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An asteroid is a minor planet—an object that is neither a true planet nor a comet—that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere. The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across to Ceres, a dwarf planet almost 1000 km in diameter.
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Lunar impact craters range from microscopic craters on lunar rocks returned by the Apollo program and small, simple, bowl-shaped depressions in the lunar regolith to large, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.
Comet Shoemaker–Levy 9 broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers worldwide. The collision provided new information about Jupiter and highlighted its possible role in reducing space debris in the inner Solar System.
Eros is a stony asteroid of the Amor group, and the first discovered, and second-largest near-Earth object. It has an elongated shape and a volume-equivalent diameter of approximately 16.8 kilometers. Visited by the NEAR Shoemaker space probe in 1998, it became the first asteroid ever studied from its own orbit.
Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. Later telescopic observations categorized Ida as an S-type asteroid, the most numerous type in the inner asteroid belt. On 28 August 1993, Ida was visited by the uncrewed Galileo spacecraft while en route to Jupiter. It was the second asteroid visited by a spacecraft and the first found to have a natural satellite.
The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids or minor planets. The identified objects are of many sizes, but much smaller than planets, and, on average, are about one million kilometers apart. This asteroid belt is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System.
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided with a Mars-sized dwarf planet of the same orbit approximately 4.5 billion years ago in the early Hadean eon, and the ejecta of the impact event later accreted to form the Moon. The impactor planet is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.
An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, as the impacting body is usually traveling at several kilometres a second, though atmospheres mitigate many surface impacts through atmospheric entry. Impact craters and structures are dominant landforms on many of the Solar System's solid objects and present the strongest empirical evidence for their frequency and scale.
Asteroid impact avoidance comprises the methods by which near-Earth objects (NEO) on a potential collision course with Earth could be diverted away, preventing destructive impact events. An impact by a sufficiently large asteroid or other NEOs would cause, depending on its impact location, massive tsunamis or multiple firestorms, and an impact winter caused by the sunlight-blocking effect of large quantities of pulverized rock dust and other debris placed into the stratosphere. A collision 66 million years ago between the Earth and an object approximately 10 kilometres wide is thought to have produced the Chicxulub crater and triggered the Cretaceous–Paleogene extinction event that is understood by the scientific community to have caused the extinction of all non-avian dinosaurs.
The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo. It was formed slightly over 66 million years ago when a large asteroid, about ten kilometers in diameter, struck Earth. The crater is estimated to be 200 kilometers in diameter and 20 kilometers in depth. It is the second largest confirmed impact structure on Earth, and the only one whose peak ring is intact and directly accessible for scientific research.
Planet V is a hypothetical fifth terrestrial planet posited by NASA scientists John Chambers and Jack J. Lissauer to have once existed between Mars and the asteroid belt. In their hypothesis the Late Heavy Bombardment of the Hadean era began after perturbations from the other terrestrial planets caused Planet V's orbit to cross into the asteroid belt. Chambers and Lissauer presented the results of initial tests of this hypothesis during the 33rd Lunar and Planetary Science Conference, held from March 11 through 15, 2002.
There is evidence that the formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.
A contact binary is a small Solar System body, such as a minor planet or comet, that is composed of two bodies that have gravitated toward each other until they touch, resulting in a bilobated, peanut-like overall shape. Contact binaries are distinct from true binary systems such as binary asteroids where both components are separated. The term is also used for stellar contact binaries.
The Baptistina family is an asteroid family of more than 2500 members that was probably produced by the breakup of an asteroid 170 km (110 mi) across 80 million years ago following an impact with a smaller body. The two largest presumed remnants of the parent asteroid are main-belt asteroids 298 Baptistina and 1696 Nurmela. The Baptistina family is part of the larger Flora clan. It was briefly speculated that the Chicxulub impactor was part of the Baptistina family of asteroids, but this was disproven in 2011 using data from the Wide-field Infrared Survey Explorer (WISE).
The Nicemodel is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies such as the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.
The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypothesis, during this interval, a disproportionately large number of asteroids and comets collided into the terrestrial planets and their natural satellites of the inner Solar System, including Mercury, Venus, Earth and Mars. These came from both post-accretion and planetary instability-driven populations of impactors. Although it used to be widely accepted, it remained difficult to provide an overwhelming amount of evidence for the hypothesis. However, recent re-appraisal of the cosmo-chemical constraints indicates that there was likely no late spike in the bombardment rate.
The five-planet Nice model is a numerical model of the early Solar System that is a revised variation of the Nice model. It begins with five giant planets, the four that exist today plus an additional ice giant between Saturn and Uranus in a chain of mean-motion resonances.
The E-belt asteroids were the population of a hypothetical extension of the primordial asteroid belt proposed as the source of most of the basin-forming lunar impacts during the Late Heavy Bombardment.
The jumping-Jupiter scenario specifies an evolution of giant-planet migration described by the Nice model, in which an ice giant is scattered inward by Saturn and outward by Jupiter, causing their semi-major axes to jump, and thereby quickly separating their orbits. The jumping-Jupiter scenario was proposed by Ramon Brasser, Alessandro Morbidelli, Rodney Gomes, Kleomenis Tsiganis, and Harold Levison after their studies revealed that the smooth divergent migration of Jupiter and Saturn resulted in an inner Solar System significantly different from the current Solar System. During this migration secular resonances swept through the inner Solar System exciting the orbits of the terrestrial planets and the asteroids, leaving the planets' orbits too eccentric, and the asteroid belt with too many high-inclination objects. The jumps in the semi-major axes of Jupiter and Saturn described in the jumping-Jupiter scenario can allow these resonances to quickly cross the inner Solar System without altering orbits excessively, although the terrestrial planets remain sensitive to its passage.
A low-aspect-ratio layered ejecta crater is a class of impact crater found on the planet Mars. This class of impact craters was discovered by Northern Arizona University scientist Professor Nadine Barlow and Dr. Joseph Boyce from the University of Hawaii in October 2013. Barlow described this class of craters as having a "thin-layered outer deposit" surpassing "the typical range of ejecta". "The combination helps vaporize the materials and create a base flow surge. The low aspect ratio refers to how thin the deposits are relative to the area they cover", Barlow said. The scientists used data from continuing reconnaissance of Mars using the old Mars Odyssey orbiter and the Mars Reconnaissance Orbiter. They discovered 139 LARLE craters ranging in diameter from 1.0 to 12.2 km, with 97% of the LARLE craters found poleward of 35N and 40S. The remaining 3% mainly traced in the equatorial Medusae Fossae Formation.
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