A near-Earth object is any small Solar System body whose orbit brings it to proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun is less than 1.3 astronomical units. If a NEO's orbit crosses the Earth's and the object is larger than 140 meters across, it is considered a hazardous object. Most known PHOs and NEOs are asteroids. There are over 19,000 known near-Earth asteroids, over a hundred short-period near-Earth comets, a number of solar-orbiting spacecraft and meteoroids large enough to be tracked in space before striking the Earth, it is now accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth. NEOs have become of increased interest since the 1980s because of greater awareness of the potential danger some of the asteroids or comets pose; when impacting the Earth, asteroids as small as 20 m cause sufficiently strong shock waves and heat to damage the local environment and populations.
Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they hit ground and tsunamis if water bodies are hit. It is in principle possible to deflect asteroids, methods of mitigation are being researched. Based on the orbit calculations of identified NEOs, their risk of future impact is assessed on two scales, the Torino scale and the more complex Palermo scale, both of which rate a risk of any significance with values above 0; some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery, but as of March 2018, more precise calculations based on subsequent observations led to a reduction of the rating to or below 0 in all cases. Since 1998, the United States, the European Union, other nations are scanning for NEOs in an effort called Spaceguard; the initial US Congress mandate to NASA of cataloging at least 90% of NEOs that are at least 1 kilometre in diameter, which would cause a global catastrophe in case of an impact with Earth, had been met by 2011.
In years, the survey effort has been expanded to objects as small as about 140 m across, which still have the potential for large-scale, though not global, damage. NEOs have low surface gravity, many have Earth-like orbits making them easy targets for spacecraft; as of January 2019, five near-Earth comets and five near-Earth asteroids have been visited by spacecraft. Two near-Earth asteroids are being orbited by spacecraft that will return asteroid samples back to Earth. Plans for commercial asteroid mining have been drafted by private companies; the major technical astronomical definition for Near-Earth objects are small Solar System bodies with orbits around the Sun that by definition lie between 0.983 and 1.3 astronomical units away from the Sun. Thus, NEOs are not currently near the Earth, but they can approach the Earth closely. However, the term is used more flexibly sometimes, for example for objects in orbit around the Earth or for quasi-satellites, which have a more complex orbital relationship with the Earth.
When a NEO is detected, like all other small Solar System bodies, it is submitted to the International Astronomical Union's Minor Planet Center for cataloging. MPC maintains separate lists of potential NEOs; the orbits of some NEOs intersect that of the Earth, so they pose a collision danger. These are considered hazardous objects. For the asteroids among PHOs, the hazardous asteroids, MPC maintains a separate list. NEOs are catalogued by two separate units of the Jet Propulsion Laboratory of the National Aeronautics and Space Administration: the Center for Near Earth Object Studies and the Solar System Dynamics Group. PHAs are defined based on parameters relating to their potential to approach the Earth dangerously closely. Objects with an Earth minimum orbit intersection distance of 0.05 AU or less and an absolute magnitude of 22.0 or brighter are considered PHAs. Objects that cannot approach closer to the Earth than 0.05 AU, or are smaller than about 140 m in diameter, are not considered PHAs.
NASA's catalog of near-Earth objects includes the approach distances of asteroids and comets. The first near-Earth objects to be observed by humans were comets, their extraterrestrial nature was recognised and confirmed only after Tycho Brahe tried to measure the distance of a comet through its parallax in 1577. The 1758–1759 return of Halley's Comet was the first comet appearance predicted in advance; the first near-Earth asteroid to be discovered was 433 Eros in 1898. The asteroid was subject to several observation campaigns, because measurements of its orbit enabled a precise determination of the distance of the Earth from the Sun. In has been said. In 1937, asteroid 69230 Hermes was discovered when it passed the Earth at twice the distance of the Moon. Hermes was considered a threat. Hermes was re-discovered in 2003, is now known to be no threat for at least the next century. On June 14, 1968, the 1.4 km diameter asteroid 1566 Icarus passed Earth at a distance of 0.042482 AU (6,355,2
A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. In the outer Solar System, comets remain frozen and are difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations, but these detections have been questioned, have not yet been independently confirmed; as a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
The streams of dust and gas thus released form a huge tenuous atmosphere around the comet called the coma, the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. The streams of dust and gas each form their own distinct tail, pointing in different directions; the tail of dust is left behind in the comet's orbit in such a manner that it forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is affected by the magnetic field of the plasma of the solar wind; the ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean. While the solid nucleus of comets is less than 50 km across, the coma may be larger than the Sun, ion tails have been observed to extend 3.8 astronomical units. The Ulysses spacecraft made an unexpected pass through the tail of the comet C/2006 P1, on February 3, 2007.
