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AST 248, Lecture 17 James Lattimer Department of Physics & Astronomy 449 ESS Bldg. Stony Brook University April 8, 2020 The Search for Intelligent Life in the Universe [email protected] James Lattimer AST 248, Lecture 17 Facts Concerning the Solar System I All the planets roughly orbit the Sun in a plane. I This plane is roughly the same as the rotational equator of the Sun. I Planetary orbits are, for the most part, circular. I The planets all revolve in the same direction about the Sun. I The Sun and the planets, with the exception of Venus and Uranus, rotate on their axis in the same direction. With the exception of Uranus, the tilt between the equator and the orbital plane of the Sun and planets is small. I The planets differ in composition: the planets nearest the Sun tend to be small, dense and metal-rich, whereas the planets farthest from the Sun tend to be large, light and hydrogen-rich. James Lattimer AST 248, Lecture 17 I Meteorites differ in chemical and geologic properties from planetary and lunar rocks, but may be similar in composition to asteroids and small moons. I Planets and most asteroids rotate with similar periods, about 5 to 10 hours, unless obvious tidal forces slow them, as in the case of the Earth and Venus. I The distances of the planets from the Sun obey the Bode's Law. I Planet-satellite systems resemble miniature solar systems. I Cometary orbits, as a group, define a large, almost spherical, cloud around the Solar System. I The Sun contains 99% of the mass in the Solar System. I The planets, not the Sun, contain nearly all the angular momentum of the Solar System. James Lattimer AST 248, Lecture 17 A Brief History of Solar System Discoveries 300 BC Aristotle { Earth is round because of circular shadow during lunar eclipses, some stars not visible from northern locations 270 BC Aristarchus { Earth revolves around Sun and Moon around Earth; estimated lunar size from eclipses and relative distances and sizes of Sun & Moon; Sun much bigger than the Earth 250 BC Eratosthenes { Measured shape & size of Earth 135 BC Hipparchus { Discovers precession of the equinoxes; better estimate of the Moon's distance; rejects Earth orbited Sun. 100 AD Ptolemy { Proposed Earth as center of Universe (Aristotle) 1473{1542 Copernicus { Earth and planets orbit Sun 1546{1604 Tycho Brahe { Measured detailed planetary positions 1571{1630 Kepler { Analyzed Brahe data and deduced 3 Laws of Motion: I Each planet moves in an ellipse with Sun at one focus (Wrong: focus is at center-of-masss) I Line between Sun and planet sweeps out equal areas in equal times 2 3 I Orbital period proportional to semi-major axis , led to 1 2 2 3 Newton's 1{2{3 Law: G(M + mplanet ) P = 4π a 1564{1642 Galileo { Laws of motion; telescopic observations proved Copernican theory 1642{1727 Newton { Law of gravity and derivation of Kepler's 3 laws 1656{1742 Halley { First good estimate of size of solar system from Venus transits; Halley's comet is periodic 1644{1710 Roemer { Measured light speed using Jupiter's moons James Lattimer AST 248, Lecture 17 Debris in the Solar System: Comets Hale-Bopp I \Dirty snowballs", a mixture of ices and dust, ejected from solar system when planets formed I Mass mostly contained in a nucleus (solid dirty snowball) with km-sized diameter Hally's nucleus I Coma, a cloud of H2O, CO2 and other gases va- porized from nucleus during close passage to Sun from Giotto I Hydrogen cloud, huge but sparse neutral H cloud I Dust tail, 1{10 million km long, dust driven off nucleus by escaping gases when close to Sun I Ion tail, 100's of millions of km long, plasma driven off by solar wind I Most comets reside far outside the orbit of Pluto, a few occasionally perturbed into close-solar orbits; some are in elliptical orbits and reappear I After a few hundred passages near the Sun, icy material is lost, Shoemaker-Levy 9 before Jupiter impact leaves dead comet which can be mis- taken for asteroid James Lattimer AST 248, Lecture 17 Comet Holmes, Halloween, 2007 V. Peris and J. L. Lamadrid Fragment G HST 7/18/1994 Shoemaker-Levy 9 HST 7/7/1994 James Lattimer AST 248, Lecture 17 Comets History I Records of comets exist at least to 1140 BC I Brahe observed comet of 1577, proved they are extraterrestrial I Halley: some comets are periodic (1531, 1607, 1682) I Halley's Comet: 2467 BC (?), 240 BC (China), 1066 (Bayeux Tapestry) I Motivated Newton to develop gravity theory Orbits I Hyperbolic: pass Sun once, depart forever I Elliptical: periodic I Peri-/Aphelion: closest/farthest solar approach Origin I Short-period come from Kuiper Belt, 35,000 icy bodies larger than 100 km, 30-100 AU I Long-period come from