1.
4179 Toutatis
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With its close approaches, and being about 4.6 km across, it is listed as a potentially hazardous object, although the probability of a collision with Earth is minute. On December 12,2012 at 06,40 UT Toutatis passed within about 18 lunar distances of Earth. On December 13, the Chinese lunar probe Change 2, launched in 2010 on a mission, flew by Toutatis at a distance of 3.2 kilometers. Toutatis approached Earth again in 2016, but will not make another close approach until 2069. 4179 Toutatis was first sighted on February 10,1934, as object 1934 CT and it remained a lost asteroid for several decades until it was rediscovered on January 4,1989, by Christian Pollas, and was named after the Celtic god of tribal protection Toutatis. The spectral properties suggest that this is an S-type, or stony asteroid and it has a moderate Bond albedo of 0.13. Radar imagery shows that Toutatis is an irregular body consisting of two distinct lobes, with maximum widths of about 4.6 km and 2.4 km respectively. It is hypothesized that Toutatis formed from two separate bodies which coalesced at some point, with the resultant asteroid being compared to a rubble pile. It has a period around its long axis of 5.38 days. This long axis is precessing with a period of 7.38 days, the asteroid may have lost most of its original angular momentum and entered into this tumbling motion as a result of the YORP effect. With a semimajor axis of 2.5294 AU, or roughly 2.5 times the distance between Earth and the Sun, Toutatis has a 3,1 orbital resonance with Jupiter and a near-1,4 resonance with Earth. It thus completes one orbit around the Sun for every 4.02 annual orbits of Earth, estimates in 1993 put the Lyapunov time horizon for predictability at around 50 years, after which the uncertainty region becomes larger with each close approach to a planet. Without the perturbations from the planets the Lyapunov time would be close to 10,000 years. The initial observations that showed its chaotic behavior were made by Wiśniewski, the low inclination of the orbit allows frequent transits, where the inner planets Mercury, Venus, Earth, and Mars can appear to cross the Sun as seen from the perspective of Toutatis. Earth did this in January 2009, July 2012 and July 2016, Toutatis makes frequent close approaches to Earth, with a currently minimum possible distance of just 0.006 AU. The approach on September 29,2004, was close, at 0.0104 AU from Earth, presenting a good opportunity for observation. A close approach of 0.0502 AU happened on November 9,2008, the most recent close approach was on December 12,2012, at a distance of 0.046 AU, with a magnitude of 10.7. At magnitude 10.7, Toutatis was not visible to the naked eye, during the 2012 encounter Toutatis was recovered on May 21,2012, by the Siding Spring Survey at apparent magnitude 18.9
2.
Near-Earth object
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A near-Earth object is any small Solar System body whose orbit brings it into proximity with Earth. By definition, a solar system body is a NEO if its closest approach to the Sun is less than 1.3 astronomical unit and it is now widely 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 increased awareness of the potential danger some of the asteroids or comets pose, and mitigations are being researched. In January 2016, NASA announced the Planetary Defense Coordination Office to track NEOs larger than 30 to 50 meters in diameter and coordinate an effective threat response, NEAs have orbits that lie partly between 0.983 and 1.3 AU away from the Sun. When a NEA is detected it is submitted to the IAUs Minor Planet Center for cataloging, some NEAs orbits intersect that of Earths so they pose a collision danger. The United States, European Union, and other nations are currently scanning for NEOs in an effort called Spaceguard. In the United States and since 1998, NASA has a mandate to catalogue all NEOs that are at least 1 kilometer wide. In 2006, it was estimated that 20% of the objects had not yet been found. In 2011, largely as a result of NEOWISE, it was estimated that 93% of the NEAs larger than 1 km had been found, as of 5 February 2017, there have been 875 NEAs larger than 1 km discovered, of which 157 are potentially hazardous. The inventory is much less complete for smaller objects, which still have potential for scale, though not global. Potentially hazardous objects are defined based on parameters that measure the objects potential to make threatening close approaches to the Earth. Mostly objects with an Earth minimum orbit intersection distance of 0.05 AU or less, objects that cannot approach closer to the Earth than 0.05 AU, or are smaller than about 150 m in diameter, are not considered PHOs. This makes them a target for exploration. As of 2016, three near-Earth objects have been visited by spacecraft, more recently, a typical frame of reference for looking at NEOs has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society, NEOs have been understood differently throughout history. Each time an NEO is observed, a different risk was posed and it is not just a matter of scientific knowledge. Such perception of risk is thus a product of religious belief, philosophic principles, scientific understanding, technological capabilities, and even economical resourcefulness.03 E −0.4 megatonnes. For instance, it gives the rate for bolides of 10 megatonnes or more as 1 per thousand years, however, the authors give a rather large uncertainty, due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination
3.
Asteroid
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Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers
4.
