Themisto also known as Jupiter XVIII is a small irregular satellite of Jupiter was discovered on September 30th 1975, the subsequently lost, and rediscovered in 2000.
|Themisto seen moving against starry background|
Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer, however, not enough observations were made to establish an orbit and it was subsequently lost. Then, in 2000, a seemingly new satellite was discovered it was soon confirmed that this was the same as the one observed in 1975.
Themisto only has a radius of about 4 kilometres and takes around 129 days to orbit Jupiter.