Gravity Probe C lock – Probing the gravitomagnetic field of the Earth by means of a clock experiment ∗

We outline a mission with the aim of directly detecting the gravitomagnetic field of the Earth. This mission is called Gravity Probe C. Gravity Probe C(lock) is based on a recently discovered and surprisingly large gravitomagnetic clock effect. The main idea is to compare the proper time of two standard clocks in direct and retrograde orbits around the Earth. After one orbit the proper time difference of two such clocks is predicted to be of the order of 2× 10 s. The conceptual difficulty to perform Gravity Probe C is expected to be comparable to that of the Gravity Probe B–mission. ∗To appear in the Proceedings of the Alpbach Summer School 1997 “Fundamental Physics in Space”, organised by the Austrian and European Space Agency, A. Wilson, ed.