Astronomy Notes
Part 8: Comets, Meteors, Asteroids, and Origin
John P. Pratt
Comets
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.
Meteors
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
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
- After Uranus was discovered in 1781, belief in Bode's Law was strengthened.
- A search began between Mars and Jupiter where a planet was missing.
- In 1801 the largest asteroid, Ceres, was discovered.
- Now about 7,000 are known; each is numbered and was named by the discoverer.
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.
Origin of the Solar System
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.