  Several space missions and instruments for UV spectropolarimetry are in
preparation, such as the proposed NASA MIDEX Polstar project, the proposed ESA
M mission Arago, and the Pollux instrument on the future LUVOIR-like NASA
flagship mission. In the frame of Polstar, we have studied the capabilities
these observatories would offer to gain information on the magnetic and plasma
properties of the magnetospheres of hot stars, helping us test the fundamental
hypothesis that magnetospheres should act to rapidly drain angular momentum,
thereby spinning the star down, whilst simultaneously reducing the net
mass-loss rate. Both effects are expected to lead to dramatic differences in
the evolution of magnetic vs. non-magnetic stars.
