Altitudes between 40 and 140 km are not accessible for conventional aircraft and spacecraft for
long-term duration. In particular the region around 100 km, the upper mesosphere and lower thermosphere
(UMLT), is of great interest to scientists and engineers since it forms the transition from earth to
space. Important physical phenomena take place at this region that have a strong influence on the atmospheric
layers below. The scientific need to explore this region is very high. So far, the UMLT region
has been accessible mainly to remote sensing observations, which are subject to vertical and horizontal
smoothing of the measured information, and require an underlying model and possibly not existing a
priori information about the subject of interest. Therefore, in-situ measurements in the UMLT region
on a global scale would be of inestimable value. However any conventional spacecraft will quickly
suffer from free molecular friction at UMLT altitudes, leading to rapid orbital decay. We intend to
highlight the scientific need for in-situ measurements in the UMLT region and discuss several mission
case studies. The impact of the special conditions at that altitude on the spacecraft design require a
careful consideration
