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Undetected asteroids and Earth


Cerebus06
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The Chelyabinsk meteor has been suggested as an example of the potential of asteroids not being detected until just prior to impacting or grazing the earth.  In this case, it was - to quote a Business Insider article in 2013 - "... because it came out of the daytime sky. These are nearly impossible to find ahead of time because telescopes can only spot asteroids during the night." (quoted from http://www.businessinsider.com/heres-why-astronomers-did-not-detect-the-russia-meteor-ahead-of-time-2013-2).

First, is this article's claim about the daytime sky as accurate as the article suggests?

My primary question, related to this, is whether or not it would be possible for a rock big enough to cause an extinction level event to remain undetectable until, at most, just days before impact.  If this is indeed possible, what are the possibilities that could cause it?  For example, perhaps a highly elliptical orbit perpendicular to the orbital plane of the solar system?  Or maybe the asteroid is composed of rock / metals that are dark enough that its albedo wasn't bright enough to be noticed until it was close?

I deeply appreciate anyone's input.

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20 hours ago, Cerebus06 said:

The Chelyabinsk meteor has been suggested as an example of the potential of asteroids not being detected until just prior to impacting or grazing the earth.  In this case, it was - to quote a Business Insider article in 2013 - "... because it came out of the daytime sky. These are nearly impossible to find ahead of time because telescopes can only spot asteroids during the night." (quoted from http://www.businessinsider.com/heres-why-astronomers-did-not-detect-the-russia-meteor-ahead-of-time-2013-2).

First, is this article's claim about the daytime sky as accurate as the article suggests?

My primary question, related to this, is whether or not it would be possible for a rock big enough to cause an extinction level event to remain undetectable until, at most, just days before impact.  If this is indeed possible, what are the possibilities that could cause it?  For example, perhaps a highly elliptical orbit perpendicular to the orbital plane of the solar system?  Or maybe the asteroid is composed of rock / metals that are dark enough that its albedo wasn't bright enough to be noticed until it was close?

I deeply appreciate anyone's input.

When Congress created the NEO Search Program in 1998 they tasked NASA with finding 90% of the near-Earth asteroids that are one kilometer or larger.  Then in 2005 Congress extended NASA's objective to include 90% of the NEOs larger than 140 meters.  While the majority of NEOs have been identified via ground-based telescopes, they are not the only sources.  Both WISE and NEOWISE were space-based infrared searches for NEOs before their cryogens were exhausted by 2010.

It is a question of size.  Since 1998 NASA has discovered ~98% of the NEOs that are one kilometer or larger.  The percentage of known NEOs drops according to their size.  At 140 meters in diameter fewer than 1% are known.  However, while a 140 meter diameter meteorite impact could easily wipe out a large city, it would not cause an extinction level event.

What was meant by the asteroid approaching from the "daytime sky" is that the asteroid approached Earth from the direction of the sun.  Even with infrared sensing satellites we would not be able to see something approaching Earth from the direction of the sun, unless the satellite was closer to the sun than the NEO.  It is our thermal blind-spot.

Depending on the density and composition of the NEO, it will need to be between 20 and 25 meters in diameter in order to impact with the surface and form a crater.  The Chelyabinsk meteor came very close to impacting the planet and it was estimated to be just over 15 meters in diameter.  In order to be the cause of an extinction level event the NEO would have to be very large indeed.  The asteroid that impacted the planet ~65 million years ago was estimated to be 12 kilometers in diameter.

Currently, the known NEO with the highest probability of impacting Earth is 2010 RF12.  Sometime between 2095 and 2117 there is a 5% chance that the asteroid may impact Earth.  At only ~7 meters in diameter the NEO does not pose much of a threat.  The odds of an unknown NEO causing an extinction level event is extremely unlikely considering the size it would have to be.  However, it is only a matter of time before a 50 to 100 meter NEO that we didn't know about impacts the planet and could very possibly kill millions if it impacts a major city.
 

Sources:
NEO Search Program - Jet Propulsion Laboratory, Center for Near-Earth Object Studies
Sentry: Earth Impact Monitoring - Jet Propulsion Laboratory, Center for Near-Earth Object Studies
Absolute Magnitudes of Asteroids and a Revision of Asteroid Albedo Estimates from WISE Thermal Observations - Icarus, Volume 221, Issue 1, September-October 2012, Pages 365-387 (free preprint)
The International Astronomical Union Minor Planet Center
Asteroid Impact Effects and their Immediate Hazards for Human Populations - Geophysical Research Letters, April 19, 2017 (free preprint)

Edited by T. McGrath
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Just a quick comment/question. It was initially predicted that Asteroid 2012 TC4, which is estimated to be 30m in size, will pass Earth at a distance of approx. 6,800kms (which is mighty close). They have since, only in July, recalculated and adjusted that estimate (based on more recent observations) to approx. 50,000kms. This asteroid will pass Earth this Thursday, merely 3 months after its orbit has been more accurately assessed. So, in theory, it could have been the other way around, no? If it was miscalculated the first time and we only find out that this asteroid was actually on route to impact Earth, what can be done in 3 months..? Not much, I assume. This is "only" a 30m asteroid, but even so it could have been extremely dangerous and might have caused major damage if it was heading towards a highly populated city.

