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WEP: Asteroids, Comets & Craters Terrestrial Craters and the Components of the Asteroid Belt In OpenSpace Ashleigh Smith Astronomy & Astrophysics Lab, North Carolina Museum of Natural Science OpenSpace Resources OpenSpace is a grant-funded NASA project that the North Carolina Museum of Natural Science is partnered on. The project is led by the American Museum of Natural History in New York City, NY, and further information can be found at www.openspaceproject.com. “OpenSpace is open source interactive data visualization software designed to visualize the entire known universe and portray our ongoing efforts to investigate the cosmos. Funded in part by NASA, OpenSpace brings the latest techniques from data visualization research to the general public. OpenSpace supports interactive presentation of dynamic data from observations, simulations, and space mission planning and operations. OpenSpace works on multiple operating systems, with an extensible architecture powering high resolution tiled displays and planetarium domes, and makes use of the latest graphic card technologies for rapid data throughput. In addition, OpenSpace enables simultaneous connections across the globe, creating opportunity for shared experiences among audiences worldwide.” I undertook this particular project of compiling the different types of small bodies found in OpenSpace and explaining them, inside the Astronomy & Astrophysics Lab at the North Carolina Museum of Natural Science under the guidance of Dr. Rachel Smith. Asteroids Asteroids can be categorized by their composition and the path of their orbit, as well as their general location in the asteroid belt. In OpenSpace, the asteroids are organized by their orbit and location in the asteroid belt, and different types of asteroids can be filtered to display the desired datasets. Throughout this section, the highlighted terms are sets of asteroids that can be found in OpenSpace. By Asteroid Orbit/Location Main Asteroid Belt (see right)1 The majority of asteroids in the solar system that we know of can be found in the main part of the asteroid belt. Asteroids that reside in this portion of the asteroid belt have an orbit with a semi-major axis of between 2.0 and 3.2 astronomical units and a perihelion distance greater than 1.6 astronomical units (see right). This means that they orbit between Mars and Jupiter. It is estimated that there are between 1.1 and 1.9 million asteroids larger than a kilometer in diameter and millions smaller than that which orbit within the Main Asteroid Belt. In OpenSpace, other related data sets that can be viewed are the Inner Main Asteroid Belt and the Outer Main Asteroid Belt, which contain asteroids with a semi-major axis greater than and less than 2 au. In the picture above, green is the Main Asteroid Belt, yellow is the Inner Main Asteroid Belt, and blue is the Outer Main Asteroid Belt. Near-Earth-Asteroids (see left)2 These are asteroids with an orbit that passes close to Earth, and there are even some that cross Earth’s path called Earth crossers. Near-Earth-Asteroids are primarily Amor, Apollo, Aten, and Atira asteroids . Similarly, there are also Mars-crossing asteroids, which cross Mars’ orbit and can be viewed in OpenSpace. There are about 861 near-Earth asteroids known that are over a kilometer in diameter and 10,003 near-Earth asteroids known total as of June 2013. There is another subset of Near-Earth-Asteroids known as Potentially Hazardous Asteroids, which are those that could be dangerous to Earth because of their close orbit, and it is estimated there are about 1,409 of them. 1 Figure 7. Main, Inner Main & Outer Main Asteroid Belt, OpenSpace 2 Figure 8. Amor, Apollo, Aten, Atira & Potentially Hazardous Asteroids, OpenSpace Trojans (see right)3 Trojans are asteroids that orbit with a larger planet but do not come into contact with them due to the L4 and L5 Lagrange points (see Vocabulary). Trojans tend to fly out of orbit naturally, so they are balanced out by the gravitational pull of the sun and the planet that is sharing its orbit; this causes them to remain on trajectory. Trojans can be found surrounding planets such as Jupiter (Jupiter Trojan Asteroids), Mars, Neptune and most recently, Earth. Trans-Neptunian Object Asteroids4 (see below) If an object is Trans-Neptunian, that means that it is beyond Neptune’s orbit entirely, or at least has a greater average distance from the Sun than Neptune does (a > 30.1 au). Trans-Neptunian Objects are often dwarf planets such as Pluto. By Asteroid Composition Asteroids can also be classified by their makeup, in categories C-Type, S-Type, and M-Type. C-Type: Accounting for more than 75 percent of known asteroids, these are simultaneously the most common type of asteroid and some of the oldest objects in the solar 3 Figure 9. Jupiter Trojan Asteroids, OpenSpace 4 Figure 10. Trans-Neptunian Object