Physics package of the 35kg space active hydrogen maser for the ACES space mission of ESA

The 35 kg space active space hydrogen maser (SHM) is part of the atomic clocks ensemble in space (ACES) mission of ESA. The physics package (PP) of the engineering model (EM) of the SHM is currently developed at the observatoire de neuchatel and for the nominal operational parameters has the long-term frequency stability of 1.5x10^-15@10´000 s. In this article we report the latest developments on the PP of the EM.