atmosphere periodically.

Cislunar space

The region outside Earth's atmosphere and extending out to just beyond the Moon’s orbit, including the Lagrangian points, is sometimes referred to as cis-lunar space.

Interplanetary space

The sparse plasma (blue) and dust (white) in the tail of comet Hale–Bopp are being shaped by pressure from solar radiation and the solar wind, respectively

Interplanetary space, the space around the Sun and planets of the Solar System, is the region dominated by the interplanetary medium, which extends out to the heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun. Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of kilometers into space. This wind has a particle density of 5–10 protons/cm³ and is moving at a velocity of 350–400 km/s (780,000–890,000 mph). The distance and strength of the heliopause varies depending on the activity level of the solar wind. The discovery since 1995 of extrasolar planets means that other stars must possess their own interplanetary media.

The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. However, this space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy. A cloud of interplanetary dust is visible at night as a faint band called the zodiacal light.

Interplanetary space contains the magnetic field generated by the Sun. There are also magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of magnetic particles such as the Van Allen radiation belt. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.

Interstellar space

Interstellar space

Spaceflight, Solar System and Interstellar, Lebo, 8-5-08 by Rishabh Sharma on Scribd

Bow shock formed by the magnetosphere of the young star LL Orionis (center) as it collides with the Orion Nebula flow

Interstellar space is the physical space within a galaxy beyond the influence of each star on the plasma. The contents of interstellar space are called the interstellar medium. Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through stellar nucleosynthesis. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula. The cataclysmic explosion of a supernova will generate an expanding shock wave consisting of ejected materials. The density of matter in the interstellar medium can vary considerably: the average is around 10⁶ particles per m³, but cold molecular clouds can hold 10⁸–10¹² per m³.

A number of molecules exist in interstellar space, as can tiny 0.1 μm dust particles. The tally of molecules discovered through radio astronomy is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the trihydrogen cation. An ionized helium atom can then split relatively abundant carbon monoxide to produce ionized carbon, which in turn can lead to organic chemical reactions.

The local interstellar medium is a region of space within 100 parsecs (pc) of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the Local Bubble, which is characterized by a lack of dense, cold clouds. It forms a cavity in the Orion Arm of the Milky Way galaxy, with dense molecular clouds lying along the borders, such as those in the constellations of Ophiuchus and Taurus. (The actual distance to the border of this cavity varies from 60 to 250 pc or more.) This volume contains about 10⁴–10⁵ stars and the local interstellar gas counterbalances the astrospheres that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000 K and radii of 0.5–5 pc.

When stars are moving at sufficiently high peculiar velocities, their astrospheres can generate bow shocks as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from Interstellar Boundary Explorer (IBEX) and NASA's Voyager probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a subsonic bow wave defines the transition from the solar wind flow to the interstellar medium. A bow shock is the third boundary of an astrosphere after the termination shock and the astropause (called the heliopause in the Solar System).

Intergalactic space

A star forming region in the Large Magellanic Cloud, perhaps the closest Galaxy to Earth's Milky Way

Intergalactic space is the physical space between galaxies. Studies of the large scale distribution of galaxies show that the Universe has a bubble-like structure, with clusters and groups of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms huge voids that are mostly empty of galaxies. Typically, a void spans a distance of (10−40) h⁻¹ MPc, where h is the Hubble constant in units of 100 km s⁻¹ Mpc.

Surrounding and stretching between galaxies, there is a rarefied plasma that is organized in a galactic filamentary structure. This material is called the intergalactic medium (IGM). The density of the IGM is 5–200 times the average density of the Universe. It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 10⁵ K to 10⁷ K, which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the plasma is very hot by terrestrial standards, 10⁵ K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm–hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 10⁸ K and above in the so-called intracluster medium.

Exploration and applications

The first image taken of the entire Earth by astronauts was shot during the Apollo 8 mission

For the majority of human history, space was explored by remote observation; initially with the unaided eye and then with the telescope. Prior to the advent of reliable rocket technology, the closest that humans had come to reaching outer space was through the use of balloon flights. In 1935, the U.S. Explorer II manned balloon flight had reached an altitude of 22 km (14 mi). This was greatly exceeded in 1942 when the third launch of the German A-4 rocket climbed to an altitude of about 80 km (50 mi). In 1957, the unmanned satellite Sputnik 1 was launched by a Russian R-7 rocket, achieving Earth orbit at an altitude of 215–939 kilometres (134–583 mi). This was followed by the first human spaceflight in 1961, when Yuri Gagarin was sent into orbit on Vostok 1. The first humans to escape Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 on board the U.S. Apollo 8, which achieved lunar orbit and reached a maximum distance of 377,349 km (234,474 mi) from the Earth.

The first spacecraft to reach escape velocity was the Soviet Luna 1, which performed a fly-by of the Moon in 1959. In 1961, Venera 1 became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of the planet Venus, although contact was lost before reaching Venus. The first successful planetary mission was the Mariner 2 fly-by of Venus in 1962. The first spacecraft to perform a fly-by of Mars was Mariner 4, which reached the planet in 1964. Since that time, unmanned spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as observation of the Earth. In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space.

The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum. This is evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from more than 13 billion years ago—almost to the time of the Big Bang—to be observed. However, not every location in space is ideal for a telescope. The interplanetary zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope out past the dust will increase the effectiveness of the instrument. Likewise, a site like the Daedalus crater on the far side of the Moon could shield a radio telescope from the radio frequency interference that hampers Earth-based observations.

Unmanned spacecraft in Earth orbit have become an essential technology of modern civilization. They allow direct monitoring of weather conditions, relay long-range communications including telephone calls and television signals, provide a means of precise navigation, and allow remote sensing of the Earth. The latter role serves a wide variety of purposes, including tracking soil moisture for agriculture, prediction of water outflow from seasonal snow packs, detection of diseases in plants and trees, and surveillance of military activities.


The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those that require ultraclean surfaces. However, like asteroid mining, space manufacturing requires a significant investment with little prospect of an immediate return. An important factor in the total expense is the high cost of placing mass into Earth orbit: $7,000–23,000 per kg in inflation-adjusted dollars, according to a 2006 estimate. Proposed concepts for addressing this issue include non-rocket spacelaunch, momentum exchange tethers, and space elevators.

