Kuiper
KUIPER BELT
Kuiper Belt (KB) is a vast reservoir of icy bodies beyond the orbit of Neptune (30 AU) to 55 AUs from Sol. Some have highly elliptical orbits; some periodically visit the inner solar system as comets. It's named after Gerard Kuiper (1905–1973), Dutch-born American astronomer.Kuiper Belt Objects (KBOs) are remnants of the original planetesimals, building blocks of the outer planets. Being well past the gas giants (Jupiter, Saturn, Uranus, Neptune), they were neither incorporated into the planets nor ejected from the solar system. KBOs are thus relics from the Solar Nebula, the original disk of gas and dust that gave rise to the Solar System.
HistorySince the discovery of Pluto, many have speculated about the region now called the Kuiper belt.
In 1930, Frederick C. Leonard was the first astronomer to suggest the existence of a trans-Neptunian population. Soon after Pluto's discovery, he pondered whether it was "… first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected".In 1943 and 1949, Kenneth Edgeworth (1880 – 1972) published articles which proposed a trans-Neptunian comet belt. He hypothesized that beyond Neptune, the material within the primordial solar nebula was too sparse to condense into planets but still dense enough to form many smaller bodies. He concluded: “… outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies" and that, occasionally one "wanders from its own sphere” and visits the inner solar system as a comet.
In 1951, Gerard Kuiper (1905 – 73) proposed a region beyond Neptune where leftover planetesimals might have existed for many eons. This was in connection with his theory of the origin of the solar system (see solar nebula). He speculated (“Astrophysics”) on a similar disc having formed early in the Solar System's evolution; ironically, he did not believe that such a belt still exists today. This was largely due to an assumption (then, commonly held by the scientific community) that Pluto was the size of the Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System.
In 1962, physicist Al G.W. Cameron postulated the existence of “a tremendous mass of small material on the outskirts of the solar system.”
In 1964, Fred Whipple (famous for "dirty snowball" hypothesis of cometary structure) thought that a "comet belt" might be massive enough to cause physical events within the inner Solar System. Examples include the purported discrepancies in the orbit of Uranus that had sparked the search for Planet X, or perhaps to affect the orbits of known comets. Since then, observations have ruled out this hypothesis.
In 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a blink comparator; the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before.
In 1980, Julio Fernandez stated that for every short period (solar orbit less than 200 years) comet sent to the inner solar system from the Oort cloud, 600 would need to exit the Solar System into interstellar space. He speculated that a comet belt from between 35 and 50 AU could account for the observed comet quantity.
In 1988, a Canadian team of astronomers followed up on Fernandez's work. Martin Duncan, Tom Quinn and Scott Tremaine ran a series of computer simulations to determine if all observed comets come from the Oort cloud. They found that the Oort cloud could not account for short-period comets, particularly as short-period comets are clustered near the plane of the Solar System, whereas Oort cloud comets tend to arrive from any point in the sky. With a belt as Fernandez described, the simulations matched observations. Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, Tremaine named this region the "Kuiper belt."
1992, after five years of searching, on August 30, 1992, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" (15760) 1992 QB1. Six months later, they discovered a second KBO, (181708) 1993 FW. Til their discoveries, the trans-Neptunian region of the solar system was thought to contain only the small planet Pluto and its satellite Charon. Dr. Jewitt later proposed the region to be renamed in honor of Dr. Fernandez who more accurately predicted the true nature of the Kuiper Belt.
On January 19, 2006, the New Horizons (NH) spacecraft mission was launched. The first space mission aimed for the KB is headed by Alan Stern of the Southwest Research Institute. NH will arrive at Pluto on July 14, 2015. By then, plans call for the New Horizons team to have selected another KBO for further study. Current selection criteria call for this KBO to be between 25 and 55 miles (40 to 90 km) in diameter and, ideally, white or grey, to contrast with Pluto's reddish color.
Physical Properties.Disk-shaped belt contains numerous small icy bodies orbiting the Sun beyond the orbit of Neptune; these KBOs orbit from 30 to 55 AU from the Sun. The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the ecliptic plane and a more diffuse distribution of objects extending several times farther. Overall it more resembles a torus or doughnut than a belt. Its mean position is inclined to the ecliptic by 1.86 degrees.
