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Western University
Microbiology and Immunology
Microbiology and Immunology 2500A/B
Derek Mc Lachlin

Near Earth Objects Ziyu Liu 250 585 137 March 28, 2013 1086G Origin and Geology of the Solar System 1 1. Introduction Near-Earth objects (NEOs) are objects in solar system, whose orbits have been brought into proximity with the Earth. Near-Earth objects include thousands of near-Earth asteroids, 8 spacecraft, meteoroids and comets. Every year, approximately 10 kg of meteoritic material falls on the Earth, from the size of dust particles to several kilometers asteroids. (Ceplecha, 1992) Near-Earth Asteroids originate from the Main belt or the Main Asteroid Belt, a region in solar system found between the orbits of Jupiter and Mars. Thousands of asteroids that have fallen on earth were collected and catalogued. Near-Earth comets come from the outer region of our solar system that was probably derived beyond the orbit of Neptune, the Kuiper belt. As a result, comets that crosses Earth’s orbit must have very high elliptical orbits in order to reach the Earth due to their distance to our planet. It is very important to investigate and pay attention to those extraterritorial objects since sometimes their impact on Earth can have drastic effect upon colliding. Asteroids with diameter of less than 10 meters enter Earth’s atmosphere with little energy, only approximated to be the equivalent of the Little Boy (The atomic bomb dropped on Hiroshima). These NEOs usually explode in the upper atmosphere and vaporized. (Clark and David, 1994) However, historical event such as the Tunguska asteroid gave out energy equivalent to 10 megatons of TNT and is estimated to occur every 2000-3000 years which can cause considerable damage if landed on Earth. (Shoemaker, 1983) This report will address the classification of various NEOs and look into details about the current NEOs as well as NEOs that have come into contact with Earth, and discuss how they are monitored and assesses in modern scientific way. 2 2. Identification and classification Near-Earth Objects can be classified as comets and asteroids base on the size and compositions. Near Earth Objects normally include only short period comets and Near Earth Asteroids can be further classified and divided into three groups, Atiras, Amor, Apollo and Atens based on their perihelion and aphelion distance as well as their semi major axes.(NASA, 2013) There are 9,683 NEOs have been discovered since February 2013, 93 were near-Earth comets and 9590 are near-Earth Asteroids. In the near-Earth Asteroids, there are 5,214 Apollo asteroids, 3,613 Amor asteroids, and 751 Aten asteroids. Also, there are 1,360 NEOs that falls in the category of Potentially Hazardous Asteroids (PHAs). (The Spaceguard Central node website, 2013) PHAs are asteroids that have the potential to cause significant regional damage when impacting on Earth due to its humongous size, having a diameter of minimum 100 to 150 meters. Specific definitions of NEOs are categorized in table 1. Near-Earth Comets have the orbital period P less than 200 years, with its perihelion distance q less than 1.3 AU, in fact, all the NEOs have the perihelion distance less than 1.3 AU and greater than 0.983.(Morbidelli A, 1989) Atiras are the assteroids that orbit inside Earth’s orbit, it’s aphelion distance (Q) is smaller than Earth’s perihelion (Q<0.983 AU), in addition to that, the asteroid’s semi-major axis is also less than Earth’s perihelion. Atens have the semi-major axis(a) of less than 1. Both Atens and Atiras are Apohele asteroids, as known as Inner Earth Objects (IEOs), due to their aphelion is entirely within the Earth’s orbit (Q<0.983 AU), some people consider Atiras to be part of the atens. Apollos have a larger semi-major axes than Earth’s (a>1.0 AU). Amors are orbiting strictly outside of Earth’s orbit, having a greater perihelion distance (q) than Earth and some of them cross the orbit of Mars. (NEO groups, 2013) Since Atens and Apollos cross Earth’s orbit, they have a chance of 3 impacting Earth, while Atiras and Amors do not cross the orbit of Earth, they are not impact threads unless their orbit changes. 3. Current NEOs 3.1 1036 Ganymed 1036 Ganymed is the largest asteroid found in Amor group with a diameter of nearly 40 km. (Abell, 2007) Ganymed was discovered on 23 October 1924 by Walter Boddle in Germany. The orbital period of Ganymed is about 4.35 years. The diameter is measured approximately 41 km.