Timeline of knowledge about the interstellar and intergalactic medium

Timeline of knowledge about the interstellar medium and intergalactic medium

• 1848 — Lord Rosse studies M1 and names it the Crab Nebula
• 1864 — William Huggins studies the spectrum of the Orion Nebula and shows that it is a cloud of gas
• 1927 — Ira Bowen explains unidentified spectral lines from space as forbidden transition lines
• 1930 — Robert Trumpler discovers absorption by interstellar dust by comparing the angular sizes and brightnesses of globular clusters
• 1944 — Hendrik van de Hulst predicts the 21 cm hyperfine line of neutral interstellar hydrogen
• 1951 — Harold I. Ewen and Edward Purcell observe the 21 cm hyperfine line of neutral interstellar hydrogen
• 1956 — Lyman Spitzer predicts coronal gas around the Milky Way
• 1965 — James Gunn and Bruce Peterson use observations of the relatively low absorption of the blue component of the Lyman-alpha line from 3C9 to strongly constrain the density and ionization state of the intergalactic medium
• 1969 — Lewis Snyder, David Buhl, Ben Zuckerman, and Patrick Palmer find interstellar formaldehyde
• 1970 — Arno Penzias and Robert Wilson find interstellar carbon monoxide
• 1970 — George Carruthers observes molecular hydrogen in space
• 1977 — Christopher McKee and Jeremiah Ostriker propose a three component theory of the interstellar medium
• 1990 — Foreground "contamination" data from the COBE spacecraft provides the first all-sky map of the ISM in microwave bands.

Earth's location in the Universe

EarthsPlaceintheUniverse Escalon by Rishabh Sharma on Scribd

https://youtu.be/1KtuChfoH88

Knowledge of Earth's location in the Universe has been shaped by 400 years of telescopic observations, and has expanded radically in the last century. Initially, Earth was believed to be the center of the Universe, which consisted only of those planets visible with the naked eye and an outlying sphere of fixed stars. After the acceptance of the heliocentric model in the 17th century, observations by William Herschel and others showed that the Sun lay within a vast, disc-shaped galaxy of stars. By the 20th century, observations of spiral nebulae revealed that our galaxy was one of billions in an expanding universe, grouped into clusters and superclusters. By the end of the 20th century, the overall structure of the visible universe was becoming clearer, with superclusters forming into a vast web of filaments and voids. Superclusters, filaments and voids are the largest coherent structures in the Universe that we can observe. At still larger scales (over 1000 megaparsecs) the Universe becomes homogeneous meaning that all its parts have on average the same density, composition and structure.

Since there is believed to be no "center" or "edge" of the Universe, there is no particular reference point with which to plot the overall location of the Earth in the universe. Because the observable universe is defined as that region of the Universe visible to terrestrial observers, Earth is, by definition, the center of the observable universe. Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still undetermined whether the Universe is infinite. There have been numerous hypotheses that our universe may be only one such example within a higher multiverse; however, no direct evidence of any sort of multiverse has ever been observed, and some have argued that the hypothesis is not falsifiable.

A diagram of our location in the observable universe

Feature	Diameter	Notes
Earth	12,756.2 km (equatorial)	Measurement comprises just the solid part of the Earth; there is no agreed upper boundary for Earth's atmosphere. The geocorona, a layer of UV-luminescent hydrogen atoms, lies at 100,000 km. The Kármán line, defined as the boundary of space for astronautics, lies at 100 km.
Orbit of the Moon	768,210 km	The average diameter of the orbit of the Moon relative to the Earth.
Geospace	6,363,000– 12,663,000 km (110-210 Earth radii)	The space dominated by Earth's magnetic field and its magnetotail, shaped by the solar wind.
Earth's orbit	299.2 million km 2 AU	The average diameter of the orbit of the Earth relative to the Sun. Encompasses the Sun, Mercury and Venus.
Inner Solar System	~6.54 AU	Encompasses the Sun, the inner planets (Mercury, Venus, Earth, Mars) and the asteroid belt. Cited distance is the 2:1 resonance with Jupiter, which marks the outer limit of the asteroid belt.
Outer Solar System	60.14 AU	Includes the outer planets (Jupiter, Saturn, Uranus, Neptune). Cited distance is the orbital diameter of Neptune.
Kuiper belt	~96 AU	Belt of icy objects surrounding the outer Solar System. Encompasses the dwarf planets Pluto, Haumea and Makemake. Cited distance is the 2:1 resonance with Neptune, generally regarded as the outer edge of the main Kuiper belt.
Heliosphere	160 AU	Maximum extent of the solar wind and the interplanetary medium.
Scattered disc	195.3 AU	Region of sparsely scattered icy objects surrounding the Kuiper belt. Encompasses the dwarf planet Eris. Cited distance is derived by doubling the aphelion of Eris, the farthest known scattered disc object. As of now, Eris's aphelion marks the farthest known point in the scattered disc.
Oort cloud	100,000–200,000 AU 0.613–1.23 pc	Spherical shell of over a trillion (1012) comets. Existence is currently hypothetical, but inferred from the orbits of long-period comets.
Solar System	1.23 pc	The Sun and its planetary system. Cited diameter is that of the Sun's Hill sphere; the region of its gravitational influence.
Local Interstellar Cloud	9.2 pc	Interstellar cloud of gas through which the Sun and a number of other stars are currently travelling.
Local Bubble	2.82–250 pc	Cavity in the interstellar medium in which the Sun and a number of other stars are currently travelling. Caused by a past supernova.
Gould Belt	1,000 pc	Ring of young stars through which the Sun is currently travelling.
Orion Arm	3000 pc (length)	The spiral arm of the Milky Way Galaxy through which the Sun is currently travelling.
Orbit of the Solar System	17,200 pc	The average diameter of the orbit of the Solar System relative to the Galactic Center. The Sun's orbital radius is roughly 8,600 parsecs, or slightly over half way to the galactic edge. One orbital period of the Solar System lasts between 225 and 250 million years.
Milky Way Galaxy	30,000 pc	Our home galaxy, composed of 200 billion to 400 billion stars and filled with the interstellar medium.
Milky Way subgroup	840,500 pc	The Milky Way and those satellite dwarf galaxies gravitationally bound to it. Examples include the Sagittarius Dwarf, the Ursa Minor Dwarf and the Canis Major Dwarf. Cited distance is the orbital diameter of the Leo T Dwarf galaxy, the most distant galaxy in the Milky Way subgroup.
Local Group	3 Mpc	Group of at least 47 galaxies of which the Milky Way is a part. Dominated by Andromeda (the largest), the Milky Way and Triangulum; the remainder are dwarf galaxies.
Local Sheet	7 Mpc	Group of galaxies including the Local Group moving at the same relative velocity towards the Virgo Cluster and away from the Local Void.
Virgo Supercluster	30 Mpc	The supercluster of which the Local Group is a part It comprises roughly 100 galaxy groups and clusters, centred on the Virgo Cluster. The Local Group is located on the outer edge of the Virgo Supercluster.
Laniakea	160 Mpc	A group connected with the superclusters of which the Local Group is a part. Comprises roughly 300 to 500 galaxy groups and clusters, centred on the Great Attractor in the Hydra-Centaurus Supercluster.
Observable universe	28,000 Mpc	More than 100 billion galaxies, arranged in millions of superclusters, galactic filaments, and voids, creating a foam-like superstructure.
Universe	Minimum 28,000 Mpc	Beyond the observable universe lie the unobservable regions from which no light has reached the Earth yet. No information is available, as light is the fastest travelling medium of information. However, uniformitarianism argues that the Universe is likely to contain more galaxies in the same foam-like superstructure.

