Near-Earth Object (NEO) - Lecture 10 PDF

Lecture 10 Near-Earth object  Comet The first near-Earth objects to be observed by humans were comets. The word comet derives from the Greek κόμη ‘the hair of the head’ and was used to mean ‘the tail of a comet’....

Lecture 10 Near-Earth object  Comet The first near-Earth objects to be observed by humans were comets. The word comet derives from the Greek κόμη ‘the hair of the head’ and was used to mean ‘the tail of a comet’. Dbot3000, CC BY-SA 4.0, via Wikimedia Commons  Outline Comet – History Efbrazil, CC BY-SA 4.0, via Wikimedia Commons  Atmospheric phenomena Comets have been noticed by humans for millennia. Aristotle believed that comets were atmospheric phenomena, due to the fact that they could vary in brightness over the course of a few days.  Great Comet of 1577 The Great Comet of 1577 is a comet that passed close to Earth in 1577. – Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. – Particularly bright examples are called “great comets”.  Precise measurements Tycho Brahe was one the most distinguished observers of this comet, making thousands of precise measurements about it.  Exist outside of Earth’s atmosphere Tycho Brahe demonstrated that comets must exist outside of Earth’s atmosphere by measuring the parallax of the Great Comet of 1577. – Parallax is a difference in the apparent position of an object viewed along two different lines of sight. It can be used to determine distances https://www.eso.org/public/events/astro-evt/hyakutake/images/phot-hya-apr03-mw/  Great Comet of 1680 The Great Comet of 1680 is also called Newton’s Comet.  Parabolic orbit In 1687, Isaac Newton proved that an object moving under the influence of gravity by an inverse square law must trace out an orbit, and he demonstrated how to fit a comet’s path through the sky to a parabolic orbit, using the Great Comet of 1680 as an example.  Three comets had similar orbital elements In 1705, Edmond Halley applied Newton’s method to 23 cometary apparitions that had occurred between 1337 and 1698. – The appearance of a comet is called an apparition. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements.  Same comet with an elliptical orbit He was confident that these three apparitions had been three appearances of the same comet. – The comet had an elliptical orbit.  Halley’s Comet He predicted that the comet would appear again in 1758–59. When the comet returned as predicted, it became known as Halley’s Comet.  Summary History of comet – Phenomenon Atmospheric (Aristotle) Outside the atmosphere (Tycho) – Orbit Parabolic (Newton) Elliptical (Halley) Efbrazil, CC BY-SA 4.0, via Wikimedia Commons  Exploration by spacecraft Comets have been visited by uncrewed probes.  Icy, small body A comet is an icy, small Solar System body. – The solid, core structure of a comet is known as the nucleus. – Cometary nuclei are composed of an amalgamation of rock, water ice, and frozen carbon dioxide, methane, and ammonia. liz west, CC BY 2.0, via Wikimedia Commons  Elongated elliptical orbit Most comets have elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.  Passing close to the Sun & Outgassing A comet warms and begins to release gases when passing close to the Sun, a process called outgassing.  Coma & tail The streams of dust and gas thus released form a huge and extremely thin 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 pointing away from the Sun.  Apparition If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope. The appearance of a comet is called an apparition. Dbot3000, CC BY-SA 4.0, via Wikimedia Commons  Wide range of orbital periods Comets have a wide range of orbital periods. The longer the period the more elongated the ellipse.  Formation Vast swarms of comet-like bodies are thought to orbit the Sun in the distant trans-Neptunian regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet.  Short-period comets Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune.  Kuiper belt The Kuiper belt is named in honor of Gerard Kuiper, who conjectured the existence of the belt in 1951. In 1992, the first Kuiper belt object since Pluto (in 1930) was discovered. Since its discovery, the number of known Kuiper belt objects has increased. Gelderen, Hugo van / Anefo, CC BY-SA 3.0 NL, via Wikimedia Commons  Long-period comets Long-period comets are thought to originate in the Oort cloud. The innermost portion of the Oort cloud is more than a thousand times as distant from the Sun as the Kuiper belt.  Oort cloud The Oort cloud is theorized to be a vast cloud of leftover icy planetesimals surrounding the Sun. The concept of such a cloud was proposed in 1950 by Jan Oort, in whose honor the idea was named. Telescopes have yet to detect objects in the Oort cloud. See File:JanOort.jpg for original file. SenseiAC for cropping the original file into this file., CC BY-SA 3.0, via Wikimedia Commons  Asteroid An asteroid is an object that orbits within the inner Solar System. They are rocky, metallic or icy bodies.  Asteroid belt Of the known asteroids, the greatest number are located between the orbits of Mars and Jupiter, in a region known as the asteroid belt.  Near-Earth asteroid Near-Earth asteroids are asteroids in a near-Earth orbit. The first near-Earth asteroid to be discovered was Eros in 1898. Tomruen, CC BY-SA 4.0, via Wikimedia Commons  Formation The near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.  Near-Earth object A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun is less than 1.3 times the Earth–Sun distance. – This definition applies to the object’s orbit around the Sun, rather than its current position. – An object is considered an NEO even at times when it is far from making a close approach of Earth.  Scientists recognised the threat of impacts Interest in NEOs has increased since the 1980s. Scientists have recognised the threat of impacts that create craters much bigger than the impacting bodies and have indirect effects on an even wider area, with mounting evidence for the theory that the Cretaceous–Paleogene extinction event was caused by a large asteroid impact.  Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K– Pg) extinction event was the mass extinction of three-quarters of the plant and animal species on Earth approximately 66 million years ago. The event caused the extinction of all non-avian dinosaurs. ScottRobertAnselmo, CC BY-SA 3.0, via Wikimedia Commons  K–Pg boundary In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks.  Iridium The boundary clay shows unusually high levels of the metal iridium. http://images-of-elements.com/, CC BY 1.0, via Wikimedia Commons  Siderophile element Iridium is a siderophile (iron- loving) element. It dissolves readily in iron. Valandil1983, CC BY-SA 3.0, via Wikimedia Commons  Extremely rare in Earth’s crust Iridium is extremely rare in Earth’s crust because it mostly sank along with iron into Earth’s core.  Common in asteroids & comets Instead, iridium is more common in comets and asteroids.  Impact hypothesis As originally proposed in 1980 by a team of scientists led by Luis Alvarez and his son Walter, it is now generally thought that the K–Pg extinction was caused by the impact of a massive asteroid.  Impact winter The impact devastated the global environment, mainly through a lingering impact winter which halted photosynthesis in plants and plankton. https://www.smithsonianmag.com/smart-news/dust-may-have-triggered-the-global-winter-that-killed-the- dinosaurs-180983165/  Chicxulub crater The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the Chicxulub crater in the early 1990s, an impact crater buried underneath Mexico. The date of the impact coincides with the K–Pg boundary. Uwe Dedering, CC BY-SA 3.0, via Wikimedia Commons  Summary Near-Earth object – Comet – Near-Earth asteroid – Cretaceous–Paleogene extinction event Impact hypothesis