1. Title: EARTH SCIENCES 2. Course Name: Origin and Geology of the Solar System
3. Course number: 1086G
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Introduction
Near-Earth Objects (NEOs) can be defined as heavenly bodies or solar system objects that have been pulled gently by the gravitational force of the Earth bringing them close to into orbits and the Earth’s neighborhood. These NEOs include Near-Earth comets, Near- Earth asteroids, meteoroids and a few solar-orbiting spacecraft huge enough to be trailed in space before hitting the Earth. The collision of the NEOs in the past has had a significant part in shaping the biological and geological history of the Earth. Near-Earth asteroids (NEA) have orbits that rest between 0.983 to 1.3 astronomical units from the sun (Shapiro, 1999). Due to this the Earth’s orbit cross with some NEA orbits posing a collision danger. By 2012 August there were 848 Near-Earth asteroids that were larger than 1km and 154 of those had were potential hazardous asteroids (PHAs).Comets are mostly composed of ice water with engrafted dust particles. They are formed on the outer system of the planet. The asteroids are mainly rocky and are formed in the warmer inner solar system between the orbits of Jupiter and Mars (Bally, Reipurth 2005).
Scientific interest in asteroids and comets is due to their status because they have unchanged debris from the solar system formation dated back to up to 4.6 billion years ago. Asteroids and comets give hints to the chemical mixture from which the planets took shape 4.6 billion years ago. NEOs are of high interest also because they present interesting scientific opportunities for both astronomical and direct investigation. They are also potential sources of extraterrestrial materials for man development economically. This is due to the fact that they can be explored through lower mission velocity due to their small gravity and their combination of less velocity with regard to the Earth (Yeomans, 2013). This makes them an appealing target for exploration. By the year 2012 three NEAs have been visited by spacecraft. This report will identify and classify current Near-Earth objects and their potential impact on the Earth.
Identification and classification of Near-Earth objects
NEOs can be classified as comets, asteroids and meteoroids depending on composition and size. Comets generate meteoroid streams that create meteor showers while asteroids are members of the asteroid family Near-Earth Comets (NECs) are further limited to include short-period comets only. In terms of orbital elements NEOs are comets and asteroids that have a perihelion distance of less than 103AU.
Near-Earth Asteroids
The majorities of NEOs are asteroids known as Near Earth Asteroids (NEAs) and are divided into groups (Apollo, Amor and Aten) depending on their aphelion distance, perihelion distance and their semi-major axes (Botke, 2002). Atens are NEAs that have semi-major axes that are smaller than the Earth’s. Apollo is an earth crossing NEA with whose semi-major axes is larger than the Earth’s. The Amor consist of earth crossing NEAs with orbits outside the earth but inside the mars while the potential hazard asteroids are NEAs that have a minimum orbit intersection distance of 0.05 AU or below or those that have an absolute magnitude of 22.0 or brighter.
Near Earth Meteoroids (NEMs)
Those objects of less than 50 meters are considered near earth meteoroids. They are many in number. Most of them are size of sand grains or small rocks though occasionally a boulder sized meteoroid can cross the Earth’s path. If this happens a fireball streak is seen across the sky even during the day. Meteoroids usually burn up in the sky and therefore pose little danger to human beings. All the NEAs smaller than 1in the measuring scale are categorized as ‘Near –Earth meteoroids’. They are usually listed as asteroids on many asteroid tables. 2008 TS26 is the smallest known Near-Earth meteoroid with an estimated size of1 meter and a magnitude of 33. Meteorite Types | Iron | primarily iron and nickel; similar to type M asteroids | | Stony Iron | mixtures of iron and stony material like type S asteroids | | Chondrite | by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets | | Carbonaceous Chondrite | very similar in composition to the Sun less volatiles; similar to type C asteroids | | Achondrite | similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites | |
Figure1. Types of meteorites (Hurd, 2009)
Near Earth Comets (NECs)
Near earth comets are objects whose orbit brings them closer to the earth. Only 84 Near-Earth comets are known to exist to date. They usually originate from the Kuiper belt or the short period comets or the Oort cloud or the long period comets. Comets that cross Earth's path will leave behind a dust trail. When Earth passes through this trail we see a meteor shower. Group | Description | Definition | NECs | Near-Earth Comets | q<1.3 AU, P<200 years | NEAs | Near-Earth Asteroids | q<1.3 AU | Atiras | NEAs whose orbits are contained entirely with the orbit of the Earth (named after asteroid 163693 Atira). | a <1.0 AU, Q<0.983 AU | Atens | Earth-crossing NEAs with semi-major axes smaller than Earth's (named after asteroid 2062 Aten). | a<1.0 AU, Q>0.983 AU | Apollo | Earth-crossing NEAs with semi-major axes larger than Earth's (named after asteroid 1862 Apollo). | a>1.0 AU, q<1.017 AU | Amor | Earth-approaching NEAs with orbits exterior to Earth's but interior to Mars' (named after asteroid 1221 Amor). | a>1.0 AU, 1.017<q<1.3 AU | PHAs | Potentially Hazardous Asteroids: NEAs whose Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less and whose absolute magnitude (H) is 22.0 or brighter. | MOID<=0.05 AU, H<=22.0 |
Figure2. Near-Earth objects (Yeomans, 2013)
Current NEOs
Near-Earth asteroids