International Space Station

The International Space Station on 23 May 2010 as seen from the departing Space Shuttle Atlantis during STS-132.
Station statistics
COSPAR ID	1998-067A
Call sign	Alpha, Station
Crew	Fully crewed: 6 Currently aboard: 3 (Expedition 49)
Launch	20 November 1998
Launch pad	Baikonur 1/5 and 81/23 Kennedy LC-39
Mass	${\displaystyle \approx }$ 419,455 kg (924,740 lb)
Length	72.8 m (239 ft)
Width	108.5 m (356 ft)
Height	${\displaystyle \approx }$ 20 m (66 ft) nadir–zenith, arrays forward–aft (27 November 2009)
Pressurised volume	931.57 m3 (32,898 cu ft) (28 May 2016)
Atmospheric pressure	101.3 kPa (29.91 inHg, 1 atm)
Perigee	409 km (254 mi) AMSL
Apogee	416 km (258 mi) AMSL
Orbital inclination	51.65 degrees
Orbital speed	7.66 kilometres per second (27,600 km/h; 17,100 mph)
Orbital period	92.69 minutes
Orbit epoch	25 January 2015
Days in orbit	17 years, 306 days (21 September)
Days occupied	15 years, 324 days (21 September)
Number of orbits	101,081 As of July 2016
Orbital decay	2 km/month
Configuration
Station elements as of August 2016

The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low Earth orbit. Its first component launched into orbit in 1998, and the ISS is now the largest artificial body in orbit and can often be seen with the naked eye from Earth. The ISS consists of pressurised modules, external trusses, solar arrays, and other components. ISS components have been launched by Russian Proton and Soyuz rockets, and American Space Shuttles.

The ISS serves as a microgravity and space environment research laboratory in which crew members conduct experiments in biology, human biology, physics, astronomy, meteorology, and other fields. The station is suited for the testing of spacecraft systems and equipment required for missions to the Moon and Mars. The ISS maintains an orbit with an altitude of between 330 and 435 km (205 and 270 mi) by means of reboost manoeuvres using the engines of the Zvezda module or visiting spacecraft. It completes 15.54 orbits per day.

The ISS is the ninth space station to be inhabited by crews, following the Soviet and later Russian Salyut, Almaz, and Mir stations as well as Skylab from the US. The station has been continuously occupied for 15 years and 324 days since the arrival of Expedition 1 on 2 November 2000. This is the longest continuous human presence in low Earth orbit, having surpassed the previous record of 9 years and 357 days held by Mir. The station is serviced by a variety of visiting spacecraft: the Russian Soyuz and Progress, the American Dragon and Cygnus, the Japanese H-II Transfer Vehicle, and formerly the Space Shuttle and the European Automated Transfer Vehicle. It has been visited by astronauts, cosmonauts and space tourists from 17 different nations.

After the US Space Shuttle programme ended in 2011, Soyuz rockets became the only provider of transport for astronauts at the International Space Station, and Dragon became the only provider of bulk cargo return to Earth services (downmass capability of Soyuz capsules is very limited).
The ISS programme is a joint project among five participating space agencies: NASA, Roscosmos, JAXA, ESA, and CSA. The ownership and use of the space station is established by intergovernmental treaties and agreements. The station is divided into two sections, the Russian Orbital Segment (ROS) and the United States Orbital Segment (USOS), which is shared by many nations. As of January 2014, the American portion of ISS is being funded until 2024.Roscosmos has endorsed the continued operation of ISS through 2024, but has proposed using elements of the Russian Orbital Segment to construct a new Russian space station called OPSEK.

On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to collaborate on the development of a replacement for the current ISS. NASA later issued a guarded statement expressing thanks for Russia's interest in future cooperation in space exploration, but fell short of confirming the Russian announcement.

Purpose

Sunrise at Zvezda

Fisheye view of several labs

CubeSats are deployed by the NanoRacks CubeSat Deployer attached to the end of the Japanese robotic arm

According to the original Memorandum of Understanding between NASA and Rosaviakosmos, the International Space Station was intended to be a laboratory, observatory and factory in low Earth orbit. It was also planned to provide transportation, maintenance, and act as a staging base for possible future missions to the Moon, Mars and asteroids. In the 2010 United States National Space Policy, the ISS was given additional roles of serving commercial, diplomatic and educational purposes.

Scientific research

The ISS provides a platform to conduct scientific research. Small unmanned spacecraft can provide platforms for zero gravity and exposure to space, but space stations offer a long-term environment where studies can be performed potentially for decades, combined with ready access by human researchers over periods that exceed the capabilities of manned spacecraft.

The ISS simplifies individual experiments by eliminating the need for separate rocket launches and research staff. The wide variety of research fields include astrobiology, astronomy, human research including space medicine and life sciences, physical sciences, materials science, space weather, and weather on Earth (meteorology). Scientists on Earth have access to the crew's data and can modify experiments or launch new ones, which are benefits generally unavailable on unmanned spacecraft. Crews fly expeditions of several months duration, providing approximately 160-man-hours per week of labour with a crew of 6.

