A gas giant is a giant planet composed of hydrogen and helium. Gas giants are sometimes known as failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System; the term "gas giant" was synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are a distinct class of giant planet, being composed of heavier volatile substances. For this reason and Neptune are now classified in the separate category of ice giants. Jupiter and Saturn consist of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass, they are thought to consist of an outer layer of molecular hydrogen surrounding a layer of liquid metallic hydrogen, with a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are composed of water and ammonia; the layer of metallic hydrogen makes up the bulk of each planet, is referred to as "metallic" because the large pressure turns hydrogen into an electrical conductor.
The gas giants' cores are thought to consist of heavier elements at such high temperatures and pressures that their properties are poorly understood. The defining differences between a low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation. Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history; the term gas giant was coined in 1952 by the science fiction writer James Blish and was used to refer to all giant planets. It is, something of a misnomer because throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form. Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases; the term has caught on, because planetary scientists use "rock", "gas", "ice" as shorthands for classes of elements and compounds found as planetary constituents, irrespective of what phase the matter may appear in.
In the outer Solar System and helium are referred to as "gases". Because Uranus and Neptune are composed of, in this terminology, not gas, they are referred to as ice giants and separated from the gas giants. Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, hence their appearance: ammonia clouds, water clouds, alkali-metal clouds, silicate clouds. Jupiter and Saturn are both class I. Hot Jupiters are class IV or V. A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 M⊕ will only be larger in volume than Jupiter. For masses above 500 M⊕, gravity will cause the planet to shrink. Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star. Although the words "gas" and "giant" are combined, hydrogen planets need not be as large as the familiar gas giants from the Solar System. However, smaller gas planets and planets closer to their star will lose atmospheric mass more via hydrodynamic escape than larger planets and planets farther out.
A gas dwarf could be defined as a planet with a rocky core that has accumulated a thick envelope of hydrogen and other volatiles, having as result a total radius between 1.7 and 3.9 Earth-radii. The smallest known extrasolar planet, a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope. A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. List of gravitationally rounded objects of the Solar System List of planet types Hot Jupiter
A circular orbit is the orbit with a fixed distance around the barycenter, that is, in the shape of a circle. Below we consider a circular orbit in astrodynamics or celestial mechanics under standard assumptions. Here the centripetal force is the gravitational force, the axis mentioned above is the line through the center of the central mass perpendicular to the plane of motion. In this case, not only the distance, but the speed, angular speed and kinetic energy are constant. There is no apoapsis; this orbit has no radial version. Transverse acceleration causes change in direction. If it is constant in magnitude and changing in direction with the velocity, we get a circular motion. For this centripetal acceleration we have a = v 2 r = ω 2 r where: v is orbital velocity of orbiting body, r is radius of the circle ω is angular speed, measured in radians per unit time; the formula is dimensionless, describing a ratio true for all units of measure applied uniformly across the formula. If the numerical value of a is measured in meters per second per second the numerical values for v will be in meters per second, r in meters, ω in radians per second.
The relative velocity is constant: v = G M r = μ r where: G, is the gravitational constant M, is the mass of both orbiting bodies, although in common practice, if the greater mass is larger, the lesser mass is neglected, with minimal change in the result. Μ = G M, is the standard gravitational parameter. The orbit equation in polar coordinates, which in general gives r in terms of θ, reduces to: r = h 2 μ where: h = r v is specific angular momentum of the orbiting body; this is because μ = r v 2 ω 2 r 3 = μ Hence the orbital period can be computed as: T = 2 π r 3 μ Compare two proportional quantities, the free-fall time T f f = π 2 2 r 3 μ and the time to fall to a point mass in a radial parabolic orbit T p a r = 2 3 r 3 μ The fact that the formulas only differ by a constant factor is a priori clear from dimensional analysis. The specific orbital energy is negative, ϵ = − v 2 2 ϵ = − μ 2 r Thus the virial theorem applies without taking a time-average: the kinetic energy of the system is equal to the absolute value of the total energy the potential energy of the system is equal to twice the total energyThe escape velocity from any distance is √2 times the speed in a circular orbit at that distance: the kinetic energy is twice as much, hence the total energy is zero.
