Black Hole

Black Hole

Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black.

II

PROPERTIES

The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity—that is, a dimensionless object of infinite density.

III

FORMATION

Stellar Evolution

Stars begin life as diffuse clouds of dust and gas. These clouds condense to form stars, after which the stars can develop into a variety of objects, depending on how much matter they contain. Stars that contain more matter experience the effects of gravity more strongly and evolve into dense bodies, such as neutron stars or even black holes.

Black holes are thought to form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their energy production is no longer available to resist contraction of the core to ever-higher densities. Two new types of pressure, electron and neutron pressure, arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the star is more than about five times as massive as the Sun, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.

Stephen William Hawking

Author of the best-selling book A Brief History of Time, physicist Stephen Hawking has strived to make difficult concepts in physics more accessible to the public. His discoveries about gravitation are regarded as some of the most important contributions to that area of physics since Albert Einstein introduced the general theory of relativity in 1915.

In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material) circling the center of the galaxy M87 with an acceleration that indicated the presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black holes in the centers of dozens of galaxies and had found that the masses of the black holes were correlated with the masses of the parent galaxies. More massive galaxies tend to have more massive black holes at their centers. Learning more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.

The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. For black holes of sufficiently small mass it is possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping (see X Ray: Pair Production). The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.

The American astronomer Kip Thorne of California Institute of Technology in Pasadena, California, has evaluated the chance that black holes can collapse to form 'wormholes,' connections between otherwise distant parts of the universe. He concludes that an unknown form of 'exotic matter' would be necessary for such wormholes to survive.

Big Bang Theory

Big Bang Theory

Big Bang Theory, currently accepted explanation of the beginning of the universe. The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 13.7 billion years ago, and the universe has since been expanding and cooling.

The theory is based on the mathematical equations, known as the field equations, of the general theory of relativity set forth in 1915 by Albert Einstein. In 1922 Russian physicist Alexander Friedmann provided a set of solutions to the field equations. These solutions have served as the framework for much of the current theoretical work on the big bang theory. American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was universally shifted toward the red end of the spectrum (see Redshift). Once “tired light” theories—that light slowly loses energy naturally, becoming more red over time—were dismissed, this shift proved that the galaxies were moving away from each other. Hubble found that galaxies farther away were moving away proportionally faster, showing that the universe is expanding uniformly. However, the universe’s initial state was still unknown.

In the 1940s Russian-American physicist George Gamow worked out a theory that fit with Friedmann’s solutions in which the universe expanded from a hot, dense state. In 1950 British astronomer Fred Hoyle, in support of his own opposing steady-state theory, referred to Gamow’s theory as a mere “big bang,” but the name stuck. Indeed, a contest in the 1990s by Sky & Telescope magazine to find a better (perhaps more dignified) name did not produce one.

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HISTORY

The overall framework of the big bang theory came out of solutions to Einstein’s general relativity field equations and remains unchanged, but various details of the theory are still being modified today. Einstein himself initially believed that the universe was static. When his equations seemed to imply that the universe was expanding or contracting, Einstein added a constant term to cancel out the expansion or contraction of the universe. When the expansion of the universe was later discovered, Einstein stated that introducing this “cosmological constant” had been a mistake.

After Einstein’s work of 1917, several scientists, including the abbé Georges Lemaître in Belgium, Willem de Sitter in Holland, and Alexander Friedmann in Russia, succeeded in finding solutions to Einstein’s field equations. The universes described by the different solutions varied. De Sitter’s model had no matter in it. This model is actually not a bad approximation since the average density of the universe is extremely low. Lemaître’s universe expanded from a “primeval atom.” Friedmann’s universe also expanded from a very dense clump of matter, but did not involve the cosmological constant. These models explained how the universe behaved shortly after its creation, but there was still no satisfactory explanation for the beginning of the universe.

