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
Tiangong Space Station in front of the Moon on February 8, 2022 (Click for full size)
Tiangong is a Chinese space station which was initially launched on April 29, 2021. It is built upon the experience gained from its preceding prototypes, Tiangong-1 and Tiangong-2. It only has the core module (Tianhe) at the moment, but two additional modules (Wentian and Mengtian) are planned to be added this year. Because Tianhe is much smaller and simpler than the ISS, I wasn't sure what to expect when photographing it. Once I did catch it crossing in front of the Moon, I noticed that it looked longer and resembled a candy in a wrapper. That was due to the cargo spacecrafts Tianzhou 2 and Tianzhou 3 docked to the ports.
Tiangong Space Station in front of the Sun on February 10, 2022 (Click for full size)
To see the solar panels on the space station and the spacecrafts, I needed to observe it crossing in front of the Sun. Fortunately, a sighting opportunity took place just two days later and I was able to get a good look. Both the large panels on the core module and the smaller ones on the spacecrafts were visible.
Tiangong Space Station zoomed in at night and day
With the shots zoomed in and stacked, you can see the individual parts more clearly. The length of the object is about 38 meters (Tianhe: 16.6m, Tianzhou: 10.6m each), which came out to be about 30 pixels long when I shot it on February 8 at 413km away. This is roughly 1/3 the size of the ISS. If there were no spacecrafts docked, it would have looked much smaller and less distinct. On February 10, it was 576km away, so it came out to be smaller at about 20 pixels. I may be able to get larger Sun-crossing shots in Summer when the maximum altitude of the Sun is higher.
Device: Nikon P1000
Settings: 3000mm - f/8 (ISO 800 - 1/800s / ISO 200 - 1/1250s)
Filters: None
Time: 2022-02-08 19:03:59 / 2022-02-10 12:11:53 KST
Location: Haenam / Gangjin, Korea
20 / 23 video frames processed with Pixelmator Pro 2.3.5
10 / 14 video frames stacked with RegiStax 6.1.0.8
Tiangong-1 (top left) and Iridium 80 (center) seen in the northern sky
Tiangong-1, the first Chinese space station, is set to fall back to Earth around April 2. Its orbit had been slowly decaying since its service was ended in 2016. I found out that it would be visible for one last time in the sky this morning, so me and my daughter Celine decided to witness its streak. I set up my iPhone X on a tripod and placed it on the window facing north. Then I launched NightCap Camera app in satellite capturing mode.
Shortly after 5:30AM, Tiangong-1 made its bright (-0.6 magnitude, 181km altitude) appearance from the west, streaking towards east and quickly dimming. Just as it disappeared from our eyesight, another bright flash appeared to the east of Polaris and then quickly disappeared. From my experience I knew that was an Iridium flare. CalSky website verified it to be Iridium 80, showing up at 5:30:36AM with a magnitude of -2.0. We were very lucky to see the two together - since the space station will disappear in a couple of days it was truly a once-in-a-lifetime experience.
On the photo, you can see, starting from the top left and looking towards bottom right, the Tiangong-1, Polaris, Iridium 80, and the Cassiopeia constellation. Click on the image to see it in full size.
Device: iPhone X
Settings: 28mm - ISO 2112 - 61s - f/1.8
Filters: None
Date/Time: 2018-03-31 05:30:01 KST
Location: Naju, Korea
