THE SOUTHWORTH PLANETARIUM
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
43.6667° N,                    70.2667° W
Founded January 1970
       "Earth with art is Eh"
                    -seen on a bumper sticker on the way to work this morning


THE DAILY ASTRONOMER
Thursday, February 4, 2016
The Wrong View of the Asteroid Belt



The Universe is stiff with belts. And, as is true with the belts once
finds in clothing stores, they appear in many varieties.  However,
unlike sartorial belts that assume one function, celestial belts
differ both in character, composition and function.   Today, we
explore the most famous of these belts: the one that separates the
inner solar system of rocky worlds from the outer region of the gas
giants:

ASTEROID BELT
The structure that 99.9% of all people, including ourselves, envision
incorrectly.  Because we watched the Millenium Falcon navigate its way
unscathed through an asteroid belt or because, in the days of our
youth, we and the Pharaoh's other tomb builders played "Asteroids" on
long afternoons, we believe that asteroid belts are crammed elbow to
pelvis with flying rocks.      The reality, as far as the solar
system's asteroid belt is concerned, is far less perilous.  Were we to
meander about the solar system, we would find this belt between the
orbits of Mars and Jupiter and if we teleported ourselves there, the
first thought we'd entertain would be,  "Where are those confounded
asteroids?  All I see is darkness!"

We'll explain:

The asteroid belt, named the "main belt," by astronomers, consists of
thousands of asteroids revolving around the Sun.   Unlike that pesky
and erroneous vision that won't go away, the asteroid are not just
randomly tossed anywhere.  They move within a disc between  2.1 AU and
3.3 AU.  (An AU is an "astronomical unit," defined as Earth's average
distance from the Sun or approximately 93 million miles.)  Within this
disk one would regions devoid of asteroids, referred to as "Kirkwood's
Gaps."   Jupiter, the giant planet far beyond the belt's outer edge,
literally created those gaps within areas of -don't hang up- "orbital
resonance."   Objects within the resonant regions would have orbital
periods equal to an integer fraction of Jupiter's orbit.  For
instance, we see a Kirkwood Gap at 2.5 AU because an asteroid at this
distance would complete exactly three orbits during the time Jupiter
required to complete one.    We'd find another gap at  3.28 AU as any
asteroid at that distance would have an orbital period exactly one
half that of Jupiter's.      Any body with these gaps would develop
unstable orbits and would move to another part of the belt.

Within the "populated" regions, one would have hundreds, perhaps a few
thousand asteroids: those we've discovered and those not yet found.
 While this number might make one think the asteroid belt is truly a
swarm of renegade mountains, it is, instead, rather diffuse.  Recall
that this belt encircles the Sun at a distance of more than 180
million miles:   we recall from geometry that one can make a rather
large circle or ellipse with a radius (or semi-major axis) that is so
long.     The circumference is almost one billion miles: about equal
to the distance between the Earth and Saturn.   The combined mass of
all asteroids within the main belt is less than that of the Moon!
By planetary standards, the Moon is quite small   Imagine demolishing
the Moon into an array of minuscule pieces and then distributing these
pieces around a billion mile loop:  one would have a diffuse belt
indeed.

That is splendid news for those would want to manuever a spacecraft
through the belt toward the outer solar system or beyond.    NASA has
deployed a few ships, such as the Voyager and Pioneer probes, through
the asteroid belt without much trouble.      There's hardly anything
there except empty space.    Then again, one could make that remark
about the solar system, itself.