[So it’s December again and time for us to do another piece on the end of the world. People worry about that at the winter solstice; I’m not sure why; maybe it’s because the days are dark and cold, and nobody’s religious these days. A friend once told me that when religion goes out superstition invades. Of course, he’s religious, so he has a bias; but there’s no denying that apocalyptic speculations are more popular in December than they were when I was a kid. Back then we concentrated on Christmas, instead.

Remember the Mayan calendar, and  how people claimed that everything would end one December because the Maya predicted it? I thought that was nonsense at the time; it rested on the notion that because the calendar ended on a certain day, that was a prediction. Perhaps there was a page 2 that we simply hadn’t found. Page 1 was a huge stone slab, weighing tons; we should have dug around for a while to find out if there were more carved stones in the area. Or perhaps the ancient Maya simply ran out of energy; after all, eventually they failed as a civilization; and hadn’t gotten around to finishing. Or, and nobody wants to admit this, perhaps the Maya were making a prediction, but were wrong. After all, we all make mistakes from time to time, and the Maya were only human. Or did you think otherwise?]

Oh, sorry, I forgot to introduce myself. I’m Phil, this blog’s resident [amateur] philosopher, and I’m going to discuss other, more scientific threats to our world, and more specifically, the possibility that something big from outer space will fall on us some day. We did a blizzard of posts last month, and one of them briefly mentioned that, according to the National Aeronautics and Space Administration (NASA), nothing like that would happen this month.

The threat’s real enough, although remote, and there’s lots of good TV that describes the general problem. I’m particularly fond of a recent program put out by National Geographic.[1] You can find it at http://www.bing.com/videos/search?q=Asteroid+Earth+Impact+Update&view=detail&mid=635CCEA3EDB6A7FAAE61635CCEA3EDB6A7FAAE61&FORM=VIRE  It says our astronomers know most of the really big things that might hurt us, but really don’t know much about the smaller, but still dangerous stuff.

Specifically, according to that program there are:

  • Approximately 940 objects 1 kilometer or larger that might threaten us; of those we have found 86%. If one of these hits, life as we know it might change dramatically, even if we survive for a time.
  • Approximately 50,000 objects 100 meters or larger that might threaten us; of those we have found 10%. That’s a wide range, don’t you know? The effects could range from a localized catastrophe to something much more deadly to us and the world.
  • Approximately 2,000,000 objects 30 meters or larger that might threaten us; of those we have found less than 1%[2]Even one of the small ones [30 meters] could make a real mess if it hits where people are.[3]

Of course, unlike the Mayan Calendar these numbers are not written in stone. They’re simply the current estimates of specialists working in the field.

As you might guess, NASA has programs to locate all of these objects, determine which ones, if any, will eventually strike us, and how much damage they might do. NASA’s current findings are summarized in its “Sentry Risk Table,” which is available the public.[4] No security clearance is required to access it.

Let’s get back to basics for a minute. Mostly we’re worried about asteroids striking our planet. An asteroid is a small body in the inner solar system that orbits the Sun. [5]   Asteroids generally are rocky or metallic in nature. So which ones threaten us? Any big one, if its orbit might intersect our own around the Sun.  These are called Potentially Hazardous Asteroids (PHAs).[6]

Why do I say “might?” Either an orbit will or won’t intersect ours; that should be simple to calculate, and once we have the answer, it shouldn’t change. Right? Not really. We’re not talking about watchmaking here. The orbits of these things are calculated based on observations of small, dim objects seen at a considerable distance. That kind of data must be verified, cross-checked, corroborated for accuracy, and so forth. Also, orbits may change. An asteroid’s path around the sun may take it to places where it collides with other objects, or is affected by their gravity. And, if one actually comes near us that alone might affect its orbit in the future.

NASA currently assigns an “impact hazard” rating to near-earth objects to quantify their potential danger to the planet. To do this it uses two different rating systems, the so-called Torino Scale, and the more recent Palermo Scale. I don’t pretend to understand the math involved[7], but the basic concepts are relatively simple.

The Torino Scale This considers (i) the likelihood that an object will actually collide with our planet, (ii) the severity of that impact, if it occurs, and (iii) assigns a hazard rating based on a combination of the two. The rating may be expressed as a color or a number. Thus, for example, if an object is unlikely to collide with the earth, or is so small that it would burn up in the atmosphere, it would be labeled “white” or rated a zero [“0”]. (no hazard). If another object definitely will hit the planet, and is so large that it may affect the future of civilization as we know it, that would be labeled “red” or rated “8 – 10”.

