Chapter 9
Asteroids
- Between
Mars and Jupiter (and a few ranging from close to the Sun out to
Jupiter and perhaps beyond) are hundreds of thousands of mostly rocky
bodies too small to be planets. Most orbit in the so-called Asteroid
Belt. The largest, Ceres, is about 1000 kilomneters
(600 miles) across, and the smallest ones
merge into the meteoroid category (perhaps as small as a few tens of
meters across)
- Asteroids
are interesting because, perhaps more even than planetary moons, they
represent a very early era in the Solar System
Comets
- Comets
are small, icy bodies usually no more than a few miles across.
Typically they orbit the Sun at great distances in two general
reservoirs. The Kuiper Belt is just outside the orbit of Pluto, whereas
the Oort Comet Cloud is perhaps halfway to the next nearest star. Due to gravitational
influences, a few comets fall in close to the Sun. When close to the
Sun, our star's strong radiation turns some of the frozen gases back
into gaseous (ionic) form. Some trapped dust is released from the gas
as well. Radiation pressure and the Solar Wind causes the gas and dust
to stream away from the comet, forming a tail. Radiation causes the gas
to fluoresce (glow), and light from the Sun reflects off the dust to
form and ion (gas) tail or a dust tail or both. The tail(s) always
points away from the Sun, regard of direction of motion of the comet
- Although
the nuclei of comets are small, they can appear very large and
impressive as they near the Sun. the gaseous head can grow to tens of
thousands of miles across, and the tail can be 10s or even 100s of
millions of miles long. Yet the only solid portion is the nucleus,
which has sometimes been referrred to as a "dirty snowball" or a "dirty
iceberg" because it is mostly ice with some sooty (carbon) or rocky
material mixed in
- Even
more than moons or asteroids, comets represent an early stage of solar
system evolution, and as such provide a window into the past and solar
system formation
Meteoroids
- Meteoroids
are small icy or rocky bodies no more than a hundred meters across at
most. (Actually there is no official dividing line between what is an
asteroid and what is a meteoroid. My definition of a hundred meters is
arbitrary and may be accepted by other authors.) Some of them have been traced to the Asteroid
Belt, whereas others appear to be related to comets (we have meteor
showers annually when Earth crosses the orbit certain comets).
- When
meteoroids collide with Earth (at up to 50,000 miles per hour), they
heat up with friction with the atmosphere. The incandescent object
streaks through the sky and is called a meteor
- If the
meteoroid survives the trip through Earth's atmosphere and reaches the
surface, it is called a meteorite
- Meteorites
come in three basic "flavors:"
- Irons,
which are typically about 90-95 percent iron and 5 -10 percent nickle.
They are very heavy and unlike Earth rocks.
- Sometimes
the heating as they pass through the atmosphere causes thumblike
depressions on their surfaces. They also usually have a thin crust,
sometimes reddish with rust
- When
iron meteorites are cut open, polished flat and then etched with acid,
they often show distinct pattering called the Widmanstatten Pattern,
which are due to iron "crystals." This indicates that the meteoroid
cooled very slowly inside another, larger object
- Stones,
which are very similar to Earth rocks in appearance. Although the
majority of meteorites that reach the Earth are of the stone variety,
fewer of them are found because they appear so much like Earth rocks
- Stony
Irons, which are the most rare of meteorites, and a combination of the
other two types. they ahve a matrix of rocky material interspersed with
iron
Pluto
and (Kuiper Belt Objects)
- Pluto is
very unlike the full planets
- Pluto
has been demoted from planethood, and is now known as a "dwarf planet".
To be a full-fledged planet, and object must be in an independent orbit
around its star; must be large enough so that its own gravity can pull
it into a spherical shape; and must have clear its orbit path of other
objects. Pluto qualifies on the first two counts, but fails in the
third. There has been an unusual amount of controversy and argument
over this move
- Pluto
is small, considerably smaller than our Moon, about 2400 km (less than
1500 miles) in diameter
- It is
cold, hundreds of degrees below zero, and dark due to the vast distance
from the Sun
- At an
average distance of 39.5 AU, Pluto is so far from the Sun that our star
is little more than an exceedingly bright star
- Pluto
has a satellite (Charon) nearly half its own size (1190 km or about 740
miles in diameter)
- It's
surface is covered with icy nitrogen, methane and carbon monoxide
- Unlike
the Gas Giants, which are mostly atmosphere, Pluto has only an
extremely thin atmosphere of nitrogen and methane. Even this freezes
out when it is too cold
- "Dwarf
Planets," Kuiper Belt Objects and TNOs
- Outward
from just past the orbit of Neptune to perhaps twice as far as the
orbit of Pluto is the Kuiper (KOY-per) belt, a region of cometary
nuclei and small icy "dwarf planets" (including Pluto and the newly
discovered Eris). Eris is just slighly larger than Pluto (about 100 km
wider) and about twice as far from the Sun. Eris was the Greek goddess
of chaos and discord, and it is said that the name was chosen to
reflect the current status in the astronomical community about changing
the status of Pluto. The only other "dwarf planet" currently is Ceres,
formerly known as an asteroid.
- The
spacecraft "New Horizons" skirted past Jupiter in February, 2007 on its
was to a rendezvous with Pluto in 2015, making it the first spacecraft
from Earth ever to visit the dwarf planet
- Pluto
has a relatively large moon and several smaller ones, and Eris is known
to have at least one moon, known as Dysnomia, the spirit of lawlessness
- Several
other In recent
years several relatively large objects have been discovered past Pluto.
Since these are too small to be planets, and are unlike anything else,
they are called Trans Neptunian Objects or TNOs and Kuiper Belt Objects
or KBOs. Although you may encounter some defintions that attempt to
distinguish differences between TNOs and KBOs, they are essentially the
same objects
- There
are now several named Kuiper Belt Objects. Typically farther from the
Sun than Pluto, there are several with names like Sedna, Orcas and
Quoar
- The
Kuiper Belt objects may well be related to comets, although manty are
much larger than any known cometary nucleus
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