To detect dark matter and answer other fundamental questions about our universe, engineers and scientists from all over the world built the Alpha Magnetic Spectrometer (AMS), which NASA compares to the Hubble space telescope, and says could not be accommodated on a free flying satellite platform partly because of its power requirements and data bandwidth needs^. On 3 April 2013, NASA scientists reported that hints of dark matter may have been detected by the Alpha Magnetic Spectrometer. According to the scientists, "The first results from the space-borne Alpha Magnetic Spectrometer confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays."

Expedition 8 Commander and Science Officer Michael Foale conducts an inspection of the Microgravity Science Glovebox

Comet Lovejoy photographed by Expedition 30 commander Dan Burbank

The space environment is hostile to life. Unprotected presence in space is characterised by an intense radiation field (consisting primarily of protons and other subatomic charged particles from the solar wind, in addition to cosmic rays), high vacuum, extreme temperatures, and microgravity. Some simple forms of life called extremophiles, including small invertebrates called tardigrades can survive in this environment in an extremely dry state called desiccation.

Medical research improves knowledge about the effects of long-term space exposure on the human body, including muscle atrophy, bone loss, and fluid shift. This data will be used to determine whether lengthy human spaceflight and space colonisation are feasible. As of 2006, data on bone loss and muscular atrophy suggest that there would be a significant risk of fractures and movement problems if astronauts landed on a planet after a lengthy interplanetary cruise, such as the six-month interval required to travel to Mars. Medical studies are conducted aboard the ISS on behalf of the National Space Biomedical Research Institute (NSBRI). Prominent among these is the Advanced Diagnostic Ultrasound in Microgravity study in which astronauts perform ultrasound scans under the guidance of remote experts. The study considers the diagnosis and treatment of medical conditions in space. Usually, there is no physician on board the ISS and diagnosis of medical conditions is a challenge. It is anticipated that remotely guided ultrasound scans will have application on Earth in emergency and rural care situations where access to a trained physician is difficult.

Microgravity

A comparison between the combustion of a candle on Earth (left) and in a microgravity environment, such as that found on the ISS (right)

The Earth's gravity is only slightly weaker at the altitude of the ISS than at the surface, but objects in orbit are in a continuous state of freefall, resulting in an apparent state of weightlessness. This perceived weightlessness is disturbed by five separate effects:

• Drag from the residual atmosphere; when the ISS enters the Earth's shadow, the main solar panels are rotated to minimise this aerodynamic drag, helping reduce orbital decay.
• Vibration from movements of mechanical systems and the crew.
• Actuation of the on-board attitude control moment gyroscopes.
• Thruster firings for attitude or orbital changes.
• Gravity-gradient effects, also known as tidal effects. Items at different locations within the ISS would, if not attached to the station, follow slightly different orbits. Being mechanically interconnected these items experience small forces that keep the station moving as a rigid body.

ISS crew member storing samples

Researchers are investigating the effect of the station's near-weightless environment on the evolution, development, growth and internal processes of plants and animals. In response to some of this data, NASA wants to investigate microgravity's effects on the growth of three-dimensional, human-like tissues, and the unusual protein crystals that can be formed in space.

The investigation of the physics of fluids in microgravity will allow researchers to model the behaviour of fluids better. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. In addition, an examination of reactions that are slowed by low gravity and temperatures will give scientists a deeper understanding of superconductivity.

The study of materials science is an important ISS research activity, with the objective of reaping economic benefits through the improvement of techniques used on the ground. Other areas of interest include the effect of the low gravity environment on combustion, through the study of the efficiency of burning and control of emissions and pollutants. These findings may improve current knowledge about energy production, and lead to economic and environmental benefits. Future plans are for the researchers aboard the ISS to examine aerosols, ozone, water vapour, and oxides in Earth's atmosphere, as well as cosmic rays, cosmic dust, antimatter, and dark matter in the universe.

Exploration

A 3D plan of the Russia-based MARS-500 complex, used for ground-based experiments which complement ISS-based preparations for a manned mission to Mars

The ISS provides a location in the relative safety of Low Earth Orbit to test spacecraft systems that will be required for long-duration missions to the Moon and Mars. This provides experience in operations, maintenance as well as repair and replacement activities on-orbit, which will be essential skills in operating spacecraft farther from Earth, mission risks can be reduced and the capabilities of interplanetary spacecraft advanced. Referring to the MARS-500 experiment, ESA states that "Whereas the ISS is essential for answering questions concerning the possible impact of weightlessness, radiation and other space-specific factors, aspects such as the effect of long-term isolation and confinement can be more appropriately addressed via ground-based simulations". Sergey Krasnov, the head of human space flight programmes for Russia's space agency, Roscosmos, in 2011 suggested a "shorter version" of MARS-500 may be carried out on the ISS.

In 2009, noting the value of the partnership framework itself, Sergey Krasnov wrote, "When compared with partners acting separately, partners developing complementary abilities and resources could give us much more assurance of the success and safety of space exploration. The ISS is helping further advance near-Earth space exploration and realisation of prospective programmes of research and exploration of the Solar system, including the Moon and Mars." A manned mission to Mars may be a multinational effort involving space agencies and countries outside the current ISS partnership. In 2010, ESA Director-General Jean-Jacques Dordain stated his agency was ready to propose to the other four partners that China, India and South Korea be invited to join the ISS partnership. NASA chief Charlie Bolden stated in February 2011, "Any mission to Mars is likely to be a global effort". Currently, American legislation prevents NASA co-operation with China on space projects.

Education and cultural outreach

Japan's Kounotori 4 berthing

The ISS crew provides opportunities for students on Earth by running student-developed experiments, making educational demonstrations, allowing for student participation in classroom versions of ISS experiments, and directly engaging students using radio, videolink and email. ESA offers a wide range of free teaching materials that can be downloaded for use in classrooms. In one lesson, students can navigate a 3-D model of the interior and exterior of the ISS, and face spontaneous challenges to solve in real time.

JAXA aims both to "Stimulate the curiosity of children, cultivating their spirits, and encouraging their passion to pursue craftsmanship", and to "Heighten the child's awareness of the importance of life and their responsibilities in society." Through a series of education guides, a deeper understanding of the past and near-term future of manned space flight, as well as that of Earth and life, will be learned. In the JAXA Seeds in Space experiments, the mutation effects of spaceflight on plant seeds aboard the ISS is explored. Students grow sunflower seeds which flew on the ISS for about nine months as a start to 'touch the Universe'. In the first phase of Kibō utilisation from 2008 to mid-2010, researchers from more than a dozen Japanese universities conducted experiments in diverse fields.

