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(Submitted September, 1998)
Due to the high number of questions we've received about the threat of an
asteroid striking the Earth, we've decided to dedicate a web page to this
issue. Most of these concerns about asteroids are probably due to the recent
summer movies "Armageddon" and "Deep Impact".
First of all, the most important thing to remember, is that these movies are
works of fiction intended to entertain audiences. They are not real!
Oftentimes, movies distort reality (especially the reality of science) to make
it more exciting.
But, sometimes there is a kernel of truth behind a movie, though it is often
hard to discern what that is. So here, we will attempt to bring you the
truth about asteroids and what they mean for Earth, as well as give you
some references to check out.
First and foremost, we know of nothing currently on a collision
course with Earth! It is true that Earth has been hit by meteors or
asteroids before and will be hit again someday. However, currently,
we are not in any immediate danger. In particular, we now know that
the asteroid which was originally announced to possibly pose a small
risk of an impact in 2028 will miss us completely.
http://impact.arc.nasa.gov/news_detail.cfm?ID=60
What are the chances of us being hit and when would it happen?
The first thing to remember is that space is big and empty. Which makes
the chance that we will be hit by anything from space very small. In much of
space, for example, large-sized objects are hundreds or thousands of light
years apart. Even the
asteroid belt has so much space in it, that we can send space probes through
it without any problems. The asteroids in the belt are spread over a ring
that is more than a billion kilometers in circumference, more than 100 million
kilometers wide, and millions of kilometers thick.
Here's what JPL's Near Earth Asteroid Tracking team has to say:
"The most dangerous asteroids, capable of a global disaster, are extremely
rare. The threshold size is believed to be 1/2 to 1 km. These bodies
impact the Earth only once every 1,000 centuries on average. Comets in
this size range are thought to impact even less frequently, perhaps once
every 5,000 centuries or so."
The Asteroid and Impact Hazard page says:
"The threshold for an impact that causes widespread global mortality and
threatens civilization almost certainly lies between about 0.5 and 5 km
diameter, perhaps near 2 km. Impacts of objects this large occur from one to
several times per million years.
"Because the risk of such an impact happening in the near future is very low,
the nature of the impact hazard is unique in our experience. Nearly all
hazards we face in life actually happen to someone we know, or we learn about
them from the media, whereas no large impact has taken place within the total
span of human history... It is this juxtaposition of the small probability
of occurrence balanced against the enormous consequences if it does happen
that makes the impact hazard such a difficult and controversial topic."
This is a difficult issue because an impact would pose enormous risk, yet
because the odds of it occurring within our lifetimes is so low, it is
unnecessary to run around believing that the sky is falling. There are two
things to consider: one is that there are many organizations with telescopes
trained to the sky, watching and tracking asteroids and comets, compiling
a list of potentially hazardous objects to keep an eye on. Many of these
objects are decades away from approaching the Earth which gives us a lot of
time to track them in order to accurately predict their orbits.
If tomorrow, we discovered an object that will intersect Earth's orbit,
what would happen?
The Near Earth Asteroid Tracking Team replies:
"Actually, some 100 bodies have already been discovered on orbits which take
them so close to the Earth's orbit, that they could hit in the far distant
future. This is because the orbits of these bodies change slowly with time.
Although their orbits do not intersect Earth's orbit at present, they could
hit in a few thousand years or more.
"The scenario you have in mind is most likely to unfold as follows. In the
course of our search for Earth-crossing asteroids, we could find one that
will hit not in the next year, or even in the next ten years, but might hit
in the next hundred years. We believe that the chance that we will find
such an object is only 1 in 1,000, even after a complete search. If we do
find such an object, we will have plenty of time to track it, measure its
orbit more precisely, and plan a system for deflecting it from its current
orbit (hopefully away from the Earth's). There will be no great hurry, and
no great panic. It would be a project for all the world's nations to take
part in. It could be a globally unifying event. Because we will have found it
long before it actually hits the Earth, it probably would take only a small
impulse (chemical rockets, or perhaps mass drivers) to divert it from a
threatening path.
"There is a much smaller chance that we would find one that could impact in
the next 10 years. The chance of that happening is 1 in 10,000. If this
were to happen, we would probably still have time to launch a crash program
of scientific and technological research, with the goal of characterizing
both the structure of the menacing asteroid, and the best means for diverting
its orbit."
Risk Analysis
Now would be a good time to look at some of the risks we face in our daily
lives - risks that we don't think twice about taking. For example, according
to the Independent Insurance Agents of America:
"Today a motor vehicle accident occurs every second. Auto accidents cause an
injury every 14 seconds, and every 13 minutes a car accident results in a
fatality. More than 31 million accidents occur per year, at an annual cost
of almost $100 billion."
Yet most people continue to drive their automobiles regardless. For the same
reason, that we can't live our lives paralyzed by the fear that something bad
may happen, we shouldn't let the remote possibility of being struck by a meteor
or asteroid rule our lives.
Risk analysis is the process of determining the risks associated with an act
(driving a car to Florida for a spring break vacation, for example) or a
product (such as a food additive), etc. When considering risks, and determining
whether something is "worth" the risks it has, many factors must be considered.
For example, an event could be fairly likely to happen, but have only mild
consequences (such as the risk of getting a ticket if you park for 2 hours and
15 minutes in a two hour zone).
Or, it could be somewhat common and moderately serious (such as the chances
that you will get in a car accident of some sort during your driving lifetime).
Or, it could be very, VERY unlikely, but have devastating consequences if it
did (such as the chances that a 5 km comet will hit the Earth next year).
Sometimes the chances are so small that something will happen that we are
willing to accept the risk, the consequences if it does happen.
Many factors enter into a person's (or a society's) decision of what are
"acceptable risks". Some people are unwilling to consider any risks
acceptable, but it is hard to live life like that, since breathing and eating
and living on the Earth all have a certain amount of risk. The important thing
is to keep it all in a balanced perspective, and to realize that risk is a
part of life. It is important to minimize it as much as possible, of course,
but carrying it to an extreme can do more harm than good, too.
Who is watching the skies?
There are agencies, NASA among them, that realize
that although we are in no immediate danger, asteroid impacts are something
that could pose a problem somewhere in the future. Panicking about something
that could happen some day is not constructive at all.
However, it is in everyone's
best interest not to ignore this, but to be watchful in order to provide the
maximum warning time should a possible threatening situation occur. The
following organizations are doing just this:
The Near Earth
Asteroid Tracking Team
SpaceWatch
The Asteroid and Comet
Impact Hazard Group
Facts about asteroids
SEDS
Asteroid Page
Asteroid Page
Asteroid Fact Sheet
What
are asteroids?
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