Radar astronomy
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Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the reflections. This research has been conducted for six decades, Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an active one. Radar systems have used for a wide range of solar system studies. The radar transmission may either be pulsed or continuous, the strength of the radar return signal is proportional to the inverse fourth-power of the distance. Upgraded facilities, increased power, and improved apparatus have increased observational opportunities. Radar techniques provide information unavailable by other means, such as testing general relativity by observing Mercury, Radar images provide information about the shapes and surface properties of solid bodies, which cannot be obtained by other ground-based techniques. Relying upon high powered terrestrial radars radar astronomy is able to provide extremely accurate astrometric information on the structure, composition and this aids in forming long-term predictions of asteroid-Earth impacts, as illustrated by the object 99942 Apophis. In particular, optical observations measure where an object appears in the sky, Radar, on the other hand, directly measures the distance to the object. The combination of optical and radar observations normally allows the prediction of orbits at least decades, there are two radar astronomy facilities that are in regular use, the Arecibo Planetary Radar and the Goldstone Solar System Radar. Control of attributes of the signal Resolve objects spatially, Delay-Doppler measurement precision, Optically opaque penetration, the maximum range of astronomy by radar is very limited, and is confined to the Solar system. This is because the signal drops off very steeply with distance to the target, the small fraction of incident flux that is reflected by the target. The distance to which the radar can detect an object is proportional to the root of the objects size. Radar could detect something ~1 km across a large fraction of an AU away, but at 8-10 AU and it is also necessary to have a relatively good ephemeris of the target before observing it. The Moon is comparatively close and was detected by radar soon after the invention of the technique in 1946, measurements included surface roughness and later mapping of shadowed regions near the poles. The next easiest target is Venus, in addition such technical prowess had great public relations value, and was an excellent demonstration to funding agencies. This led to early claims which are now known to be incorrect, all of these agreed with each other and the conventional value of AU at the time. The first un-ambiguous detection of Venus was made by Jet Propulsion Laboratory on 10 March 1961, a correct measurement of the AU soon followed. Once the correct value was known, other groups found echos in their archived data that agreed with these results, the following is a list of planetary bodies that have been observed by this means, Mars - Mapping of surface roughness from Arecibo Observatory
5.
Goldstone Deep Space Communications Complex
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The Goldstone Deep Space Communications Complex, commonly called the Goldstone Observatory, is located in the Mojave Desert near Barstow in the U. S. state of California. Operated by the US National Aeronautics and Space Administrations Jet Propulsion Laboratory, its purpose is to track. It is named after Goldstone, California, a nearby gold-mining ghost town, the complex includes the Pioneer Deep Space Station, which is a U. S. National Historic Landmark. The current communications complex is one of three in the NASA Deep Space Network, the others being the Madrid Deep Space Communications Complex and the Canberra Deep Space Communication Complex. Five large parabolic antennas are located at the Goldstone site to handle the workload, the antennas function similarly to a home satellite dish. The radio frequencies used for communication are in the microwave part of the radio spectrum, S band, X band. In addition to receiving radio signals from the spacecraft, the antennas transmit commands to the spacecraft with high power radio transmitters powered by klystron tubes. A major goal in the design of the station is to reduce interference with the weak incoming downlink radio signals by natural, the remote Mojave Desert location was chosen because it is far from manmade sources of radio noise such as motor vehicles. The RF front ends of the receivers at the dishes use ruby masers. It is commonly believed that the first American satellite, Explorer 1, was confirmed to be in orbit by the use of the phrase Goldstone has the bird. However, Goldstone was not in operation at the time of Explorer 1, others claim that the actual phrase was Gold has it. Incorrectly identifying Gold as a tracking station at Earthquake Valley, east of Julian. In fact, Gold Station was located at the Air Force Missile Test Center in Florida, the Goldstone Deep Space Communications Complex conducts tours of its facility and museum to middle and high schools, as well as individuals interested in visiting. The DSS14, Mars telescope is specifically quoted in the webcomic Wondermark, episode 881 In which a Standard is Questioned
6.
Comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of 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 collections of ice, dust. The coma may be up to 15 times the Earths diameter, 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 observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, 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, 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 coma and 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. As of November 2014 there are 5,253 known comets, 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. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model
7.
