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ASTA01 @ UTSC – Lecture 13. Chapter 12 The Origin of the Solar System Asteroids Comets Meteors and meteoroids The story of planet formation: ages. Comets.
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ASTA01 @ UTSC – Lecture 13 • Chapter 12The Origin of the Solar System • Asteroids • Comets • Meteors and meteoroids • The story of planet formation: ages
Comets • In contrast to the rocky asteroids, the brightest comets are impressively beautiful objects. Most comets are faint and are difficult to locate even at their brightest. • But in 2013 a very bright comet is expected (mV= -14m !)
Comets • A comet may take months to sweep through the inner solar system. • During this time, it appears as a glowing head with an extended tail of gas and dust.
Comet Hartley 2, visited in Nov. 2010 by EPOXI • The beautiful tail of a comet can be longer than 1 AU. • However, it is produced by an icy nucleus only a few tens of kilometres in diameter. Nucleus is covered with a dark crust, which breaks at places allowing gas (H2O +CO+…) + dust + sand + stones, to escape in jets.
Comets: Halley’s comet The nucleus remains frozen and inactive while it is far from the Sun. • As the nucleus moves along its elliptical orbit into the inner solar system, the Sun’s heat begins to vapourize the ices, releasing gas and dust.
Comets • The pressure of sunlight and the solar wind push the gas and dust away, forming a long tail. Peter Apian’s drawing from 1532 shows that the tail always points away from the sun. (He drew comet Halley.)
Comets • The gas and dust respond differently to the forces acting on them. • So, they often separate into two separate sub-tails. Comet Hale-Bopp in 1995
Comets • Comet nuclei contain ices of water and other volatile compounds such as carbon dioxide, methane, and ammonia. • These ices are the kinds of compounds that should have condensed from the outer solar nebula. The comets never fully melted. • That makes astronomers think that comets are ancient samples of the gases and dust from which the outer planets formed.
Comets • Five spacecraft flew past the nucleus of Comet Halley when it visited the inner solar system in 1985 and 1986. • Since then, spacecraft have visited the nuclei of several other comets. • Images show that comet nuclei are irregular in shape and very dark, with jets of gas and dust spewing from active regions on the nuclei.
Comets • In general, crusts of these nuclei are darker than a lump of coal. • This suggests that they have composition similar to certain dark, water- and carbon-rich meteorites.
Comets • Since 1992, astronomers have discovered roughly a thousand small, dark, icy bodies orbiting in the outer fringes of the solar system beyond Neptune.
Comets • This collection of objects is called the Kuiper belt. • It is named after the Dutch-American astronomer Gerard Kuiper, who predicted their existence in the 1950s.
Comets • There are probably 100 million bodies larger than 1 km in the Kuiper belt. • Any successful theory should explain how they came to be where they are. • We will return to them later in this course
Comets • Astronomers believe that some comets,those with the shortest orbital periodsand orbits in the plane of thesolar system, come from the Kuiper belt.
Comets • The longer-period comets do not always orbit in the plane of the solar system, but can approach the Sun from random directions. • They are believed to originate from the Oort cloud, a roughly spherical cloud of comets that lies much farther away at a distance of almost one light-year from the Sun. • The solar nebula theory can explain these two different sources of comets.
Comets • American astronomer Carolyn Shoemaker holds the record for being the most successful comet hunter alive, although, remarkably, she only took up astronomy at age 51. • Canadian amateur astronomer David Levy is also one of the most successful comet discoverers in history.
Comet SL9 • Carolyn and Eugene Shoemaker, David Levy, and Philippe Bendjoya were co-discoverers of Comet Shoemaker–Levy 9, which flew by Jupiter, disintegrated under its tidal force in 1992, and in 1994 collided with Jupiter
Meteoroids, Meteors, and Meteorites • Unlike the stately comets, meteors flash across the sky in momentary streaks of light. • They are commonly called “shooting stars.”
Meteoroids, Meteors, and Meteorites • They are not stars but small bits of rock and metal falling fast into Earth’s atmosphere. They burst into incandescent vapour about 80 km above the ground because of friction with the air. • This hot vapour condenses to form dust, which settles slowly to the ground, adding ~40 000 tons/yr of mass. 3 different terms: • Meteoroid – in space • Meteor – in the atmosphere • Meteorite – on the ground
Large Impacts of meteoroids: Tunguska event 1908 A 60 m body hit over Siberia and disintegrated 7-8 km above almost unpopulated Taiga forest on June 30, 1908. No meteorite found on the ground near river Podkamennaya Tunguska • Flattened area 2000 km2 • 80 mln trees • Shock waves, heat • Isotopes 14N, 13C recently found, • typical of asteroids prof. L. Kulik's 1927 expedition photo
Large Impacts of meteoroids: Tunguska event 1908 Tunguska catastrophe – originally thought due to a small comet, however, both comets & asteroids can provide the required energy: 1986 • E = 5-30 Mton TNT (equiv.) • ~1000 Hiroshima bombs • Could power Canada for a month • Energy was kinetic, E~mV2 • If V=30 km/s (Earth’s speed)… • ..then 1kg of ice = 107 kg TNT • Tunguska-type events happen every ~103 yr, while smaller impacts more often (E=1 kt TNT, 1-10 per yr) 1986
Large Impacts of meteoroids Tunguska event was caused by a smaller asteroid of a type similar to a stony asteroid Itokawa 60 m (Tunguska asteroid) 500 x 300 x 250 m (Itokawa)
Are solar system debris dangerous? • If Tunguska body had come 5 hrs later on June 30, 1908, then it would have killed 1 mln people in St. Petersburg, Russia • Still, chances of hitting populated areas are very small • SpaceWatch – Arizona initiative to track near-Earth asteroids (NEA) • On June 14, 2012 a 500m NEA ZL1 passed relatively close, but not dangerously close to Earth