Maneuvering into a large circular orbit, e.g. a geostationary orbit, requires a larger delta-v than an escape orbit, although the latter implies getting arbitrarily far away and having more energy than needed for the orbital speed of the circular orbit. It is a matter of maneuvering into the orbit. See Hohmann transfer orbit. In Schwarzschild metric, the orbital velocity for a circular orbit with radius r is given by the following formula: v = G M r − r S where r S = 2 G M c 2 is the Schwarzschild radius of the central body. For the sake of convenience, the derivation will be written in units in which c = G = 1; the four-velocity of a body on a circular orbit is given by: u μ
A spacecraft is a vehicle or machine designed to fly in outer space. Spacecraft are used for a variety of purposes, including communications, earth observation, navigation, space colonization, planetary exploration, transportation of humans and cargo. All spacecraft except single-stage-to-orbit vehicles cannot get into space on their own, require a launch vehicle. On a sub-orbital spaceflight, a space vehicle enters space and returns to the surface, without having gained sufficient energy or velocity to make a full orbit of the Earth. For orbital spaceflights, spacecraft enter closed orbits around the Earth or around other celestial bodies. Spacecraft used for human spaceflight carry people on board as crew or passengers from start or on orbit only, whereas those used for robotic space missions operate either autonomously or telerobotically. Robotic spacecraft used to support scientific research are space probes. Robotic spacecraft that remain in orbit around a planetary body are artificial satellites.
To date, only a handful of interstellar probes, such as Pioneer 10 and 11, Voyager 1 and 2, New Horizons, are on trajectories that leave the Solar System. Orbital spacecraft may be recoverable or not. Most are not. Recoverable spacecraft may be subdivided by method of reentry to Earth into non-winged space capsules and winged spaceplanes. Humanity has achieved space flight but only a few nations have the technology for orbital launches: Russia, the United States, the member states of the European Space Agency, China, Taiwan (National Chung-Shan Institute of Science and Technology, Taiwan National Space Organization, Israel and North Korea. A German V-2 became the first spacecraft when it reached an altitude of 189 km in June 1944 in Peenemünde, Germany. Sputnik 1 was the first artificial satellite, it was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957. The launch ushered in new political, military and scientific developments. Apart from its value as a technological first, Sputnik 1 helped to identify the upper atmospheric layer's density, through measuring the satellite's orbital changes.
It provided data on radio-signal distribution in the ionosphere. Pressurized nitrogen in the satellite's false body provided the first opportunity for meteoroid detection. Sputnik 1 was launched during the International Geophysical Year from Site No.1/5, at the 5th Tyuratam range, in Kazakh SSR. The satellite travelled at 29,000 kilometers per hour, taking 96.2 minutes to complete an orbit, emitted radio signals at 20.005 and 40.002 MHz While Sputnik 1 was the first spacecraft to orbit the Earth, other man-made objects had reached an altitude of 100 km, the height required by the international organization Fédération Aéronautique Internationale to count as a spaceflight. This altitude is called the Kármán line. In particular, in the 1940s there were several test launches of the V-2 rocket, some of which reached altitudes well over 100 km; as of 2016, only three nations have flown crewed spacecraft: USSR/Russia, USA, China. The first crewed spacecraft was Vostok 1, which carried Soviet cosmonaut Yuri Gagarin into space in 1961, completed a full Earth orbit.
There were five other crewed missions. The second crewed spacecraft was named Freedom 7, it performed a sub-orbital spaceflight in 1961 carrying American astronaut Alan Shepard to an altitude of just over 187 kilometers. There were five other crewed missions using Mercury spacecraft. Other Soviet crewed spacecraft include the Voskhod, flown uncrewed as Zond/L1, L3, TKS, the Salyut and Mir crewed space stations. Other American crewed spacecraft include the Gemini spacecraft, Apollo spacecraft, the Skylab space station, the Space Shuttle with undetached European Spacelab and private US Spacehab space stations-modules. China developed, but did not fly Shuguang, is using Shenzhou. Except for the Space Shuttle, all of the recoverable crewed orbital spacecraft were space capsules. Crewed space capsules The International Space Station, crewed since November 2000, is a joint venture between Russia, the United States and several other countries; some reusable vehicles have been designed only for crewed spaceflight, these are called spaceplanes.
