Download Comets and asteroids

Comets and asteroids (Chapter 12)
I.
Asteroids - rocky or metallic objects orbiting the Sun; smaller than a planet and show no
evidence of an atmosphere or of other types of activity.
 Mostly found in the broad space between Mars and Jupiter
 Too small to be seen without a telescope
 First of them discovered at the beginning of 19 th century
 First discovered asteroid - Ceres - at 2.8 AU from Sun - 1801 - Giovanni Piazzi
 By 1890 - about 300 asteroids discovered
 Now - more than 20 000 asteroids with well determined orbits are known
 We don’t see all asteroids - some of them are very small to be seen even with large
telescope, but we can estimate the total number - 1 million with diameter larger than
1 km
 Ceres (diameter 1000 km), Pallas and Vesta (diameter 500 km), 15 more are with
diameter larger than 250 km
 The total mass of all 1 million asteroids is smaller than the mass of the Moon
 Orbital characteristics of the asteroids
 Revolve about the Sun in the same west-to-east direction as the planets
 Plane of the orbits
 Asteroid belt - the region where all asteroids with distance from Sun in the
range 2.2 to 3.3 AU are found
 Orbital periods - 3.3 to 6 yrs
 75 % of the known asteroids are in the belt
 not closely spaced - typical spacing between objects - 5 million km
 groups of asteroids with similar orbital characteristics can be separated probably formed together
 Composition and Classification
 Asteroids with low reflectivity (3-4%) - look very dark - old, composed of
silicates mixed with carbon (C-asteroids) - almost all objects in the outer
third of the belt (Ceres and Pallas)
 Asteroids with moderate reflectivity (15-20 %) - stony (silicate
composition) - S-asteroids
 Asteroids with high reflectivity (60%) - look bright - composed primary of
metal - M-asteroids - not very large group
 Different classes of asteroids are grouped together at different distances
from Sun
 Asteroids Far and Near - special groups of asteroids that stray outside the belt's
boundaries
 The Trojans - located far beyond the main belt, orbiting the Sun at 5.2 AU this is at the distance similar to the orbit of Jupiter
 The gravitational force from Jupiter makes most orbits of asteroids near it
unstable.
 There are 2 points in the orbit of Jupiter near which an asteroid can remain
almost indefinitely - two concentrations of asteroids around these points eastern group and western group.
 Trojans are dark primitive objects like those in the outer part of the belt,
size 200 km for the largest of them
 Asteroids in the outer solar system - orbit far beyond Jupiter - the most
distant is found at 33 AU from the Sun - beyond the orbit of Neptune - has
the reddest surface of any object in the Solar System and still unknown
composition
II.
Comets - small bodies of icy and dusty matter that revolve about the Sun. When a comet comes
near the Sun, some of its material vaporizes, forming a large head of tenuous gas, and often a tail.
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Seen as faint diffuse spots of light, with angular size smaller than the Moon
Small pieces of icy material, develop an atmosphere as they get closer to the Sun
Later there may be a faint nebulous tail extending several degrees away from the
main body of the comet
Orbital characteristics
 Newton: their orbits should be extremely elongated
 At that time 3 comets have been known that appeared in the years 1531,
1607 and 1682.
 Edmund Halley (1656-1742) predicted that this has been the same object,
returning to the perihelion at average interval of 76 years - so the object
should return next about 1758 - was right - comet Halley
 We now know from historical records that this comet has been observed
and recorded since 239 BC at intervals from 74 to 79 years - the orbital
period is not stable because of the gravitational pull of the giant planets
 Last appeared in the sky 1986; it will return in 2061, this is a short-period
comet
 Most comets have long periods - don’t return in a time measurable in
human terms - takes thousands of years to return, if they return at all
 Every few years a comet may apper to be seen with a naked eye
 March 1996 - Hyakutake (very long tail); 1997 - Hale-Bopp
The comet's structure
 Comet's head or coma - a temporary atmosphere of gas and dust illuminated
by the Sun (this is actually all we see looking at a comet). The atmosphere
is rapidly escaping all the time (because the gravity is small) and are
replaced by new one, coming from the nucleus.
 Comet's nucleus - small (few km across), solid, dark, hidden by the
atmosphere
 Composition of the nucleus (water ice mixed with other ices, silicates,
dust) - like a "dirty snowball"
 the comet's atmosphere - when a comet approaches the Sun, the ices start to
evaporate - need temperature 200 K - (beyond the orbit of Mars). The
evaporating ice releases the dust component - both flow into space at speed
1 km/s
 comet atmosphere starts to expand - can reach a diameter of 1 million km
 many comets develop tails when they approach the Sun - - an extension of
its atmosphere, consisting of the same gas and dust that make up its head
 tails always point away from Sun
The Kuiper Belt - A reservoir of cometary material just beyond the orbit of Pluto
Oort comet cloud: The large spherical region around the Sun from which most
"new" comets come; a reservoir of objects with aphelia at about 50,000 AU, or
extending about a third of the way to the nearest other stars.
Origin and evolution of comets and their fate
III. Examples - pages 286 and 287 - Figuring for yourself + examples 15, 16, 20, 22, 23.