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05/16/2002
Defending Ourselves from Asteroids
It was deja vu all over again. A couple weeks ago I mentioned
attending a New York Philharmonic Orchestra concert that ended
with Scriaban''s First Symphony and watching the Joseph
Flummerfelt Choral Group sit stoically, maintaining a tight
lipped silence for five of the six movements. Last week, it was a
concert by our local symphony orchestra. Sure enough, the
concert ended with Debussy''s Nocturnes and featured a choral
group. The piece had three movements - Nuages, Fetes and
Sirenes. This time the first two movements not only had no
vocal elements but the singers were nowhere in sight. Finally,
before Sirenes, the kettledrums were shifted aside to allow about
15 to 20 neatly attired women to file on stage in back of the
orchestra and the last movement commenced. But what''s going
on here? Scriaban gave the Flummerfelt group just one line to
sing. These gals didn''t even have a line! As best I can transcribe
it, all they sang was "Aaaahaaauuh" over and over. Were these
compositions the forerunners of the limited lyrics found in much
of the so-called music typical of the past few decades?
Speaking of deja vu, I''m afraid you''re in for another column on
asteroid impacts. Immediately after posting last week''s column
on the subject, I stumbled upon several articles dealing with
these visitations by objects from outer space. One article in the
March 2002 issue of Scientific American is titled "Repeated
Blows" by Luann Becker a geochemist at the University of
California, Santa Barbara. Her specialty is studying the markers,
or tracers, that can be used to identify impact sites from the
distant past. We''ll get back to her later.
The other two articles were more current and/or forward looking.
One was a very short article by Fenella Saunders in the February
2002 Discover magazine that really heightened my concern
about incoming missiles from outer space. According to the
article, there is roughly 500 tons of debris from space raining
down on us Earthlings every day. Not only that, but it seems that
a meteor packing the punch of an atomic bomb hits us about
once a year! This statement rather shocked me. I''m assuming
that this energy is dissipated in the atmosphere or the ocean most
of the time or else we would hear about these meteors a lot more
frequently. Saunders mentions the work of Douglas ReVelle and
colleagues, who use low-frequency monitoring stations around
the world to pick up the sounds of the meteors as they charge
through our atmosphere. This permits them to determine the
location and speed of these wandering space objects. It heartens
me to know that someone is watching out for us, even if the
likelihood is nil that we can do anything about these meteors that
show up and disappear very quickly.
Three articles that really shook me up appeared in the April 5,
2002 issue of Science. They dealt with Asteroid 1950 DA. On
March 16, 2880, there is perhaps a 1 in 300 chance that we''ll be
clobbered by this 1-kilometer size asteroid and its 10,000-
megaton wallop. Ok, I agree, that''s over 800 years away and we
probably shouldn''t lose any sleep worrying about it. I wouldn''t
even have mentioned it except for the Yarkovsky effect. I never
heard of Yarkovsky until I read these articles by Richard Kerr,
by Joseph Spitale and by J. D. Giorgini and 13 other authors!
(With that many authors on a single paper, you know this has got
to be hot stuff.) I felt better after reading these articles.
Apparently, there''s hope that we can defend ourselves from
against these extinction-producing objects.
Yarkovsky is a Russian engineer who published a paper in
Science back in 1999 that discussed what happens when a
spinning asteroid goes in and out of the sunlight, just as we do
every day here on earth. What he showed was that the
"afternoon" section of an asteroid, which has been heated the
most by the sun will give off thermal radiation. How much heat
it emits depends on how hot the asteroid gets and this in turn is
quite dependent on the surface. For example, if the surface is
black it will absorb more sunlight than if it''s white. Who cares if
it gives off thermal radiation? You might care a lot if you know
the asteroid is headed your way!
This thermal radiation, in the form of photons of various
wavelengths, acts like a bunch of teensy rockets pushing the
asteroid ever so gently as that hot section of the asteroid spins
into dusk and darkness. This tiny push, over a long period of
time can steer an asteroid like our 1950 DA toward or away from
us over the next 800 plus years. The Yarkovsky effect is a major
reason that astronomers can''t say definitely that the asteroid will
hit or miss us. To calculate more exactly what the effect will be,
they need to know such things as the shape, color, surface
roughness and spin rate. Fortunately, they have some time to get
the needed data before 2880 arrives.
But what happens if they get the data and calculate that it''s going
to hit us dead on? Spitale proposes that we use the Yarkovsky
effect to get us out of this jam. He argues that it would only take
a modification of the upper inch or so of the asteroid''s surface to
modify how hot the surface gets. If we''re clever, we could
change the Yarkovsky effect enough to nudge the asteroid out of
its earthbound path. How do you accomplish such a feat?
Spitale suggests that one could ferry a couple hundred thousand
tons of dirt and dump it on 1950 DA. This, he figures, would
take only about 90 Saturn rockets full of dirt. Other possibilities
he considers are detonating conventional explosive to change the
texture and color of the surface.
The article by Kerr suggests possibly covering the surface with
soot or powdered chalk might do the trick. You can see that all
these approaches require some degree of planning and execution,
to put it mildly. Spitale suggests it could take a century to alter
the course enough to make the difference. All this speculation is
obviously for the future.
When it comes to past impacts, the situation is becoming much
clearer, thanks to work such as that by Luann Becker and her
colleagues. How does one identify the existence of an impact?
The most obvious one is the existence of a very visible crater
such as the one in Arizona that you may have seen in person.
But more typically the impact crater is not so obvious and it
takes some good detective work to pin the impact down. That''s
where some sort of tracer is needed. We''ve discussed before the
work by Luis Alvarez and his son Walter, who led the Berkeley
team that in 1980 found unusually high amounts of the element
iridium in a layer of 65 million-year old clay in Italy. Knowing
that iridium is found in many meteorites and that the dinosaurs
became extinct 65 million years ago, Alvarez proposed the
connection between the two and the hunt was on for other tracers
to confirm the hypothesis.
Soon it was clear that in the same time period there were tiny
droplets of glass known as microspherules and of quartz that had
been rudely pounded ("shocked" quartz). Both were attributable
to a tremendous impact. Also, excessive amounts of soot and ash
were found. All this circumstantial evidence provided by the
tracers was neatly confirmed by geophysicists from an oil
company who found that there was a circular formation in the
Gulf of Mexico about 110 miles in diameter, just the size impact
crater that would be expected.
Becker and her group have found another convincing tracer. As
we''ve discussed earlier, one of the hottest fields around today is
the study of fullerenes, new forms of carbon that include the so-
called buckyball. This is a form of carbon in which the unit of
crystal structure is 60 carbon atoms arranged as in a soccer ball.
If you picture a soccer ball, you can visualize that there is room
inside the soccer ball to trap a tennis ball or two. Similarly, a
carbon buckyball can trap other atoms or molecules inside.
Becker''s group found that, at a known impact crater in Canada,
there were fullerenes and that inside the structure there was
helium. Just as there is ordinary hydrogen and also a heavier
form (isotope) known as deuterium, there are different isotopes
of helium. On Earth, there is a certain ratio of the amounts of
two of these isotopes. In the Canadian crater, the buckyballs
contained helium in which the ratio was vastly different. In fact,
the ratio was similar to that found for helium in some meteorites
and cosmic dust. As usual there are skeptics who think
volcanoes may be involved but I''m a believer.
I promise no more asteroid impacts for the next few months
unless, of course, one hits us! Next week, the foul shot and other
weighty matters.
Allen F. Bortrum
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