Direct string magnetic gradiometer for space applications
  Recently, a novel Direct String Magnetic Gradiometer (DSMG) has been developed, where a vibrating wire, driven by an AC current, is used as a single sensitive element. It is designed to directly measure the local off-diagonal components of the magnetic gradient tensor, Bxz, Byz and Bxy, provided the distance to an object creating magnetic anomalies is much larger than the length of the string. This requirement is well satisfied in space, if the sensor is deployed from a satellite platform orbiting near the planet under surveillance. Current instruments operating at $1{kPa}$ pressure achieve sensitivity of $1.8 \times 10^{-10}{T}/{m}$ in the band $0.0001{Hz}$ to $0.1{Hz}$. In this paper we show that proposed modifications to the current gradiometer design, specifically aimed at the deployment in space, could have a magnetic gradient sensitivity better than $10^{-13}{T}/{m}/\sqrt{Hz}$ in the frequency range of interest for specific missions both for fundamental research and for such applications as geophysical exploration on Mars and other solar system planets. Also, by combining a few single-axis magnetic gradiometer modules, it is possible to deploy a full tensor magnetic gradiometer. 
Andrew Sunderland, Alexey V. Veryaskin, Wayne McRae, Li Ju, David G.   Blair