Four  (4) Passes of the ISS & Cygnus Cargo Spacecraft
Wednesday Night, May 23-24, 2018


On Wednesday night, May 23-24, 2018, there would be four (4) passes of the ISS (International Space Station) visible from Maple Shade, NJ, if weather permitted. This Bob King online article at Sky & Telescope explains the orbital geometry of the earth and the ISS that briefly produces the all-night visibility this time of the year. In addition, the Cygnus cargo spacecraft, launched Monday morning, May 21, should be visible as it was catching up to its ISS destination. It had been spotted once on Tuesday morning at 3:27 am during one of six ISS passes seen. Tuesday night, May 22-23, was cloudy, so no passes were seen.

The four ISS passes would peak at 8:46 pm, 10:43 pm, midnight, and finally, 1:38 am EDT. I still didn't have any good information on the position of Cygnus relative to the ISS, other than having seen it on Tuesday morning around 3:30 am, five or six minutes behind the ISS. I did find it listed at Cal-Sky, but besides Cal-Sky being somewhat inscrutable, it had the Cygnus ahead of the ISS, so I dismissed it. It seemed the Cygnus should be behind, but getting close to the ISS as their rendezvous was scheduled for 5:20 am on Thursday, May 24. In brief, the weather was clear, all the ISS passes were seen with unaided eyes and the Cygnus was seen all four times with 15x56 binoculars from my front yard (or the street out front).

On the first pass, the ISS would go just above Denebola in Leo, which in turn was not far above the gibbous Moon, which provided a handy guide to finding Denebola in the binoculars during relatively bright twilight. As the bright (and therefore easily visible with unaided eyes) ISS approached Leo, I aimed the binoculars at Leo's hindquarters and waited for it. The ISS burst brightly through the field. I kept Denebola at the bottom of the field after the ISS left, and a fainter object (≈ magnitude +2) went through on nearly the same path, perhaps 10 to 15 seconds later. I didn't think to count, so later, as the ISS dropped in the east, I noted when it passed a tree branch and counted 15 seconds until the Cygnus passed the same branch (counting one-thousand-one, one-thousand-two, etc.).

The subsequent passes were in darkness (after the end of twilight), so I had field stars visible. Counting the interval between the ISS passing a particular star and Cygnus passing the same star, Cygnus was 13 seconds behind on the second pass, 8 seconds behind on the third pass and 5 seconds behind on the fourth pass (roughly four hours before the scheduled rendezvous). Since these times were not precision measurements, and there are no correction factors for apparent altitude or the distance, don't make too much out of them besides suggesting Cygnus was catching up to the ISS (no surprise in that).

So, it was a good few nights of ISS and Cygnus spotting this week. On three nights, May 20/21, May 21/22 and May 23/24, I saw 13 of 15 visible ISS passes, and the accompanying Cygnus cargo spacecraft 5 times. I also set a personal record of six (6) ISS passes in one night, May 21/22 (probably the most that could be seen in one night).



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Last Update: Monday, May 28, 2018 at 09:08 PM Eastern Time