The two biggest objects orbiting the low earth orbit, International Space Station (ISS) and the Tiangong Space Station (a.k.a. Chinese Space Station or CSS) can be seen transiting the Sun every now and then. But because their orbital characteristics are different, it's hard to see the two like that on the same day within the 200 km range. And it's much rarer for the two to be seen in front of the Sun at the same time. But earlier this month, I found out that I could see the next best thing - seeing the two make the transit from the same location within 14 minutes of each other.
Composite of ISS and Tiangong passing in front of the Sun on 11 AM, April 7, 2023The result of this observation is shown here as a multiple-frame composite. Thanks to the weather, both space stations were photographed clearly and you can make out their distinct shapes - the less dense one is Tiangong. And because the observations occurred within a short timeframe, it's easy to gauge the relative apparent size difference between each other.
Comparison of the ISS and Tiangong's apparent sizeClose-ups of the two space stations make this more apparent. The ISS was 53 arc-seconds wide at 521 km away, while Tiangong was 37 arc-seconds wide at 460 km away. The difference in distance shows that Tiangong's orbit is lower than that of ISS. In fact, Tiangong's nominal orbit is around 375 km, while ISS is at about 420 km.
If the ISS was observed at the same distance, it would have been 60 arc-seconds wide. This means that Tiangong, which was fully built a few months ago, is roughly 60% as wide as the ISS and much bigger than what the transit finder website claims. I think that it's showing the size of just the core module (Tianhe), not the entire structure.
Maybe if I get lucky I will be able to spot the two making the transit at the same time in the future. Until then, I'll keep looking.
Device: Nikon P1000
Settings: 3000mm - ISO 200 - 1/1000s - f/8
Filters: ICE N100000 (Neutral Density 16.5 Stop)
Time: 2023-04-07 11:00 - 11:14 KST
Location: Dangjin, Korea
28 photos (14 each) processed with Pixelmator 3.3.2, RegiStax 6.1.0.8, and PIPP 2.5.9
Crew Dragon approaches Int'l Space Station on April 5, 2021
With clear skies yesterday evening, it was a perfect time to watch the International Space Station make a pass near zenith. Like last month, I set my P1000 camera to video mode while I tracked manually. When I reviewed the recording, I was happy to see that the frames were mostly in focus. The quality was actually one of the best I got with the camera, with most of the modules easily distinguishable. But then I noticed a faint dot next to the station in much of the frames, so I looked up the news.
It turned out that the SpaceX Crew Dragon spacecraft was moving from one docking port to another during those moments. The move only took 39 minutes from undocking (19:31:02) to soft capture (20:10:12), so it was out of sheer coincidence and luck that I was recording while spacecraft was near, but not docked to the space station. The fact that the Crew Dragon's was just big enough to show up in the frames helped, too. The 109-meter ISS was 83 pixels wide at the closest approach, so the Dragon being 4 meters in diameter appeared as a 3-pixel dot. In any case, I was pleasantly surprised to find out that I photographed this occasion. It was something I had on my bucket list.
Device: Nikon P1000
Settings: 3000mm - ISO 200 - 1/1000s - f/8
Filters: None
Time: 2021-04-05 20:05:26-20:06:12 KST
Location: Naju, Korea
432 video frames processed with PIPP 2.5.9, RegiStax 6.1.0.8, and Pixelmator Pro 2.0
Illuminated ISS passes in front of the first quarter Moon on March 22, 2021 (50% - click for full size)
While the Sun is up in the sky, the Moon is still visible at the right phases due to its brightness. Although the Int'l Space Station can also become bright, it's only visible shortly before to about an hour after sunset. So when I found out that it was to pass in front of the Moon while the sky was fairly bright (36 minutes before sunset), I wasn't sure of the results. Would it look brighter or darker than the Moon? Would it even be visible when it's not in front of the Moon?
These questions were cleared up when I got the Moon at that moment on video. Because of the uncertainties and the quickness of the pass, I used 4k 30fps recording instead of the usual 7-frame burst mode on the P1000 camera. The prediction calculation was slightly off and the ISS was not centered, but I still managed to get more than 10 useful frames. The fuzziness is due to video compression, but it's far better than nothing. Interestingly enough, the radiators on the space station were quite a bit brighter than the surface of the Moon in general.
Shots of ISS caught on the video and the resulting stacked image
When we take a closer look, the prominent features of the ISS are easily discernible. The solar panels are stretching from top to bottom, while the Russian modules Zvezda (right) and Zarya (center) are more illuminated than the multi-national modules on the left. Considering this success, it looks like I should try the video recording again for the next ISS pass.
Device: Nikon P1000
Settings: 3000mm - ISO 400 - 1/1000s - f/8
Filters: None
Time: 2021-03-22 18:10:36 KST
Location: Buan, Korea
12 video frames processed with Pixelmator Pro 2.0
Brightly lit ISS passes in front of the Moon on November 11, 2020 (click for full resolution)
The International Space Station is usually in the Earth's shadow when it makes a pass in front of the Moon. But when the conditions are just right, you can see both illuminated by the Sun just like the photo I took above. When seen with the naked eyes, it looked as if a bright meteor was darting across the sky towards the Moon and eventually pass right through it. To witness this, I drove about 40km northeast to the edge of Damyang in the early morning.
Device: Nikon P1000
Settings: 3000mm - ISO 400 - 1/500s - f/8
Filters: None
Time: 2020-11-11 05:36:43 KST
Location: Damyang, Korea
6 photos processed with RegiStax 6.1.0.8 and Pixelmator Pro 1.8
