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Transcript
Vagabonds of the Solar System – Asteroids – Comets – Meteoroids, Meteors, Meteorites Asteroids • As solar system was forming, matter with too much angular momentum to fall into the sun coalesced at varying distances from the sun into planetesimals. Asteroids • Many collided, forming planets and larger moons. • Others were captured whole by planets as small, irregularly shaped moons • Many still orbit the Sun. These are asteroids. Discovery of asteroids Part 1 • Ceres was the first asteroid discovered on New Year’s Day in 1801. – It was discovered because it changed position every night – Ceres is huge. – Ceres accounts for 30% or the mass of all known asteroids • Pallas was the next asteroid discovered 1803 – It was discovered because it changed position every night • Only two more, Juno and Vesta, were found until mid 1800’s Discovery of asteroids Part 2 • By mid 1800’s, improved telescopes were available – Most asteroids are very small compared to Ceres – New telescopes made it possible to see smaller asteroids – Most are <1km across • About 300 asteroids found lying between orbits of Mars and Jupiter • This region is called the asteroid belt; the asteroids are called belt asteroids Discovery of asteroids Part 3 • Applied photographic technique in 1891 introduced by Max Wolf • Take long exposures and look for tracks made by asteroids • Wolf discovered 228 asteroids this way Discovery of asteroids Part 4 • Improved digital image processing • Don’t waste telescope time on long exposures • Take multiple images and subtract – LINEAR – NEAT LINEAR (Lincoln Near Earth Asteroid Research) • The GTS-2 telescope is a 1 meter folded prime focus Cassegrain design identical to that of the Ground-based Electro-Optical Deep Space Surveillance (GEODSS) telescope used by the Air Force for space surveillance. It is located at the Experimental Test Site on White Sands Missile Base in NM • Take several images of same view each night & subtract to detect new objects NEAT (Near Earth Asteroid Tracking) • NEAT detects moving objects asteroids and comets - by observing the same part of the sky 3 times during an interval of about 1 hour. The automatic data analysis system searches for moving objects by comparing the 3 images. • The NEAT system is now mounted on the Maui Space Surveillance Site (MSSS) 1.2-m GEODSS telescope (same as LINEAR). Discovery of asteroids Part 5 • Estimate there are >1,000,000,000 asteroids in the solar system Origin of asteroids • Most likely the force of Jupiter on the planetesimals kept them from coalescing into one object • Less likely that they had been one object and were split into many – If all of the asteroids in the asteroid belt were put together to form a planet, it would be very small, smaller than Pluto. But some may be fragments of larger asteroids • Some may break apart but not have enough speed to escape each others gravitational attraction, so reassemble • Some large fragments end up orbiting near each other or even in contact 4 asteroids have been imaged at close range • The Galileo spacecraft (while on its way to Jupiter) imaged Gaspra and Ida. • Gaspra and Ida have different amounts of cratering, so may have been broken apart from their parent asteroids at different times Ida Gaspra 4 asteroids have been imaged at close range • The NEAR (Near Earth Asteroid Rendezvous) mission did a flyby of Mathilde and orbited Eros. Eros Mathilde NEAR landed on Eros • The touchdown speed of less than 4 miles per hour was one of the softest planetary landings ever. • Team members then commanded craft's gamma-ray spectrometer to gather data on the elemental composition on and just below the asteroid's surface. Eros landing site Some asteroids have satellites • At least 2 asteroids have moons Dactyl Ida and Dactyl Asteroids outside the asteroid belt • Some have highly elliptical orbits that bring them inside the orbits of some planets – Apollo asteroids cross Earth’s orbit • There are about 300 known Earth-crossing asteroids – Amor asteroids cross Mars’ orbit – Some asteroid orbits extend beyond Pluto’s orbit Asteroids outside the asteroid belt • Some are located at stable Lagrange points – There are ~460 asteroids in Jupiter’s Lagrange points, called Trojan asteroids – An asteroid has also been discovered at one of the Earth’s Lagrange points L4 and L5 are stable L1, L2, and L3 are unstable