Original Jules Verne manuscripts displayed by crew inside Jules Verne ATV

Cultural activities are another major objective. Tetsuo Tanaka, director of JAXA's Space Environment and Utilization Center, says "There is something about space that touches even people who are not interested in science."

Amateur Radio on the ISS (ARISS) is a volunteer programme which encourages students worldwide to pursue careers in science, technology, engineering and mathematics through amateur radio communications opportunities with the ISS crew. ARISS is an international working group, consisting of delegations from nine countries including several countries in Europe as well as Japan, Russia, Canada, and the United States. In areas where radio equipment cannot be used, speakerphones connect students to ground stations which then connect the calls to the station.
First Orbit is a feature-length documentary film about Vostok 1, the first manned space flight around the Earth. By matching the orbit of the International Space Station to that of Vostok 1 as closely as possible, in terms of ground path and time of day, documentary filmmaker Christopher Riley and ESA astronaut Paolo Nespoli were able to film the view that Yuri Gagarin saw on his pioneering orbital space flight. This new footage was cut together with the original Vostok 1 mission audio recordings sourced from the Russian State Archive. Nespoli, during Expedition 26/27, filmed the majority of the footage for this documentary film, and as a result is credited as its director of photography. The film was streamed through the website firstorbit.org in a global YouTube premiere in 2011, under a free license.

In May 2013, commander Chris Hadfield shot a music video of David Bowie's "Space Oddity" on board the station; the film was released freely on YouTube. It was the first music video ever to be filmed in space.

Assembly

Partially constructed ISS in December 2002

S3-S4 Truss Installed in 2007

Soyuz TMA-19 departs in 2010

ISS in 2007, with fewer solar arrays

The assembly of the International Space Station, a major endeavour in space architecture, began in November 1998. Russian modules launched and docked robotically, with the exception of Rassvet. All other modules were delivered by the Space Shuttle, which required installation by ISS and shuttle crewmembers using the Canadarm2 (SSRMS) and EVAs; as of 5 June 2011, they had added 159 components during more than 1,000 hours of EVA. 127 of these spacewalks originated from the station, and the remaining 32 were launched from the airlocks of docked Space Shuttles. The beta angle of the station had to be considered at all times during construction, as the station's beta angle is directly related to the percentage of its orbit that the station (as well as any docked or docking spacecraft) is exposed to the sun; the Space Shuttle would not perform optimally above a limit called the "beta cutoff". Many of the modules that launched on the Space Shuttle were integrated and tested on the ground at the Space Station Processing Facility to find and correct issues prior to launch.
The first module of the ISS, Zarya, was launched on 20 November 1998 on an autonomous Russian Proton rocket. It provided propulsion, attitude control, communications, electrical power, but lacked long-term life support functions. Two weeks later a passive NASA module Unity was launched aboard Space Shuttle flight STS-88 and attached to Zarya by astronauts during EVAs. This module has two Pressurized Mating Adapters (PMAs), one connects permanently to Zarya, the other allows the Space Shuttle to dock to the space station. At this time, the Russian station Mir was still inhabited. The ISS remained unmanned for two years, during which time Mir was de-orbited. On 12 July 2000 Zvezda was launched into orbit. Preprogrammed commands on board deployed its solar arrays and communications antenna. It then became the passive vehicle for a rendezvous with the Zarya and Unity. As a passive "target" vehicle, the Zvezda maintained a stationkeeping orbit as the Zarya-Unity vehicle performed the rendezvous and docking via ground control and the Russian automated rendezvous and docking system. Zarya's computer transferred control of the station to Zvezda's computer soon after docking. Zvezda added sleeping quarters, a toilet, kitchen, CO₂ scrubbers, dehumidifier, oxygen generators, exercise equipment, plus data, voice and television communications with mission control. This enabled permanent habitation of the station.

The first resident crew, Expedition 1, arrived in November 2000 on Soyuz TM-31. At the end of the first day on the station, astronaut Bill Shepherd requested the use of the radio call sign "Alpha", which he and cosmonaut Krikalev preferred to the more cumbersome "International Space Station". The name "Alpha" had previously been used for the station in the early 1990s, and following the request, its use was authorised for the whole of Expedition 1. Shepherd had been advocating the use of a new name to project managers for some time. Referencing a naval tradition in a pre-launch news conference he had said: "For thousands of years, humans have been going to sea in ships. People have designed and built these vessels, launched them with a good feeling that a name will bring good fortune to the crew and success to their voyage." Yuri Semenov, the President of Russian Space Corporation Energia at the time, disapproved of the name "Alpha"; he felt that Mir was the first space station, and so he would have preferred the names "Beta" or "Mir 2" for the ISS.
 
Expedition 1 arrived midway between the flights of STS-92 and STS-97. These two Space Shuttle flights each added segments of the station's Integrated Truss Structure, which provided the station with Ku-band communication for US television, additional attitude support needed for the additional mass of the USOS, and substantial solar arrays supplementing the station's existing 4 solar arrays.
Over the next two years, the station continued to expand. A Soyuz-U rocket delivered the Pirs docking compartment. The Space Shuttles Discovery, Atlantis, and Endeavour delivered the Destiny laboratory and Quest airlock, in addition to the station's main robot arm, the Canadarm2, and several more segments of the Integrated Truss Structure.

Aft view showing a Progress spacecraft docked to Zvezda

The expansion schedule was interrupted by the Space Shuttle Columbia disaster in 2003, with the resulting two year hiatus in the Space Shuttle programme halting station assembly. The space shuttle was grounded until 2005 with STS-114 flown by Discovery.

Assembly resumed in 2006 with the arrival of STS-115 with Atlantis, which delivered the station's second set of solar arrays. Several more truss segments and a third set of arrays were delivered on STS-116, STS-117, and STS-118. As a result of the major expansion of the station's power-generating capabilities, more pressurised modules could be accommodated, and the Harmony node and Columbus European laboratory were added. These were followed shortly after by the first two components of Kibō. In March 2009, STS-119 completed the Integrated Truss Structure with the installation of the fourth and final set of solar arrays. The final section of Kibō was delivered in July 2009 on STS-127, followed by the Russian Poisk module. The third node, Tranquility, was delivered in February 2010 during STS-130 by the Space Shuttle Endeavour, alongside the Cupola, closely followed in May 2010 by the penultimate Russian module, Rassvet. Rassvet was delivered by Space Shuttle Atlantis on STS-132 in exchange for the Russian Proton delivery of the Zarya Module in 1998 which had been funded by the United States. The last pressurised module of the USOS, Leonardo, was brought to the station by Discovery on her final flight, STS-133, followed by the Alpha Magnetic Spectrometer on STS-134, delivered by Endeavour.

