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Asteroids Discovery Titus-Bode Law – During the 18th century, astronomers were intrigued by a numerical progression called the Titius-Bode law (also referred to as just Bode’s law). It appeared to predict the distances of the known planets from the Sun, but with one exception, the law suggested there should be a planet at a distance of 2.8 AU from the Sun (between the orbits of Mars and Jupiter). When Sir William Herschel discovered Uranus at a distance that corresponded to the Titius-Bode law, scientific excitement about the validity of this numerical progression reached an all-time high. Many astronomers were absolutely convinced that a planet must exist between Mars and Jupiter. “The Outlaw Neptune” – The Titus-Bode law was widely accepted until Neptune was discovered in 1846 and found not to comply. Ceres – By the end of the 1700s, a group of astronomers had banded together to use the observatory at Lilienthal, Germany to hunt down the missing planet. They called themselves the Celestial Police. However, despite their efforts, they were beaten. On January 1, 1801, Giuseppe Piazzi discovered what he believed to be the missing planet. The new object was named Ceres, after the Roman goddess of the harvest (thus the word cereal). However, subsequent observations swiftly established that it could not be classed a planet, since its diameter is only about 600 miles. The search for the “real” planet continued. “Star-Like” – Between 1801 and 1807, members of the Celestial Police tracked down three other bodies: Pallas, Juno and Vesta, each smaller than Ceres. Because they were too small to be seen as more than star-like points of light, these objects were called “asteroids”, which actually means star-like. It became obvious that there was no single large planet between Mars and Jupiter. Moreover, after eight more years of unsuccessful searches, most astronomers assumed that there were no more asteroids and abandoned any further searches. However, astronomer Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. In 1845, he found Astraea. He also found Hebe less than two years later. Population – As of September 2013, the Minor Planet Center – the official worldwide organization in charge of collecting observational data for minor planets (asteroids) and comets – had data on more than one million objects in the inner and outer Solar System, of which 625,000 had enough information to be given formal designations. Calculations indicate that the total mass of all the asteroids is still quite small (less than the Earth’s Moon). 1 Discovery Methods – The method of discovering asteroids by differentiating their motion from that of stars has not changed since 1801. However, the technology available to apply that original method has evolved significantly (from hand-drawn star charts to astrophotography to automated systems). Time-Exposure Photographs 1 – In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids. In time-exposure photographs, asteroids appear as short streaks due to their planetary motion with respect to fixed stars. This drastically increased the rate of detection compared with previous visual methods. Wolf alone discovered 248 asteroids, whereas only slightly more than 300 had been discovered up to that point. “Vermin of the Skies” – Up until the 1980s, asteroids catalogued were discovered accidentally on telescopes’ photographic plates, showing up as small streaks. Most astronomers viewed an asteroid streak as a nuisance, blemishing an otherwise perfect photographic plate, especially if the asteroid was streaking across the image of interest to the astronomer. Most streaks were unreported. Automated Systems – An increasing interest in identifying asteroids whose orbits cross Earth’s orbit, and that could, given enough time, collide with Earth, helped spur the launch of highly efficient automated systems that consist of ChargeCoupled Device (CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. Nomenclature Formal Designation – Once an asteroid’s orbit has been confirmed, it is given a number, and later may also be given a name (433 Eros). However, because modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. Names – The first few asteroids were named after figures from Greco-Roman mythology, but as these names started to run out, others were used: famous people, literary characters, the names of the discoverer’s wives, children, and even television characters. There was little controversy about this until 1971, when an asteroid was named Mr. Spock, which was not even named after the Star Trek character, but after the discoverer’s cat who supposedly bore a resemblance to him. Although the International Astronomical Union subsequently banned pet names as sources, unusual asteroid names are still being proposed and accepted, such as Cheshire Cat, James Bond (which has the number designation of 9007), and Mister Rogers. 