In the 1940s George Gamow was joined by his students Ralph Alpher and Robert Herman in working out details of Friedmann’s solutions to Einstein’s theory. They expanded on Gamow’s idea that the universe expanded from a primordial state of matter called ylem consisting of protons, neutrons, and electrons in a sea of radiation. They theorized the universe was very hot at the time of the big bang (the point at which the universe explosively expanded from its primordial state), since elements heavier than hydrogen can be formed only at a high temperature. Alpher and Hermann predicted that radiation from the big bang should still exist. Cosmic background radiation roughly corresponding to the temperature predicted by Gamow’s team was detected in the 1960s, further supporting the big bang theory, though the work of Alpher, Herman, and Gamow had been forgotten.

III

THE THEORY

The big bang theory seeks to explain what happened at or soon after the beginning of the universe. Scientists can now model the universe back to 10^-43 seconds after the big bang. For the time before that moment, the classical theory of gravity is no longer adequate. Scientists are searching for a theory that merges gravity (as explained by Einstein's general theory of relativity) and quantum mechanics but have not found one yet. Many scientists have hope that string theory, also known as M-theory, will tie together gravity and quantum mechanics and help scientists explore further back in time (see Physics: Unified Field Theory).

Because scientists cannot look back in time beyond that early epoch, the actual big bang is hidden from them. There is no way at present to detect the origin of the universe. Further, the big bang theory does not explain what existed before the big bang. It may be that time itself began at the big bang, so that it makes no sense to discuss what happened “before” the big bang.

According to the big bang theory, the universe expanded rapidly in its first microseconds. A single force existed at the beginning of the universe, and as the universe expanded and cooled, this force separated into those we know today: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. A theory called the electroweak theory now provides a unified explanation of electromagnetism and the weak nuclear force theory (see Unified Field Theory). Physicists are now searching for a grand unification theory to also incorporate the strong nuclear force. String theory seeks to incorporate the force of gravity with the other three forces, providing a theory of everything (TOE).

One widely accepted version of big bang theory includes the idea of inflation. In this model, the universe expanded much more rapidly at first, to about 10⁵⁰ times its original size in the first 10^-32 second, then slowed its expansion. The theory was advanced in the 1980s by American cosmologist Alan Guth and elaborated upon by American astronomer Paul Steinhardt, Russian American scientist Andrei Linde, and British astronomer Andreas Albrecht. The inflationary universe theory (see Inflationary Theory) solves a number of problems of cosmology. For example, it shows that the universe now appears close to the type of flat space described by the laws of Euclid’s geometry: We see only a tiny region of the original universe, similar to the way we do not notice the curvature of the earth because we see only a small part of it. The inflationary universe also shows why the universe appears so homogeneous. If the universe we observe was inflated from some small, original region, it is not surprising that it appears uniform.

Once the expansion of the initial inflationary era ended, the universe continued to expand more slowly. The inflationary model predicts that the universe is on the boundary between being open and closed. If the universe is open, it will keep expanding forever. If the universe is closed, the expansion of the universe will eventually stop and the universe will begin contracting until it collapses. Whether the universe is open or closed depends on the density, or concentration of mass, in the universe. If the universe is dense enough, it is closed.

IV

SUPPORTING EVIDENCE

The universe cooled as it expanded. After about one second, protons formed. In the following few minutes—often referred to as the “first three minutes”—combinations of protons and neutrons formed the isotope of hydrogen known as deuterium as well as some of the other light elements, principally helium, as well as some lithium, beryllium, and boron. The study of the distribution of deuterium, helium, and the other light elements is now a major field of research. The uniformity of the helium abundance around the universe supports the big bang theory and the abundance of deuterium can be used to estimate the density of matter in the universe.

From about 380,000 to about 1 million years after the big bang, the universe cooled to about 3000°C (about 5000°F) and protons and electrons combined to make hydrogen atoms. Hydrogen atoms can only absorb and emit specific colors, or wavelengths, of light. The formation of atoms allowed many other wavelengths of light, wavelengths that had been interfering with the free electrons prior to the cooling of the universe, to travel much farther than before. This change set free radiation that we can detect today. After billions of years of cooling, this cosmic background radiation is at about 3 K (-270°C/-454°F).The cosmic background radiation was first detected and identified in 1965 by American astrophysicists Arno Penzias and Robert Wilson.