The Cupola arrived in 2010

As of June 2011, the station consisted of fifteen pressurised modules and the Integrated Truss Structure. Five modules are still to be launched, including the Bigelow Expandable Activity Module, the Nauka with the European Robotic Arm, the Uzlovoy Module, and two power modules called NEM-1 and NEM-2. When attached, the Bigelow Expandable Activity Module will be the first inflatable module to become part of a space station. As of November 2015, Russia's future primary research module Nauka is set to launch in February 2017, along with the European Robotic Arm which will be able to relocate itself to different parts of the Russian modules of the station. After the Nauka module is attached, the Uzlovoy Module will be attached to one of its docking ports. When completed, the station will have a mass in excess of 400 tonnes (440 short tons).

The gross mass of the station changes over time. The total launch mass of the modules on orbit is about 417,289 kg (919,965 lb) (as of 3 September 2011). The mass of experiments, spare parts, personal effects, crew, foodstuff, clothing, propellants, water supplies, gas supplies, docked spacecraft, and other items add to the total mass of the station. Hydrogen gas is constantly vented overboard by the oxygen generators.

Station structure

Russian Orbital Segment Windows

USOS International Space Station window locations

The ISS is a third generation modular space station. Modular stations can allow the mission to be changed over time and new modules can be added or removed from the existing structure, allowing greater flexibility.

Below is a diagram of major station components. The blue areas are pressurised sections accessible by the crew without using spacesuits. The station's unpressurised superstructure is indicated in red. Other unpressurised components are yellow. Note that the Unity node joins directly to the Destiny laboratory. For clarity, they are shown apart.

Russian docking port

Solar array

Zvezda DOS-8 (service module)

Solar array

Russian docking port

Poisk (MRM-2) airlock

Pirs airlock

Russian docking port

Nauka lab to replace Pirs

European robotic arm

Solar array

Zarya FGB (first module)

Solar array

Rassvet (MRM-1)

Russian docking port

PMA 1

Cargo spacecraft berthing port

Leonardo cargo bay

BEAM habitat

Quest airlock

Unity Node 1

Tranquility Node 3

PMA 3 docking port

ESP-2

Cupola

Solar array

Solar array

Heat radiator

Heat radiator

Solar array

Solar array

ELC 2, AMS

Z1 truss

ELC 3

S5/6 Truss

S3/S4 Truss

S1 Truss

S0 Truss

P1 Truss

P3/P4 Truss

P5/6 Truss

ELC 4, ESP 3

ELC 1

Dextre robotic arm

Canadarm2 robotic arm

Solar array

Solar array

Solar array

Solar array

ESP-1

Destiny laboratory

Kibō logistics cargo bay

Cargo spacecraft berthing port

Cargo spacecraft berthing port

Kibō robotic arm

External payloads

Columbus laboratory

Harmony Node 2

Kibō laboratory

Kibō external platform

PMA 2 docking port

IDA 2 docking adapter

 Pressurised modules

Zarya

Zarya as seen by Space Shuttle Endeavour during STS-88

Zarya (Russian: Заря́; lit. dawn), also known as the Functional Cargo Block or FGB (from the Russian "Функционально-грузовой блок", Funktsionalno-gruzovoy blok or ФГБ), was the first module of the International Space Station to be launched. The FGB provided electrical power, storage, propulsion, and guidance to the ISS during the initial stage of assembly. With the launch and assembly in orbit of other modules with more specialized functionality, Zarya is now primarily used for storage, both inside the pressurized section and in the externally mounted fuel tanks. The Zarya is a descendant of the TKS spacecraft designed for the Soviet Salyut program. The name Zarya was given to the FGB because it signified the dawn of a new era of international cooperation in space. Although it was built by a Russian company, it is owned by the United States. Zarya weighs 19,300 kg (42,500 lb), is 12.55 m (41.2 ft) long and 4.1 m (13 ft) wide, discounting solar arrays.
Built from December 1994 to January 1998 in Russia at the Khrunichev State Research and Production Space Center (KhSC) in Moscow, Zarya's control system was developed by the Khartron Corp. (Kharkiv, Ukraine).

Zarya was launched on 20 November 1998, on a Russian Proton rocket from Baikonur Cosmodrome Site 81 in Kazakhstan to a 400 km (250 mi) high orbit with a designed lifetime of at least 15 years. After Zarya reached orbit, STS-88 launched on 4 December 1998, to attach the Unity Module.
Although only designed to fly autonomously for six to eight months, Zarya did so for almost two years because of delays with the Russian Service Module, Zvezda, which finally launched on 12 July 2000, and docked with Zarya on 26 July using the Russian Kurs docking system.

Unity

Unity as pictured by Space Shuttle Endeavour

Unity, or Node 1, is one of three nodes, or passive connecting modules, in the US Orbital Segment of the station. It was the first US-built component of the Station to be launched. Cylindrical in shape, with six berthing locations facilitating connections to other modules, Unity was carried into orbit by Space Shuttle Endeavour as the primary cargo of STS-88 in 1998. Essential space station resources such as fluids, environmental control and life support systems, electrical and data systems are routed through Unity to supply work and living areas of the station. More than 50,000 mechanical items, 216 lines to carry fluids and gases, and 121 internal and external electrical cables using six miles of wire were installed in the Unity node. Unity is made of aluminium. Prior to its launch aboard Endeavour, conical Pressurized Mating Adapters (PMAs) were attached to the aft and forward berthing mechanisms of Unity. Unity and the two mating adapters together weighed about 11,600 kg (25,600 lb). The adapters allow the docking systems used by the Space Shuttle and by Russian modules to attach to the node's hatches and berthing mechanisms.

Unity was carried into orbit as the primary cargo of the Space Shuttle Endeavour on STS-88, the first Space Shuttle mission dedicated to assembly of the station. On 6 December 1998, the STS-88 crew mated the aft berthing port of Unity with the forward hatch of the already orbiting Zarya module.

