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Asteroids

Asteroids. The orbits of most of the asteroids lie between those of Mars and Jupiter Asteroid belt More than 10,000 asteroids have well-determined orbits Asteroids 2410 and 4859 are named for the two of the authors of our book, Morrison and Fraknoi

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Asteroids

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  1. Asteroids • The orbits of most of the asteroids lie between those of Mars and Jupiter • Asteroid belt • More than 10,000 asteroids have well-determined orbits • Asteroids 2410 and 4859 are named for the two of the authors of our book, Morrison and Fraknoi • There are about a million asteroids with a diameter greater than 1 km • The largest asteroid is Ceres and was the first to be discovered in 1801 • Diameter just under 1000 km • The total mass of the asteroids sums to about the mass of the Moon • Many are probably missing from the original distribution Lecture 14

  2. The Asteroid Belt Trojans Mars Jupiter • The asteroids all orbit the Sun in the same direction as the planets • The asteroid belt contains orbits with semimajor axes between 2.2 and 3.3 AU • The asteroids are not particularly close together • Typical spacing is millions of km • The asteroids seem to group into families that have similar physical characteristics and probably resulted from collisions between asteroid Lecture 14

  3. Composition and Classification • Asteroids are not all alike • Some are very dark like a lump of coal • Reflectivity = 3% • C class, most numerous • Carbonaceous or carbon-rich • Some reflect like the Moon • Reflectivity = 20% • S class, second most populous • Stony composition • Some are very bright • Reflectivity = 60% • M class, much less numerous • Metal The Largest Asteroids Lecture 14

  4. Where Different Asteroids are Found Orbit of Mars Orbit of Jupiter • The different classes of asteroids are grouped together at different distances from the Sun • Apparently the asteroids are still located near their birthplaces Lecture 14

  5. Vesta, A Volcanic Asteroid • Vesta is a very unusual asteroid • Much brighter than other main belt objects • Surface is covered with basalt • Indicates volcanism in spite of its small size • Some meteorites have been found with compositions similar to Vesta • 4.4 to 4.5 billion years old • Whatever process created Vesta was early and short lived • Hubble found a crater on Vesta deep enough to expose the mantle Lecture 14

  6. Asteroids Up Close • To get to Jupiter, Galileo had to traverse the asteroid belt • Galileo has close encounters with two main-belt asteroids, Gaspra and Ida • Gaspra is and S-type asteroid 16 km long • Cratering suggests it is 200 million years old • Ida is a larger S-type asteroid 56 km in length • Cratering shows it is 1 billion years old • Ida has a satellite, Dactyl, whose orbit was used to calculate the mass and hence the density of Ida • 2.5 g/cm3, similar to primitive rocks Lecture 14

  7. Portraits of Asteroids Gaspra Ida Galileo images of the small main-belt asteroid, Gaspra. The dimensions of Gaspra are 16 x 11 x 10 km. Asteroid Ida from Galileo images. Ida is 56 km in length. Lecture 14

  8. As Close as it Gets Picture taken by NEAR Shoemaker at 120 m. The vertical lines at the bottom resulted when the spacecraft struck the surface while transmitting the picture Picture taken by NEAR Shoemaker at 170 m. • One February 12, 2001 the NEAR (Near Earth Asteroid Rendezvous) Shoemaker spacecraft landed on the surface of the asteroid Eros Lecture 14

  9. The Moons of Mars • The moons of Mars, Deimos and Phobos, are thought to be captured asteroids Deimos Phobos Lecture 14

  10. The Trojans • The Trojan asteroids are located far beyond the main belt at about the same distance as Jupiter • The Trojan asteroids are dark and sizable • There is one group ahead and one group behind Jupiter Lecture 14

  11. Asteroid in the Outer Solar System • Asteroids exist with orbits that carry them far outside the orbit of Jupiter • Chiron is one such asteroid • Diameter of 200 km • During closest approach to the Sun, brightened by a factor of 2 • Pholus is another such asteroid • Ventures out past the orbit of Neptune • Is the reddest object ever observed Lecture 14

  12. Earth Approaching Asteroids • In 1989, a 200 m object passed with 800,000 km of Earth and in 1994 a 10 m object passed with 105,000 km of Earth • About 500 NEOs (near earth objects) are known • The orbits of NEO are unstable • Will collide with terrestrial planet • Will be ejected from the solar system • We are naturally interested in NEOs since an encounter with one would be unpleasant Lecture 14

