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Asteroids are small, rocky bodies that orbit the Sun •Most are smaller than 300 km in diameter •Asteroids have eccentric orbits and spend the majority of their ...
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•Interplanetary debris Leftover from the formation of the Solar System Ranges from large asteroids and comets to microscopic dust Rocky material resembles the outer layers of the terrestrial planets
•Interplanetary debris The total mass of the debris is less than that of the Moon These objects are nearly unchanged since the formation of the Solar System
•Comets fall into two major groups, depending on the period of their orbits (short-period comets and long-period comets)
•Comets are made of ice and rock, similar to the moons of the outer planets – they are also composed of ancient material
•The first asteroid discovered was Ceres, detected by Giuseppe Piazzi in 1801
•The orbit of Ceres has semi-major axis of 2.8 AU
•Within a few years, Pallas, Juno, and Vesta were discovered
•Several hundred thousand asteroids have been discovered
•Several thousand new discoveries are made each year
•The largest asteroids are:
Name Diameter Semi-major axis Eccentricity Ceres 940 km 2.8 AU 0. Pallas 580 km 2.8 AU 0. Vesta 525 km 2.4 AU 0. Juno 240 km 2.7 AU 0.
•We are currently aware of over 4,000 asteroids with determined orbits
•Most asteroids spend all their time between Mars and Jupiter in the Asteroid Belt, between 1.5 – 3.5 AU from the Sun
•These large asteroids have relatively small eccentricities
•NASA maintains an “Asteroid Fact Sheet” for selected objects:
http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html
•Asteroids that cross the orbit of Mars are called Amor Asteroids
•Asteroids that cross the orbit of Earth are called Apollo Asteroids
•There are five locations in the solar system, called Lagrange points, where an asteroid can orbit in equilibrium with the Sun and Jupiter
1
•Asteroids located at L 1 , L 2 , or L 3 would drift away…
•Asteroid surfaces display evidence for complex reprocessing •Vesta is rich in pyroxine, which is commonly observed in lava flows on Earth •Many meteorites have the same chemical composition, indicating that they are “chips” of Vesta
•Many asteroids, with eccentricities larger than about 0.4, cross the orbit of Earth – these are the Apollo Asteroids
•The minimum and maximum distances from the Sun are given by
D perihelion = (1 - e)a
D aphelion = (1 +e)a
•For the asteroid to cross Earth’s orbit, we must have
Earth’s orbit
•Most Apollo Asteroids are 1-10 km in size
•The Apollo Asteroids are the most dangerous for life on Earth due to the possibility of catastrophic impacts
•A collision with Earth would devastate an area 100 km in diameter
•The explosion would be equivalent to 1,000,000 1-megaton nuclear bombs
•Large asteroids strike the Earth every few hundred thousand years
•We are currently aware of almost 2000 Apollo Asteroids
•Harvard maintains a list of all known Apollo Asteroids:
http://cfa-www.harvard.edu/iau/lists/Apollosq.html
•In 1990 a smaller explosion was detected over the Pacific •In 1972, a 1000-ton object skipped off the atmosphere over Wyoming – and was photographed by tourists •This would have caused a 10-kiloton explosion in Canada if it had hit the Earth •A 100-meter iron asteroid hit Arizona about 20,000 years ago, leaving the “Arizona Meteor Crater” •A 10-km asteroid probably struck Earth about 65 million years ago, wiping out the dinosaurs •This was a typical-size Apollo Asteroid
Name Year Distance 2002 NY40 2002 150,000 km 2003 DW10 2003 543,000 km 2003 HW10 2003 331,000 km 2003 SW130 2003 162,000 km 2003 SQ222 2003 84,555 km
•The closest approach tends to occur in the year the object is discovered
•This is because the objects are difficult to detect until they are very close to the Earth
•Hence we may have little or no warning about a collision with a killer asteroid
•We can compute the surface acceleration on a planet or moon using Newton’s laws of motion and gravitation:
2 2 eros
eros eros 0.^001 ms R
− •We find that = =
http://www.johnstonsarchive.net/astro/asteroidmoons.html
•Despite their low gravity, some asteroids have there own moons!
2 2 earth
earth earth 9.^8 ms R
− = =
•Using values for the Earth, Moon, Mercury, and Jupiter, we obtain for the surface accelerations
2 2 moon
moon moon 1.^7 ms R
GM A =− = −
2 2 eros
eros eros 0.^001 ms R
− = =
2 2 mercury
mercury
•The acceleration at the surface of Eros is 10,000 times smaller than at the Earth’s surface!!!
•Comets show extreme variation in their distance from the Sun during each orbit
•The minimum and maximum distances from the Sun are given by
D perihelion = (1 - e)a
D aphelion = (1 +e)a
•For Halley’s Comet, we find that
•The large variation in solar distance leads to extreme changes in the amount of solar heating experienced by the comet
(^23)
years AU
⎟ ⎠
⎞ ⎜ ⎝
⎛ ⎟⎟^ = ⎠
⎞ ⎜⎜ ⎝
⎛ P a
years
•This is a relatively short period for a comet
3 / 2
years AU
⎟ ⎠
⎞ ⎜ ⎝
⎛ ⎟⎟^ = ⎠
⎞ ⎜⎜ ⎝
⎛ P a
•Kepler’s second law (equal areas in equal times) implies that comets move very quickly when near the Sun, and slowly when far away
•Hence, Comets spend most of their time moving very slowly, far from the Sun
Red dots indicate the location of the comet at equally spaced time intervals
•In 1708, Edmund Halley predicted a reappearance of a bright comet in 1758, after a previous visit in 1682
•The comet reappeared as predicted in 1758, making Halley famous! (unfortunately, Halley was dead by then)
•Once the period of 76 years was known, historical records were searched to check for previous visits
