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Roche Sphere
An astronomical body's Hill sphere
Hill sphere
is the region in which it dominates the attraction of satellites. The outer shell of that region constitutes a zero-velocity surface. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere
Hill sphere
of its own. Any object within that distance would tend to become a satellite of the moon, rather than of the planet itself. One simple view of the extent of the Solar System
Solar System
is the Hill sphere
Hill sphere
of the Sun
Sun
with respect to local stars and the galactic nucleus.[1] In more precise terms, the Hill sphere
Hill sphere
approximates the gravitational sphere of influence of a smaller body in the face of perturbations from a more massive body
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Hills Cloud
In astronomy, the Hills cloud (also called the inner Oort cloud[1] and inner cloud[2]) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well-defined, is hypothetically located at 7013373994676750000♠250–1500 AU,[citation needed] well beyond planetary and Kuiper-belt object orbits - but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.[3] Oort cloud comets are continually perturbed by their environment. A non-negligible fraction leaves the Solar System or finds its way into the inner system. It should therefore have been depleted long ago, but it has not. The Hills cloud theory addresses the longevity of the Oort cloud by postulating a densely populated inner Oort region
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Asteroid Belt
The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars
Mars
and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets
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Yarkovsky Effect
The Yarkovsky effect
Yarkovsky effect
is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies.Contents1 History of discovery 2 Mechanism 3 Measurement 4 See also 5 References 6 External linksHistory of discovery[edit] The effect was discovered by the Russian civil engineer Ivan Osipovich Yarkovsky (1844–1902), who worked on scientific problems in his spare time. Writing in a pamphlet around the year 1900, Yarkovsky noted that the daily heating of a rotating object in space would cause it to experience a force that, while tiny, could lead to large long-term effects in the orbits of small bodies, especially meteoroids and small asteroids
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Retrograde Orbit
Retrograde motion is motion in the direction opposite to the movement of something else and the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such as precession or nutation of the axis. In reference to celestial systems, retrograde motion usually means motion that is contrary to the rotation of the primary, that is, the object that forms the system's hub. Rotation
Rotation
is determined with respect to an inertial frame of reference, such as distant fixed stars. In our Solar System, all of the planets and most of the other objects that orbit the Sun, with the exception of many comets, do so in the "prograde" direction, i.e
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Prograde Orbit
Retrograde motion is motion in the direction opposite to the movement of something else and the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such as precession or nutation of the axis. In reference to celestial systems, retrograde motion usually means motion that is contrary to the rotation of the primary, that is, the object that forms the system's hub. Rotation
Rotation
is determined with respect to an inertial frame of reference, such as distant fixed stars. In our Solar System, all of the planets and most of the other objects that orbit the Sun, with the exception of many comets, do so in the "prograde" direction, i.e
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Space Shuttle
The Space Shuttle
Space Shuttle
was a partially reusable low Earth orbital spacecraft system operated by the U.S. National Aeronautics
Aeronautics
and Space Administration (NASA), as part of the Space Shuttle
Space Shuttle
program. Its official program name was Space Transportation System
Space Transportation System
(STS), taken from a 1969 plan for a system of reusable spacecraft of which it was the only item funded for development.[10] The first of four orbital test flights occurred in 1981, leading to operational flights beginning in 1982. In addition to the prototype whose completion was cancelled, five complete Shuttle systems were built and used on a total of 135 missions from 1981 to 2011, launched from the Kennedy Space Center (KSC) in Florida
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Tonnes
The tonne (/tʌn/ ( listen)) (Non-SI unit, symbol: t), commonly referred to as the metric ton in the United States, is a non-SI metric unit of mass equal to 1,000 kilograms;[1][2][3][4] or one megagram (Mg); it is equivalent to approximately 2,204.6 pounds,[5] 1.102 short tons (US) or 0.984 long tons (imperial). Although not part of the SI, the tonne is accepted for use with SI units and prefixes by the International Committee for Weights and Measures.[6]Contents1 Symbol and abbreviations 2 Origin and spelling 3 Conversions 4 Derived units 5 Alternative usage5.1 Use of mass as proxy for energy 5.2 Unit of force6 See also 7 Notes and references 8 External linksSymbol and abbreviations[edit] The SI symbol for the tonne is "t", adopted at the same time as the unit in 1879.[2] Its use is also official for the metric ton in the United States, having been adopted by the United States National Institute of Standards and Technology.[7] It
