Asteroids and Meteorites
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Asteroids Small rocky bodies in orbit about the Sun Semantics Comets Small bodies that orbit the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail Meteoroids Small asteroids Meteorites the debris collected on Earth Meteors A brief flash of light (i.e. a shooting star)
Asteroids (starlike) 1. How were asteroids discovered? 2. What is their origin? 3. What do they look like? 4. What are near-earth asteroids? 5. What happens when an asteroid intersects Earth? Ceres, the largest asteroid in the asteroid belt
Recall Bode s Law As we discussed previously, Bode s law is mathematical representation of the semi-major axis of each planet Start with the simple sequence 0 3 6 12 24 48 96 192 384 Then Add 4 4 7 10 16 28 52 100 196 388 Then divide by 10 0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.2 38.8 Not a scientific law in the usual sense
Bodes Law prediction vs. actual (semi-major axis in AU) OBJECT PREDICTED ACTUAL Mercury 0.4 0.39 Venus 0.7 0.72 Earth 1.0 1.0 Mars 1.6 1.52??? 2.8??? Jupiter 5.2 5.2 Saturn 10 9.5 Uranus 19.6 19.2 Neptune 38.8 30.1 Pluto 78 39.6
Discovery of Ceres Giuseppe Piazzi searched for the missing planet Found Ceres
Discovery of Ceres Ceres disappeared behind the Sun and then reappeared just like a planet Ceres is about 2.8 AU from the Sun just as predicted by Bode s law However, Ceres looked like a star must be a very small planet! It turns out that Ceres is smaller than Pluto, but, by far, the largest of the asteroids in the asteroid belt
Soon after Ceres was discovered, other small planets were found All about 2.8 AU from the Sun New technique (ca. 1800 s): Invented by Max Wolfe long-exposure photography Look for asteroid trails on photos
Soon after Ceres was discovered, other small planets were found All about 2.8 AU from the Sun New technique (ca. 1800 s): Invented by Max Wolfe long-exposure photography Look for asteroid trails on photos Did the Missing Planet Even Exist?
Discovery of the Asteroid Belt Astronomers were finding many asteroids with the new technique Wolfe alone discovered over 200 Modern Technique CCD images at various times, have a computer look for differences Have most likely found all objects greater than 1km across Year # known 1800 0 1844 4 1890 287 1900 452 1979 2125 1988 4044 2000 >10,000
Searching for Asteroids and Comets IMAGE 1 IMAGE 2 Difference between 2 images highlights differences that may be asteroids/comets! IMAGE 1 IMAGE 2
Asteroid Names Many asteroids are discovered by amateurs The process can take decades After discovery, it is reported to the Smithsonian Astrophysical Observatory Minor Planet Center Given a temporary name (1980 JE) If the object is found at the same location at 4 successive oppositions, the discover gives it a name and it is assigned a number 1 Ceres 2 Pallas 3834 Zappafrank
Asteroids 1. How were asteroids discovered? 2. What is their origin? 3. What do they look like? 4. What are near-earth asteroids? 5. What happens when an asteroid intersects Earth? Ceres, the largest asteroid in the asteroid belt
There is no missing planet Origin of Asteroids Asteroids are not the remains of a long-destroyed large planet (not much combined mass) The combined mass of all asteroids in the asteroid belt is less than the mass of our Moon Asteroids are relics of planetesimals that failed to accrete into a full-sized planet effect of Jupiter
Jupiter s effect on the Asteroid Belt Numerical simulations indicate that the existence of Jupiter makes it unlikely that a planet would have formed in the asteroid belt If Jupiter were not there, it is likely that a fifth terrestrial planet would have formed Jupiter also depleted the asteroid belt removing any that got close to Jupiter (and were flung out into the solar system) Note: Asteroid belt doesn t look like movie asteroid belts, it is mostly empty space
Even today, gravitational perturbations by Jupiter deplete certain orbits within the asteroid belt The resulting gaps, called Kirkwood gaps, occur at simple fractions of Jupiter s orbital period Similar to gaps in planetary rings (but keep in mind eccentricity!) Kirkwood Gaps
Located at the Lagrange points of Jupiter Stable orbits, same period as Jupiter Named for Trojan war characters About 50 Trojan s are known, many more are suspected Trojan Asteroids Earth has no Trojans However we have spacecraft at Earth s Lagrange points (ACE/SOHO/Genesis and others)
Asteroids 1. How were asteroids discovered? 2. What is their origin? 3. What do they look like? 4. What are near-earth asteroids? 5. What happens when an asteroid intersects Earth? Ceres, the largest asteroid in the asteroid belt
Asteroids Physical Structure Size and Shape All are smaller than 1000 km Ceres is the largest with a diameter of 934 km (more than 2 times smaller than Pluto) Only the largest are round Uniform brightness Most are irregular shaped Elongated asteroids vary in brightness as seen from Earth as they rotate and present varying amount of cross-sectional area Gaspra Some have moons
Physical Structure: Composition Some are differentiated Iron cores? Density of some are consistent with them being made of rocky silicates Dark (low albedos) Carbon rich? Others have very low densities indicating that they are probably rubble piles Ida and its moon Dactyl
Some Asteroids are Rubble Piles A survey of asteroid rotation rates show that many rotate quite slowly If an asteroid is a collection of smaller chunks (i.e. a rubble pile), it would not be able to rotate very quickly without flying apart Spherical (or really small!) asteroids rotate more rapidly They are solid
