(X-ray space telescope plan aiming for 2031 launch)
Arcus is a plan (developed c2010-2015) for an X-ray space
observatory using a gratingspectrometer in the soft
X-ray range, intending to provide astronomical data on spectral lines
previously unavailable.
It has been proposed but not selected for NASA missions and
remains a possibility for a future mission.
Or possibly the plan will offer ideas for other future
X-ray missions.