Earlier, I was looking for the current distance from Earth to 67P/Churyumov-Gerasimenko, in order to do a "back of envelope" calculation of signal delay. Not important, but I was somewhat puzzled to find quite a large discrepancy between values reported by different sites, and would love to understand why.

ESA's Where is Rosetta page currently gives the distance as around 719m km, which, if I'm using it correctly, is backed up by JPL Horizons tool, which gives me a delta of around 4.81 AU. Which is great.

But when I look at The Sky Live's 67P live tracker, it is reporting around 941m km, which their comet info page gives as around 6.3 AU. Am I missing something blindingly obvious?

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    $\begingroup$ In fact, none of the other numbers match up well either, and HORIZONS says the comet is receding while the live tracker (which claims to use HORIZONS data or similar) says the comet it approaching. 99+% sure that the tracker is wrong and is either using an older ephemeris or computing from a body other than Earth. $\endgroup$ – user21 Oct 1 '16 at 0:48

I suspect that The Sky Live got their comet IDs mixed up. The orbital elements show perihelia in Aug 2015 and Jan 2022, aphelion 5.69 AU, period 6.45 years, consistent with other sources e.g. Minor Planet Center. However, the distance chart shows a perihelion in mid 2018, aphelion about 11 AU, period about 16 years, clearly not the same object. Report the bug and they'll probably fix it.


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