Asteroid Collisions Some asteroids have huge craters, like 253 Mathilde shown here Being a rubble pile, it more-easily absorbs energy from the collision and remains intact
Asteroids 1. How were asteroids discovered? 2. What is their origin? 3. What do they look like? 4. What are near-earth asteroids? 5. What happens when an asteroid intersects Earth? Ceres, the largest asteroid in the asteroid belt
Near-Earth Asteroids Asteroids that are within the orbit of Mars are called Near-Earth Objects (NEOs) 2500 are currently known Fairly recently, it was thought that Asteroid 2004 MN4 had a 1 in 60 chance of hitting Earth in 2029 An impact has now been ruled out It is about a 400-meter sized asteroid (would cause a lot of damage) The larger asteroid 1950 DA (1-km) has a 1:300 chance of striking Earth in 2880 Image by Jim Scotti of LPL s Spacewatch program
Possibility of an Impact Calculating asteroid trajectories, precisely, can be tricky Need a detailed mapping of the Sun s gravitational field Need a better understanding of the characteristics of the asteroid (rotation, orbit, shape, etc.) Yarkovsky effect Diurnal and seasonal variations in the thermal radiation emitted by the asteroid can act as a miniature propulsion system Radar image of 1950 DA
NEO s and Impacts with Earth 30-meter sized asteroids come close to Earth about every 2 years They strike Earth every 6000 years or so A recent close call There was an explosion over the Mediterranean on June 6, 2002 (a 10-meter sized asteroid) Have probably found all objects larger than 1km, so probably no global effects without advance warning
Asteroids 1. How were asteroids discovered? 2. What is their origin? 3. What do they look like? 4. What are near-earth asteroids? 5. What happens when an asteroid intersects Earth? Ceres, the largest asteroid in the asteroid belt
Collision Course Burns up in atmosphere (outer layers vaporize, very little heating of rock occurs) If large enough it slows down to terminal velocity, then falls to Earth as a meteorite If larger still, will strike the surface with a tremendous amount of kinetic energy creating an explosion This is what created Berringer Meteor Crater, and flung ejecta out to a distance of 1-2 km. Events like this occur ~6000 yrs
Meteor
Collision Course Burns up in atmosphere (outer layers vaporize, very little heating of rock occurs) If large enough it slows down to terminal velocity, then falls to Earth as a meteorite If larger still, will strike the surface with a tremendous amount of kinetic energy creating an explosion This is what created Berringer Meteor Crater, and flung ejecta out to a distance of 1-2 km. Events like this occur ~6000 yrs
Impact with surface
Ejecta Blanket
Ejecta Blanket
Ejecta Blanket
Ejecta Blanket
Collision Course Burns up in atmosphere (outer layers vaporize, very little heating of rock occurs) If large enough it slows down to terminal velocity, then falls to Earth as a meteorite If larger still, will strike the surface with a tremendous amount of kinetic energy creating an explosion This is what created Berringer Meteor Crater, and flung ejecta out to a distance of 1-2 km. Events like this occur ~6000 yrs
Meteorites
Meteorites All meteorites were broken off from larger objects and have fallen to Earth from space, but not all are from asteroids. Some are from the Moon and Mars (but not many). No Earth meteorites have been recognized. Falls are observed to land, finds are just found lying on ground 3 main categories Stony Chondrites (contain chondrules) Achondrite Stony-Iron Iron Many other sub-classes of meteorites
Stony Most abundant meteorite falls, but only about 10% of collection Hard to distinguish from Earth rocks, erode quickly in Earth s atmosphere Fusion Crust can make identification easier Some of them are the oldest rocks ever found (4.56 Ga) This is one way we get the age of the solar system One class has almost an identical composition to that of the Sun (outside of H and He) Building blocks of the planets
Stony-Iron and Iron Stony-Iron rarest in both fall and in collections Iron not very abundant fall, most abundant in collection Easy to find Made of metal (uncommon on Earth s surface) Only source of metal to humans before 2000BC Take longer to erode
Meteorite/Asteroid Connection Undifferentiated asteroids Chondrites Differentiated asteroids Achondrites (mantle) Stony-Iron (mantlecore boundary) Iron (core)
Widmanstätten pattern in Iron Meteorites Provides a conclusive test that the material is indeed meteoritic The pattern is produced as the iron-nickel material is cooled slowly (millions of years) like the core of an asteroid parent body that remains molten for a long time after its formation
most abundant type of stony meteorite Chondrites very primitive chemistry not been modified by melting or differentiation of a parent body Part of bodies that accreted right at the time the solar system formed Most contain chondrules (small spherical-shaped glassy-like objects embedded within the meteorite) They can also contain some of the material that existed prior to the formation of the Sun
Carbonaceous chondrites A small class of chondritic meteorites that contain high levels of water and organic compounds The presence of volatile elements and water indicate that the object was not heated significantly Material straight from the original solar-nebula