Comets, Meteors, and Asteroids

 

Part I

 

Comets

 

The leading theory of the nature of comets is called the dirty snowball model.  Far from the Sun a comet is a very small (5-10 km diameter) nucleus composed of various ices (H2O, CH4, NH3, CO2) mixed with a little dust and rock.

 

As the comet approaches the Sun (generally by the orbit of Mars) some of the ices begin to sublime, forming a very thin but very extensive cloud of molecules and ions around the nucleus called the coma.  The coma may reach a diameter of 100,000 km, but would make a good vacuum in Earth based laboratories.  The coma is large, however, and can reflect a lot of sunlight, making it easy to see.

 

Well-developed comets will develop a tail as they get closer to the Sun.  The tail may come in two parts:

 

 

Comets are discovered at a rate of 15-20 per year but there is no evidence that they do not belong to the solar system.  Their aphelion distances seem to cluster around 50,000 AU.  For all these reason Jan Oort proposed a cold storage region for comets that we now call the Oort Cloud.  These "potential comets" very likely represent the origin solar system material left behind as the solar nebula began its original collapse.  The interest is comets today is simply that comets offer our only chance to examine pristine solar system material.

 

Most comets are sporadic comets - they are on open orbits (parabolas or hyperbolas) and will pass the Sun only once.  A few (like Halley) are periodic - visiting the vicinity of the Sun regularly.  These likely originated in the Oort Cloud as well, but had their orbit changed by a close approach of Jupiter.

 

The ancient Greeks taught that comets were phenomena of the upper Earth's atmosphere.  This assumption was to preserve their belief that the Celestial Realm could not change.  It wasn't until 1577 that Tycho Brahe demonstrated that comets are members of the solar system.

 

Meteors

 

Comets shed material in their path as they orbit the Sun.  Occasionally, the orbit of a comet intersects the orbit of the Earth.  As the Earth plows through the debris left in the comet's path, we experience a meteor shower.  Since the position of the Earth in its orbit can be identified with a certain calendar date, meteor showers return regularly on certain days of the year.  As we go through the stream, we seem to see the particles moving at us in parallel lines.  Any student of art will understand that this gives rise to a perspective point.  Showers appear to emanate from a certain point in the sky that we can identify with a constellation.  The brightness of a meteor is related to its speed as it enters the atmosphere.  For this reason shower meteors are best seen after midnight, when the rotational and orbital motions of the Earth add together.

 

Sporadic meteors likely emanate from the main asteroid belt.  Collisions between asteroids not only produce small fragments, but also change the orbit of the fragments so that some may come into the inner solar system.  The most common sporadic meteors are the stony meteors, although the most easily found are the irons because they look extraterrestrial.  Complex molecules have been discovered on meteorites, including amino acids.  Some have even suggested that life on Earth was seeded by collisions of meteors.

 

Large meteorites have impacted the Earth.  The reason that we have very few impact craters is the very rapid erosion on the surface in the form or wind and water, as well as plate tectonics.  Large impacts occur less frequently, but can alter the history of the planet when they occur.

 

Asteroids

 

Most asteroids reside in the main belt between the orbits of Mars and Jupiter.  Several significant groups are not in the main belt:

 

 

Asteroids are typically very small and, therefore, irregularly shaped.  There are gravitational resonances in the main belt, induced by the combined gravity of Mars and Jupiter.  This produces gaps in the asteroid belt (Kirkwood gaps).