Asteroids are also called planetoids especially the huge ones. They exist in million of numbers, many are thought to be remains of planetsimals. They orbit in the asteroid belt between the orbits of mars and Jupiter. They are classified by their spectra characteristics with many falling in the c-type, s-type and m-type according to the carbon-rich, stony and metallic characteristics respectively. Asteroid 4 Vesta is normally seen by the naked eye. Asteroids differ greatly in sizes from rocks just tens of meters to nearly 1000 kilometers for the largest. The dwarf planet Ceres is the largest asteroid having a diameter of 975km. This is followed by 4 Vesta and Pallas both with diameters of over 500km. all objects of the asteroid belt are estimated to have a mass of 2.8-3.2*1021kg. Of this Ceres comprises of 0.95*1021kg which is a third of the total. Vesta contributes 9%, Pallas 7%, and Hygeia 3% bringing the total to 51 %. ( Yabushita, Henrard 1998) Ceres is the only large asteroid with a huge gravitational force that can force it in to a spherical shape. It has a higher absolute magnitude than the rest at around 3.32 and has a layer of ice. It also has a core, a crust and a mantle just like planets. The physical composition of asteroids is varied. Ceres has a rocky core covered with ice mantle. Nickel-iron core, ballast crust and olivine mantle is found in Vesta. The small asteroids are thought to be rubbles held together by gravitational force (Botke, 2002). Some asteroids have moons. They have traces of amino acids plus other inorganic compounds. The composition is calculated from density, albedo and surface spectrum. The color of asteroids becomes redder and darker with age as a result of space weathering.
Near-Earth comets
Comets are usually dirty and dusty objects. They are a few kilometers across and are composed mainly of carbon dioxide, water, methane ices and ammonia with dust mixed in. they interact gravitationally with the sun. Most of their orbits are elliptical and parabolic in some cases. They are more likely to be seen than asteroids and most of them belong to the population of the short comets. They have bright tails almost 150 km in length. The tails are composed of dust and gas emitted from the nucleus and is very diffuse.
Dangers of Near-Earth asteroids and comets
Near-Earth objects pose a variety of dangers to the earth. Collisions of asteroids occur randomly. Due to this localized destruction, equivalent to a hydrogen bomb occur every couple of hundred years. The last occurred in Siberia at Tunguska River. The number of casualties will depend on the size of the object and place of impact. Asteroids usually cause tsunamis when they occur in oceans that cause destruction in the adjacent seaside. When an asteroid struck a city the casualties could be more than one million hence they are very dangerous. Global destructions due to asteroids occur every 10 million years and involve an impact of a 10km asteroid creating a 100 and over km crater. The nature of destruction depends on the rock properties of the crater. Some may be acidic leading to mass extinction (Marov, Rickman 2001). This is dangerous if it occurs today as it may lead to millions of casualties.
It is now clear that dinosaurs and over 70% of all species endured mass extinctions as a result of an impact of a comet or asteroid about 65 million years ago. The speed at which comets and asteroids enter the earth is very high. A 10 km asteroid moving at a speed of 30km/s has energy equal to 15 billion times of the Hiroshima nuclear bomb. This will lead to shock waves in the earth crust, destruction of the ozone layer, firestorms over large areas of the continent, alteration of the of the atmosphere as a result of the large amounts of materials ejected into the upper atmosphere and so on (Erwin, 2006). The probability of an impact by a comet is much lower compared to an asteroid but cannot be ignored as Comet Hyakutake, which passed near to the Earth in late March this year, had only been discovered two months before. There relative speed is higher than that of a small objects on average and can pose more danger.
Russian meteor explosion
On the morning of February 15, 2013 a large fire ball was observed in the skies near Chelyabinsk, Russia. It was caused by small asteroid measuring 17 to 20 meters in size entering the Earth’s atmosphere at a shallow angle and at a high speed (Bronshten, 2013). It fragmented at high altitude, released huge amounts of energy and created a shower of pieces of numerous sizes that dropped to the ground as meteorites. It was observed by video cameras, low frequency infrasound detectors and the U.S government sensors. This made the details of the fire ball clearer. The geographic location of the fire ball at maximum brightness was latitude54.8 degrees N and longitude 61.1 degrees E. the altitude of maximum brightness of the fireball was 23.3 km approximated to 14.5 miles. Its velocity at peak brightness was 18.6km/sec approximately 11.6 miles / sec (Bronshten, 2013). The approximate total radiated energy of the fireball was 3.75*1014 joules which is equivalent of 90 kilotons of TNT explosives. It was an extra ordinary large fire ball with the most energetic impact recognized since the 1908 Tunguska blast in Russia. The recovered meteorites from the fireball are ordinary chondrites with a density of about 3.6g/cm3. Given the energy of 440kt the diameter of the asteroid was 18 meters and it’s mass roughly 11000 tons. The Chelyabinsk fireball is not in any way related to the asteroid 2012 DA14 which made a close fly of the Earth over 16 hours after the Russian fire ball event passing within 17200 miles of the Earth’s surface. This is because the two fire balls entered the earth from different directions and had different orbits from the sun. There is also no physical connection between the two.