Evidence of the encounter was published in the October 2007 issue of the Astrophysical Journal. The observation of antitails contributed to the discovery of solar wind; the ion tail is the result of ultraviolet radiation ejecting electrons off particles in the coma. Once the particles have been ionised, they form a plasma which in turn induces a magnetosphere around the comet; the comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. The comet is supersonic relative to the solar wind, so a bow shock is formed upstream of the comet, in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions congregate and act to "load" the solar magnetic field with plasma; the field lines "drape" around the comet forming the ion tail. If the ion tail loading is sufficient the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs; this leads to a "tail disconnection event".
This has been observed on a number of occasions, notable among, on the 20th. April 2007 when the ion tail of comet Encke was severed as the comet passed through a coronal mass ejection; this event was observed by the STEREO spacecraft. A disconnection event was seen with C/2009 R1 on May 26, 2010. On January 29, 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions." Comets at Curlie Comets Page at NASA's Solar System Exploration Source of useful comet-related material on the Web Did you know that if you condensed the entire 100 thousand mile long tail of a comet such as Hale-Bopp to the density of water, it would not fill a swimming pool
In astronomy, a rubble pile is a celestial body, not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make them up. Asteroids Bennu and Ryugu have a measured bulk density which suggests a rubble pile internal structure. Many comets and most smaller minor planets are thought to be composed of coalesced rubble. Most smaller asteroids are thought to be rubble piles. A close analogy to a rubble pile is a melted shot and wax slug projectile fired from a shotgun as opposed to a monolithic pure lead slug, with the former having a lower density as it is loosely held together by wax and is composed of numerous individual lead bird shot spheres. Rubble piles form when an asteroid or moon is smashed to pieces by an impact, the shattered pieces subsequently fall back together due to self-gravitation; this coalescing takes from several hours to weeks. When a rubble-pile asteroid passes a much more massive object, tidal forces change its shape.
Scientists first suspected that asteroids are rubble piles when asteroid densities were first determined. Many of the calculated densities were less than those of meteorites, which in some cases had been determined to be pieces of asteroids. Many asteroids with low densities are thought to be rubble piles, for example 253 Mathilde; the mass of Mathilde, as determined by the NEAR Shoemaker mission, is far too low for the volume observed, considering the surface is rock. Ice with a thin crust of rock would not provide a suitable density; the large impact craters on Mathilde would have shattered a rigid body. However, the first unambiguous rubble pile to be photographed is 25143 Itokawa, which has no obvious impact craters and is thus certainly a coalescence of shattered fragments; the asteroid 433 Eros, the primary destination of NEAR Shoemaker, was determined to be riven with cracks but otherwise solid. Other asteroids including Itokawa, have been found to be contact binaries, two major bodies touching, with or without rubble filling the boundary.
Large interior voids are possible because of the low gravity of most asteroids. Despite a fine regolith on the outside, the asteroid's gravity is so weak that friction between fragments dominates and prevents small pieces from falling inwards and filling up the voids. All the largest asteroids are solid objects without any macroscopic internal porosity; this may be because they have been large enough to withstand all impacts, have never been shattered. Alternatively and some few other of the largest asteroids may be massive enough that if they were shattered but not dispersed, their gravity would collapse most voids upon recoalescing. Vesta, at least, has withstood intact one major impact since its formation and shows signs of internal structure from differentiation in the resultant crater that assures that it is not a rubble pile; this serves as evidence for size as a protection from shattering into rubble. Observational evidence suggest that the cometary nucleus may not be a well-consolidated single body, but may instead be a loosely bound agglomeration of smaller fragments, weakly bonded and subject to occasional or frequent disruptive events, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris as in the asteroid case.
