| Orba | |
| Mission Type: | Atmospheric |
| Operator: | RAE |
| Mission Duration: | Failed to orbit |
| Launch Date: | UTC |
| Launch Rocket: | Black Arrow R2 |
| Launch Site: | Woomera LA-5B |
| Orbit Epoch: | Planned |
| Orbit Reference: | Geocentric |
| Orbit Regime: | Low Earth |
| Apsis: | gee |
Orba, also known as X-2, was intended to be the first satellite launched by a British rocket. It was launched at 00:34 GMT on 2 September 1970, atop a Black Arrow rocket from Launch Area 5B at Woomera, but failed to reach orbit after the second stage of the carrier rocket shut down 13 seconds early.[1] Orba was built from spare parts due to funding restrictions,[2] and was to have been used to measure upper atmosphere density by monitoring the decay of its orbit.[3]