Telescope Reviews: Satellite Passes in Front of the Moon

The YouTube videos would be more useful if there was some documentation as to when and where they were taken and what the frame rate is.

I know very little about this, but I'm surprised that more respondents haven't cautioned against being too quick to interpret dark objects seen drifting across the Moon as Earth-orbiting satellites seen in silhouette. None of the objects depicted in these videos look like satellites or space junk to me. Some are out of focus compared to the Moon, which means they must be extremely close to the telescope, and one particularly oddly shaped one (although sharply focused and therefore somewhat distant) is followed by a wake of bad seeing (which surely indicates, if the video is even real, that it is a powered aircraft). Most appear to me to be drifting balloons or clumps of balloons.

To understand the problem, note that everything in low earth orbit resolves at a speed of about 7 to 8 km/sec. That means that when passing overhead they move, as "Ptarmigan" points out, very quickly. At its rather low elevation of 330 km, the ISS (unlike anything in the YouTube videos) crosses the entire diameter of Sun or Moon literally in the blink of an eye (~0.5 sec).

Objects at higher altitudes than the ISS will take longer to cross the Moon, just as a distant plane seems to be moving more slowly than one moving at the same speed nearby. But if they are farther away they will also look smaller. An extreme example would be a geostationary communication satellite which, being by definition at a fixed point in the sky, would, like a non-tracking telescope, sweep over the 0.5 deg diameter of the Moon in about 2 minutes. But to accomplish this degree of slowness it has to be about 36,000 km away (nearly 1/10th the distance to the Moon), meaning it will look small indeed. At that distance even an extremely large satellite with a 10 m wingspan would be only 0.06 arc-sec across -- barely big enough to blot out a 100 m crater on the Moon and much too small for any amateur telescope to detect as a dark dot in transit.

The biggest problem with the YouTube objects is that they move across the Moon extremely slowly compared to the ISS -- meaning that (if satellites) they are in much higher orbits -- yet in angle they look comparable to the ISS. This leads to the improbable conclusion that if they are satellites and/or space junk they must be larger than the ISS; or to the much more likely conclusion that they are mundane nearby objects (like balloons) floating across the telescope's field of view.

Another way to look at this is that moving (like most other things in orbit) at 7-8 km/sec, even the largest piece of space junk, the ISS, with a maximum dimension ~75 m, moves by 100x its own diameter in 1 second. A more conventional, but still very large orbiting object with a diameter of 8 m will move by 1000x its own diameter in 1 sec. Smaller pieces of space junk will move (in inverse proportion to their size) by many thousands of their own diameters in 1 sec. It would seem to me, then, that if you watch the dark silhouette of something drifting across an overhead Moon, and in 1 second it hasn't moved by many 100's of its own diameter, you can be pretty sure it isn't in orbit.

The objects in the YouTube videos are all many pixels across: if they are Earth-orbiting satellites viewed overhead it would seem to me they should cross the field in a fraction of a second.

-- Jim