29:50 Modern Astronomy
Fall 1997
Lecture 33 ...November 12, 1999
Comets, Meteors, and the Leonids
Today I will talk about things other than major planets. The subjects will be:
Comets
Comets are among the more spectacular astronomical objects. We are fortunate in having had two good
ones in the last couple of years. Pictures of comets are on p344 and 345 of your textbook.
Throughout history, comets were considered portents of doom. We now know that they are ordinary
citizens of the solar system.
The basic structure consists of two tails; the dust tail which reflects sunlight and the ion tail
or plasma tail, which glows by emission lines of the ionized molecule 
plus other ionized
molecules.
Hand drawing of structure of comet
The structure also contains the coma, or bright head of a comet.
The essence of a comet is the nucleus which is solid mass of ice, rock, and black hydrocarbons at the center. The nuclei of comets are often described as ``dirty snowballs''. Dimensions of these nuclei range from a few kilometers to a small one to perhaps 50 kilometers for a spectacular comet like Hale-Bopp. (See Figure 15.17)
It is an amazing fact about comets that a structure which can be seen to stretch over tens of millions of kilometers in space originates in an object only a few miles in diameter.
Since the nucleus is so small, and is hidden by the much larger coma, the nuclei are essentially never seen from Earth. An exception was 1986 when Halley's comet was visited by a spacecraft which photographed the nucleus close-up. This is shown on Figure 15-30 on p351 of your textbook.
An important feature to note about comets is that they ``light up''
only when they come within a few 
astronomical units. At such a distance, solar heating
heats them up so they begin to sublimate, i.e. the solid ices make the transition to gases.
The evaporating gases become ionized, producing the ion tail, and they also blow off piece of ice,
rock, etc, which forms the dust tail.
Diagram with comet orbit, phases of disintegration
The orbits of comets are extremely interesting. They are good illustrations of the the validity
of Kepler's Laws. In almost all cases, comet orbits are highly elliptical.
Drawing of cometary orbit on blackboard.
It is common for comets to have eccentricities in excess of 0.99.
The fact that these comets have eccentricities so large, and typical perihelia of close to an a.u. leads to an extremely interesting fact: comets come from very deep in space.
The Oort Cloud
In the 1920's the orbits of enough comets had been analysed to yield an interesting fact. Most comets
were on elliptical orbits with eccentricities 
and extremely large semimajor
axes. Values were of order 10,000 to 100,000 astronomical units. Recall that the most distant planet
we have discussed is Neptune with a mean distance from the sun of 30 astronomical units.
The orbital periods for such comets are millions of years to tens of millions of years.
From this we can deduce that the total number of comets must be huge. Since we only see a given comet for a few months while its orbit takes it through the inner solar system, the total number of comets must be larger than the number of observed ones by at least the ratio of 10 million years to one year. It is actually bigger than that, because we only see comets which ``light up'' when they come within 3 a.u. There must be innumerable comets which pass within 5 or 10 or 20 or 100 a.u. and are never observed by us.
From this, astronomers deduce that the total number of comets must be of order 
located in
a huge deep freeze for comets called the Oort cloud.
Figure 15-20 from book.
Meteors
Pieces of matter from the solar system actually come to us. Meteors are solid objects coming
into the atmosphere at speeds of several to as high as 50 kilometers per second. They heat up and
glow, and also produce a glowing column of ionized air.
If the object entering the atmosphere is large enough, a piece of it survives and makes it to the
ground. These are called meteorites. About 95 percent of meteorites are stony objects called
chrondites. The remaining 5 % are blobs of nearly pure metal and are called iron-nickel meteorites.
Actual meteorites.
It is possible to determine the age of formation of meteorites using radioisotope dating schemes. This shows most of them are extremely old, with ages dating up to 4.5 to 4.6 billion years. This is very close to the oldest Moon rocks and Martian meteorites, and have helped us establish a solid age for the solar system.
Other facts about meteors/meteorites
Meteorites have various sources. Some are comet debris, particularly those associated with
meteor showers. Others have been blasted off the planet Mars, the Moon, or the asteroid Vesta.
Still others are ``planetesimals'', or material left over from the formation of the solar
system.
Meteor showers originate when the Earth crosses the orbit of a comet.