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Low Earth Orbit
A low Earth
Earth
orbit (LEO) is an orbit around Earth
Earth
with an altitude of 2,000 km (1,200 mi) or less, and with an orbital period of between about 84 and 127 minutes. Objects below approximately 160 km (99 mi) will experience very rapid orbital decay and altitude loss due to atmospheric drag.[1][2] With the exception of the 24 astronauts who flew lunar flights in the Apollo program
Apollo program
during the four-year period spanning 1968 through 1972, all human spaceflights have taken place in LEO or below. The International Space Station
International Space Station
conducts operations in LEO. The altitude record for a human spaceflight in LEO was Gemini 11
Gemini 11
with an apogee of 1,374.1 kilometres (853.8 mi)
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Lead
Lead
Lead
is a chemical element with symbol Pb (from the Latin
Latin
plumbum) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead
Lead
is soft and malleable, and has a relatively low melting point. When freshly cut, lead is bluish-white; it tarnishes to a dull gray color when exposed to air. Lead
Lead
has the highest atomic number of any stable element and three of its isotopes each conclude a major decay chain of heavier elements. Lead
Lead
is a relatively unreactive post-transition metal. Its weak metallic character is illustrated by its amphoteric nature; lead and lead oxides react with acids and bases, and it tends to form covalent bonds. Compounds of lead
Compounds of lead
are usually found in the +2 oxidation state rather than the +4 state common with lighter members of the carbon group
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Geostationary Satellite
A geostationary orbit, geostationary Earth
Earth
orbit (often referred to as geosynchronous equatorial orbit)[1] (GEO) is a circular geosynchronous orbit 35,786 kilometres (22,236 mi) above the Earth's equator and following the direction of the Earth's rotation. An object in such an orbit appears motionless, at a fixed position in the sky, to ground observers. Communications satellites and weather satellites are often placed in geostationary orbits, so that the satellite antennas (located on Earth) that communicate with them do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located
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Neptune
Neptune
Neptune
is the eighth and farthest known planet from the Sun
Sun
in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune
Neptune
is 17 times the mass of Earth
Earth
and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth
Earth
and slightly larger than Neptune.[d] Neptune
Neptune
orbits the Sun
Sun
once every 164.8 years at an average distance of 30.1 AU (4.5 billion km)
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Asteroid
Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun
Sun
that did not show the disc of a planet and was not observed to have the characteristics of an active comet
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1 Ceres
7020939300000000000♠(9.393±0.005)×1020 kg[6] 6996150000000000000♠0.00015 Earths 0.0128 MoonsMean density7003216100000000000♠2.161±0.009 g/cm3[8]Surface gravity0.28 m/s2[7] 0.029 gMoment of inertia factor6999370000000000000♠0.37[9][b] (estimate)Escape velocity0.51 km/s[7]Sidereal rotation period7004326678400000000♠0.3781 d 7004326670120000000♠9.074170±0.000002 h[10]Equatorial rotation velocity7001926100000000000♠92.61 m/s[7]Axial tilt4°[11]North pole right ascension291.42744°[12]North pole declination66.764°[11]Albedo 6998900000000000000♠0.090±0.0033 (V-band geometric)[13]Surface temp. min mean maxKelvin≈ 168 K[17] 235 K[18]Spectral typeC[14]Apparent magnitude6.64[15] to 9.34[16]Absolute magnitude (H)70
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Apocenter
An apsis (Greek: ἁψίς; plural apsides /ˈæpsɪdiːz/, Greek: ἁψῖδες) is an extreme point in an object's orbit
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(66391) 1999 KW4
(66391) 1999 KW4, provisional designation 1999 KW4, is a binary[6] asteroid, classified as near-Earth object and potentially hazardous asteroid of the Aten group, approximately 1.3 kilometers in diameter. It was discovered on 20 May 1999, by Lincoln Near-Earth Asteroid Research (LINEAR) at the Lincoln Laboratory's Experimental Test Site in Socorro, New Mexico, United States.[2] It is also a Mercury-crosser and the closest known binary system to the Sun with a perihelion of just 0.2 AU.Contents1 Orbit 2 Physical characteristics2.1 Satellite 2.2 Diameter and shape 2.3 Lightcurves3 Numbering and naming 4 References 5 External linksOrbit[edit] The asteroid orbits the Sun at a distance of 0.2–1.1 AU once every 6.18 months (188 days)
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