NExtUP: The Normal-incidence Extreme Ultraviolet Photometer

The Normal-incidence Extreme Ultraviolet Photometer (NExtUP) is a smallsat mission concept designed to measure the EUV radiation conditions of of F-M type stars and exoplanet host stars. The evolution and loss of exoplanetary atmospheres depend critically on the host stars’ extreme ultraviolet (EUV) spectra and fluxes. EUV radiation is absorbed at high altitude, in the exosphere and upper thermosphere, where the gas can be readily heated to high temperatures conducive to escape. EUV heating is thought to be a dominant atmospheric loss mechanism during most of a planet’s life. There are only a handful of accurately measured EUV stellar fluxes, all dating from Extreme Ultraviolet Explorer (EUVE) observations in the ‘90s. These observations were mostly single snapshots of what are highly variable and often flaring sources. Consequently, current models of stellar EUV emission are uncertain by more than an order of magnitude and are the largest uncertainty in planetary atmospheric loss models. NExtUP will use periodic and aperiodic multilayers on off-axis parabolic mirrors and a prime focus microchannel plate detector to image stars in 5 bandpasses between 150 and 900 A, down to flux limits two orders of magnitude lower than reached by EUVE. NExtUP may also accomplish a compelling array of secondary science goals, including using line-of-sight absorption measurements to understand the structure of the local interstellar medium, and imaging EUV emission from energetic processes on solar system objects at unprecedented spatial resolution. NExtUP is well within smallsat weight limits, requires no special orbital conditions, and draws on decades of mission heritage and expertise, including similar instruments successfully launched and operated to observe the Sun.