2 Physical Characteristics General Description – Asteroids are predominantly rocky bodies, ranging in size from about 600 miles (Ceres) to tens of feet across. The number of asteroids increases rapidly with decreasing size. This size distribution, with high numbers of small sizes, is probably produced by collisional fragmentation. The largest asteroids are probably solid. However, most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity. Some asteroids have moons or are co-orbiting binaries (two asteroids orbiting their common center of mass) 2. Asteroid shapes range from spherical to elongated and irregular (indicating a fragmental origin). Largest Asteroids – The three largest asteroids, Ceres (≈600 miles), Pallas (≈340 miles), and Vesta (≈325 miles) share many characteristics common to planets, and are atypical compared to the majority of “potato-shaped” asteroids. NASA’s Dawn spacecraft was launched in 2007 to study Ceres and Vesta. Ceres 3 – Ceres is the only asteroid spherical in shape. It is differentiated into a rocky core, icy mantle, and crust composed of water ice and hydrated minerals such as carbonates and clays. It has been suggested that a layer of liquid water within the interior may exist. If the mantle is about 25% of Ceres mass, it would contain more fresh water than is found on Earth. Ceres appears to be geologically inactive, with a surface sculpted by impacts. The surface also displays several bright spots (possibly fresh ice) and a pyramid-shaped mountain 4 (over 3 miles high) of unknown origin. Pallas – Pallas is unusual in that, like Uranus, it rotates on its side. Its composition is similar to that of Ceres, and perhaps partially differentiated. Vesta 5 – Vesta is the only intact asteroid resurfaced by basaltic lava flows. This contradicts the conventional idea that asteroids are essentially cold, rocky fragments left behind from the early days of planetary formation. Vesta is thought to have accreted from material that included the radioactive isotope aluminum-26, originating from a nearby supernova explosion. The decay of this short-lived isotope caused the asteroid to differentiate, followed by the eruption of molten rock onto its surface. Occurring more than four billion years ago, Vesta’s surface has remained relatively unchanged, except for the occasional crater-forming impact. Eucrites – Vesta is the probable parent body of a specific group of meteorites called the Eucrites. They must have originated from a major impact of the asteroid causing some of its surface material to be ejected out into space and eventually land on Earth as meteorites. Life – Asteroids contain traces of organic compounds, and some scientists speculate that asteroid (and comet) impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth. 3 Asteroid vs Comet – The main difference between an asteroid and a comet is that a comet shows a coma (discussed later) due to sublimation of surface ice by solar radiation. A few objects have ended up being dual-listed because they were first classified as asteroid but later showed evidence of cometary activity. Conversely, some comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most asteroids with particularly eccentric orbits are probably dormant or extinct comets Classification – The classification of asteroids is based on composition, derived from measurements of reflectivity (albedo), surface spectrum, density, and similarities to known meteorite types. Most asteroids are categorized in one of three classes: 1. Carbonaceous or C-Type Asteroids – C-type asteroids are very dark, and composed of silicates (stony materials), carbon compounds, and hydrated minerals (chemically combined with water molecules). They are similar to carbonaceous chondrite meteorites (discussed later). C-type asteroids comprise 75% of the asteroid population. Mathilde 6 – The Near-Shoemaker spacecraft obtained close-up views of the C-type asteroid Mathilde. It measures 33 miles across, and the surface exhibits many large craters, including the deeply shadowed one at the center, which is estimated to be more than 6 miles deep. The angular shape is believed to be the result of a violent history of impacts. 2. Silicaceous or S-Type Asteroids – S-type asteroids are moderately bright, and composed of silicates (stony materials) and small amounts of nickel-iron. They are similar to a variety of stony meteorites (discussed later). S-type asteroids comprise 17% of the asteroid population. Ida 7 – The Galileo spacecraft encountered the S-type asteroid Ida. It is an irregularly shaped asteroid, about 35 miles across. Its surface is heavily cratered suggesting that it has existed in its present form for at least a billion years, perhaps much longer. An unexpected discovery was a small satellite, named Dactyl, measuring less than 1 mile across. YORP Effect – Initially, it was speculated that Dactyl is large boulder ejected from the surface of Ida in a recent impact. However, astronomers now believe that the YORP effect may be responsible for the formation of Dactyl and satellites of other smaller asteroids. Specifically, when sunlight hits one of these smaller asteroids, the material absorbs some of the radiation and then re-emits it at a slightly different angle, causing the asteroid to spin up slightly. (The YORP effect is also thought to cause the rotation rates of some asteroids to slow down, while others to rotate chaotically.) Subjected to the YORP effect over millions of years, the asteroid can increase in rotation enough that the force outward (centrifugal force) will overpower the gravitational pull inward, causing material to eject along the equator to form a new small satellite. 