However, in situ observations by the Rosetta mission, indicates that it may be more complex than that. The moon Phobos, the larger of the two natural satellites of the planet Mars, is thought to be a rubble pile bound together by a thin regolith crust about 100 m thick. Spectroscopy of Phobos' composition suggests. Comet nucleus List of slow rotators Close-up images of Itokawa, a rubble pile asteroid NASA Astronomy Picture of the Day: Saturn's Moon Calypso, another possible rubble pile Hyper-Velocity Impacts on Rubble Pile Asteroids pdf online @ kent.ac.uk
A comet is an icy, small Solar System body that, when passing close to the Sun and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, sometimes a tail; these phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice and small rocky particles; the coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been recorded since ancient times by many cultures. Comets have eccentric elliptical orbits, they have a wide range of orbital periods, ranging from several years to several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune.
Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space; the appearance of a comet is called an apparition. Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus; this atmosphere has parts termed the tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System; the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets.
In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3. 27 Manx comets were found from 2013 to 2017. As of July 2018 there are 6,339 known comets, a number, increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. One comet per year is visible to the naked eye, though many of those are faint and unspectacular. Bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet, NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior; the word comet comētēs. That, in turn, is a latinisation of the Greek κομήτης, the Oxford English Dictionary notes that the term κομήτης meant "long-haired star, comet" in Greek.
Κομήτης was derived from κομᾶν, itself derived from κόμη and was used to mean "the tail of a comet". The astronomical symbol for comets is ☄; the solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, water ice, frozen carbon dioxide, carbon monoxide and ammonia; as such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs". Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense; the surface of the nucleus is dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde and ethane and more complex molecules such as long-chain hydrocarbons and amino acids.
In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components may have been formed on asteroids and comets; the outer surfaces of cometary nuclei have a low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it, Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be dark, like tar or crude oil; the low reflectivity of cometary surfaces causes them to absorb t
The term apsis refers to an extreme point in the orbit of an object. It denotes either the respective distance of the bodies; the word comes via Latin from Greek, there denoting a whole orbit, is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, the vertices, thus the periapsis and the apoapsis; the major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e. a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g. the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger the orbital parameters are independent of the smaller mass. For general orbits, the terms periapsis and apoapsis are used. Pericenter and apocenter are equivalent alternatives, referring explicitly to the respective points on the orbits, whereas periapsis and apoapsis may refer to the smallest and largest distances of the orbiter and its host.
For a body orbiting the Sun, the point of least distance is the perihelion, the point of greatest distance is the aphelion. The terms become apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee and greatest distance the apogee, from Ancient Greek Γῆ, "land" or "earth". For objects in lunar orbit, the point of least distance is sometimes called the pericynthion and the greatest distance the apocynthion. Perilune and apolune are used. In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are used to refer to the orbital altitude of the spacecraft above the surface of the central body; these formulae characterize the pericenter and apocenter of an orbit: Pericenter Maximum speed, v per = μ a, at minimum distance, r per = a. Apocenter Minimum speed, v ap = μ a, at maximum distance, r ap = a.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit: Specific relative angular momentum h = μ a Specific orbital energy ε = − μ 2 a where: a is the semi-major axis: a = r per + r ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 Note t
University of Arizona Press
The University of Arizona Press, a publishing house founded in 1959 as a department of the University of Arizona, is a nonprofit publisher of scholarly and regional books. As a delegate of the University of Arizona to the larger world, the Press publishes the work of scholars wherever they may be, concentrating upon scholarship that reflects the special strengths of the University of Arizona, Arizona State University, Northern Arizona University; the Press has some 1,400 books in print. These include scholarly titles in American Indian studies, archaeology, environmental studies, Chicano studies, Latin American studies, the space sciences; the UA Press publishes general interest books on Arizona and the Southwest borderlands. In addition, the Press publishes books of personal essays, such as Nancy Mairs's Plaintext and two series in literature: Sun Tracks: An American Indian Literary Series and Camino del Sol: A Chicana/o Literary Series; the University of Arizona began their Camino del Sol Series in 1994, focusing on Chicanx and Latinx Literature.
