I posted this on Astromart on Sunday, but so far no-one has tackled my question. So, let's try here.


I'm usually not very interested in tracking satellites, but I spotted something unusual (to me) this weekend and would like some help figuring out what I saw.

First, some background:

I am a long-time member of Central Valley Astronomers in Fresno, California. I spent Friday and Saturday night at my club's high elevation observing site near Courtright Reservoir in the Sierra Nevada mountains east of Fresno (3704'54" N, 11857'47" W, elevation 8140 ft ....and primitive camping).

At about 00:50 on August 3 (i.e. very early Saturday morning), I was attempting to find asteroid (15) Eunomia. My SkyTools 3 observing plan gave a location of RA 21h32m15.8s / DE -0540'33" (JDATE at the transit time of 01:42). The SkyTools 3 finder chart I printed gave a location of RA 21h30m03.5s / DE -0555'48" (J2000 at 01:00).

I was using a Celestron CPC-1100 HD and a 41mm Panoptic eyepiece, which gives a calculated FOV of 59.7 arcminutes. I entered the JDATE coordinates into the hand controller (appropriate since the mount was aligned on stars that themselves are at JDATE coordinates) and found the exact star field shown in the J2000 finder chart. I always find this match to be encouraging.

I marked on the finder chart about two dozen of the brighter stars (and a few fainter ones) that I saw in the eyepiece, including Beta Aqr. However, I didn't spot the asteroid, which at magnitude 8.4 would have been brighter than about half of the stars I marked. Unfortunately, sometimes a SkyTools 3 finder chart for an asteroid completely misses the mark, even with updated orbital elements. It's happened to me maybe 6 times out of 100. Also, most of the time, the asteroid that is plotted in the center of the FOV is based on "extrapolated orbital elements" and a second version of the asteroid is plotted nearby that is based on a recent epoch for the orbital elements (in this case 2017.13). I have found that the "off center" version of the asteroid is almost always where I actually see the asteroid when I look, so my current practice is to recenter the finder chart to the latter version of the asteroid.

Now, what I saw:

As I was marking stars on the finder chart (some as faint as 13th magnitude) I spotted a faint, equal brightness "double star" that wasn't on the finder chart. I roughly estimated both "stars" to be about 11th magnitude and the separation to be maybe 45 arcseconds. Then I noticed that the "double stars" were moving....in a south-to-north direction....very nearly through the center of the FOV....at about the speed that I have seen other polar satellites move. Intriguing.

When I got home Sunday afternoon, I did a Google search for "twin polar satellites" and the only thing that came up were the Grace and Grace Follow-on missions. The Grace satellites are no longer in orbit, while the Grace-FO satellites were launched in May 2018 and are still active.

Except for ISS, I have limited experience tracking satellites. The satellite tracking app on my iPhone has at least 100 satellites, but Grace-FO isn't included. There is probably something on the Internet that would allow me to translate the ground track into the sky for my location, time, and FOV, but I don't know where to start. I did a few Google searches using relevant terms, but didn't find anything that would allow me to verify which satellite pair went through my FOV.

So, CosmoQuest experts, any ideas? Thanks in advance.