Asteroids Physical Properties. Solar System Debris. Missions to Asteroids. Types of Asteroids (based on composition)

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1 Solar System Debris Asteroids Physical Properties Spacecraft Missions Origin Orbits Risk to Earth Tens to hundreds of km in diameter Comets History Structure Orbits Origin Missions Meteoroids & Meteor shower Asteroids Physical Properties Asteroids are relatively small, rocky objects that revolve around the Sun, also referred as "minor planets," or sometimes "planetoids. They are too small even to be classified as planets. To date, more than 20,000 asteroids have been catalogued. Though, the total number of known asteroids now exceeds 100,000 (unofficially, because orbits are still not known). - too small to hold an atmosphere - surface gravity is very low! Irregular in shape - too small for self-gravity to make the spherical All show craters and dust, some show fractures - most likely due to collisions Rotation periods vary - measured from variation in light intensity Some have satellites! Volcanism Vesta is unique among asteroids in that, despite its small size, it appears to have undergone volcanism in its past. (May be a part of larger body) Types of Asteroids (based on composition) C-type(or carbonaceous): large fraction of carbon. - dark (low reflectivity), 75% asteroids are of this type. - remnants of solar system formation (consist of very primitive material that have not experienced significant heating or chemical evolution since they first formed 4.6 billion years ago.) - their number increases as we go outwards in the asteroid belt. S-type: large fraction of silicates. - standard rocky material, 15% of asteroids. - lie predominantly in the inner portion of the asteroid belt. M-type: large fraction of iron, nickel. - standard rocky material, ~10% of asteroids. Missions to Asteroids Galileo (en route to Jupiter): - passed twice through the asteroid belt, making close encounters with asteroid Gaspra in October 1991 and asteroid Ida in August Both are S- type asteroids. NEAR (Near Earth Asteroid Rendezvous): - flew by Mathilde (C- type) in June orbited Eros (and landed! --- Feb 12, 2001) 1

2 Gaspra and Ida Mathilde The largest craters are about 20 km across. Gaspra Diameter: 20 km Age: 200 million years - colors have been enhanced Ida Diameter: 60 km, Age: 1 billion years Density: kg/m 3 Dactyl (1.5 km across, orbiting the asteroid at a distance of about 90 km.) Both are irregular in shape pitted with craters ranging in size from a few hundred meters to 2 km across and are covered with a layer of dust of variable thickness. - dimensions: ~ 60 x 50 km - rotation period: 17.5 Earth days - C type, low density (1400 kg/m 3 ) - loosely packed (interior must be quite porous) NEAR Descent to Eros Feb. 12, 2001 Eros is a heavily cratered body. Mass: ~ 7 x kg Density: ~ 2700 kg/m 3. Interior: solid rock not rubble, as in the case of Mathilde. But extensively fractured. It is primitive, unevolved sample of material from the early solar system. Asteroids - Origin Originally thought to be a planet that broke up under Jupiter s gravitational pull. However, not enough material to make a planet. Current theory: left-over material from formation of solar system. Jupiter prevents asteroids from coalescing (accumulating to form a planet). 2

3 Asteroid Orbits Locked into a 1:1 orbital resonance Main asteroid belt between orbits of Mars and Jupiter: AU (eccentricities: ) - includes the big four : Ceres (940 km in diameter), Pallas (580 Km), Juno, and Vesta (540 km, it is unique as it appears to have undergone volcanism) Apollo (if semimajor axis exceeds 1 A.U.) or Aten asteroids: cross Earth s orbit (eccentricities greater than 0.4) Amor asteroids: cross Mars orbit Trojan asteroids: in Jupiter s orbit at Lagrangian points (L 4 and L 5 ) Risk to Earth The asteroids whose paths may intersect Earth s orbit, leading to the possibility of a collision with our planet are collectively known as Earth-crossing asteroids (Apollo or Aten asteroids). As of early 2001, more than 1200 Earth-crossing asteroids are known. Apollo asteroids are 1 km in diameter or less. However, an asteroid with a diameter of 1 km can do a lot of damage! times more energy than all the Earth s nuclear weapons. - would cause a global dust storm, killing off much of the Earth s plant (and eventually animal) life. Since 1990, 2 asteroids have passed inside Moon s orbit! (~ 40 asteroids passed within 10 million Km of our planet) However, chances of striking Earth are relatively small (one every few million years). NASA currently has programs looking for near-earth asteroids. Comets Comets are usually discovered as faint, fuzzy patches of light on the sky that develop an extended tail as they reach near the Sun. They are generally small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains (dust) and frozen gases (e.g. dry ice). They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto. Hale-Bopp by John Laborde History of Comets Before 1600, comets were essentially considered to be heavenly omens. Thought to be atmospheric (meteorological phenomena) in medieval times. Now known to be solar system objects. In 1705, Sir Edmond Halley realized that the same comet appeared every 76 years. He calculated its orbit, and predicted its reappearance in Last appearance of Halley s Comet was in 1986 visited by a number of spacecraft e.g. Vega 2, Giotto etc. The Bayeux tapistry records the bad omen Halley's Comet Sir Edmond Halley ( ) 3

