Astronomy Notes

Part 8: Comets, Meteors, Asteroids, and Origin

John P. Pratt

Swords in the Sky

Comets are the most spectacular of the small bodies in the solar system.
They take weeks or months to go across the sky.
They have a nucleus which is only about 10 miles across, too small to be seen in any telescope.
They have a head which is the glowing part around the nucleus.
They have at least one tail, which points away from the sun, being blown by the solar wind.
The tail can extend for many degrees across the sky, or even all across the sky. Best seen in a dark sky.

Discovery of Comet Orbits

Tycho showed in 1577 that comets do not arise in the atmosphere by observing one from two locations.
Edmond Halley in 1704 found that 4 comets with similar orbits had a 75-year periodicity.
He predicted they were all the same comet and that it would return in 1758.
When it did, it was named Halley's Comet.

Long Period Comets from the Oort Cloud

Most comets are long period comets, and have periods up to millions of years.
In 1950's a Dutch astronomer named Oort showed that most long period comets come from 100,000 a.u. from sun.
They are spherically distributed in a huge cloud around the sun, called the Oort Cloud.
It is estimated that there are about 100 billion comets in the Oort Cloud.
The short period comets are in a belt in the plane of the ecliptic just beyond Neptune called the Kuiper Belt.

Short Period Comets from the Kuiper Belt

Short period comets have periods of a century or less.
In 1950's a Dutch astronomer named Oort showed that most long period comets come from 100,000 a.u. from sun.
They are spherically distributed in a huge cloud around the sun, called the Oort Cloud.
It is estimated that there are about 100 billion comets in the Oort Cloud.
The short period comets are in a belt in the plane of the ecliptic just beyond Neptune called the Kuiper Belt.

Dirty Icebergs

Comet nuclei are only about 10 miles in diameter.
Emission spectra from the tail reveal many organic molecules.
Comets often have two tails.
A gas tail which blows straight back away from the sun, and which looks blue like our gaseous sky.
A dust tail which curves away from the gas tail.

Annual Meteor Showers

Meteors are shooting stars that streak across the sky.
There are about 3 per hour every night.
On certain nights of every year there are meteor showers with about 1 meteor per minute.
The best meteor shower in most years is the Perseids on the early morning of Aug. 12.

Meteors are Tiny Pieces of Debris from Disintegrated Comets

Once when a comet failed to reappear, it was replaced by a new annual meteor shower.
Comets get smaller every time they orbit the sun.
Eventually the pieces get spread out all along the orbit.
Every year when the earth passes that old comet orbit on the same day, there is a meteor shower.
When the earth passes through the head of the old comet there can be 10-100 meteors per second.
The average meteor is only the size of a matchhead.

Meteorites are meteors that hit the ground.

Very few are big enough to survive all the way to the ground.
Only a few brick size meteors are recovered every years.
There are two main kinds of meteorites: stony and iron.
There are two main subtypes of stony meteorites.
- Chondrites contain little glassy spheres.
- Achrondrites don't.

Asteroids are rocky moon-sized interplanetary bodies.

They are mostly located in the gap between Jupiter and Mars.
They differ from comets in having almost no ice.
They vary in size from a few miles across to one third the diameter of our moon.

Discovery

Asteroids Outside the Belt

Apollo Asteroids cross inside the earth's orbit and could collide with the earth.
Trojan asteroids orbit 60° ahead or behind Jupiter, and stable orbital points.
Chiron is an example of an asteroid which orbits as far out as Saturn.

What Needs Explaining

Why do the planetary orbits nearly all lie in a plane?
Why is the Sun's equator also in this plane?
Why are the planetary orbits nearly circular?
Why do the planets all revolve around the sun in the same west-to-east direction?
Why are there both terrestrial and giant gas planets?
Why do most of the planets rotate on their axes in the same direction as they revolve?
Why do planets follow Bode's Rule?
Why do satellites behave in a similar manner?

Catastrophic Theories

These theories postulate that some unusual big event caused the solar system.
The first such theory (1745) was that a star came near the sun and pulled off pieces.
The problem with catastrophic theories is that random accidents do not explain all of the order.

Evolutionary Theories

These theories postulate that slow, gradual changes caused the solar system to form.
The first such theory (1644) by Descartes was that the solar system began as a local eddies in diffuse matter.
This is the most popular, that interstellar particles came together slowly by gravity.
These theories explain most of the order seen, like why the sun's equator aligns with the ecliptic.
This also explains the circular orbits because small particles would collide when elliptical.

The Current Theory

A huge interstellar gas and dust cloud could begin contracting to form a protosun.
As it contracted it would speed up in rotation rate and also heat up.
It would also flatten out because there is no force to stop if from contracting in the plane of rotation.
At a certain distance out, the rotational rate could have been fast enough to leave a ring.
One weak part of the theory is just how did gas and dust particles form into a planet?
Another difficulty is to explain why is the sun rotating so slowly on its axis (about once a month)?
- It should be rotating much faster because it should have kept speeding up as it contracted.
- Perhaps magnetic braking slowed it down from the reaction of particle ejected in its strong magnetic field.