The first example of such was the North American X-15 spaceplane, which conducted two crewed flights which reached an altitude of over 100 km in the 1960s. The first reusable spacecraft, the X-15, was air-launched on a suborbital trajectory on July 19, 1963; the first reusable orbital spacecraft, a winged non-capsule, the Space Shuttle, was launched by the USA on the 20th anniversary of Yuri Gagarin's flight, on April 12, 1981. During the Shuttle era, six orbiters were built, all of which have flown in the atmosphere and five of which have flown in space. Enterprise was used only for approach and landing tests, launching from the back of a Boeing 747 SCA and gliding to deadstick landings at Edwards AFB, California; the first Space Shuttle to fly into space was Columbia, followed by Challenger, Discovery and Endeavour. Endeavour was built to replace Challenger when it was lost in January 1986. Columbia broke up during reentry in February 2003; the first automatic reusable spacecraft was the Buran-class shuttle, launched by the USSR on November 15, 1988, although it made only one flight and this was uncrewed.
This spaceplane was designed for a crew and resembled the U
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft. For spacecraft far from Earth an orbital maneuver is called a deep-space maneuver; the rest of the flight in a transfer orbit, is called coasting. The Tsiolkovsky rocket equation, or ideal rocket equation is an equation, useful for considering vehicles that follow the basic principle of a rocket: where a device that can apply acceleration to itself by expelling part of its mass with high speed and moving due to the conservation of momentum, it is a mathematical equation that relates the delta-v with the effective exhaust velocity and the initial and final mass of a rocket For any such maneuver: Δ v = v e ln m 0 m 1 where: m 0 is the initial total mass, including propellant, m 1 is the final total mass, v e is the effective exhaust velocity, Δ v is delta-v - the maximum change of speed of the vehicle. The applied change in speed of each maneuver is referred to as delta-v; the total delta-v for all and each maneuver is estimated for a mission and is called a delta-v budget.
With a good approximation of the delta-v budget designers can estimate the fuel to payload requirements of the spacecraft using the rocket equation. An "impulsive maneuver" is the mathematical model of a maneuver as an instantaneous change in the spacecraft's velocity as illustrated in figure 1, it is the limit case of a burn to generate a particular amount of delta-v, as the burn time tends to zero. In the physical world no instantaneous change in velocity is possible as this would require an "infinite force" applied during an "infinitely short time" but as a mathematical model it in most cases describes the effect of a maneuver on the orbit well; the off-set of the velocity vector after the end of real burn from the velocity vector at the same time resulting from the theoretical impulsive maneuver is only caused by the difference in gravitational force along the two paths which in general is small. In the planning phase of space missions designers will first approximate their intended orbital changes using impulsive maneuvers that reduces the complexity of finding the correct orbital transitions.
Applying a low thrust over a longer period of time is referred to as a non-impulsive maneuver. Another term is finite burn, where the word "finite" is used to mean "non-zero", or again: over a longer period. For a few space missions, such as those including a space rendezvous, high fidelity models of the trajectories are required to meet the mission goals. Calculating a "finite" burn requires a detailed model of the spacecraft and its thrusters; the most important of details include: mass, center of mass, moment of inertia, thruster positions, thrust vectors, thrust curves, specific impulse, thrust centroid offsets, fuel consumption. In astronautics, the Oberth effect is where the use of a rocket engine when travelling at high speed generates much more useful energy than one at low speed. Oberth effect occurs because the propellant has more usable energy and it turns out that the vehicle is able to employ this kinetic energy to generate more mechanical power, it is named after Hermann Oberth, the Austro-Hungarian-born, German physicist and a founder of modern rocketry, who first described the effect.
