Mars + phobos + deimos + interloper Image 2016/04/26(UT)
Anthony Wesley (406mm Newton)
There was a transit of Phobos this morning, so I grabbed some images of it just before it crossed in front of Mars. Deimos was also in the frame and - to my surprise - so was another faint object on the opposite side of Mars which was approximately the same brightness as Deimos.
Here's an image showing Phobos just before it enters transit in front of Mars - you can see it at lower left very close to Mars, and also Deimos further out on approximately the same path. The unidentified object is at upper right.
I've also included a 2-frame animation showing a timelapse of 17 minutes, the interloper moves quite some distance relative to Mars over this time - at a guess I'd say it's moved about 5 arc seconds on a track almost perpendicular to the orbital plane of the moons.
Note: Phobos is not visible in the second frame of the animation as it's too close to Mars to see and lost in the glare.
Mars' proper motion over this timelapse is very small, much less than 1 arc second, I'm not sure the interloper could be a background star - anybody have any ideas? My guess is that it must be an asteroid but I'm happy to be corrected.
To be sure there were no hot pixels or camera artifacts present, Mars was moved constantly left<->right across the frame at slow speed during the captures. You can see the residual noise is spread out in this way, so anything that stays put must be signal and not noise.
A quick update to my previous email - the interloper is indeed a star (thanks Phil). USNO J1625340-213402 with magnitude approximately the same as Deimos.