Kuiper belt is much larger than the asteroid belt, 20 times as wide and 20 to perhaps 200 times as massive. Like the asteroid belt, it consists mainly of small bodies (remnants from the Solar System's formation). Unlike the asteroid belt (objects composed primarily of rock and metal), the Kuiper belt contains frozen volatiles (i.e. "ices"), such as methane, ammonia and water.
A KBO was first discovered in 1992; since then, over a thousand more Kuiper Objects (KBOs) have been discovered; more than 70,000 KBOs over 100 km in diameter are believed to exist. It contains at least three dwarf planets: Pluto, Haumea and Makemake.
KB contains many binary objects (two objects of similar mass which orbit "each other"). The most notable example is the Pluto-Charon binary. Scientists estimate over 1 percent of KBOs (a high percentage) exist as binaries. No longer considered a planet, Pluto is now classified as a dwarf planet. The orbit, icy composition, and diminutive size of Pluto qualify it as a giant KBO, the largest known member of the Kuiper belt. Many KBOs share Pluto’s orbital resonance with Neptune, these are called "Plutinos".
Gravitational disturbances by Neptune are now thought to cause most short-period comets, solar orbits last less than 200 years. Recent studies show the farther scattered disc, a dynamically active region created by the outward motion of Neptune, 4.5 billion years ago, to be their true place of origin. Neptune's moon Triton is believed to be a captured KBO during Neptune's outward migration.
Scattered disc objects such as Eris are KBO-like bodies with extremely large orbits that take them as far as 100 AU from the Sun.
The centaurs, comet-like bodies that orbit among the gas giants, are believed to have originated from the KB.
The Kuiper belt is distinct from the hypothesized Oort Cloud, a thousand times more distant. KBOs, members of the scattered disc, potential objects from the Hills cloud or Oort cloud are collectively referred to as trans-Neptunian objects (TNOs).
Other Stars. As of 2006, astronomers have resolved dust disks believed to be Kuiper belt-like structures around nine other stars. Beyond this, 15-20% of solar-type stars have observed infrared excess which is believed to indicate massive Kuiper Belt like structures
KBO Orbits.
Kuiper Belt Objects can be grouped on the basis of orbits. These groups include: Resonance, Classical and Scattered.
1. Resonance. Kuiper Belt contains several mean motion resonances. Resonances are stable areas where member bodies can survive indefinitely. When an object's orbital period is an exact ratio of Neptune's (a situation called a mean motion resonance), then it will synchronize with Neptune to stabilize the orbit.
Example: When Neptune completes three orbits around Sol, Pluto completes two orbits. Thus, Pluto is in the 2:3 (also called 3:2) resonance, and it corresponds to a characteristic semi-major axis of about 39.4 AU. This resonance protects Pluto from close encounters with Neptune; it also protects other members of the same resonance (Plutinos). This and other resonance areas are further discussed below.
· The 2:3 resonance area contains about 200 known objects (Plutinos) including Pluto and its moons. While Pluto and other Plutinos might intersect Neptune’s orbit, their resonance ensures they will never collide with Neptune. Plutinos have high orbital eccentricities, suggesting that they are not native to their current positions but were instead thrown haphazardly into their orbits by the migrating Neptune.
· The 1:2 resonance objects complete half an orbit for each of Neptune's; this resonance corresponds to a semi-major axis of ~47.7AU, and is sparsely populated. Its residents are sometimes called “twotinos”.
· Other resonances also exist at 3:4, 3:5, 4:7 and 2:5.
· Neptune possesses a number of Trojan objects, which occupy its L4 and L5 points, gravitationally stable regions leading and trailing it in its orbit by 60°. Neptune Trojans are often described as being in a 1:1 resonance with Neptune. These objects are remarkably stable in their orbits and are unlikely to have been captured by Neptune; it’s more likely they were formed alongside it.
2. Classical. The classical Kuiper Belt objects lie outside the resonances and are characterized by near-circular orbits near the ecliptic plane. These orbits are what would be expected from the first-generation planetesimals in the solar nebula, suggesting that the classical Kuiper Belt objects are indeed primordial planetesimals that have managed to preserve their original orbits.
Between approximately 42–48 AU, however, the gravitational influence of Neptune is negligible, and objects can exist with unmolested orbits. This region is known as the classical Kuiper belt, and its members comprise roughly two thirds of KBOs observed to date. Because the first modern KBO discovered, 1992 QB1, is considered the prototype of this group, classical KBOs are often referred to as cubewanos ("Q-B-1-os").
