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March 11, the day my last column ran, will no doubt be
remembered as one of the most significant dates in science this
year. The importance of this date, of course, has nothing to do
with my column. At the same time one or two of you may have been
reading Relatively Speaking, It's All About Time, members of the
press around the world were reading a disturbing release written
by Dr. Brian G. Marsden of the International Astronomical Union.
Orbit calculations for a recently discovered asteroid predicted
that on October 26, 2028, the mile-wide asteroid would pass
within 50,000 kilometers of Earth (closer than the moon),
possibly even colliding with Earth. Such a collision would
likely be the worst disaster ever to strike humankind!
The very next day, though, NASA's Jet Propulsion Laboratory
released a statement saying new calculations had shown that in
2028 the asteroid would miss Earth by nearly a million
kilometers, so there was no chance of a collision. Many
headlines around the world were not kind to scientists after
this "false" alarm--Is the sky falling? Let's check the math
first (Houston Chronicle), Asteroid experts postpone the end of
the world (The London Times), Okay, so they were a little off
(U.S. News), Big asteroid, even bigger mistake (SF Examiner).
How could the first release have been so wrong?
Science and the Press
Well, actually, it wasn't. Orbit calculations are based on
measurements of certain observations. The more observations we
have of an object in space, the greater the accuracy of orbit
calculations based on those observations. The asteroid was just
discovered this past December, so the observations were somewhat
limited. The calculations (unless they truly contained an
error), were the most accurate available at that time. In the
original release, Marsden suggested that the asteroid may have
been noticable in certain past years but had gone undetected;
older photographs might give more accurate data. The next day,
following the suggestion, NASA scientists found the asteroid in
1990 photos and included that data to calculate the more
accurate orbit.
The press tended to report the more sensational parts of the
original release. Emphasized were a collision "is not entirely
out of the question," and "The object could come significantly
closer than 30,000 miles." Seldom reported were the phrases "The
chance of an actual collision is small," and "There is still
some uncertainty to the computation." The image then, portrayed
by many (but not all) media was that scientists had bungled
calculations and scared everybody with a fasle alarm.
On the contrary, though, I think this is a good example of how
science works. Observations were made about something in the
natural world. A conclusion was made based on the observations.
New observations, which would support or invalidate the original
conclusion were suggested. Those observations led to a new, more
reliable conclusion. This is how science is supposed to work.
Scientists are human, of course, and must take some of the blame
for the "Chicken Little" take on the asteroid story. Sometimes,
the competitive nature of scientists (there is much glory in
being the first to discover something of scientific importance)
causes scientists to rush to release preliminary findings. This
asteroid was not due for thirty years, what harm would there
have been in checking for prediscovery observations before
issuing an official press release? Finally, the response of the
press, right or wrong, was predictable. Press releases could be
written in a manner that maintains the "factual" nature of the
information but anticipates and minimizes potential sensational
responses.
But enough commentary on the nature of science, scientists, and
the press. Let's get back to asteroids.
How real is the threat?
A large, massively destructive asteroid impact is very unlikely
in our lifetime. Yet in the long run, such an event is a
certainty! A pair of binoculars and a full moon are all that are
needed to confirm this. Unlike the Earth, the moon has a stable
surface. With no atmosphere, the moon's surface is not subjected
to the erosive forces of wind and water. The moon has been
geologically stable for three billion years (no new mountains
formed, no major faulting or shifting on the surface).
About one-sixth of the moon's surface consists of smooth regions
known as "seas." These seas show a visible record of surface
disturbances over time. Impact craters from five asteroids big
enough to destroy much of life as we know it on Earth can be
clearly seen in these seas. Since such collisions are random, it
is reasonable to assume that around thirty (five per sixth)
major asteroids have hit the moon since its surface stabilized.
And the moon is a much smaller target than the Earth!
The Danger of Asteroids
The real danger with a potential asteroid collision is the speed
with which asteroids travel. Truly faster than a speeding
bullet, a large asteroid would release enough energy in its
explosion to cause a tremendous loss of life. Convincing
evidence indicates such an impact was responsible for the
extinction of the dinosaurs 65 million years ago. In fact, a
major asteroid may hit the Earth on average every ten million
years. Although slim, those are better odds than some lottery
players face!
Detecting Asteroids
Astronomers are focusing now more than ever on finding and
mapping potential hazardous asteroids. Only a small percentage
have been discovered, and the process is slow. The recent public
focus on the possibility of an asteroid impact, though, may lead
to more funding for asteroid detection research. Two upcoming
summer movies, Armageddon and Deep Impact (basic plots:
big-hearted heroes versus bigger asteroids) attest to the
timeliness of planning for the possibility of an asteroid
collision.
Can We Stop an Asteroid?
Suppose scientists discover an asteroid that really is likely to
collide with Earth. What can we do to stop it? No one really
knows, but there are two lines of thought: move it or destroy
it. Perhaps, by landing on it and attaching rockets to it or
bombarding it with rockets from the side, we can push the
asteroid off course enough to miss us. Or, if that does not
work, then perhaps we would try to destroy the asteroid with
nuclear weapons. In all likelihood, we will test some of these
ideas on a close-approaching asteroid sometime in the future.
We can look at the moon and count the impact craters. The
fossil record whispers to us about an asteroid that may have
killed the dinosaurs. Other mass extinctions in Earth's history
hint of asteroid involvement. We all saw comet Shoemaker-Levy 9
crash into Jupiter. In 1989, an asteroid surprised scientists
with a close pass by the Earth. We do not need to panic, but we
do need to be prepared. Let me paraphrase a leading astronomical
group's position paper on the potential threat of an asteroid
impact: What greater feat could there be than to perhaps someday
alter the course of an asteroid to prevent an impending
disaster? And what greater tragedy, given our knowledge and
abilities, than to be caught unprepared?
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