The Cosmic Background Explorer (COBE) spacecraft, a project of the National Aeronautics and Space Administration (NASA), mapped the cosmic background radiation between 1989 and 1993. It verified that the distribution of intensity of the background radiation precisely matched that of matter that emits radiation because of its temperature, as predicted for the big bang theory. It also showed that cosmic background radiation is not uniform, that it varies slightly. These variations are thought to be the seeds from which galaxies and other structures in the universe grew.

Evidence indicates that the matter that scientists detect in the universe is only a small fraction of all the matter that exists. For example, observations of the speeds at which individual galaxies move within clusters of galaxies show that a great deal of unseen matter must exist to exert sufficient gravitational force to keep the clusters from flying apart. Cosmologists now think that much of the universe is dark matter—matter that has gravity but does not give off radiation that we can see or otherwise detect. One kind of dark matter theorized by scientists is cold dark matter, with slowly moving (cold) massive particles. No such particles have yet been detected, though astronomers have made up fanciful names for them, such as Weakly Interacting Massive Particles (WIMPs). Other cold dark matter could be nonradiating stars or planets, which are known as MACHOs (Massive Compact Halo Objects).

An alternative theory that explains the dark-matter model involves hot dark matter, where hot implies that the particles are moving very fast. Neutrinos, fundamental particles that travel at nearly the speed of light, are the prime example of hot dark matter. However, scientists think that the mass of a neutrino is so low that neutrinos can only account for a small portion of dark matter. If the inflationary version of big bang theory is correct, then the amount of dark matter and of whatever else might exist is just enough to bring the universe to the boundary between open and closed.

Scientists develop theoretical models to show how the universe’s structures, such as clusters of galaxies, have formed. Their models invoke hot dark matter, cold dark matter, or a mixture of the two. This unseen matter would have provided the gravitational force needed to bring large structures such as clusters of galaxies together. The theories that include dark matter match the observations, although there is no consensus on the type or types of dark matter that must be included. Supercomputers are important for making such models.

V

REFINING THE THEORY

Astronomers continue to make new observations that are also interpreted within the framework of the big bang theory. No major problems with the big bang theory have been found, but scientists constantly adjust the theory to match the observed universe. In particular, a “standard model” of the big bang has been established by results from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001 (see Cosmology). The probe studied the anisotropies, or ripples, in the temperature of cosmic background radiation at a higher resolution than COBE was capable of. These ripples indicate that regions of the young universe were very slightly hotter or cooler, by a factor of about 1/1000, than adjacent regions. WMAP’s observations suggest that the rate of expansion of the universe, called Hubble’s constant, is about 71 km/s/Mpc (kilometers per second per million parsecs, where a parsec is about 3.26 light-years). In other words, the distance between any two objects in space that are separated by a million parsecs increases by about 71 km every second in addition to any other motion they may have relative to one another. In combination with previously existing observations, this rate of expansion tells cosmologists that the universe is “flat,” though flatness here does not refer to the actual shape of the universe but rather that the geometric laws that apply to the universe match those of a flat plane.

To be flat, the universe must contain a certain amount of matter and energy, known as the critical density. The distribution of sizes of ripples detected by WMAP show that ordinary matter—like that making up objects and living things on Earth—accounts for only 4.4 percent of the critical density. Dark matter makes up an additional 23 percent. Astoundingly, the remaining 73 percent of the universe is composed of something else—a substance so mysterious that nobody knows much about it. Called “dark energy,” this substance provides the antigravity-like negative pressure that causes the universe's expansion to accelerate rather than slow down. This “accelerating universe” was detected independently by two competing groups of astronomers in the last years of the 20th century. The ideas of an accelerating universe and the existence of dark energy have caused astronomers to substantially modify previous ideas of the big bang universe.

WMAP's results also show that cosmic background radiation was set free about 389,000 years after the big bang, later than was previously thought, and that the first stars formed about 200 million years after the big bang, earlier than anticipated. Further refinements to the big bang theory are expected from WMAP, which continues to collect data. An even more precise mission to study the beginnings of the universe, the European Space Agency’s Planck spacecraft, is scheduled to be launched in 2007.