Red [“8 – 10”] events occur perhaps once every 100 thousand years.[8]

The Palermo Scale[9]  The newer Palermo Scale considers the same factors as the Torino Scale – i.e., (i) an object’s orbit and the likelihood that it will hit the Earth at some point and (ii) if it does the amount of damage it might do. It also compares that result with so-called background hazard, i.e., with “the average risk posed by objects of the same size or larger over the years until the date of the potential impact,” [10] and from that derives a single hazard rating.

  • A rating of +2 or above indicates the hazard presented by the object is 100 times [or more] greater than a random background event;
  • A rating less than −2 indicates the object will have no likely consequences for us; and
  • A Palermo Scale value of 0 indicates that the event “is just as threatening as the background hazard.”[11]

Some criticize the Palermo scale as confusing to the layperson, i.e., to you and me, because it often produces negative ratings that imply “a less than zero chance of impact.” [12] That’s not the intention. Negative numbers indicate that risk is lower than the so-called “background hazard,” not non-existent. Nevertheless, under Palermo only “[p]ositive values suggest that some level of concern is merited.”[13]

Now let’s go back to the NASA Sentry Table[14] and see what threatens us.            If you open it and look to the right, you’ll see that NASA obligingly records both the Torino and Palermo values for each object listed, and that, for the most part, the Torino values are “0” and the Palermo values are comfortably negative. That’s pretty much the case for all items on the list [18 pages worth] except, of course, where ratings have yet to be calculated. So are we home free?

Yes, for the moment. But remember, we don’t know everything about what’s up there in the sky, and that’s why we keep looking.[15] There could be bad news lurking. On the other hand, don’t be surprised if something pops up as potentially dangerous, stays on the list for a while, is studied and examined, then is taken off [the list]. Over time that’s happened to quite a few NEOs.[16]

So for now the situation in outer space is favorable to humans. There’s nothing really big scheduled to fall on us. If civilization as we know it is really going to end, it looks like we’ll have to do the deed ourselves.

[1] See National Geographic, Star Staff, Asteroid Attack (documentary) (May 27, 2016). The hypertext Link to the program is given in the text, above.

[2] You can get this information by scrolling about a third of the way through the program.

[3] The surface of the Earth is 70% water, more or less.

[4] See NASA, Near Earth Object (NEO) Program, Sentry Risk Table (updated periodically), available at http://neo.jpl.nasa.gov/risk/

[5] Inner solar system means “inside the orbit of Jupiter.” See the discussion in Wikipedia at Asteroid, http://en.wikipedia.org/wiki/Asteroid

[6] See NASA, Near Earth Object Program, NEO Groups at http://neo.jpl.nasa.gov/neo/groups.html  Although we’re discussing asteroids this time around, NEOs also include comets.

[7] Want to try? Take a look, for example, at Prezi.com, O’Leary, Palermo Technical Impact Hazard Scale (Presentation) (11 January 2013), available at https://prezi.com/qmahb4iwkbom/palermo-technical-impact-hazard-scale/

[8] If you want to know more about the Torino Scale, check out the Wikipedia entry on it at https://en.wikipedia.org/wiki/Torino_scale

[9] See NASA, Near Earth Object Program, The Palermo Technical Impact Hazard Scale, available at http://neo.jpl.nasa.gov/risk/doc/palermo.html

[10] If you want another view of the Palermo Scale, check out the Wikipedia entry on it at https://en.wikipedia.org/wiki/Palermo_Technical_Impact_Hazard_Scale

[11] See note 8.

[12]  See World Wide Words, Investigating the English language across the globe, Palermo Scale, available at http://www.worldwidewords.org/turnsofphrase/tp-pal1.htm

[13] Id.

[14] See note 3.

[15] See NASA, Near Earth Object Program, NASA Releases Near-Earth Object Search Report (last update, 12/03/2016), available at http://neo.jpl.nasa.gov/neo/report.html

[16] See, NASA, Near Earth Object Program, Objects Removed, (no current update) available at http://neo.jpl.nasa.gov/risk/removed.html

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