4 3. Metallic or M-Type Asteroids – M-type asteroids are relatively bright and composed of nickel-iron (pure or mixed with stone). They are thought to be the remains of the cores of differentiated bodies, produced by collisional fragmentation. This type of asteroid is very similar to iron meteorites (discussed later). M-type asteroids comprise most of the remaining 8% of the asteroid population. Psyche – Measuring over 125 miles across, Psyche is likely the largest M-type asteroid. Other Types – In addition to the common types described above, astronomers have identified more than a dozen other asteroid types. The number of types continues to grow as more asteroids are studied. Orbital Groupings Main Belt Asteroids 8 – The great majority of the asteroids orbit the Sun in the main asteroid belt from 2 to 4 AU (between the orbits of Mars and Jupiter). They are believed to be leftover fragments from the formation of the solar system about 4.6 billion years ago. The main asteroid belt is estimated to contain nearly two million asteroids larger than a half mile in diameter, and millions of smaller ones. Scientists suspect that asteroids were prevented from accreting into larger objects because of Jupiter’s massive gravitational pull. Specifically, as Jupiter orbited outside the asteroid belt, its gravity would tug on the small bodies, perturbing their orbits and preventing them from coalescing into a single body. Zoning of Types – The compositional differences among asteroids show a remarkably distinct distribution with distance from the Sun. While examples of most asteroid types span a large range of distances from the Sun, there is a clear progression from S-type in the inner portion of the main asteroid belt, to M-type within the central portion, and C-type within the outer portion. This gradation with solar distance likely reflects properties of the primordial solar nebula, with high-temperature minerals condensing nearer the Sun and mixtures of ices and organics farther out. Asteroid Families – An asteroid family is defined as group of asteroids with similar orbits, suggesting a common origin. Although not clustered together in space at the present, the members of an asteroid family were all at the same place at some undetermined time in the past. Members of the same family tend to have similar reflectivites and spectra. Apparently, the family members are fragments of broken asteroids shattered in some ancient collision and still following similar orbital paths. Most asteroid families are found in the main asteroid belt. Family names include: Hungaria, Flora, Phocaea, Koronos, Eos, Themis, Cybele, and Hilda. 5 Main Belt Comets – In 2006, scientists reported that at least three bodies in the outer part of the main belt demonstrate classical cometary behavior, and defined a new class of bodies called the main belt comets. This followed the discovery in 2005 that the largest main belt asteroid, Ceres, contains a significant amount of water-ice. In fact, many main belt asteroids may contain a lot of ice, especially in the outer part of the belt. The line separating asteroids and comets is growing increasingly fuzzy. Kirkwood Gaps – Kirkwood gaps are regions in the main asteroid belt where few asteroids are found. First observed by Daniel Kirkwood in 1886, these gaps are cleared of asteroids by the orbital resonance effects with Jupiter. Centaur Asteroids – Centaur asteroids orbit far from the Sun. For example: Hidalgo, the first to be discovered, orbits from the inner edge of the main belt out almost as far as Saturn, Chiron orbits between Saturn and Uranus 9, the orbit of Damocles ranges from near Mars to beyond Uranus, and Pholus orbits from Saturn to past Neptune. Their planet-crossing orbits are unstable and are frequently perturbed. Thus, Centaurs will ultimately collide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets (particularly Jupiter). The composition of these objects is probably more like that of comets or Kuiper Belt objects, rather than that of ordinary asteroids. In fact, Chiron has been reclassified as both an asteroid and a comet. Moreover, any Centaur that is perturbed close enough to the Sun is expected to become a comet. Trojan (Langrangian) Asteroids 10 – Trojan asteroids are located in two gravitationally stable regions of Jupiter’s orbit – 60º preceding and following the planet. These two regions, referred to as Langrangian points (named for the French mathematician Joseph Louis Comte Lagrange who demonstrated their existence in 1772), each contain several hundred known asteroids. Several small asteroids have also been discovered in the Lagrangian points of Mars, Saturn and Neptune. Near-Earth