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How
well do we know our neighbors? Not very well, but we're getting
better. Perhaps it's better to start with the question of how well
we know the neighborhood and then work toward the individuals. In
the case of our cosmic neighborhood we know it fairly well, but
it certainly looks different from what we thought it did just a
few years ago.
The
old picture of the solar system showed the Sun in the middle, surrounded
by the four planets of the inner solar system, Mercury, Venus, Earth
and Mars. Then came the somewhat mysterious asteroid belt between
Mars and Jupiter, followed by the outer planets, Jupiter, Saturn,
Uranus, Neptune and Pluto. That was it! Nice and clean. Everything
well behaved and orderly.
Now
things have gotten messy it seems. Of course, it’s always been
this way, but we’re getting more observant, and that’s made a real
difference in our outlook. Starting at its outer edge the neighborhood
extends into the Kuiper Belt,
a disk-shaped halo of comets outside Neptune and Pluto that probably
contains hundreds of millions of icy cometary bodies. Some even
argue that Pluto doesn’t belong in the planetary family at all,
but rather should be seen as the king of the Kuiper Belt objects.
Much further out lies the Oort Cloud, pictured
as a sphere of comet-like objects numbered in the billions.
But
of particular interest is our new understanding of the inner solar
system. It was thought by early astronomers that there “ought to
be” a planet at about 2.8 AU (1 AU is the distance from the Sun
to the Earth). At the beginning of the 19th century
the asteroid Ceres was discovered and shortly thereafter it was
recognized that there were many other asteroids that populated this
“belt” around 2.8 AU.
 It was right at the end of the 19th
century that Eros,
the first near Earth asteroid (NEA), was discovered. Eros remained
a fairly lonely anomaly until well past the middle of the 20th
century when it was finally realized that there were whole classes
of objects in near Earth orbit around the Sun. As the 20th
century drew to a close we had discovered over 2,000 NEAs, we knew
that over 630 of them were larger than 1 kilometer in diameter,
and that the population of those that could create terrible damage
if they were to hit the Earth numbered in the hundreds of thousands!
That’s quite a different picture
of our inner solar system neighborhood than most of us grew up with.
Today
we not only know that there is a substantial population, but also
that its members occasionally hit the Earth with potentially very
serious consequences. We have in our sister planet, the Moon, an
excellent history of the visitation record of NEAs and comets to
our local neighborhood. Given no atmosphere and the absence of
most other erosional influences, the Moon has retained the calling
cards left by these visitors, large and small. Were it not for
Earth’s atmosphere and its geophysically active systems, the record
of craters here would be over 10 times that evident on the Moon’s
surface. As it is we have now identified over 160 impact
craters on the Earth.
What
is missing in the current knowledge base is the vastly larger numbers
of NEAs smaller than 1 km. The members of this cohort of particular
interest are those above approximately 100 meters in diameter.
At this size we are dealing with an energy level of about 80 – 100
megatons, more than enough to create serious local devastation.
If such an impact event were to occur near a populated area it could disrupt
not only the local environment, but also the global economy. One
has only to look at the consequences of the terrorist strike on
New York City and recall the massive economic disruption caused
be a dramatically smaller physical scale event.
At
the moment, NEAs smaller than 1 km. in size are being regularly
discovered. However, these smaller objects are being found incidental
to the primary purpose of the Spaceguard Survey to discover 90%
of the NEAs over 1 km. in size by 2008. In essence what is happening
is that we are only discovering the smaller bodies when they are
very close to the Earth. At the current rate of discovery it will
take many decades to make a dent in the total population of the
NEAs between 100 and 1000 meters in diameter. Even though an impact
would be randomly located around the Earth, no individual or public
official wants to see a 100 megaton bomb going off that might have
been prevented. We therefore believe that it is time to restructure
the current detection program.
Current
estimates give a population of between 200,000 and 400,000 to the
NEAs in the 100 meter class. This is a massive database to handle,
yet the current system run by the Minor Planet Center at the Smithsonian
Astrophysical Observatory would probably be able to grow to handle it were funding provided.
More challenging are the larger and/or more sensitive survey telescopes
needed to scan the skies on a regular basis. There is progress
being made here, both on the ground and in space. The Pan-STARRS system
of the University of Hawaii is planned for operation beginning in
2007. Even more capable is the proposed Large Synoptic Survey Telescope
(LSST) that would
enable detection of NEAs down to the size of interest. Once these
instruments begin operation there is no reason that we should not
have an excellent inventory of 90% of objects down to 100 meters
in diameter within 10 – 15 years. At that point we will have the
basic knowledge to terminate most of the threat to life from cosmic
collisions.
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