Oort comet cloud, ∼ 1012 comets, 30,000 AU{1 lt. yr. I Most formed within inner solar system, then pushed outwards by repeated near encounters James Lattimer AST 248, Lecture 17 Asteroids I Also called minor planets or planetoids, largely lying within the orbits of Mars and Jupiter (main asteroid belt). I Objects lying outside Neptune are called trans-Neptunian objects, centaurs or Kuiper-Belt objects. I The first asteroid to be discovered was Ceres on Jan 1, 1801. I Total asteroid mass is about 4% of the Moon's mass. Ceres is about 32% of this. The first 4 asteroids have 51% of the total. I Can be roughly classified by spectral type: C-type carbonaceous, 75% S-type silicaceous, 17% L-type metallic, 8% These percentages do not reflect true proportions, but only observed proportions; some are easier to see than others. I About a third of asteroids are members of families that have similar orbits. Probably they form as a result of collisions between asteroids. They last for about a billion years. I Asteroids are slowly lost due to collisions with planets and to gravitational encounters that eject them from solar system. I Trojan asteroids collect in Lagrangian points (stable gravitational minima) 60◦ ahead of and behind Jupiter in its orbit. Similarly for Mars, Earth and Venus. I Asteroids are given a number in order of their discovery and the discoverer or the IAU may name them. James Lattimer AST 248, Lecture 17 James Lattimer AST 248, Lecture 17 James Lattimer AST 248, Lecture 17 951 Gaspra 243 Ida 433 Eros James Lattimer AST 248, Lecture 17 What is a Kuiper-Belt Object? The Kuiper Belt is a disk- shaped region past the orbit of Neptune extending roughly from 30 to 50 AU from the Sun containing many small icy bodies. It is now consid- ered to be the source of the short-period comets. About 1000 are known, and many (like Pluto) are in solarviews.com a 3:2 orbital resonance with Neptune. Centaur objects, of which 9 are known, orbit between Jupiter and Neptune. Their orbits are unstable, and are probably displaced from the Kuiper Belt. James Lattimer AST 248, Lecture 17 solarviews.com Kuiper Belt Objects Name Diameter (km) 2003 UB313 2400 ± 100 Pluto 2320 2003 EL61 1200? 2005 FY9 1250? Charon 1270 Sedna < 1500? 2004 DW ∼ 1500 Quaoar 1200 ± 200 Ixion 1065 ± 165 2002 AW197 890 ± 120 Varuna 900 ± 140 solarviews.com James Lattimer AST 248, Lecture 17 They Have Moons! solarviews.com James Lattimer AST 248, Lecture 17 What are Near-Earth Objects? I Comets and asteroids deflected into orbits with perihelion distance q < 1:3 AU I Made of water ice plus embedded dust I Originated in cold outer solar system I Primordial, leftover building blocks of planets Classes of Near-Earth Asteroids: I Atens - Earth-crossing, semi-major axis a < 1:0 AU I Apollos - Earth-crossing, a > 1:0 AU I Amors - Earth-approaching but interior to Mars, a > 1:0 AU and 1 AU < q < 1:3 AU, NASA/JPL-Caltech I PHAs - Potentially Hazardous Asteroids I Closest approach to Earth less than 0.05 AU I Absolute magnitude less than 22.0 I Assuming albedo is 13%, this means diameter greater than 150 m (500 ft) I There are currently 1885 known PHAs, 157 are larger than 1 km James Lattimer AST 248, Lecture 17 James Lattimer AST 248, Lecture 17 How Do We Find Near-Earth Objects? I Early efforts relied on comparison photographs taken several minutes apart I Vast majority of objects recorded were stars and galaxies, which don't move I Special stereo viewing microscopes pick out NEOs I Current efforts use CCD cameras I Computer-aided comparisons of digital images I NASA's goal: Find > 90% of NEAs > 1 km within 10 years I Total population estimated to be about 1000 I Lincon Near-Earth Asteroid Research (LINEAR) I Near-Earth Asteroid Tracking (NEAT) I Spacewatch I Lowell Observatory Near-Earth Object Search (LONEOS) I Catalina Sky Survey I Japanese Spaceguard Association (JSGA) I Asiago DLR Asteroid Survey (ADAS) James Lattimer AST 248, Lecture 17 James Lattimer AST 248, Lecture 17 Studying Near-Earth Objects in Space Typical Instruments: I Imager: Camcorder I IR and UV spectrometers to infer mineralocgical and gaseous compositions I Lidar: Optical equivalent of radar, useful to define shape of target and aids in navigation I X-ray, γ-ray and αX-ray spectrometers to infer chemical/elemental composition of surfae. X-rays come from Sun, γ-rays from cosmic rays, α-particles from a spacecraft Curium source I Dust mass spectrometer detects high-velocity dust whose impacts produce ions I Magnetometer to infer magnetic field of target I Package to measure temperatures/densities of plasma clouds created as sun's radiation ionizes cometary atmosphere.
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