Zvezda

Zvezda (Russian: Звезда́, meaning "star"), also known as DOS-8, Service Module or SM (Russian: СМ). Early in the station's life, Zvezda provided all of its critical systems. It made the station permanently habitable for the first time, adding life support for up to six crew and living quarters for two. Zvezda's DMS-R computer handles guidance, navigation and control for the entire space station. A second computer which performs the same functions will be installed in the Nauka module, FGB-2.

The hull of Zvezda was completed in February 1985, with major internal equipment installed by October 1986. The module was launched by a Proton-K rocket from Site 81/23 at Baikonur, on 12 July 2000. Zvezda is at the rear of the station according to its normal direction of travel and orientation, and its engines may be used to boost the station's orbit. Alternatively Russian and European spacecraft can dock to Zvezda's aft port and use their engines to boost the station.

Destiny

Destiny interior in 2001

Destiny is the primary research facility for United States payloads aboard the ISS. In 2011, NASA chose the not-for-profit group Center for the Advancement of Science in Space (CASIS) to be the sole manager of all American science on the station which does not relate to manned exploration. The module houses 24 International Standard Payload Racks, some of which are used for environmental systems and crew daily living equipment. Destiny also serves as the mounting point for the station's Truss Structure.

Quest

Quest is the only USOS airlock, and hosts spacewalks with both United States EMU and Russian Orlan spacesuits. It consists of two segments: the equipment lock, which stores spacesuits and equipment, and the crew lock, from which astronauts can exit into space. This module has a separately controlled atmosphere. Crew sleep in this module, breathing a low nitrogen mixture the night before scheduled EVAs, to avoid decompression sickness (known as "the bends") in the low-pressure suits.

Pirs and Poisk

Pirs (Russian: Пирс, meaning "pier"), (Russian: Стыковочный отсек), "docking module", SO-1 or DC-1 (docking compartment), and Poisk (Russian: По́иск; lit. Search), also known as the Mini-Research Module 2 (MRM 2), Малый исследовательский модуль 2, or МИМ 2. Pirs and Poisk are Russian airlock modules. Each of these modules have 2 identical hatches. An outward opening hatch on the MIR space station failed after it swung open too fast after unlatching, because of a small amount of air pressure remaining in the airlock. A different entry was used, and the hatch repaired. All EVA hatches on the ISS open inwards and are pressure sealing. Pirs was used to store, service, and refurbish Russian Orlan suits and provided contingency entry for crew using the slightly bulkier American suits. The outermost docking ports on both airlocks allow docking of Soyuz and Progress spacecraft, and the automatic transfer of propellants to and from storage on the ROS.

Harmony

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Tranquility node in 2011

Harmony is the second of the station's node modules and the utility hub of the USOS. The module contains four racks that provide electrical power, bus electronic data, and acts as a central connecting point for several other components via its six Common Berthing Mechanisms (CBMs). The European Columbus and Japanese Kibō laboratories are permanently berthed to the starboard and port radial ports respectively. The nadir and zenith ports can be used for docking visiting spacecraft including HTV, Dragon, and Cygnus, with the nadir port serving as the primary docking port. American Shuttle Orbiters docked with the ISS via PMA-2, attached to the forward port.

Tranquility

Tranquility is the third and last of the station's US nodes, it contains an additional life support system to recycle waste water for crew use and supplements oxygen generation. Like the other US nodes, it has six berthing mechanisms, five of which are currently in use. The first one connects to the station's core via the Unity module, others host the Cupola, the PMA docking port #3, the Leonardo PMM and the Bigelow Expandable Activity Module. The final zenith port remains free.

Columbus

Columbus module in 2008

Columbus, the primary research facility for European payloads aboard the ISS, provides a generic laboratory as well as facilities specifically designed for biology, biomedical research and fluid physics. Several mounting locations are affixed to the exterior of the module, which provide power and data to external experiments such as the European Technology Exposure Facility (EuTEF), Solar Monitoring Observatory, Materials International Space Station Experiment, and Atomic Clock Ensemble in Space. A number of expansions are planned for the module to study quantum physics and cosmology. ESA's development of technologies on all the main areas of life support has been ongoing for more than 20 years and are/have been used in modules such as Columbus and the ATV. The German Aerospace Center DLR manages ground control operations for Columbus and the ATV is controlled from the French CNES Toulouse Space Center.

Kibō

Not large enough for crew using spacesuits, the airlock on Kibō has a sliding drawer for external experiments.

Kibō (Japanese: きぼう, "hope") is the largest single ISS module. This laboratory is used to carry out research in space medicine, biology, Earth observations, materials production, biotechnology, communications research, and has facilities for growing plants and fish. During August 2011, the MAXI observatory mounted on Kibō, which utilises the ISS's orbital motion to image the whole sky in the X-ray spectrum, detected for the first time the moment a star was swallowed by a black hole. The laboratory contains a total of 23 racks, including 10 experiment racks and has a dedicated airlock for experiments. In a 'shirt sleeves' environment, crew attach an experiment to the sliding drawer within the airlock, close the inner, and then open the outer hatch. By extending the drawer and removing the experiment using the dedicated robotic arm, payloads are placed on the external platform. The process can be reversed and repeated quickly, allowing access to maintain external experiments without the delays caused by EVAs.

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  Pressurized Module
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  Experiment Logistics Module
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  Exposed Facility
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  Experiment Logistics Module
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  Remote Manipulator System

A smaller pressurised module is attached to the top of Kibō, serving as a cargo bay. The dedicated Interorbital communications system allows large amounts of data to be beamed from Kibō's ICS, first to the Japanese KODAMA satellite in geostationary orbit, then to Japanese ground stations. When a direct communication link is used, contact time between the ISS and a ground station is limited to approximately 10 minutes per visible pass. When KODAMA relays data between a LEO spacecraft and a ground station, real-time communications are possible in 60% of the flight path of the spacecraft. Ground staff use telepresence robotics to conduct on-orbit research without crew intervention.

Cupola

The Cupola's design has been compared to the Millennium Falcon from Star Wars.

Dmitri Kondratyev and Paolo Nespoli in the Cupola. Background left to right, Progress M-09M, Soyuz TMA-20, the Leonardo module and HTV-2.

