Friday, February 5, 2016

Certain of uncertainty for near earth asteroids

In my continuing efforts to de-mystify the art of Asteroid Astrometry, I thought I'd follow up last week's article on about 2016 BE with a deeper examination of the Uncertainty parameter when its listed in orbital elements.

This week there is some attention on 2013 TX68 which will possibly make a record close pass of 11,000 klms or possibly be 40 times further away than the moon on March the 5th. I can see that puzzled look on your face ;-) 2013 TX68 is also a Virtual Impactor in 2017, a term we discussed last week.

So firstly lets get some perspective on this uncertainty thing.

2011 CF66 was also listed as a virtual impactor for Feb 2nd 2016, it didn't hit us, no-one was worried if it would, and in fact no-one has any idea where it actually is. It is only a tiny asteroid about 3-9m in diameter and wouldn't have done any damage even if it did. In fact there are 20 other virtual impactors listed in the Risk Table this year, the next one might approach on Feb 18th, is 2009 VZ39, and is slightly smaller than 2013 TX68. 2009 VZ39 is also in the daytime sky and not observable for follow up and further confirmation. I only highlight this to emphasise the point here - all asteroids once they are discovered need to be tracked for sometime, to improve the precision of the orbit before any pronouncements about where they are going to be at a certain point in time. The difference between 2013 TX68 and 2009 VZ39 for example is that 2013 TX68 was observed for 31 data positions (astrometry) over 3 nights where as 2009 VZ39 was observed on only one night with 8 astrometric data positions. If you look at the orbital elements for 2013 TX68 the uncertainty parameter is listed as 7, where as for 2009 VZ39 there is not even enough data to start that calculation. For 2011 CF66, there is a 1.1e-8 chance of a collision between 2016 and 2114, so its mathematically possible, but highly unlikely.

NASA/JPL produced this nice graph with it's press release this week which illustrates the point well.

Image Credit: P. Chodas (NASA/JPL)

What you see is a graphical representation of the "error bars" or the zone of uncertainty, based on the orbit elements that we currently know. This asteroid will be picked up again in future surveys and the zone of uncertainty will reduce further.

Uncertainty Parameter is quite a complex calculation, but it essentially always starts off being a "9" and reduces over ensuing months as more data is collected. Uncertainty is a table of the "Runoff" of arcsecs per decade and Level 7 just means essentially there will be less than 33,121 arcsecs of "runoff" over the next decade. You can think of this as being: in a decade the place to look will be 33,121 arcsecs bigger than the range of uncertainty we need to factor in when we look for it now.

Many of the "click-bait" bloggers and conspiracy theory followers regularly confuse Uncertainty with the Torino Scale - they shouldn't, as even WIKIpedia has a great explanation of the Uncertainty factor, however they google "Asteroid rating 7" and get a hit on what the Torino Scale is and confuse the two, without doing any further investigation. The Torino scale is a risk weighted table and ALL current Virtual Impactors are listed as Torino Level 0. The virtual impactors that are listed have "potential" collisions over a range of years and are only there because we largely don't have enough data yet to remove them.

Asteroid Apophis (99942) 2004 MN4 briefly shot up the Torino Scale to a record high Level 4, before subsequent radar imaging and 4452 observations over 10 oppositions reduced its Uncertainty Parameter to 0 and its Torino Level to 0, with an asterisk that it needs to be carefully watched. Asteroid Apophis will be a naked eye object on April 13th 2029 when it makes a very close but HIGHLY CERTAIN pass of the earth. This level of certainty is only refined by many hundreds of hours of dedicated work from professional and amateur astronomers.

Lastly, Uncertainty should not be confused with don't know, don't care, have no idea what we are doing. Its just a case of not YET having enough data, more needs to be collected.

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