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The Universe comprises everything we perceive to physically exist, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them. However, the term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or Nature.
The word Universe is usually defined as encompassing everything. However, using an alternative definition, some have speculated that the "Universe" composed of expanding space-as-we-know-it, is just one of many disconnected "universes", which are collectively denoted as the multiverse. For example, in the many-worlds hypothesis, new "universes" are spawned with every quantum measurement. These universes are usually thought to be completely disconnected from our own and therefore impossible to detect experimentally. Observations of older parts of the universe (which are far away) suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. However, in bubble universe theory, there may be an infinite variety of "universes" created in various ways, and perhaps each with different physical constants.
Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the Universe. The earliest quantitative geocentric models were developed by the ancient Greeks, who proposed that the Universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size – corresponding to the fixed stars, the Sun and various planets – rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the characterization of the Milky Way, and the discovery of other galaxies and the microwave background radiation; careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.
According to the prevailing scientific model of the Universe, known as the Big Bang, the Universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a brief period (less than 10-32 seconds) of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of the dark energy, and that most of the matter and energy in the Universe is fundamentally different from that observed on Earth and not directly observable. The imprecision of current observations has hindered predictions of the ultimate fate of the Universe.
Current interpretations of astronomical observations indicate that the age of the Universe is 13.73 (± 0.12) billion years, and that the diameter of the observable universe is at least 93 billion light years, or 8.80 × 1026 metres. According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. It is uncertain whether the size of the Universe is finite or infinite.
The Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the retinue of objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly-flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.
The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these regions, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
Earth (or the Earth) is the third planet from the Sun, and the fifth-largest of the eight planets in the Solar System. It is also the largest, most massive, and densest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet, or Terra.
Home to millions of species, including humans, Earth is the only place in the Universe where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land. The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.
Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface. Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.
Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days. The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. Between approximately 4.1 and 3.8 billion years ago, asteroid impacts during the Late Heavy Bombardment caused significant changes to the surface environment.
Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth or in Earth being the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.
Aeronautics is the science involved with the study, design, and manufacture of flight-capable machines, or the techniques of operating aircraft. While the term—literally meaning "sailing the air"—originally referred solely to the science of operating the aircraft, it has since been expanded to include technology, business and other aspects related to aircraft. One of the significant parts in aeronautics is a branch of physical science called aerodynamics, which deals with the motion of air and the way that it interacts with objects in motion, such as an aircraft. Aviation is a term sometimes used interchangeably with aeronautics, although "aeronautics" includes lighter-than-air craft such as airships, while "aviation" does not.
The first mention of aeronautics in history was in the writings of ancient Egyptians who described the flight of birds. It also finds mention in ancient China where people were flying kites thousands of years ago. The medieval Islamic scientists were not far behind, as they understood the actual mechanism of bird flight. Before scientific investigation of aeronautics started, people started thinking of ways to fly. In a Greek legend, Icarus and his father Daedalus built wings of feathers and wax and flew out of a prison. Icarus flew too close to the sun, the wax melted, and he fell in the sea and drowned. When people started to scientifically study how to fly, people began to understand the basics of air and aerodynamics.
One of the earliest scientists to study aeronautics was Ibn Firnas who studied the dynamism of flying and carried out a number of experiments in 8th century in Cordoba, Al-Andalus. Roger Bacon and Leonardo da Vinci were some of the first modern Europeans to study aeronautics. Leonardo studied the flight of birds in developing engineering schematics for some of the earliest flying machines in the late fifteenth century AD. His schematics, however, such as the ornithopter ultimately failed as practical aircraft. The flapping machines that he designed were either too small to generate sufficient lift, or too heavy for a human to operate. Although the ornithopter continues to be of interest to hobbyists, it was replaced by the glider in the 19th century. Sir George Cayley was one of the most important people in the history of aeronautics. Many consider him the first true scientific aerial investigator and the first person to understand the underlying principles and forces of flight. A pioneer of aeronautical engineering, he is credited as the first person to separate the forces of lift and drag which are in effect on any flight vehicle,
An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft. While generally reserved for professional space travelers, the term is sometimes applied to anyone who travels into space, including scientists, politicians, journalists, and tourists.
Until 2003, astronauts were sponsored and trained exclusively by governments, either by the military, or by civilian space agencies. With the sub-orbital flight of the privately-funded SpaceShipOne in 2004, a new category of astronaut was created: the commercial astronaut.
The criteria for what constitutes human spaceflight vary. The Fédération Aéronautique Internationale (FAI) Sporting Code for astronautics recognizes only flights that exceed an altitude of 100 kilometers (62 mi). In the United States, professional, military, and commercial astronauts who travel above an altitude of 50 miles (80 km) are awarded astronaut wings.
As of September 19, 2009, a total of 505 humans from 38 countries have reached 100 km or more in altitude, of which 502 reached Low Earth orbit or beyond. Of these, 24 people have traveled beyond Low Earth orbit, to either lunar or trans-lunar orbit or to the surface of the moon; three of the 24 did so twice (Lovell, Young and Cernan).
Under the U. S. definition, 496 people qualify as having reached space, above 50 miles (80 km) altitude. Of eight X-15 pilots who exceeded 50 miles in altitude, seven reached above 50 miles (80 km) but below 100 kilometers (about 62 miles). Space travelers have spent over 30,400 person-days (or a cumulative total of over 83 years) in space, including over 100 astronaut-days of spacewalks. As of 2008, the man with the longest time in space is Sergei K. Krikalev, who has spent 803 days, 9 hours and 39 minutes, or 2.2 years, in space. Peggy A. Whitson holds the record for most time in space by a woman, 377 days.