Astrophysics

Astrophysics

Life of a Star

A star begins life as a large, relatively cool mass of gas in a nebula, such as the Orion Nebula (left). As gravity causes the gas to contract, the nebula’s temperature rises, eventually becoming hot enough to trigger nuclear reactions in its atoms and form a star. A main sequence star (middle) shines because of the massive, fairly steady output of energy from the fusion of hydrogen nuclei to form helium. The main sequence phase of a medium-sized star is believed to last as long as 10 billion years. The Sun is just over halfway through this phase. Stars eventually use up their energy supply, ending their lives as white dwarfs, which are extremely small, dense globes, or in the case of larger stars, as spectacular explosions called supernovas. A supernova is shown within the Large Magellanic Cloud at the bottom right of the rightmost photo.

Astrophysics, the branch of astronomy that seeks to understand the birth, evolution, and end states of celestial objects and systems in terms of the physical laws that govern them. For each object or system under study, astrophysicists observe radiations emitted over the entire electromagnetic spectrum and variations of these emissions over time (see Electromagnetic Radiation; Spectroscopy; Spectrum). This information is then interpreted with the aid of theoretical models. It is the task of such a model to explain the mechanisms by which radiation is generated within or near the object, and how the radiation then escapes. Radiation measurements can be used to estimate the distribution and energy states of the atoms, as well as the kinds of atoms, making up the object. The temperatures and pressures in the object may then be estimated using the laws of thermodynamics.

Models of celestial objects in equilibrium are based on balances among the forces being exerted on and within the objects, with slow evolution taking place as nuclear and chemical transformations occur. Cataclysmic phenomena are interpreted in terms of models in which these forces are out of balance.

II

THE STUDY OF STARS

Hertzsprung-Russell Diagram

The H-R diagram compares the brightness of a star with its temperature. The diagonal line running from the upper left to the lower right is called the Main Sequence. Stars lying on the Main Sequence are blue when they are bright and red when they are dim. The Sun lies in the center of the Main Sequence.

Stars are among the best understood celestial objects. If the light of a star is dispersed into its wavelength spectrum, the relative intensities at various wavelengths yield considerable information about the star. The surface temperature can be estimated, using the laws of thermal radiation.

If the distance of the star is known, its luminosity can be found by summing the observed intensities over all wavelengths. Its radius can then be found using the fact that the luminosity is the product of the energy emitted per unit area (which depends only on the surface temperature) and the total surface area.

If the spectrum of a star is studied under high resolution, many dark lines are seen at specific wavelengths. These lines are due to the absorption of light from deeper layers by atoms in the cooler layers above. The kinds of atoms present in the star can then be identified by comparing stellar absorption lines with those produced in the laboratory by known gases, and the temperature and pressure of the atmosphere as well as the relative abundances of the chemical elements can be calculated.

Most stars are classified as part of a “main sequence” in which both temperature and luminosity increase with mass. Some stars are much brighter and hence much larger than main-sequence stars of the same temperature, and are called red giants. Many stars are much fainter and hence much smaller than main-sequence stars of the same temperature, including white dwarfs (1 percent the size of the Sun) and neutron stars (0.001 percent the size of the Sun). Black holes emit no light, but absorb all light that passes within a few kilometers (see Black Hole).

Theoretical models of stellar interiors have been calculated based on the theory that an equilibrium exists between the force of gravity, which tends to cause the star to collapse, and the pressure of superheated gases, which tend to expand. High stellar temperatures also drive a flow of heat from inside the star to the outside. If the star is to be in equilibrium, this heat loss must be compensated by the energy released by nuclear reactions in the core. As various nuclear fuels are exhausted, the star slowly evolves, and the core contracts to increasingly higher densities.

For stars of low mass, this process ends when the outer layers are gently ejected to form a planetary nebula; the core then cools down to form a white dwarf. More massive stars become unstable; as they evolve, this core suddenly collapses to form a neutron star or black hole, and the energy thereby released ejects the outer layers at very high speed, forming a supernova (see Supernova).

III

THE STUDY OF GALAXIES

Colliding Galaxies

A collision between two spiral galaxies that began millions of years ago created the so-called Antennae galaxies, named for the antenna-like arms thrown out by the encounter. The two galaxies are merging together, causing billions of new stars to form in the blue regions. Our Milky Way and the Andromeda galaxy will collide in a similar way billions of years from now.

