ISS vs Tiangong — how to tell them apart
Two space stations now orbit Earth at similar altitudes. Here is how they differ, and how to know which one you just saw cross your sky.
For the first time in decades, there are two large, crewed space stations circling Earth at the same time. The International Space Station has been up there since 1998. China’s Tiangong (literally “Heavenly Palace”) completed assembly in late 2022 and has been permanently crewed since. If you saw a bright, steady dot drifting across the sky tonight, it could be either one.
Size and orbit
| ISS | Tiangong | |
|---|---|---|
| Mass | ~420 tonnes | ~100 tonnes |
| Length | ~109 m (truss tip to tip) | ~55 m |
| Altitude | ~410 km | ~385 km |
| Inclination | 51.6° | 41.5° |
| Orbit period | ~92 min | ~91 min |
| Crew | Usually 7 | Usually 3 |
The ISS is roughly four times heavier and twice as long. Its massive solar arrays make it dramatically brighter — magnitude −4 on a good pass, rivalling Venus. Tiangong is smaller and dimmer, typically around magnitude −1 to −2 at best — still clearly visible, but not the unmistakable blaze the ISS can be.
How to tell which one you saw
The quickest test:
- Check the time. Our pass predictions tell you exactly when the ISS crosses your sky. If the dot you saw matches the predicted time, direction, and elevation, it was the ISS. If it was at a different time, it may have been Tiangong.
- Check the inclination. The ISS reaches up to 51.6° north and south, so observers at high latitudes see it regularly. Tiangong’s lower 41.5° inclination means it only passes over locations between roughly 43°N and 43°S. If you are in Scandinavia, Scotland, or southern Patagonia and you saw a bright pass, it was almost certainly the ISS.
- Brightness. A truly brilliant pass — brighter than any star — is most likely the ISS. A visible but clearly dimmer pass, still steady and smooth, could be Tiangong.
Different programmes, different goals
The ISS is a multinational partnership (NASA, Roscosmos, ESA, JAXA, CSA). Tiangong is built and operated entirely by the China Manned Space Agency (CMSA). China is not part of the ISS programme — by US law, NASA is prohibited from bilateral cooperation with China on space activities.
Both stations run microgravity science, but Tiangong is also designed as a technology demonstrator for longer-duration Chinese space ambitions.
Will they overlap?
The ISS is currently planned to operate until 2030, with deorbit in early 2031 (see our guide on ISS deorbit). Tiangong is expected to operate for at least ten years, potentially into the mid- 2030s. That means the window of two stations overhead at once is roughly this decade — a genuinely unusual moment in human spaceflight.
To track the one we know best, jump to the live ISS tracker or read what the Station is.
Frequently asked
How can I tell the ISS apart from Tiangong?
Check the predicted time first: if a bright pass matches the time, direction and elevation in an ISS pass prediction, it was the ISS. The ISS also reaches up to 51.6 degrees north and south, so high-latitude observers see it regularly, while Tiangong's lower 41.5-degree inclination keeps it between roughly 43 north and 43 south.
Is the ISS brighter than Tiangong?
Yes, considerably. The ISS reaches about magnitude minus 4 on a good pass, rivalling Venus, because of its huge solar arrays. Tiangong is smaller and typically peaks around magnitude minus 1 to minus 2, still clearly visible but not the unmistakable blaze the ISS can be.
Who operates Tiangong?
Tiangong is built and operated entirely by the China Manned Space Agency (CMSA). China is not part of the ISS programme; by US law NASA is prohibited from bilateral cooperation with China on space activities.
Are there really two space stations in orbit at once?
Yes. The ISS has been up since 1998 and China's Tiangong completed assembly in late 2022 and has been permanently crewed since. With the ISS planned to operate until 2030, the window of two crewed stations overhead at once is roughly this decade.
Last updated