Comets • Comets formed near Uranus and Neptune, where water was plentiful and the temperature was low enough for ice to condense with roughly equal amounts of rocky and metallic material into bodies that still orbit the sun • Gravitational forces from Uranus and Neptune flung the comets in every direction Kuiper belt • Kuiper belt – Centered on the ecliptic extending beyond the orbit of Pluto. – Largest comet is 1/5 size of Pluto – ~200,000,000 comets Oort cloud • Oort cloud – NOT centered on the ecliptic but is a spherical distribution around the Sun extending about about 50,000AU. 1/5 of the distance to the nearest star – Can even have orbits perpendicular to the ecliptic – ~Several billion comets – A few pass through the inner solar system as Hale-Bopp and Hyakutake Comets • Structure – Because the Kuiper belt and the Oort clouds are far from the Sun, comets are completely frozen. – As a comet approaches the sun, some of the ice vaporizes forming an atmosphere around the nucleus called the coma. – The comet nucleus is only 10km across, but the coma can extend a million km across – The hydrogen envelope surrounds the nucleus Comet tails • Tail – The tail develops from coma gases that are pushed outward by the solar wind – This is why the solar wind was initially predicted – The existence of the solar wind was verified by Mariner 2 Comet tails • Two types of tails – Gas (or ion) tail • Positively charged ions are pushed away from the Sun by the solar wind • Always points away from the Sun – Dust Tail • Formed when photons strike dust particles that have been freed from the comet’s evaporating nucleus • These particles are massive enough not to flow straight away from the sun, so lie between the gas tail and the direction of the comet’s motion Comet tails Hale-Bopp Compositon of comets • Stardust – The goal of the Stardust mission is to return both particle samples from a comet and interstellar dust. By returning these samples to Earth for analysis a great deal is expected to be learned about the composition of the early universe. Comet orbits • Long period comets – Most move so fast that they leave the inner solar system after one pass by the Sun and take millions of years to return • Short period comets – Some pass near a planet which changes their orbit, slows it, and traps it in an orbit in the inner solar system. – These comets may then have orbits of a few hundred years or less (as Halley) Comet lifetime • Comets lose mass ( about 1/60 - 1/100 of its mass) every time they pass the Sun • After its ices have all evaporated, the remaining dust and rock spread out in a loose collection the continues to circle the Sun along the comet’s original orbit – When the earth passes through this collection, we see a meteor shower – ~30 meteor showers can be seen each year Comet lifetime • A comet can be torn apart if it comes too close to a planet, like Comet Shoemaker-Levy 9 which fragmented in the tidal force from Jupiter Comet lifetime • Comet LINEAR Breaks Up Meteoroids, Meteors, Meteorites • Meteoroids – Rocky and metallic debris smaller than asteroids (10’s of meters across to microscopic) scattered throughout the solar system • Some are broken off from asteroids of from planets, but some were never part of a larger body • Meteors – When a meteoroid is pulled by Earth’s gravity into Earth’s atmosphere, air friction creates so much heat that the outer layer vaporizes • Common names are shooting stars, bolides, and fireballs • Meteorites – Meteors that reach the ground before completely vaporizing Impact craters • Any meteor that survives passage through the atmosphere may leave an impact crater a comet back to Earth – Barringer (or Meteor) Crater in Arizona which formed about 50,000 years ago – Tunguska mystery – Allende meteorite – Alvarez discovery 95 Worlds and Counting • Film • Tour of the satellites of planets in our solar system Vagabonds of the Solar System • On the morning of his departure he put his planet in perfect order. He carefully cleaned out his active volcanoes. He possessed two active volcanoes; and they were very convenient for heating his breakfast in the morning. He also had one volcano that was extinct. But, as he said, "One never knows!" So he cleaned out the extinct volcano, too. If they are well cleaned out, volcanoes burn slowly and steadily, without any eruptions. Volcanic eruptions are like fires in a chimney. The Little Prince