Asteroids (NEAs) – Asteroids with orbits that bring them within 1.3 AU of the Sun are referred to as Near-Earth Asteroids (together with the comets whose orbits take them through near-Earth space form the Near-Earth Objects or NEOs). Asteroids that actually cross Earth’s orbital path are known as Earth-crossers. Scientists believe that many NEAs are objects nudged from the main asteroid belt. The nudging can result from collisions between objects, the gravitational tug of the Jovian planets, or a subtle force known as the Yarkovsky effect. The Yarkovsky effect occurs when an asteroid’s Sun-warmed surface re-radiates its heat on its afternoon side. The photons departing the surface of the asteroid create a tiny change in the asteroid’s orbital velocity. NEAs are grouped into three categories, named for famous members of each: 6 1. Amors 11 – Amors cross Mars’ orbit, but do not quite reach the orbit of the Earth (perihelion distances between 1.017 and 1.3 AU). Eros 12 – A member of the Amors, Eros, is the second largest NEA. It is also one of the most elongated asteroids, with estimated dimensions of 20 by 8 by 8 miles. Eros exhibits a heavily cratered surface with one side dominated by a huge gouge, and the opposite side by a sharp, raised rimmed crater. On February 12, 2001, the Near-Shoemaker spacecraft touched down on Eros. The last image of Eros received from the spacecraft was taken from a range of 394 feet, and measures 20 feet across 13. 2. Atens 14 – Atens orbit largely inside the orbit of the Earth. They cross the Earth’s orbit when they are near their greatest distance (aphelion) from the Sun. 3. Apollos 15 – Apollos include most Earth-crossing asteroids. They cross the Earth’s orbit when they are near their closest distance (perihelion) from the Sun. Toutatis 16 – Measuring nearly 3 miles in length, the Apollo asteroid named Toutatis is a highly elongated body consisting of two distinct “lobes”. It is hypothesized that Toutatis formed from two originally separate bodies which coalesced at some point, with the resultant asteroid being compared to a pile of rubble. Observations also reveal that Toutatis has an unusual spin state, consisting of two apparent simultaneous rotations around different axes, with periods of 5.4 and 7.3 days. So while most asteroids rotate somewhat like a football thrown in a perfect spiral, Toutatis tumbles like a flubbed pass. NEAs’ Fate – The orbits of NEAs are unstable as a result of the constantly varying gravitational influence of Earth, Mars, and Venus. A NEA will experience one of two fates: (1) it may be gravitationally ejected as the result of near-miss with a planet or (2) it may terminate its existence dramatically in a crater-forming impact. Calculations indicate that about one-third of the asteroids whose orbits currently cross that of the Earth will eventually hit our planet. NEA Populations –There are about one thousand NEAs larger than a half-mile in diameter, 4 to 8 thousand larger than 1500 feet in diameter, and 0.5 to 1.5 million larger than 150 feet in diameter that pose a potential threat to Earth. NEA Impacts – NEA impacts become a significant hazard when they are larger than 150 feet in diameter. Asteroids of this size can reach the Earth’s lower atmosphere or its surface and release a substantial amount of energy, equal to or greater than the energy released in the detonation of a nuclear weapon. In fact, like a nuclear explosion, the energy of an impact is measured in megatons of TNT. However, a significant impact on the Earth is rare. Moreover, large impacts occur much less frequently than small ones. Nevertheless, it is a random event. For example, in March 1989 an asteroid (designated 1989 FC) approximately 1500 feet in diameter, missed the Earth by only a few hours. The table on the following page summarizes the energy yield, average frequency, and environmental consequences according to a range of impact sizes of NEAs. 7 Impact Effects Of A Near-Earth Asteroid Diameter Energy Yield (Megatons of TNT) Average Frequency (Years) 150 to 300 feet 10 to 100 1,000 600 to 1500 feet 1,000 to 10,000 100,000 0.5 to 3 miles 100,000 to 10,000,000 1,000,000 5 miles 100,000,000 to 1,000,000,000 100,000,000 8 Environmental Consequences Referred to as “city-busters”. The Tunguska blast in 1908 is believed to have been caused by an airburst of a 180-foot NEA 17, which killed herds of Reindeer and leveled trees out to 12 miles 18. A land impact could destroy an area the size of a small to moderate state. An ocean impact produces tsunamis capable of devastating all associated coasts. A land impact could destroy an area the size of a country, approaching continental scale. Both land and ocean impacts raise dust, causing climatic changes. Destruction of ozone layer. Referred to as “planet-busters”. Impact ejecta triggers global-scale fires. Prolonged “nuclear winter”. Threatens most species of life. The K/T Mass Extinction 65 million years ago was most likely caused by an impact of a 6-mile NEA. The probable impact site was identified as the Chicxulub Crater, Northern Yucatan, Mexico 19.