The classical Kuiper belt appears to be a composite of two separate populations. The first, known as "dynamically cold" population, has orbits much like the planets; nearly circular, with an orbital eccentricity of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). The second, the "dynamically hot" population, has orbits much more inclined to the ecliptic, by up to 30°. The two populations have been named this way not because of any major difference in temperature, but from analogy to particles in a gas, which increase their relative velocity as they become heated up.[44] The two populations not only possess different orbits, but different compositions; the cold population is markedly redder than the hot, suggesting it formed in a different region. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giants. The cold population, on the other hand, is believed to have formed more or less in its current position although it may also have been later swept outwards by Neptune during its migration.
3. Scattered. The scattered Kuiper Belt objects stand out from the rest of the Kuiper Belt with their very large, elliptical orbits. The origin of the scattered Kuiper Belt is unknown, but it may be a by-product of the same scattering process that produced the Oort Cloud.The scattered disc is a sparsely populated region beyond the Kuiper belt, extending as far as 100 AU and farther. Scattered disc objects (SDOs) travel in highly elliptical orbits, usually also highly inclined to the ecliptic. Most models of solar system formation show both KBOs and SDOs first forming in a primordial comet belt, while later gravitational interactions, particularly with Neptune, sent the objects spiraling outward; some into stable orbits (the KBOs) and some into unstable orbits, becoming the scattered disc.[6] Due to its unstable nature, the scattered disc is believed to be the point of origin for many of the Solar System's short-period comets.[6]
Scattered objectsAccording to the Minor Planet Center, which officially catalogues all trans-Neptunian objects, a KBO, strictly speaking, is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. Objects found outside the belt are classed as scattered objects. However, in some scientific circles the term "Kuiper belt object" has become synonymous with any icy planetoid native to the outer solar system believed to have been part of that initial class, even if its orbit during the bulk of solar system history has been beyond the Kuiper belt (e.g. in the scattered disk region). They often describe scattered disc objects as "scattered Kuiper belt objects." Eris, the recently discovered object now known to be larger than Pluto, is often referred to as a KBO, but is technically an SDO. A consensus among astronomers as to the precise definition of the Kuiper belt has yet to be reached, and this issue remains unresolved.The centaurs, which are not normally considered part of the Kuiper belt, are also believed to be scattered objects, the only difference being that they were scattered inward, rather than outward. The Minor Planet Center groups the centaurs and the SDOs together as scattered objects.[74]
In 1992, another object 5145 Pholus, was discovered in a similar orbit.[16] Today, an entire population of comet-like bodies, the centaurs, is known to exist in the region between Jupiter and Neptune. The centaurs' orbits are unstable and have dynamical lifetimes of a few million years. From the time of Chiron's discovery, astronomers speculated that they therefore must be frequently replenished by some outer reservoir.
Further evidence for the belt's existence later emerged from the study of comets. That comets have finite lifespans has been known for some time. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space, eating them gradually away. In order to still be visible over the age of the Solar System, they must be frequently replenished.
[19]One such area of replenishment is the Oort Cloud; the spherical swarm of comets extending beyond 50 000 AU from the Sun first hypothesised by astronomer Jan Oort in 1950.[20] It is believed to be the point of origin for long period comets, those, like Hale-Bopp, with orbits lasting thousands of years.
In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System." He encouraged then-graduate student Jane Luu to help locate another object beyond Pluto's orbit, because, as he told her, "If we don't, nobody will." Using telescopes at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a blink comparator.
Initially, examination of each pair of plates took about eight hours,[27] but the process was sped up with the arrival of electronic Charge-coupled devices or CCDs, which, though their field of view was narrower, were not only more efficient at collecting light (they retained 90 percent of the light that hit them, rather than the ten percent achieved by photographs) but allowed the blinking process to be done virtually, on a computer screen. Today, CCDs form the basis for most astronomical detectors.
