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.
The Normal-incidence Extreme Ultraviolet Photometer (NExtUP) is a smallsat design that will use innovative, efficient normal-incidence multilayer technology, both periodic and aperiodic, to form sharp images of stars in 5 EUV bandpasses between 150 and 900 Å, down to flux limits two orders of magnitude lower than EUVE. A prime focus microchannel plate detector will measure the EUV fluxes formed at different temperatures in stellar outer atmospheres during quiescent and flaring states. 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.
Proposed for the recent NASA Pioneers opportunity, NExtUP is low-cost and efficient, requiring no mechanisms or special orbital conditions during operation. It draws on decades of mission heritage expertise at SAO and LASP, including similar instruments successfully launched and operated to observe the Sun. NExtUP would be flown on a spacecraft supplied by MOOG Industries, with a mission design developed in collaboration with NASA AMES.