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Clearing Up Confusion About The Number Of Potentially Hazardous Asteroids

Updated on June 19, 2014

Depending on which news report you might read, you most likely will encounter different numbers that represent how many potentially Earth-killing asteroids exist.

What Exactly Is An Asteroid?

The term "asteroid", at first, appears to refer to any of the million or so relatively small, metallic, rocky bodies without atmospheres that orbit the sun. On closer inspection, however, the term "asteroid" appears to be ill defined. One source ... suggests that an asteroid must be at least eleven yards across. Another source ... suggests that a yard or so is the minimum size, and that larger ones range upward to 20 miles across. A third source ... goes so far as to imply that asteroids can be as small as dust particles.

Regardless of what name you might choose, the most important thing to realize is that there are various sized pieces of naturally occurring outer space debris, left over from the solar system's formation.

In this article, we are concerned with pieces of outer space debris large enough to pose potential catastrophic collision threats to humanity.

Humanity As An Asteroid Target

The surface of Earth is about 72% water and 28% land, where humans populate only 3% of the land surface (or 0.8% of the entire planetary surface). Earth rotates on an axis, and it revolves in an orbit around the sun, which makes this 0.8% populated surface area a target that never sits statically in one place for very long. In other words, when you add it all up –

  • Earth’s 0.8% populated surface area
  • Earth's axial rotation
  • Earth's orbital revolution

– you can see that any relatively small chunk of outer space debris randomly entering and surviving passage through our atmosphere most likely will strike an unpopulated area of the globe.

But what is "relatively small", and what sorts of space rocks typically hit our planet?

According to NASA ...

  • Every day, more than 100 tons of dust and sand-sized particles bombards Earth from outer space.
  • About once a year, an automobile-sized asteroid hits Earth's atmosphere, creates an impressive fireball, and burns up before reaching the surface.
  • Every 2,000 years or so, a meteoroid the size of a football field hits Earth and causes significant damage to the area.
  • Only once every few million years, an object large enough to threaten Earth's civilization comes along.

As you can see, even NASA ping pongs between different terms to name this Earth-assaulting outer space debris (using the term, "asteroid" to describe an "automobile-sized" object in one instance, only to revert to the term, "meteoroid" to refer to a much larger "football-field" sized object in another instance). Again, regardless of the terminology we might use to describe it, "stuff" from outer space regularly hits the Earth. Some of this "stuff" is harmless to humanity, while some of it is potentially catastrophic to humanity.

NASA Map of 1483 Known Potentially Hazardous Asteroids

NASA map of known potentially hazardous asteroids, showing 1,483 overlapping orbits
NASA map of known potentially hazardous asteroids, showing 1,483 overlapping orbits

A Ball Park Figure For The Number Of Potentially Hazardous Asteroids

According to NASA's official Near Earth Object Program ... "There are currently 1483 known potentially hazardous asteroids."

According to an earlier, 2012 Jet Propulsion Laboratory ... report ... about NASA's Wide-field Infrared Survey Explorer (WISE), there are between 3,200 and 6,200 potentially hazardous asteroids.

In stark contrast to these numbers, a 2012 editorial by Joseph N. Pelton ... , President of the International Space Safety Foundation, provides an estimate of around 20,000 potentially hazardous asteroids.

This number, however, pales in comparison to the estimate by Ed Lu and Martin Rees in a February 13, 2013 article, A Warning From The Asteroid Hunters, in THE WALL STREET JOURNAL. Lu is CEO, and Rees is Strategic Advisor for the B612 Foundation ... , whose mission is to find and to stop dangerous asteroids from colliding with the Earth. According to this article, "The number of asteroids in dangerous orbits near Earth, large enough to do great damage if they struck, is about one million, but scientists have calculated trajectories for fewer than 10,000 of these (less than 1%)."

So, which number, readers might ask, is the most truthful ball park figure – 1,483 or 3,200 or 6,200 or 20,000 or 1,000,000? There is quite a disparity here.

NASA's 1,483 appears to be the number of KNOWN potentially hazardous asteroids (i.e., charted orbits). The other intermediate numbers appear to come from statistical determinations. Lu's and Rees's number of a MILLION, on the other hand, seems difficult to reconcile with the lower numbers. Are Lu and Reese simply wrong, or do they know something that the other sources are not fully disclosing? I do NOT know the answer.

One thing seems clear: Regardless of the precise number of potentially hazardous asteroids, there appear to be enough to be of real concern.

Map Of Asteroid Impacts In Earth's Atmosphere Since Year 2000

B612 Foundation Video Visualization Of Asteroid Impacts In Earth's Atmosphere Since Year 2000

Should We Be Afraid?

Any number between 1,500 and 1,000,000 potentially hazardous asterioids is cause for concern. But should we be afraid?

Referring again to THE WALL STREET JOURNAL article by Ed Lu and Martin Rees (cited previously):

  • The chance of an impact somewhere on Earth this century by an asteroid 140 feet across and explosive energy of 700 Hiroshima atomic bombs is about 30%.
  • The chance of an even bigger asteroid impact on Earth—with explosive energy of over 6 THOUSAND Hiroshima atomic bombs—is about 1%. This risk to humanity is similar to an individual's odds of dying in a car accident. “That risk is small, but would you drive a car without air bags and seat belts? The question is apt because our society is effectively doing so with regard to the risk of a devastating asteroid strike.”
  • The chance of a three-thousand-foot wide asteroid impact—with explosive energy of over 2 ½ MILLION Hiroshima atomic bombs—is about 0.001%, which is about the odds of the average person’s dying in an earthquake. Such an asteroid would wipe out humanity, no matter where it hit. NASA has found and is tracking 90% of these, but 10% remain undiscovered.

Consequently, I think the answer is that we should be not so much "afraid" as concerned enough to consider dangerous asteroid discovery and impact prevention a priority.

Simulation Of A Huge Asteroid HItting The Earth

Simulation Of A Huge Asteroid Hitting The Earth is a video clip from MIRACLE PLANET - THE VIOLENT PAST, a six-part documentary series, co-produced by Japan's NHK and the National Film Board of Canada, published February 23, 2013. The original, full length video can be found at ...


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