Studies since the trans-Neptunian region was first charted have shown that in fact, the region now called the Kuiper belt is not the point of origin for short-period comets, but that they instead derive from a separate but linked population called the scattered disc. The scattered disc was created when Neptune migrated outward into the proto-Kuiper belt, which at the time was much closer to the Sun, and left in its wake a population of dynamically stable objects which could never be affected by its orbit (the Kuiper belt proper), and a separate population whose perihelia are close enough that Neptune can still disturb them as it travels around the Sun (the scattered disc). Because the scattered disc is dynamically active and the Kuiper belt relatively dynamically stable, the scattered disc is now seen as the most likely point of origin for periodic comets.[6]The presence of Neptune has a profound effect on the Kuiper belt's structure due to orbital resonances. Over a timescale comparable to the age of the Solar System, Neptune's gravity destabilizes the orbits of any objects which happen to lie in certain regions, and either sends them into the inner Solar System or out into the Scattered disc or interstellar space. This causes the Kuiper belt to possess pronounced gaps in its current layout, similar to the Kirkwood gaps in the Asteroid belt. In the region between 40 and 42 AU, for instance, no objects can retain a stable orbit over such times, and any observed in that region must have migrated there relatively recently.[39].The 1:2 resonance appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the Classical belt or just the beginning of a broad gap.
Objects have been detected at the 2:5 resonance at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation.
Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU; so this sudden drastic falloff, "Kuiper cliff", was completely unexpected, and its cause, to date, is unknown. Bernstein and Trilling et al. have found evidence that the rapid decline in objects of 100 km or more in radius beyond 50 AU is real, and not due to observational bias. Possible explanations include that material at that distance is too scarce or too scattered to accrete into large objects, or that subsequent processes removed or destroyed those which did form.[54] Patryk Lykawka of Kobe University has claimed that the gravitational attraction of an unseen large planetary object, perhaps the size of Earth or Mars, might be responsible.[55][56]CompositionStudies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices; a mixture of light hydrocarbons (such as methane), ammonia, and water ice, a composition they share with comets.[57] The temperature of the belt is only about 50K,[58] so many compounds that would remain gaseous closer to the Sun are solid.
Due to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine. The principal method by which astronomers determine the composition of a celestial object is spectroscopy. When an object's light is broken into its component colors, an image akin to a rainbow is formed. This image is called a spectrum. Different substances absorb light at different wavelengths, and when the spectrum for a specific object is unraveled, dark lines (called absorption lines) appear where the substances within it have absorbed that particular wavelength of light. Every element or compound has its own unique spectroscopic signature, and by reading an object's full spectral "fingerprint", astronomers can determine what it is made of.Initially, such detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their colour.[59] These first data showed a broad range of colours among KBOs, ranging from neutral grey to deep red.[60] This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons. This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of their volatile ices to the effects of cosmic rays. Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing. However, Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in color was too extreme to be easily explained by random impacts.
Mass and size distributionDespite its vast extent, the collective mass of the Kuiper belt is relatively low. The upper limit to the total mass is estimated at roughly a tenth the mass of the Earth, with some estimates placing it at a thirtieth an Earth mass.[66] Conversely, models of the Solar System's formation predict a collective mass for the Kuiper belt of 30 Earth masses.[3] This missing >99% of the mass can hardly be dismissed, as it is required for the accretion of any KBOs larger than 100 km in diameter. At the current low density, these objects simply should not exist. Moreover, the eccentricity and inclination of current orbits makes the encounters quite "violent," resulting in destruction rather than accretion. It appears that either the current residents of the Kuiper belt have been created closer to the Sun or some mechanism dispersed the original mass. Neptune’s current influence is too weak to explain such a massive "vacuuming", though the Nice model proposes that it could have been the cause of mass removal in the past. While the question remains open, the conjectures vary from a passing star scenario to grinding of smaller objects, via collisions, into dust small enough to be affected by solar radiation.[67]
Bright objects are rare compared with the dominant dim population, as expected from accretion models of origin, given that only some objects of a given size would have grown further. This relationship N(D), the population expressed as a function of the diameter, referred to as brightness slope, has been confirmed by observations. The slope is inversely proportional to some power of the diameter D.where the current measures[68] give q = 4 ±0.5.
Less formally, there are for instance 8 (=2³) times more objects in 100–200 km range than objects in 200–400 km range. In other words, for every object with the diameter of 1000 km there should be around 1000 (=10³) objects with diameter of 100 km.The law is expressed in this differential form rather than as a cumulative cubic relationship, because only the middle part of the slope can be measured; the law must break at smaller sizes, beyond the current measure.Of course, only the magnitude is actually known, the size is inferred assuming albedo (not a safe assumption for larger objects).