4 Sparse envelope released by breakup of molecules (such as H 2 0), not seen in visible light (millions of km in length) Structure of Comets Dust tail blown off the nucleus by Sun s radiation pressure; made up of dust particles; broad,diffuse and curved. Ion tail blown off by the Solar wind ( ionized particles from the Sun); made up of plasma; mostly straight. both tails can reach ~1 AU (10 8 km) Comets tails (a) comet Giacobini-Zinner (1959): coma - 70,000 km across; ion tail - 500,000 km long. (b) comet Hale-Bopp in At closest approach to the Sun, its tail stretched nearly 40 across the sky. Dense sphere of material evaporated when the comet reaches the inner Solar System (~100,000 km across) Relatively stable & solid dirty snowball, mixed dust/ice (H 2 0, methane, amonia, CO 2 ) (~10 km across) 4

5 Nucleus of Halley s Comet Spacecrafts visited: Vega 2 (Soviet craft ), Giotto (European craft) Nucleus: Irregular Size : 15 x 10 km Very dark several jets streaming out to form coma & tail Picture taken by Giotto spacecraft Speed: 60 km/s 5

6 Comet Orbits Comet orbits are highly elliptical (extend far beyond Pluto, even as far as 50,000 A.U.) and tend to be inclined relative to the ecliptic. But the tails are in all cases directed away from the Sun by the solar wind. The ion tails and dust tails differ in shape because of the different responses of gas and dust to the forces acting on them. Ion tail (Type I) pushed rapidly, straight back by Solar wind -straight tail Dust tail (Type II) gently pushed back by Solar radiation. Dust particles left behind are in longer orbits curved tail Origin of Comets Meteors, Meteoroids & Meteorites Meteor shooting star : is a sudden streak of light in the night sky. It caused by friction (or collision) between air molecules in Earth s atmosphere and an incoming piece of interplanetary matter, for example an asteroid, comet, or meteoroid. Oort cloud: long-period comets (hundred of thousands, some even million of years long orbit) all inclinations, all orientations, both pro & retrograde. (never come anywhere near Sun) - occasionally pulled into inner Solar System due to gravitational tug by nearby star. Kuiper belt: short-period (less than 200 years) prograde orbits lying close to the ecliptic. circular orbits between A.U. Meteoroids: are chunks of rocky interplanetary debris, having size less than 100 m in diameter (otherwise called asteroids). Meteorites: Any piece of debris that survives its fiery passage through our atmosphere and falls on the Earth is called a meteorite. For example Fragments from asteroids, Moon, Mars, etc. can survive if they are large enough. Meteorites are used to study the composition of the early Solar System 6

7 Meteors, Meteoroids & Meteorites Meteoroids Meteor Meteorite EETA79001 from Mars Stony meteorite Iron meteorite The world s second largest meteorite, the Ahnighito, on display at the American Museum of Natural History in New York (Corbis-Blair; J. Mandaville/Aramco Magazine) The Wabar meteorite, discovered in the Arabian desert. Although small fragments of the original meteor had been collected more than a century before, the 2000-kg main body was not found until Meteor shower Each time a comet rounds the Sun, some cometary material becomes dislodged (at the rate of about 30 tons per second if the comet is within an A.U. from the Sun), forming a meteoroid swarm. Over the course of time, the swarm gradually disperses along the orbit, and become more or less smoothly spread all the way around the parent comet s orbit Following the same path. These small objects are then called as the micrometeoroids. If Earth s orbit happens to intersect the orbit of such a young cluster of meteoroids, a spectacular meteor shower can result. Ha-ha Fireball very bright meteor 7

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