2003-May-3, IRTF

Where do the meteorites come from?

Alan Tokunaga and Bobby Bus 
Institute for Astronomy 
Univ. of Hawaii

Every day about 3000 tons of small pieces of rock and iron hit the
Earth's atmosphere.  Fortunately for us, most of this material burns up
in the atmosphere before hitting the Earth's surface.  These small
chunks of rock are called "meteoroids" and as they burn up in the
atmosphere, they leave behind streaks of light that you can see crossing
the night sky, called "meteors".  Not surprisingly, these streaks of
light are often referred to as "falling stars".  While  the idea of
stars falling to the Earth has some romantic appeal to it, thinking
about these small bits of rock and iron speeding through the Earth's
atmosphere can be very sobering.  We are fortunate indeed that the
atmosphere is protecting us from nearly all of these infalling objects.
We would find it disconcerting to be dodging meteors every day!

Several thousand of these meteoroids are large enough, however, that
they do reach the Earth's surface every year.  So you could say that the
Earth is being enriched continuously by new material from space.  This
"gathering" of new material is very slow compared to what happened when
the Earth first formed about 4.5 billion years ago.  At that time large
objects half a mile in diameter and larger collided together and
accreted to form the Earth itself.  The rate at which accretion is
occurring has dropped off significantly,  but the process has not yet
ended. The meteoroids that hit the Earth every day are the smallest of
the class of bodies that are comprised of rock and iron  in space.  The
largest members of this class are the asteroids-- objects that are
larger than about one tenth of a mile in diameter.  Even now asteroids
do hit the Earth on occasion, but the ones that can cause real damage
hit someplace on the Earth only about once per century and the ones that
can cause global damage hit the Earth about once every 100 million
years. So the threat of impact is actually very low, and is not
something we need to worry too much about.  Keep paying your taxes and
plan to enjoy your morning coffee.

But coming back to the question of this article.  Those meteoroids that
are large enough to survive their passage through the atmosphere and
reach the Earth's surface are called "meteorites".  We'll bring some
samples of meteorites to the talk and you can hold a piece of the solar
system in your hand.  These pieces of rock are some of the oldest
material in the solar system (about 4.5 billion years old), and have
been circling the sun for all of those years until they finally collided
with the Earth.  Astronomers and scientists who study meteorites view
these important objects as containing clues that may help us understand
the formation and evolution of the solar system.

So we know that meteorites were once in orbit around the Sun and were
unfortunate enough to hit the Earth.  Does this answer the question of
where the meteorites come from?  Not really.  If you have followed the
logic of this article so far, you now know that to answer this question
we need to know where the meteoroids themselves come from.  As is often
the case in science, answering one question almost always leads us to
ask another.

And so where do the meteoroids come from?  Many are thought to come from
the main asteroid belt, that region of the solar system between the
orbits of Mars and Jupiter.  We know that the region of space around the
Earth is not empty.  Not only do we see meteors streaking across the sky
at night, but we also are learning much about the larger Near-Earth
Asteroids.  It is these objects that we hear about in the news whenever
one makes a particularly close pass by the Earth. The trick for
astronomers is to understand the connections between the meteorites that
we pick up on the Earth's surface, the Near-Earth Asteroids, and main
asteroid belt.  At the IRTF and other telescopes on Mauna Kea, there are
active research programs aimed at understanding the asteroids,
especially the ones that come very close to the Earth.

Now we are getting closer to answering the question of "Where do the
meteorites come from".  We just need to know where the Near-Earth
Asteroids come from.  Here we are reaching the limits of our knowledge. 
It seems that we can see connections between some of the Near-Earth
Asteroids-- there are strong similarities in their orbits or surface
composition.  We know that these Near-Earth Asteroids cannot survive
longer than a few million years in their current orbits, and so there
must be a source that is producing Near-Earth Asteroids all the time. 
We suspect that the population of Near-Earth Asteroids may be regularly
replenished from the main belt of asteroids, but we don't know for sure.

The seemingly simple question of where the meteorites come from is a
profound one.  While we think we understand the basic relationships
between meteorites, meteoroids, Near-Earth and main-belt asteroids,
there are many questions for which the answers remain elusive.  For
example, what is the physical process that can take asteroids in the
main belt, between Mars and Jupiter, and "fling" them toward the Earth? 
By finding out answers to questions like this, we may be able to
formulate a more complete picture of the Earth's history.  Impacts on
the Earth, whether from small meteorites or large Near-Earth Asteroids,
have had a great affect on the evolution of life on the Earth. The
research being carried out on Mauna Kea will help answer these 
questions, and perhaps some of you still in school will work on this
problem in the future.  We hope so since we would like to know the
answer to this question.