New Horizons
John Spencer, an astronomer on the New Horizons mission team, says that no target for a post-Pluto Kuiper belt encounter has yet been selected, as they are awaiting data from the Pan-STARRS survey project to ensure as wide a field of options as possible.[79] The Pan-STARRS project, due to come fully online by 2009,[80] will survey the entire sky with four 1.4 gigapixel digital cameras to detect any moving objects, from near-earth objects to KBOs.[81]
Gerard Peter Kuiper
Father of Modern Planetary Science
G. P. Kuiper, 1905–73, American astronomer, b. the Netherlands. Kuiper is the father of modern planetary science due to his wide ranging studies of the solar system. He is best known for his namesake, the Kuiper Belt. He proposed (1951) the existence of a disk-shaped region of minor planets outside the orbit of Neptune (now called the Kuiper belt) as a source for short-period comets—those making complete orbits around the sun in less than 200 years.
During the 1960s Kuiper served as chief scientist for the Ranger lunar-probe program, choosing crash-landing sites on the moon; by analyzing Ranger photographs, he also participated in the the Surveyor and Apollo programs.
A pioneer in the field of infrared astronomy, he was honored posthumously when the National Aeronautics and Space Administration (NASA) named its airborne infrared telescope the Kuiper Airborne Observatory (1975). Kuiper was the editor of two encyclopedic works, The Solar System (4 vol., 1953–58) and Stars and Stellar Systems (9 vol., 1960–68).
Early life
Kuiper, the son of a tailor in the village of Tuitjenhorn in North Holland, had an early interest in astronomy. He had extraordinarily sharp eyesight; he could see stars of magnitude 7.5 stars, about four times fainter than visible to normal eyes. He went to Leiden University in 1924, where he befriended fellow students Bart Bok and Pieter Oosterhoff and was taught by Ejnar Hertzsprung, Antonie Pannekoek, Willem de Sitter, Jan Woltjer, Jan Oort and the physicist Paul Ehrenfest. He received his B.Sc. in Astronomy in 1927 and continued straight on with his graduate studies. Kuiper finished his doctoral thesis on binary stars with Hertzsprung in 1933, after which he immediately traveled to California to become a fellow under Robert Grant Aitken at the Lick Observatory.
In 1935, he left to work at the Harvard College Observatory where he met Sarah Parker Fuller, whom he married on June 20, 1936. Although he had planned to move to Java to work at the Bosscha Observatory, he took a position at the Yerkes Observatory of the University of Chicago and became an American citizen in 1937. In 1949, Kuiper initiated the Yerkes - McDonald asteroid survey (1950 - 1952).
Discoveries
Kuiper discovered
1. Uranus's satellite Miranda (1948).
2. Neptune's satellite Nereid (1949).
3. Carbon dioxide in the atmosphere of Mars (1948).
4. Methane in atmosphere of Saturn's satellite Titan (1944).
5. Airborne infrared observing (he used a Convair 990 in the 1960s).
6. Several binary stars which received "Kuiper numbers" to identify them, i.e., KUI 79.
Kuiper spent most of his career at the University of Chicago, but moved to Tucson, Arizona in 1960 to found the Lunar and Planetary Laboratory at the University of Arizona. Kuiper was the laboratory's director until his death in 1973 while on vacation with his wife in Mexico. In the 1960s, Kuiper helped identify landing sites on the moon for the Apollo program.
Honors
In 1947, Kuiper was awarded the Janssen Medal of the Astronomical Society of France.
In 1959, Kuiper won the Henry Norris Russell Lectureship of the American Astronomical Society.
In 1971, Kuiper received the Kepler Gold Medal from the American Association for the Advancement of Science and the Franklin Institute.
Besides the Kuiper Belt, his other namesakes include following:
1. Minor planet 1776 Kuiper.
2. Lunar crater Kuiper,
3. Martian craters
4. Mercury crater,
5. Now-decommissioned Kuiper Airborne Observatory .
6. One of the three LPL buildings at University of Arizona is named in his honor.
Most astronomers refer to a region of small planets beyond Neptune as the "Kuiper belt", since Kuiper had suggested that such small planets or comets may have formed there. Ironically, he believed that few such objects now exist because he thought they would have been swept clear by planetary gravitational perturbations.
The Kuiper Prize, named in his honor, is the most distinguished award given by the American Astronomical Society's Division for Planetary Sciences, an international society of professional planetary scientists. The prize recognizes outstanding contributors to planetary science, and is awarded annually to scientists whose lifetime achievements have most advanced our understanding of planetary systems. Winners of this award include Carl Sagan, James Van Allen, and Eugene Shoemaker.
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