Galaxies are giant systems of stars at very great distances from each other. Many galaxies also contain interstellar material in the form of diffuse gas and dust particles, permeated by weak magnetic fields in which are trapped energetic charged particles called cosmic rays (see Cosmic Rays; Galaxy).

Elliptical galaxies are spheroidal in shape and have little interstellar matter; spiral galaxies are highly flattened rotating disks composed of interstellar matter and large numbers of massive stars, as well as the less massive stars that are also common in ellipticals. The matter in the disk forms a spiral pattern, usually with two spiral arms.

Galaxies M86 and M84

The elliptical galaxies M86 (center) and M84 (right) are members of the Virgo cluster of galaxies, located about 50 million light-years away from our smaller cluster, the Local Group. Elliptical galaxies are populated by older stars and contain little interstellar matter. They are usually the brightest galaxies.

In the nucleus of some galaxies active sources of relativistic particles (particles with speeds approaching that of light) emit radio waves and X rays as well as visible light. This phenomenon is observed in both elliptical and spiral galaxies; objects called quasars seem to be extreme forms of such activity, with luminosities ranging up to 100 times that of all the stars in the galaxy. At present the explanation of the energy source in active galaxies is unknown (see Quasar; Radio Astronomy).

Theoretical models of galaxies are based on the exchange of matter and energy between stars and interstellar matter. When a galaxy forms, it consists at first entirely of interstellar matter; but stars then form from this gas. From the supernovas occurring among these stars, matter enriched in heavy elements is injected back into space. Thus, interstellar matter is progressively enriched with heavy elements, which then become part of new generations of stars. In ellipticals, the process is largely complete, and little interstellar matter remains. In spirals, however, much interstellar matter remains; in these galaxies the rate of star formation is much higher in the spiral arms than in the core. Apparently, spiral density waves compress interstellar matter to form dark clouds, and these subsequently collapse to form new stars.

IV

THE STUDY OF THE UNIVERSE

Cosmology seeks to understand the structure of the universe. Modern cosmology is based on the American astronomer Edwin Hubble's discovery in 1929 that all galaxies are receding from each other with velocities proportional to their distances. In 1922 the Russian astronomer Alexander Friedmann proposed that the universe is everywhere filled with the same amount of matter. Using Albert Einstein's general theory of relativity to calculate the gravitational effects, he showed that such a system must originate in a singular state of infinite density (now called the big bang) and expand from that state in just the way Hubble observed. Most astronomers today interpret their data in terms of the big bang model, which in the early 1980s was further refined by the so-called inflationary theory, an attempt to account for conditions leading to the big bang. According to the theory, the big bang occurred about 14 billion years ago. The discovery in 1965 of cosmic background microwave radiation, a faint glow or radio transmission almost identical in all directions, fulfilled a prediction of the big bang model that radiation created in the big bang itself should still be present in the universe.

Theorists now believe that the universe will continue to expand forever. There does not appear to be enough mass in the universe for the attraction of gravity to slow and eventually reverse the universe’s expansion. Observations of extremely distant supernovas indicate instead that the universe’s expansion is accelerating. Astronomers have invented the name “dark energy” for the cause of this expansion, but they do not know what dark energy is.

Astronomy of the 20th Century

Astronomy of the 20th Century

The 20th century brought many new developments to the study of space. Huge telescopes, such as the 10-m (400-in) Keck telescopes, came into use. Astronomers began to use the Hubble Space Telescope and other space-based observatories. They also began to measure and record radiation that is not visible to the human eye.