Orbit
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In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet about a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating path around a body, to a close approximation, planets and satellites follow elliptical orbits, with the central mass being orbited at a focal point of the ellipse, as described by Keplers laws of planetary motion. For ease of calculation, in most situations orbital motion is adequately approximated by Newtonian Mechanics, historically, the apparent motions of the planets were described by European and Arabic philosophers using the idea of celestial spheres. This model posited the existence of perfect moving spheres or rings to which the stars and it assumed the heavens were fixed apart from the motion of the spheres, and was developed without any understanding of gravity. After the planets motions were accurately measured, theoretical mechanisms such as deferent. Originally geocentric it was modified by Copernicus to place the sun at the centre to help simplify the model, the model was further challenged during the 16th century, as comets were observed traversing the spheres. The basis for the understanding of orbits was first formulated by Johannes Kepler whose results are summarised in his three laws of planetary motion. Second, he found that the speed of each planet is not constant, as had previously been thought. Third, Kepler found a relationship between the orbital properties of all the planets orbiting the Sun. For the planets, the cubes of their distances from the Sun are proportional to the squares of their orbital periods. Jupiter and Venus, for example, are respectively about 5.2 and 0.723 AU distant from the Sun, their orbital periods respectively about 11.86 and 0.615 years. The proportionality is seen by the fact that the ratio for Jupiter,5. 23/11.862, is equal to that for Venus,0. 7233/0.6152. Idealised orbits meeting these rules are known as Kepler orbits, isaac Newton demonstrated that Keplers laws were derivable from his theory of gravitation and that, in general, the orbits of bodies subject to gravity were conic sections. Newton showed that, for a pair of bodies, the sizes are in inverse proportion to their masses. Where one body is more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body. Lagrange developed a new approach to Newtonian mechanics emphasizing energy more than force, in a dramatic vindication of classical mechanics, in 1846 le Verrier was able to predict the position of Neptune based on unexplained perturbations in the orbit of Uranus. This led astronomers to recognize that Newtonian mechanics did not provide the highest accuracy in understanding orbits, in relativity theory, orbits follow geodesic trajectories which are usually approximated very well by the Newtonian predictions but the differences are measurable. Essentially all the evidence that can distinguish between the theories agrees with relativity theory to within experimental measurement accuracy
8.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
9.
Impact event
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An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have found to regularly occur in planetary systems, though the most frequent involve asteroids. Impact craters and structures are dominant landforms on many of the Solar Systems solid objects and present the strongest empirical evidence for their frequency, Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Notable impact events include the Chicxulub impact,66 million years ago, throughout recorded history, hundreds of Earth impacts have been reported, with some occurrences causing deaths, injuries, property damage, or other significant localised consequences. One of the best-known recorded impacts in modern times was the Tunguska event, the Comet Shoemaker–Levy 9 impact provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. Impact events have been a plot and background element in science fiction, Impact structures are the result of impact events on solid objects and, as the dominant landforms on many of the Systems solid objects, present the most solid evidence of prehistoric events. Small objects frequently collide with Earth, there is an inverse relationship between the size of the object and the frequency of such events. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the craters diameter. Asteroids with a 1 km diameter strike Earth every 500,000 years on average, large collisions – with 5 km objects – happen approximately once every twenty million years. The last known impact of an object of 10 km or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago, the energy released by an impactor depends on diameter, density, velocity, and angle. The diameter of most near-Earth asteroids that have not been studied by radar or infrared can generally only be estimated within about a factor of 2 based on the asteroid brightness, the density is generally assumed because the diameter and mass are also generally estimates. The minimum impact velocity on Earth is 11 km/s with asteroid impacts averaging around 17 km/s, the most probable impact angle is 45 degrees. Stony asteroids with a diameter of 4 meters enter Earths atmosphere approximately once per year. Asteroids with a diameter of 7 meters enter the atmosphere about every 5 years with as much energy as the atomic bomb dropped on Hiroshima. These ordinarily explode in the atmosphere and most or all of the solids are vaporized. However, asteroids with a diameter of 20 m, and which strike Earth approximately twice every century, the 2013 Chelyabinsk meteor was estimated to be about 20 m in diameter with an airburst of around 500 kilotons, an explosion 30 times the one over Hiroshima. Much larger objects may impact the solid earth and create a crater, objects with a diameter less than 1 m are called meteoroids and seldom make it to the ground to become meteorites. Although no human is known to have been killed directly by an impact, in 2005 it was estimated that the chance of a single person born today dying due to an impact is around 1 in 200,000
10.
Minimum orbit intersection distance
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Minimum orbit intersection distance is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the orbits of two bodies. Of greatest interest is the risk of a collision with Earth, Earth MOID is often listed on comet and asteroid databases such as the JPL Small-Body Database. MOID values are defined with respect to other bodies as well, Jupiter MOID, Venus MOID. An object is classified as a hazardous object – that is, posing a possible risk to Earth – if, among other conditions. A low MOID does not mean that a collision is inevitable as the planets frequently perturb the orbit of small bodies. It is also necessary that the two bodies reach that point in their orbits at the time before the smaller body is perturbed into a different orbit with a different MOID value. Two Objects gravitationally locked in orbital resonance may never approach one another, numerical integrations become increasingly divergent as trajectories are projected further forward in time, especially beyond times where the smaller body is repeatedly perturbed by other planets. MOID has the convenience that it is obtained directly from the elements of the body. The only object that has ever been rated at 4 on the Torino Scale and this is not the smallest Earth MOID in the catalogues, many bodies with a small Earth MOID are not classed as PHOs because the objects are less than roughly 140 meters in diameter. Earth MOID values are more practical for asteroids less than 140 meters in diameter as those asteroids are very dim. It is even smaller at the more precise JPL Small Body Database
[astro-ph.EP].
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