Complex sentence: Detecting satellites less than 10 km in diameter orbiting Jupiter is difficult and requires some of the best telescopes in the world; it is only since 1999 with the discovery of Callirrhoe that astronomers have been able to discover many of Jupiter's smallest moons.
Split into simpler sentences: Detecting satellites less than 10 km in diameter orbiting Jupiter is difficult and requires some of the best telescopes in the world. It is only since 1999 with the discovery of Callirrhoe that astronomers have been able to discover many of Jupiter's smallest moons.