  13. Comets • A comet is a relatively small chuck of icy material that develops an atmosphere as it approaches the Sun • Comets can develop tails • Comets move with respect to the background stars but are much more unpredictable than planets • Comets are the best preserved, most primitive material available in the solar system • May provide unique access to the material that formed the planets 4.5 billion years ago • Comets spend most of their lives very far away from the Sun where it is very cold Lecture 14

  14. The Orbits of Comets • Newton recognized that the orbits of comets were highly eccentric • Edmund Halley published calculations in 1705 for the orbits of 24 comets and predicted that a particular comet would return in 1758 • It did and was named Halley’s Comet • Halley’s Comet last appeared in 1986 and was studied by several spacecraft Lecture 14

  15. The Comet’s Structure • When we see a comet, we see its temporary atmosphere of gas and dust • This material comes from the nucleus of the comet • The comet has • Nucleus (1-10 km) • Coma (105 km) • Hydrogen envelope (107 km) • Ion tail (directly away from the Sun) • Dust tail (away from comet’s motion) Lecture 14

  16. Location and Origin of Comets • Most comets exist in the Oort cloud • Huge spherical cloud surrounding the solar system • Extends out to 50,000 AU • The gravitational sphere of influence of the Sun • Orbits are stable • Occasionally a comet will be perturbed and enter the solar system • Only then is a comet visble • About 1013 comets may exist, 1000 times the mass of the Earth • Comets also are found in the Kuiper cloud • Flattened disk just outside the orbit of Pluto Lecture 14

  17. The Fate of Comets • Comets spend nearly their entire life in the Oort cloud at a temperature near absolute zero • It a comet, enters the inner solar system then several things can happen • May survive passing the Sun and return to the Oort cloud • May hit the Sun or come so close that it is vaporized • May interact with a planet • Impact the planet • Get speeded up and ejected from the solar system • Perturbed into an orbit with a shorter period • Comet will rather quickly end its life Lecture 14

  18. Comet Shoemaker-Levy 9 Comet Shoemaker-Levy 9 broken up into 21 pieces photographed by Hubble Hubble photo showing the impact of fragment G Lecture 14

  19. Meteors • Meteors are the result of solid particles entering the Earth’s atmosphere from space • These particles vaporize in the atmosphere at heights of 80 to 130 km • The typical bright meteor is produced by a particle with a mass less than 1 gram • No larger than a pea • If a particle the size of a golf hits the atmpshere, a much brighter trail is created • Fireball • If a bowling ball size object hits the atmosphere, it has a good chance of reaching the ground Lecture 14

  20. Meteor Showers • Most of the meteors that strike the Earth can be associated with specific comets • Some visible some not visible • A meteor shower consists of passing through the debris of a comet • These meteor showers seem to come from one spot in the sky • Radiant • Meteor showers are often designated by the constellation they seem to come from Lecture 14

  21. Major Annual Meteor Showers Lecture 14

  22. Nature of Meteor Showers • No shower meteor has ever reached the surface • From the flight paths, one can deduce that the particles are very light or porous • Comet dust is apparently fluffy, inconsequential stuff • The most reliable meteor shower is the Perseid shower (comes from the Perseus constellation on August 11) • One can estimate that total mass of of the particles in the Perseid swarm is nearly a billion tons from the Swift-Tuttle comet • Comet Swift-Tuttle was last seen in 1992 and is predicted to return in 2126 and will have a close pass with Earth Lecture 14

  23. Meteorites • A meteor that survives its fall through the atmosphere is called a meteorite • Hundreds fall on the Earth every year • Meteorites do not come from comets • First documented case in modern times was recorded in 1803 • Meteorites are discovered in two ways • Observed meteorite falls • Meteorite finds • About 25 per year are found • Antarctica is a fertile ground for finding meteorites • Ice cap collects over a large area and preserves the meteorites Lecture 14

  24. Meteorite Classification • Traditionally meteorites have been placed into three broad classes • Irons • Nearly pure nickel-iron • Stones • Silicate or rocky • Stony-irons • Mixture of stone and metallic iron Lecture 14

  25. Ages and Compositions of Meteorites • Meteorites include the oldest and most primitive materials available for direct study • Using radioactive dating, the average age of meteorites is between 4.54 ± 0.1 billion years • Usually taken as the age of the solar system (4.5 billions years) • Meteorites almost certainly originate from asteroids • Two famous meteorites (both fell in 1969) • Murchison (Australia) • Carbonaceous. Contained complex organic molecules, amino acids • Allende (Mexico) • Contained material older than the solar system • Material formed by previous generations of stars Lecture 14

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