Asteroids, OpenSpace system. They generally are composed of rocks made from clay and silicate, are darker in appearance due to their low albedo, and reside in the outer regions of the main asteroid belt.. S-Type: These make up about 17 percent of known asteroids and are known for having a higher albedo and are therefore brighter and more reflective. They reside in the inner asteroid belt and are generally composed of iron-silicates, magnesium silicates, metallic iron, or some combination thereof. M-Type: These asteroids are metallic in composition, but can be different from one another depending on how far from the sun they are formed. M-Type asteroids are less predictable than other types of asteroids because their close proximity to Jupiter or Mars’ gravitational pull can force them out of the Main Asteroid Belt in any direction-- even potentially towards Earth, which would make them potentially hazardous Earth-crossing asteroids. They are fairly bright, with an average albedo, and primarily reside in the middle of the main belt if they are not knocked out of orbit. Comets Comets are frozen chunks of dust, rock, and ice that are left from the solar system’s origins, and they can be as small as a few or as large as dozens of miles wide. As comets get closer to the sun, they begin to get hotter and typically form a giant glowing head with a tail that can stretch for millions of miles. Throughout this section, the highlighted terms are sets of comets that can be found in OpenSpace. OpenSpace has four comet data sets: Chiron-type, Encke-type, Halley-type, and Jupiter-family comets. Periodic Comets (P) Period Comets are comets that have historically been observed more than once at their perihelion, indicating a reliable orbit over time. A famous example of this is the Halley comet, which astronomers have been observing since 240 BCE. Periodic comets can further be organized into 3 categories: Short Period comets, Intermediate Period comets, and Long Period comets. Short Period Comets have a round trip orbit of less than 20 years, Intermediate Period comets have a round trip orbit of between 20 and 200 years, and Long Period comets have an orbit over 200 years. There are also Single-Apparition comets that do not have a circular orbit, instead following a parabolic or hyperbolic path. The following data sets of various comet types and their defining parameters can be found in OpenSpace and are listed below. TJupiter represents Tisserand’s Parameter with respect to Jupiter, a represents the semi-major axis of the orbit in astronomical units (au), aJupiter represents the semi-major axis of Jupiter’s orbit, and y represents the number of years in an orbit. 5 6 Encke-Type Comets : TJupiter > 3; a < aJupiter Halley-Type Comets : 20 y < P < 200 y 7 8 Jupiter-Family Comets : 2 < TJupiter < 3 Chiron-Type Comets : TJupiter > 3; a < aJupiter Non-Periodic Comets Non-Periodic Comets are ostensibly the opposite of Periodic Comets, in that they are typically seen passing through our solar system only once. A more well-known example of non-periodic comets is the Comet Hale-Bopp, which was seen on April 1, 1997. Other Comets There are also Lost Comets and Comets with No Meaningful Orbit, which are further classifications that denote comets which were not observed during their expected perihelion, and comets for which there was not sufficient data to plot a defined orbit. 5 Figure 10. Encke-Type Comets, OpenSpace 6 Figure 10. Encke-Type Comets, OpenSpace 7 Figure 12. Jupiter-Family Comets, OpenSpace 8 Figure 13. Chiron-Type Comets, OpenSpace Centaur Objects9 Centaur Objects (referred to as Centaur Asteroids in OpenSpace) can be classified as both asteroids and comets because they are more like asteroids in the way of size, but more like comets in composition. They primarily revolve between Jupiter and Neptune (5.5 au < a < 30.1 au). The first type of Centaur Object to be discovered was Chiron, found in 1977, and the JPL Chiron-type datasets can be seen in OpenSpace. Background Information: Terrestrial Craters There are approximately 170 known impact craters on Earth that can still be identified, as impact craters can be millions of years old and the toll that erosion takes over time often erases evidence of their existence. Impact crater creation happens in three steps: Contact & Compression, Excavation, and Modification. The first stage of an impact is the contact/compression phase, beginning when the impactor first contacts the surface. Shock waves begin to form at the meteorite-surface interface and propagate into the meteorite and surface. The material at the impact site is compressed from the impact. The high internal pressures generated during this event create a phenomenon called "jetting", where material is ejected from the impact site in the form of hydrodynamic jets with a velocity several times that of the impactor.
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