Telescope Reviews: New SkySafari version and 2012 DA14

It's time to spill the beans. 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.

So that's it for now. Have fun with it, and let us know how it works for you.

-Tim & Bill