Chapter 9 Asteroids, Comets, and Dwarf Planets Their Nature, Orbits, and Impacts
Asteroid Facts Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1,000 km. There are 150,000 in catalogs, and probably over a million with diameter >1 km. Small asteroids are more common than large asteroids. All the asteroids in the solar system wouldn t add up to even a small terrestrial planet.
Asteroids with Moons Some large asteroids have their own moon. Asteroid Ida has a tiny moon named Dactyl.
Asteroid Orbits Most asteroids orbit in a belt between Mars and Jupiter. Trojan asteroids follow Jupiter s orbit. Orbits of near-earth asteroids cross Earth s orbit.
Which explanation for the asteroid belt seems the most plausible? 1. The belt is where all the asteroids happened to form. 2. The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. 3. The belt is where all the asteroids happened to survive.
Asteroids 1. Are rocky and small typically the size of a grain of rice or a marble 2. Are rocky, with a wide range of sizes, up to hundreds of miles in diameter 3. Have only thin atmospheres 4. Are made mostly of metals 5. Are mostly found in the inner solar system
If you keep pushing a person on a swing with little pushes, at just the right frequency, they will swing very high. This is an example of: 1. Newton s second law 2. Newton s first law 3. Energy conservation 4. Resonance 5. Conservation of angular momentum
Orbital Resonances Asteroids in orbital resonance with Jupiter experience periodic nudges. Eventually those nudges move asteroids out of resonant orbits, leaving gaps in the belt.
Orbits of asteroids in the asteroid belt 1. Often intersect planets 2. Are mostly between Mars and Jupiter 3. Are grouped into patterns by resonances with Jupiter 4. Are mostly inside the frost line 5. All except # 1
Meteorites come from 1. Stars they are falling stars 2. Destroyed planets 3. Asteroids 4. The Moon and Mars 5. Volcanic ejecta
A typical meteorite is 1. About the size of a house, and makes a crater when it lands 2. About the size of a pea or grain of rice and is invisible when it lands 3. About the size of a pea or grain of rice. It makes a bright streak in the sky and burns up 4. Made of ice
Meteor Terminology Meteorite: A rock from space that falls through Earth s atmosphere. Meteor: The bright trail left by a meteorite.
Meteorites from Moon and Mars A few meteorites arrive from the Moon and Mars Composition differs from the asteroid fragments A cheap (but slow) way to acquire moon rocks and Mars rocks
How do comets get their tails?
Comet Facts Formed beyond the frost line, comets are icy counterparts to asteroids. The nucleus of a comet is like a dirty snowball. Most comets do not have tails. Most comets remain perpetually frozen in the outer solar system. Only comets that enter the inner solar system grow tails.
Anatomy of a Comet Coma is atmosphere that comes from heated nucleus. Plasma tail is gas escaping from coma, pushed by solar wind. Dust tail is pushed by light.
Growth of Tail
If Earth passed through the tail of a comet, what would happen? 1. People would die from the gasses such as methane and ammonia 2. It depends on if it was the gas tail or the dust tail 3. Earth might be knocked out of its orbit or its axis might get tilted 4. Nothing. Halley s comet did this and nothing happened
What is plasma (in astronomy)? 1. An element commonly found in space 2. A constituent of blood 3. An ionized or charged gas, made when atoms lose one or more electrons 4. Another name for the solar wind
Why do comet tails always point away from the Sun? 1. They are left behind as the comet moves 2. Newton s third law: Comet goes one way, tail goes the other 3. The solar wind blows on them 4. They don t; this is just a perspective effect of how we view them
Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet s orbit.
How big can a comet be?
Is Pluto a Planet? Much smaller than the eight major planets Not a gas giant like the outer planets Has an icy composition like a comet Has a very elliptical, inclined orbit Pluto has more in common with comets than with the eight major planets.
Discovering Large Iceballs In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto. Eris even has a moon: Dysnomia.
Other Kuiper Belt Objects Most have been discovered very recently so little is known about them. NASA s New Horizons mission will study Pluto and a few other Kuiper Belt objects in a planned flyby.
Have we ever witnessed a major impact?
Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter.
This crater chain on Callisto probably came from another comet that tidal forces tore to pieces.
Dusty debris at an impact site
Artist s conception of SL9 impact
Several impact sites
Impact sites in infrared light
Did an impact kill the dinosaurs?
Mass Extinctions Fossil record shows occasional large dips in the diversity of species: mass extinctions. The most recent was 65 million years ago, ending the reign of the dinosaurs.
Iridium Layer No dinosaur fossils in upper rock layers Thin layer containing the rare element iridium Dinosaur fossils in lower rock layers
Consequences of an Impact A meteorite 10 km in size would send large amounts of debris into the atmosphere. Debris would reduce the amount of sunlight reaching Earth s surface. The resulting climate change may have caused mass extinction.
Likely Impact Site Geologists found a large subsurface crater about 65 million years old in Mexico.
Comet or asteroid about 10 km in diameter approaches Earth
Is the impact threat a real danger or just media hype?
Facts About Impacts Asteroids and comets have hit the Earth. A major impact is only a matter of time: not IF but WHEN. Major impacts are very rare. Extinction level events ~ millions of years Major damage ~ tens to hundreds of years
Tunguska, Siberia: June 30, 1908 A ~40 meter object disintegrated and exploded in the atmosphere
Meteor Crater, Arizona: 50,000 years ago (50 meter object)
Frequency of Impacts Small impacts happen almost daily. Impacts large enough to cause mass extinctions are many millions of years apart.
The Asteroid with Our Name on It We haven t seen it yet. Deflection is more probable with years of advance warning. Control is critical: breaking a big asteroid into a bunch of little asteroids is unlikely to help. We get less advance warning of a killer comet.
What are we doing about it? Stay tuned to http://impact.arc.nasa.gov
Surprising discovery?: A small asteroid that orbits within the asteroid belt has an active volcano. 1. Plausible. Several small objects in the solar system have active volcanoes (e.g. Io). 2. Plausible. Several asteroids are known to be composed of basaltic (lava) material. 3. Implausible. Only planets, not moons or asteroids, have volcanoes. 4. Implausible. Asteroids are too small to be geologically active now.
Surprising discovery?: A small asteroid that orbits within the asteroid belt has an active volcano. 1. Plausible. Several small objects in the solar system have active volcanoes (e.g. Io). 2. Plausible. Several asteroids are known to be composed of basaltic (lava) material. 3. Implausible. Only planets, not moons or asteroids, have volcanoes. 4. Implausible. Asteroids are too small to be geologically active now.