Another event that has impacted the earth in the past 200 years was on March 8 2002 when an asteroid went hurling pass the earth. Its distance was slightly more than 1.2 times the distance to the moon. It came from the suns direction and was not detected until after four days it’s near- earth approach. Asteroid 2002 EM7 was two thirds the size of a football field. It had a destruction capacity of a 4-megaton nuclear bomb. It was one of the closest known asteroids to approach the earth (Great Britain., & British National Space Centre 2003). Its direction from the sun made it difficult to be noticed taking everyone by surprise. Predictions have been made that asteroid 2002 EM7 could hit the earth in 2093.
Future investigations of NEOs
Canada’s NEOSSat (Near-Earth Object Surveillance Satellite) project
NEOSSat is a Canadian microsatellite mission funded by department of National Defense/ Defense Research and Development Canada and the Canadian Space Agency (CSA). It was developed on February 25, 2013 and it’s the world’s first space telescope dedicated to detecting and tracking asteroids and satellites. It moves around the globe every 100 minutes to pinpoint asteroids that may one day pass close to earth. NEOSSat has an eye on asteroids. It orbits approximately 800 km above the earth identifying near- earth asteroids that are difficult to spot through ground-based telescopes. It is not limited to day-night cycle and can operate 24/7. The images generated by NEOSSat will be analyzed by University of Calgary’s which is its centre for science operations. The overall aim of the mission is to determine and discover orbits of NEAs that cannot be detected effectively on the ground. This involves monitoring of the trajectories of the asteroids.
The OSIRIS-Rex (Origins Spectral Interpretation Resource Identification Security Regolith Explorer) mission
The mission is set to launch in 2016. It’s an unmanned mission to gather samples of grievous near- earth asteroid 1999 RQ36 which is 1500 feet wide and bring them to earth. The mission is also aimed to gather the best measurements of the small forces that act on asteroids. There are more than 1300 space rocks classified by NASA as potentially hazardous asteroids. They have an orbital path that brings them closer to the earth and they measure at least 140 meters making them dangerous (Hurd, 2009). They carry enormous energy as a result of their velocity as they move at an average speed of 12 to 15 km/sec. The spacecraft will travel for two years and rendezvous with asteroid 1999 RQ36. It will then begin 505 days surface mapping at a distance of 4.8km. The results will be used to select sample site and the gradual process of approaching the asteroid by the mission.
Conclusion
Near-Earth objects include Near -Earth asteroids (NEAs), Near -Earth comets, meteoroids and a number of solar-orbiting spacecraft whose orbits brings them in proximity with the earth. They have the potential to cause danger if not detected and tracked in advance due to their collisions. The asteroids are the most dangerous of all NEOs due to their huge size and high velocity with which they enter the earth. There effects could be compared to that of a nuclear bomb. Examples of recent objects that have come into contact with Earth include the Russia meteor explosion which occurred on the 15 of February 2013. Future investigations into the NEOs are majorly concerned with the asteroids due to the grievous threat they pose to the earth. This includes the Canadian NEOSSat project that aims to identify Near-Earth objects that are difficult to spot on the ground and the OSIRIS-Rex mission that is set to launch in 2016.

References
Bally, J., & Reipurth, B. (2005). The birth of stars and planets. Cambridge: Cambridge University Press.
Yeomans, D. K. (2013). Near-Earth objects: Finding them before they find us. Princeton: Princeton University Press.
Erwin, D. H. (2006). Extinction: How life on earth nearly ended 250 million years ago. Princeton, N.J: Princeton University Press.
Hurd, B. (2009). Near earth object. S.l.: Publishamerica Inc.
Shapiro, R. (1999). Planetary dreams: The quest to discover life beyond earth. New York: J. Wiley.
Bronshten, V. A. (January 01, 2000). Nature and destruction of the Tunguska cosmical body. Planetary and Space Science, 48, 9, 855-870.
Yabushita, S., & Henrard, J. (1998). Dynamics of comets and asteroids and their role in earth history: Proceedings of a workshop held at the Dynic Astropark 'Ten-Kyu-Kan', August 14-18, 1997. Dordrecht, the Netherlands: Kluwer Academic.
Bottke, W. F. (2002). Asteroids III. Tucson: University of Arizona Press.
Marov, M. I. A., & Rickman, H. (2001). Collisional processes in the solar system. Dordrecht: Kluwer Academic Publishers.
Great Britain., & British National Space Centre. (2000). Report of the Task Force on Potentially Hazardous Near Earth Objects. Lobdon: Information Unit, British National Space Centre.