(RASNZ, 2011) Ganymed also passes the orbit of Mars, the next approach to Mars will be on 16 December 2150 at a distance of 0.0957 AU. (NASA, 2011) The absolute magnitude was published to be 9.24 in 1931, but the present confirmed value to be 9.45. Ganymed is categorized in S-type asteroid, S-type asteroid composed of iron and magnesium silicates and has a relatively reflective appearance, and the surface of Ganymed is rich in orthopyroxenes and some metals.(Bus and Binzel, 2002)_ The Arecibo radio telescope observed Ganymed in the year of 1998 and produced several images of this asteroid and found that it is roughly a spherical object. On august 22, 1985, an occultation of star by Ganymed was observed in California. The rotation period was confirmed to be10.314 ± 0. 004 h and it has a lightcurve amplitude of 0.12 mag. (Krugly, 2006) Ganymed approaches Earth about every 13 years. The previous approach was on October 13, 2011 at a distance of 0.359 AU and the next approach will be on October 13, 2024 at a distance of 0.374. (Minor Planet Center, 2013) Since 1036 Ganymed only seemed to have close approaches to Earth, there is not much potential danger to us. 4 3.2 Comet Encke Comet Encke is a Near Earth Comet that orbit around the sun every three years. It has the shortest period of any comets that we have known. Encke was discovered by Pierre Mechain in 1786, however it was not found to be periodic until Johann Franze Encke uses more advanced calculator in 1819. Encke comet was the first periodic comet after the discovery of Halley’s comet. The next closest approach of the Encke to the sun will be on November 21, 2013, which can be observed by binoculars at the end of September in the evening skies. It is one of the brightest comets in 2013. (Fall of Thousand suns, 2012) The diameter of Encke was calculated to be 4.8 km. It was found to be having unstable orbits due to perturbation and outgassing, and the orbit is often perturbed by the inner planets. (NASA, 2013) Encke usually approaches Earth every 33 years, the closest distance of the comet’s orbit to Earth is 0.1731 AU. The last closest approach was on 4 July, 1997, Encke reaches the Earth at a distance of 0.19 AU. The next approach will be on June 29, 2172 and predicted to be at a distance of 0.48. (NASA, 2013) Scientist suggests that the Tunguska event was probably caused by the fragments of comet Encke. (Kresak, 1978) 5 4. Recent objects contacted with Earth 4.1 Chelyabinsk Meteor th Chelyabinsk Meteor entered Earth’s atmosphere over Russia on 15 February 2013 at about 09:20 YEKT with a speed of approximately 18 km/s (40,000 mph)(Near Earth Object Prgrame, 2013). After passing through the atmosphere, it became a bright superbolide meteor of the sky of southern Ural region. The energy of the asteroid was so great that the light was bright enough to cast moving shadows in the daylight, the heat also can be felt by the eyewitnesses from the fireball. A powerful shock wave was generated when Chelyabinsk meteor exploded in an air burst over Chelyabinsk Oblast on the sky, shown in figure 1. Most of the object’s energy was absorbed by the atmosphere, with a total kinetic energy estimated to be 440 kilotons of TNT, the energy released was roughly 20 times greater than the Big Boy atomic bomb. The shockwave breaks windows in the residence and about 1,500 people were injured, over 7,200 structures in six cities across the region were damaged by the explosion. (RBTH, 2013) Chelyabinsk meteor is the largest object that have entered Earth’s atmosphere since the Tunguska event occurred in 1908. (Today, 2013) Chelyabinsk meteor was expected to have an initial mass of 11,000 tonnes and about 17 to 20 meters across, however, this object was not detected by radar before its atmospheric entry and there was no defense against it. (Today, 2013) Scientists from the Ural Federal University suspects that the 6-metre-wide hole in Lack Chebarkul’s surface was resulted from the impact of the Chelyabinsk meteor. Meteor shatters into pieces that are all under 1 cm in size and laboratory analysis confirmed their meteoric origin. Fragments found are officially named to be Chlyabinsk meteorite. 6
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