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.

Asteroid 2016 BE Virtual Impactor for now in 2053

UPDATE 31/1/2016: The 2053 pass has been eliminated as a risk. There are still 5 virtual impacts from 2076 - 2111. These will also likely be removed as the precision of the orbit is improved.

Tonight, tracking potential virtual impactor Asteroid 2016 BE. Its a 79m wide Asteroid discovered on Jan 16th by the Catalina Sky Survey(703).

78 positions have so far been reported from 18 different observatories, including both H06 and I89, the iTelescope.net observatories. I managed to grab another 3 positions tonight. The asteroid is starting to speed up (in terms of relative velocity against the background stars) and tonight is moving quite quickly at 19 Arcsecs per minute. As a result of the fairly bright magnitude and the fast speed, you need to take as short an exposure time as possible. However the full moon is still pretty bright I had to use 30 sec images which means the asteroid is still slightly streaked. I have used a stacking technique to make sure my residuals are still reasonably good (given it is travelling at fairly high speed).

So what actually is a "Virtual Impactor". As asteroids are tracked the length of the recorded "arc" increases - now 10 days for 2016 BE. Of course the level of precision for future positions of the asteroid increases the longer the "known arc" and therefore the ability to determine future positions of the asteroid improves as the "zone of uncertainty" reduces. As long as the "zone of uncertainty" overlaps the earth's future orbit position, the object is listed as a virtual impactor for that pass. This happens from time to time without much concern, because as the precision of the orbit improves with more observations, the object usually drops out of the risk table quite quickly.

Whilst the asteroid will make a moderately close pass in the first week of February at 5.7 Lunar distances, its important to understand that the "virtual impact" currently listed in the Sentry Risk Table is for the 3rd February 2053 .... NOT ON THIS PASS!!!! As you would expect the "zone of uncertainty" for 2053 is much large than the "zone of uncertainty" for next week. So it will take many further observations before it is (most likely) removed from the risk table. Its current risk table score of Torino-0 just means that at this point the chance of a collision in 2053 has not been able to be eliminated at this stage.

Measured position:

K16B00E KC2016 01 28.46044 12 04 01.15 +65 50 49.0 16.9 R H06

K16B00E KC2016 01 28.46319 12 04 09.44 +65 49 54.8 17.3 R H06

K16B00E KC2016 01 28.46604 12 04 18.01 +65 48 58.7 17.4 R H06