In 2010, Rigoberto Gonzalez edited an anthology honoring the series published by the University of Arizona press. Camino del Sol authors include: Farid Matuk, Pat Mora, Daniel A. Olivas, Vickie Vértiz, Tim Z. Hernandez, Emmy Pérez, Ray Gonzalez, Carmen Giménez Smith, Roberto Tejada, more. Fu, Lo-shu. Fu, Lo-shu, ed. A Documentary Chronicle of Sino-Western Relations, 1644-1820: Translated texts. Volume 22 of Monographs of the Association for Asian Studies, Volume 1 of A Documentary Chronicle of Sino-Western Relations, 1644-1820. Translated by Lo-shu Fu. Published for the Association for Asian Studies by the University of Arizona Press. Retrieved 24 April 2014. University of Arizona Press website
The National Aeronautics and Space Administration is an independent agency of the United States Federal Government responsible for the civilian space program, as well as aeronautics and aerospace research. NASA was established in 1958; the new agency was to have a distinctly civilian orientation, encouraging peaceful applications in space science. Since its establishment, most US space exploration efforts have been led by NASA, including the Apollo Moon landing missions, the Skylab space station, the Space Shuttle. NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle, the Space Launch System and Commercial Crew vehicles; the agency is responsible for the Launch Services Program which provides oversight of launch operations and countdown management for unmanned NASA launches. NASA science is focused on better understanding Earth through the Earth Observing System. From 1946, the National Advisory Committee for Aeronautics had been experimenting with rocket planes such as the supersonic Bell X-1.
In the early 1950s, there was challenge to launch an artificial satellite for the International Geophysical Year. An effort for this was the American Project Vanguard. After the Soviet launch of the world's first artificial satellite on October 4, 1957, the attention of the United States turned toward its own fledgling space efforts; the US Congress, alarmed by the perceived threat to national security and technological leadership, urged immediate and swift action. On January 12, 1958, NACA organized a "Special Committee on Space Technology", headed by Guyford Stever. On January 14, 1958, NACA Director Hugh Dryden published "A National Research Program for Space Technology" stating: It is of great urgency and importance to our country both from consideration of our prestige as a nation as well as military necessity that this challenge be met by an energetic program of research and development for the conquest of space... It is accordingly proposed that the scientific research be the responsibility of a national civilian agency...
NACA is capable, by rapid extension and expansion of its effort, of providing leadership in space technology. While this new federal agency would conduct all non-military space activity, the Advanced Research Projects Agency was created in February 1958 to develop space technology for military application. On July 29, 1958, Eisenhower signed the National Aeronautics and Space Act, establishing NASA; when it began operations on October 1, 1958, NASA absorbed the 43-year-old NACA intact. A NASA seal was approved by President Eisenhower in 1959. Elements of the Army Ballistic Missile Agency and the United States Naval Research Laboratory were incorporated into NASA. A significant contributor to NASA's entry into the Space Race with the Soviet Union was the technology from the German rocket program led by Wernher von Braun, now working for the Army Ballistic Missile Agency, which in turn incorporated the technology of American scientist Robert Goddard's earlier works. Earlier research efforts within the US Air Force and many of ARPA's early space programs were transferred to NASA.
In December 1958, NASA gained control of the Jet Propulsion Laboratory, a contractor facility operated by the California Institute of Technology. The agency's leader, NASA's administrator, is nominated by the President of the United States subject to approval of the US Senate, reports to him or her and serves as senior space science advisor. Though space exploration is ostensibly non-partisan, the appointee is associated with the President's political party, a new administrator is chosen when the Presidency changes parties; the only exceptions to this have been: Democrat Thomas O. Paine, acting administrator under Democrat Lyndon B. Johnson, stayed on while Republican Richard Nixon tried but failed to get one of his own choices to accept the job. Paine was confirmed by the Senate in March 1969 and served through September 1970. Republican James C. Fletcher, appointed by Nixon and confirmed in April 1971, stayed through May 1977 into the term of Democrat Jimmy Carter. Daniel Goldin was appointed by Republican George H. W. Bush and stayed through the entire administration of Democrat Bill Clinton.
Robert M. Lightfoot, Jr. associate administrator under Democrat Barack Obama, was kept on as acting administrator by Republican Donald Trump until Trump's own choice Jim Bridenstine, was confirmed in April 2018. Though the agency is independent, the survival or discontinuation of projects can depend directly on the will of the President; the first administrator was Dr. T. Keith Glennan appointed by Republican President Dwight D. Eisenhower. During his term he brought together the disparate projects in American space development research; the second administrator, James E. Webb, appointed by President John F. Kennedy, was a Democrat who first publicly served under President Harry S. Truman. In order to implement the Apollo program to achieve Kennedy's Moon la