Period

Year

Event

1900-1919	1905	Einstein states the special theory of relativity.
	1908	Hertzsprung classifies stars by their brightness: dwarfs and giants.
	1912	The period-luminosity correlation of Cepheids is calculated by H. Leavitt.
	1916	Einstein publishes the general theory of relativity.
	1917	The mirror of the Mount Wilson telescope (2.5 m) is put into use.
1920-1939	1923	Hubble discovers that spiral nebulas are galaxies outside the Milky Way.
	1929	Hubble suggests that the universe is expanding.
	1930	Tombaugh discovers Pluto.
	1931	The first radio waves in space are observed by Jansky.
	1937	With a radio telescope, Reber observes radio radiation from the Milky Way.
	1938	Hans Bethe states the theory of nuclear energy, source of star radiation.
1940-1949	1942	Discovery of radio emissions from the Sun by the team of Hey.
	1946	Identification of the most powerful radio source in the sky (Cygnus A) by Hey, Phillips, and Parsons.
	1948	Use of the 5-m telescope on Mount Palomar (California) begins. Cosmological theory of the stationary universe presented by Bond and Gold. Big bang theory and the origin of elements is developed by Alpher and Gamow.
1950-1959	1952	The scale of the distance to the galaxies is doubled by Baade.
	1957	Launch of the first Sputnik: The era of space conquest opens.
	1959	The Soviet probe Luna 3 takes the first pictures of the hidden side of the Moon.
1960-1969	1961	Gagarin takes the first piloted space flight.
	1962	The first planetary mission succeeds: The United States probe Mariner 2 flies past Venus.
	1963	The first quasar is discovered by Schmidt at Mount Palomar.
	1965	Discovery of radio radiation in deep space at 3 K by Penzias and Wilson.
	1967	Bell Burnell discovers pulsars at Cambridge (Great Britain). V. Komarov (USSR) is the first human victim of a space flight.
	1969	The mission Apollo 11 (Armstrong and Aldrin) puts the first human on the Moon.
1970-1974	1971-1972	The U.S. probe Mariner 9 orbits Mars and gathers the first images.
	1973	First flight past Jupiter by Pioneer 10. A telescope 4 m in diameter is put into service at Kitt Peak (Arizona).
	1974	The probe Mariner 10 registers the first surface details of Mercury and of the atmosphere of Venus.
1975-1979	1975	Soviet space probes Verena 9 and 10 take the first photographs of the Sun from Venus. U.S.-Soviet space docking of Apollo-Soyuz.
	1976	U.S. probes Viking 1 and 2 land on Mars (first measurements of the atmosphere and surface).
	1977	Kowal discovers the asteroid Chiron within the solar system. Discovery of Uranus's rings.
	1978	Christy discovers Charon, Pluto's satellite.
	1979	Launch of two U.S. probes, Voyager 1 and 2, which fly past Jupiter. Pioneer 11 achieves the first flyby of Saturn.
1980-1984	1980	Long baseline interferometer is put into service in New Mexico.
	1980-1981	First detailed study of Saturn and its rings by U.S. probes Voyager 1 and 2.
	1981	First flight of the U.S. space shuttle. Second flyby of Saturn by Voyager 2.
	1983	First infrared scanning of space, by the Infrared Astronomy Satellite (IRAS), starts.
	1984	First in-space repair of an artificial satellite.
1985-1989	1985-1986	Observation of Halley's Comet by different Soviet, European, and Japanese probes.
	1986	Flyby of Uranus by Voyager 2. U.S. space shuttle Challenger explodes in flight.
	1987	Supernova 1987A appears in the Large Magellanic Cloud.
	1988	Record human stay in space: V. Titov and M. Manarov return to Earth after a one-year-long space flight.
	1989	Voyager 2 flies by Neptune. Discovery of dense 'walls' and empty spaces in the spatial distribution of galaxies, by Geller and Uchra. Two U.S. probes are launched: Magellan towards Venus, and Galileo towards Jupiter.
1990-1999	1990	Launch of the Hubble Space Telescope; defectiveness of mirror is discovered. First radar cartography of Venus made by Magellan.
	1992	Two signals almost as old as the universe itself are registered by the satellite COBE. Service of the telescope Keck, 10 m in diameter, begins.
	1993	In-space repair of the Hubble Space Telescope.
	1994	Fragmented comet Shoemaker-Levy collides with Jupiter.
	1995	European Solar and Heliophysical Observatory (SOHO) launched to study the Sun.
	1996	Mars Pathfinder and Sojourner rover explore the surface of Mars.
	1998	Voyager 1 becomes most distant human-made object from Earth. Construction of the International Space Station begins.
	1999	Mars Global Surveyor begins systematic mapping of Mars.