| XEUS | — | — | — |
 Ximenian Observatory | Italia | Florencia | — |
| Xingming observatory | República Popular China | Urumqi | — |
| Xingming Observatory | República Popular China | Urumqi | — |
| Xingming Observatory #2, Nanshan | República Popular China | Urumqi | — |
| Xingming Observatory #3, Nanshan | República Popular China | Urumqi | — |
| Xingming Observatory-KATS | República Popular China | Urumqi | — |
 XMM-Newton | — | — | — |
 XRISM | — | — | — |
 Xujiahui Observatory | República Popular China | Shanghái | — |
 Xuntian | República Popular China | — | — |
| Xuyi Astronomical Observatory of Purple Mountain Observatory | República Popular China | — | — |