Cupola is a seven window observatory, used to view Earth and docking spacecraft. Its name derives from the Italian word cupola, which means "dome". The Cupola project was started by NASA and Boeing, but cancelled because of budget cuts. A barter agreement between NASA and the ESA resulted in the Cupola's development being resumed in 1998 by the ESA. It was built by Thales Alenia Space in Torino, Italy. The module
comes equipped with robotic workstations for operating the station's main robotic arm and shutters to protect its windows from damage caused by micrometeorites. It features 7 windows, with an 80-centimetre (31 in) round window, the largest window on the station (and the largest flown in space to date). The distinctive design has been compared to the 'turret' of the fictitious Millennium Falcon from the motion picture Star Wars; the original prop lightsaber used by actor Mark Hamill as Luke Skywalker in the 1977 film was flown to the station in 2007.

Rassvet

Rassvet (Russian: Рассве́т; lit. "dawn"), also known as the Mini-Research Module 1 (MRM-1) (Russian: Ма́лый иссле́довательский модуль, МИМ 1) and formerly known as the Docking Cargo Module (DCM), is similar in design to the Mir Docking Module launched on STS-74 in 1995. Rassvet is primarily used for cargo storage and as a docking port for visiting spacecraft. It was flown to the ISS aboard NASA's Space Shuttle Atlantis on the STS-132 mission and connected in May 2010, Rassvet is the only Russian owned module launched by NASA, to repay for the launch of Zarya, which is Russian designed and built, but partially paid for by NASA. Rassvet was launched with the Russian Nauka Laboratory's Experiments airlock temporarily attached to it, and spare parts for the European Robotic Arm.

Leonardo

Leonardo installed

Leonardo Permanent Multipurpose Module (PMM) is a storage module attached to the Tranquility node. The three NASA Space Shuttle MPLM cargo containers—Leonardo, Raffaello and Donatello—were built for NASA in Turin, Italy by Alcatel Alenia Space, now Thales Alenia Space. The MPLMs were provided to NASA's ISS programme by Italy (independent of their role as a member state of ESA) and are considered to be US elements. In a bartered exchange for providing these containers, the US gave Italy research time aboard the ISS out of the US allotment in addition to that which Italy receives as a member of ESA. The Permanent Multipurpose Module was created by converting Leonardo into a module that could be permanently attached to the station.

Bigelow Expandable Activity Module

On 16 January 2013, Bigelow Aerospace was contracted by NASA to provide a Bigelow Expandable Activity Module (BEAM) for a two-year technology demonstration. BEAM was delivered to the ISS aboard SpaceX CRS-8 on 10 April 2016, and was berthed to the aft port of the Tranquility node on 16 April.

During its two-year test run, instruments are expected to measure its structural integrity and leak rate, along with temperature and radiation levels. The hatch leading into the module will remain mostly closed except for periodic visits by space station crew members for inspections and data collection. Following the test run, the module will be detached and jettisoned from the station.

Scheduled additional modules

Nauka

Nauka (Russian: Нау́ка; lit. "science"), also known as the Multipurpose Laboratory Module (MLM) or FGB-2 (Russian: Многофункциональный лабораторный модуль, МЛМ), is the major Russian laboratory module. It was scheduled to arrive at the station in 2014, docking to the port that was occupied by the Pirs module. Due to deteriorations following many years spent in storage, it proved necessary to build a new propulsion module, and the launch date was postponed to November 2017. Prior to the arrival of the Nauka module, a Progress spacecraft will remove Pirs from the station, deorbiting it to reenter over the Pacific Ocean. Nauka contains an additional set of life support systems and attitude control. Originally it would have routed power from the single Science-and-Power Platform, but that single module design changed over the first ten years of the ISS mission, and the two science modules, which attach to Nauka via the Uzlovoy Module, or Russian node, each incorporate their own large solar arrays to power Russian science experiments in the ROS.
Nauka's mission has changed over time. During the mid-1990s, it was intended as a backup for the FGB, and later as a universal docking module (UDM); its docking ports will be able to support automatic docking of both spacecraft, additional modules and fuel transfer. Nauka has its own engines. Smaller Russian modules such as Pirs and Poisk were delivered by modified Progress spacecraft, and the larger modules; Zvezda, Zarya, and Nauka, were launched by Proton rockets. Russia plans to separate Nauka, along with the rest of the Russian Orbital Segment, before the ISS is deorbited, to form the OPSEK space station.

Uzlovoy Module

The Uzlovoy Module (UM), or Node Module is a 4 metric ton ball shaped module that will support the docking of two scientific and power modules during the final stage of the station assembly and provide the Russian segment additional docking ports to receive Soyuz TMA and Progress M spacecraft. UM is to be incorporated into the ISS in 2016. It will be integrated with a special version of the Progress cargo ship and launched by a standard Soyuz rocket. The Progress would use its own propulsion and flight control system to deliver and dock the Node Module to the nadir (Earth-facing) docking port of the Nauka MLM/FGB-2 module. One port is equipped with an active hybrid docking port, which enables docking with the MLM module. The remaining five ports are passive hybrids, enabling docking of Soyuz and Progress vehicles, as well as heavier modules and future spacecraft with modified docking systems. The node module was conceived to serve as the only permanent element of the future Russian successor to the ISS, OPSEK. Equipped with six docking ports, the Node Module would serve as a single permanent core of the future station with all other modules coming and going as their life span and mission required. This would be a progression beyond the ISS and Russia's modular MIR space station, which are in turn more advanced than early monolithic first generation stations such as Skylab, and early Salyut and Almaz stations.
Science Power Modules 1 & 2 (NEM-1, NEM-2) (Russian: Нау́чно-Энергетический Модуль-1 и -2)

The cancelled Habitation module under construction in 1997

Cancelled components

Several modules planned for the station have been cancelled over the course of the ISS programme, whether for budgetary reasons, because the modules became unnecessary, or following a redesign of the station after the 2003 Columbia disaster. The US Centrifuge Accommodations Module was intended to host science experiments in varying levels of artificial gravity. The US Habitation Module would have served as the station's living quarters. Instead, the sleep stations are now spread throughout the station. The US Interim Control Module and ISS Propulsion Module were intended to replace functions of Zvezda in case of a launch failure. The Russian Universal Docking Module, to which the cancelled Russian Research modules and spacecraft would have docked. The Russian Science Power Platform would have provided the Russian Orbital Segment with a power supply independent of the ITS solar arrays, and two Russian Research Modules that were planned to be used for scientific research.