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Arguably equal, or greater risks may come from comets. Such risks are of two types:

  • Centaurs
  • Long period comets

Centaurs

The Centaurs, sharing chracteristics of asteroid and comet, have unstable, eccentric orbits between Jupiter and Neptune. They are larger than NEAs, meausred in 10's of kilometres.(The largest identified so far is around 260 km in diameter.) Their unstable orbits and interaction with one or more of the giant planets deflects them towards the inner solar system (and Earth crossing orbits) where they break up.

Recently researchers have pointed out that the resultant debris is still very large and more than one component is likely to impact the Earth. Such impacts could continue over a period of around 100,000 years.

Long Period Comets

Most comets that visit the inner solar system are short period comets, with periods from a few years up to 200 years. Halley's comet is an example. They are thought to have originated in the Keuper Belt. There are orbits are reasonably well known.

In contrast long period comets come from the Oort cloud and have periods measured in hundreds, thousands and - in some cases - millions of years. The danger they present compared with NEAs is threefold:

  • Larger than most NEAs
  • Traveling two per three times faster and therefore possessing much higher kinetic energy
  • Unknown until they are already en route, giving very little reaction time for mitigation

 

 

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On 10/10/2017 at 1:17 AM, Memammal said:

Just a quick comment/question. It was initially predicted that Asteroid 2012 TC4, which is estimated to be 30m in size, will pass Earth at a distance of approx. 6,800kms (which is mighty close). They have since, only in July, recalculated and adjusted that estimate (based on more recent observations) to approx. 50,000kms. This asteroid will pass Earth this Thursday, merely 3 months after its orbit has been more accurately assessed. So, in theory, it could have been the other way around, no? If it was miscalculated the first time and we only find out that this asteroid was actually on route to impact Earth, what can be done in 3 months..? Not much, I assume. This is "only" a 30m asteroid, but even so it could have been extremely dangerous and might have caused major damage if it was heading towards a highly populated city.

Your assertion is correct.  The more observations that are made, the more accurately the asteroid's orbit can be calculated.  Asteroids of such small size (~30m) are extremely difficult to detect.  Even when found trying to find it again can be problematic.  Yet these are the asteroids that are most likely to impact Earth in the short term.  Approximately 100 tons of dust and sand impacts the planet daily.  Asteroids the size of automobiles burn up in our atmosphere annually on average.  NASA estimates that only once every 2,000 years an asteroid 300 meters in diameter impacts the planet.

In order to come up with a means of mitigating the problem, a great deal about the asteroid needs to be known.  Such as its density and composition.  Is the asteroid a stereotypical carbonaceous chondrite (C-type), made of mostly metals (M-type), or a rubble-pile loosely held together (S-type)?  Each different type would require a different form of mitigation on our part.  That is assuming we had enough time to make such observations and time enough to act which, considering the size of the NEO, seems highly unlikely.

NASA has the asteroid 2012 TC4 passing Earth today at 26,000 miles (~42,000 km) with a diameter between 15 meters and 30 meters.  As the name suggests, it was first discovered in 2012 by a ground-based telescope in Hawaii, but then they lost it.  It was rediscovered again, as you say, in July 2017.  I was able to find out a great deal about asteroid 2012 TC4's orbit, however, I was unable to find out anything about the composition of the asteroid.

Sources:
Asteroid Fast Facts - NASA, March 31, 2014
Asteroid Tracking Network Observes Oct. 12 Close Approach - NASA, October 10, 2017

Edited by T. McGrath
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On 13/10/2017 at 0:04 AM, T. McGrath said:

I was able to find out a great deal about asteroid 2012 TC4's orbit, however, I was unable to find out anything about the composition of the asteroid.

Try this article. Based ujpon observations made during the 2012 approach the authors conclude that the small diameter, elongate shape and fast rotation make it unlikely that the body is a "rubble pile". This narrows the compostional options. This paper notes the probability that the asteroid is tumbling, reinforcing the suspicion that it is not a "rubble pile".

 

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