[AZ-Observing] Re: [SPAM] Asteroid 2012 DA14 and SkySafari

  • From: Bob Christ <bchrist@xxxxxxxxxxxxxx>
  • To: az-observing@xxxxxxxxxxxxx
  • Date: Tue, 12 Feb 2013 11:37:37 -0700

Thanks for this Tom.  I had regenerated the ephemeris again yesterday using 
Starry Night Pro + and there was still no congruence with the data returned by 
Horizons.  Now I can stop "scratching my head."
Bob 


On Feb 12, 2013, at 11:21 AM, Tom Polakis wrote:

> This is only sort of related to observing in Arizona, so delete if you're not 
> interested.
> 
> People who use desktop and tablet planetarium software/apps have been 
> noticing that the predicted position of Friday's close-approaching asteroid 
> is many tens of degrees off.  The error is due to most software neglecting 
> the perturbation of the orbit due to Earth's gravity.
> 
> The authors of the SkySafari for iOS decided to do something about it.  The 
> latest release of SkySafari nails the position within arcseconds.  The 
> method, which includes some high math and understanding of orbital mechanics, 
> is explained below by co-author Tim Debenedictis.  I copied and pasted his 
> message from the southern-stars mailing list.
> 
> The mention of "4th order Runge-Kutta" made me flash back to my engineering 
> numerical methods class, in which the professor interrupted his lecture to 
> tell us that we'll never use this stuff.  Thankfully, in my case, he was 
> right.
> 
> Tom
> 
> =========
> 
> Apple has just released SkySafari Plus/Pro 3.7.3 for iOS, and we've released 
> SkySafari 1.7.3 for Mac OS X and Android.
> 
> About 2 weeks ago, we started to get these questions about 2012 DA14, and why 
> our apps fail to predict its position accurately.  The problem was that 
> SkySafari - like basically all other mobile and desktop planetarium apps - 
> models the motion of comets and asteroids as having a simple Keplerian, 
> elliptical orbit around the Sun.  We did not take pertubrations  by Earth's 
> gravity into account.  So any orbit that describes the asteroid's motion 
> before the flyby will completely fail to predict its position afterwards, and 
> vide-versa.  After answering this question the 327th time, I finally decided 
> to do something better.  
> 
> The solution was to build an "orbit integrator" into SkySafari.  Instead of 
> modelling the trajectory of the asteroid as a simple Keplerian ellipse, we 
> now model it using true N-body newtonian physics, taking the gravitational 
> perturbations of Earth (and the Moon, Venus, Mars, Jupiter, and Saturn) into 
> account, along with a decent numerical method (4th order Runge-Kutta).  It 
> took a few tries to get this right.  After reaching out to some professionals 
> in the field of solar system dynamics, we got some assistance JPL as well.  
> JPL has made clear that they cannot officially endorse any commercial 
> product, but I can confidently say they were impressed that we got this 
> working on a smartphone at all.
> 
> We did take some shortcuts.  Our model does not take relativity into account, 
> nor the oblateness of the Earth.  We also don't include Mercury, Uranus, or 
> Neptune in the set of asteroid-perturbing masses.  (Smartphones, whiie very 
> capable, are still not supercomputers.  The code still has to run fast enough 
> to be useable.)  Nevertheless, even with these shortcuts, our model predicts 
> the position of 2012 DA14 to within arcseconds of JPL's positions on the day 
> of the flyby and for many days before and after.  We've even run our 
> integrator out 9 years into the past or future, for a selection of different 
> asteroids and comets, and in nearly all cases it does much better than a 
> simple Keplerian orbit.  So we're very confident that we're doing the math 
> right.
> 
> Anyhow, that's the theory.  In practice, here's what you need to know.
> 
> 1) Plus and Pro only.  The integrator is not in the Basic version.
> 
> 2) You still need to update your minor body orbit data, just like you did 
> before.  The orbit integrator is useless if it doesn't have a decent orbit to 
> start from.
> 
> 3) The integrator only gets "turned on" when you select a particular comet or 
> asteroid, and it's only used for the asteroid/comet you selected.
> 
> 4) We run the integrator for one year in the past and future from the date of 
> the orbital elements, using a 1-hour timestep.  All that math takes about 1.5 
> seconds on my iPhone 4, which is the slowest device I own.  (You'll feel a 
> brief pause when first selecting a new asteroid or comet).
> 
> 5) Since we only run the integrator one year from the date of issue of the 
> orbital elements, don't try to use it to find out if asteroid Apophis will 
> hit Earth in 2036.  It won't work.  At least not in this release.  Yet.
> --
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> 

 




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