Oberth effect is used in a powered flyby or Oberth maneuver where the application of an impulse from the use of a rocket engine, close to a gravitational body can give much more change in kinetic energy and final speed than the same impulse applied further from the body for the same initial orbit. Since the Oberth maneuver happens in a limited time, to generate a high impulse the engine needs to achieve high thrust, thus the Oberth effect is far less useful for low-thrust engines, such as ion thrusters. A lack of understanding of this effect led investigators to conclude that interplanetary travel would require impractical amounts of propellant, as without it, enormous amounts of energy are needed. In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy; the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t
Heliocentrism is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. Heliocentrism was opposed to geocentrism, which placed the Earth at the center; the notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos, but at least in the medieval world, Aristarchus's heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic Era. It was not until the 16th century that a mathematical model of a heliocentric system was presented, by the Renaissance mathematician and Catholic cleric Nicolaus Copernicus, leading to the Copernican Revolution. In the following century, Johannes Kepler introduced elliptical orbits, Galileo Galilei presented supporting observations made using a telescope. With the observations of William Herschel, Friedrich Bessel, other astronomers, it was realized that the Sun, while near the barycenter of the Solar System, was not at any center of the universe.
While the sphericity of the Earth was recognized in Greco-Roman astronomy from at least the 4th century BC, the Earth's daily rotation and yearly orbit around the Sun was never universally accepted until the Copernican Revolution. While a moving Earth was proposed at least from the 4th century BC in Pythagoreanism, a developed heliocentric model was developed by Aristarchus of Samos in the 3rd century BC, these ideas were not successful in replacing the view of a static spherical Earth, from the 2nd century AD the predominant model, which would be inherited by medieval astronomy, was the geocentric model described in Ptolemy's Almagest; the Ptolemaic system was a sophisticated astronomical system that managed to calculate the positions for the planets to a fair degree of accuracy. Ptolemy himself, in his Almagest, points out that any model for describing the motions of the planets is a mathematical device, since there is no actual way to know, true, the simplest model that gets the right numbers should be used.
However, he rejected the idea of a spinning Earth as absurd as he believed it would create huge winds. His planetary hypotheses were sufficiently real that the distances of the Moon, Sun and stars could be determined by treating orbits' celestial spheres as contiguous realities; this made the stars' distance less than 20 Astronomical Units, a regression, since Aristarchus of Samos's heliocentric scheme had centuries earlier placed the stars at least two orders of magnitude more distant. Problems with Ptolemy's system were well recognized in medieval astronomy, an increasing effort to criticize and improve it in the late medieval period led to the Copernican heliocentrism developed in Renaissance astronomy; the non-geocentric model of the Universe was proposed by the Pythagorean philosopher Philolaus, who taught that at the center of the Universe was a "central fire", around which the Earth, Sun and planets revolved in uniform circular motion. This system postulated the existence of a counter-earth collinear with the Earth and central fire, with the same period of revolution around the central fire as the Earth.
The Sun revolved around the central fire once a year, the stars were stationary. The Earth maintained the same hidden face towards the central fire, rendering both it and the "counter-earth" invisible from Earth; the Pythagorean concept of uniform circular motion remained unchallenged for the next 2000 years, it was to the Pythagoreans that Copernicus referred to show that the notion of a moving Earth was neither new nor revolutionary. Kepler gave an alternative explanation of the Pythagoreans' "central fire" as the Sun, "as most sects purposely hid their teachings". Heraclides of Pontus said that the rotation of the Earth explained the apparent daily motion of the celestial sphere, it used to be thought that he believed Mercury and Venus to revolve around the Sun, which in turn revolves around the Earth. Macrobius Ambrosius Theodosius described this as the "Egyptian System," stating that "it did not escape the skill of the Egyptians," though there is no other evidence it was known in ancient Egypt.
The first person known to have proposed a heliocentric system, was Aristarchus of Samos. Like Eratosthenes, Aristarchus calculated the size of the Earth, measured the sizes and distances of the Sun and Moon. From his estimates, he concluded that the Sun was six to seven times wider than the Earth, thought the larger object would have the most attractive force, his writings on the heliocentric system are lost, but some information about them is known from a brief description by his contemporary and from scattered references by writers. Archimedes' description of Aristarchus's theory is given in The Sand Reckoner; the entire description comprises just three sentences, which Thomas Heath translates as follows: You are aware that "universe" is the name given by most astronomers to the sphere, the centre of, the centre of the earth, while its radius is equal to the straight line between the centre of the sun and the centre of the earth. This is the common account, but Aristarchus brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the "universe" just mentioned.
His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun on the circumference of a circle, the sun lying in the middle of the orbit, that the sphere of the fixed stars