NASA's UVEX Ultraviolet Telescope Set to Launch in 2031 to Map the Universe's Hidden Light

For most of us, a lifetime of looking up at the night sky turns up very little that changes. The moon waxes and wanes, planets drift along their slow paths, satellites and the International Space Station cross the view for a few moments, and now and then a meteor streaks past. With luck, a person might catch a prominent comet, whether a returning visitor like Halley's or a once-in-several-millennia arrival like Hale-Bopp. A very few might see a distant supernova.

That sense of quiet, the reporting argues, is an illusion, a selection bias created by the fact that human eyes see only visible light. Even a typical backyard telescope shares the limitation. Violent, short-lived events happen almost continuously, and many phenomena, among them stellar flares and exploding stars, show themselves fully only in ultraviolet light, which requires technological help to see.

NASA has now announced that UVEX, the UltraViolet EXplorer, will launch in 2031. The mission is led by Caltech and will carry out an all-sky survey in near- and far-ultraviolet light. Caltech President and astronomy professor Ray Jayawardhana called it a bold leap forward in exploring the ultraviolet universe. It will be the first new UV telescope in 20 years, and the first UV mission to build deep-sky maps.

UVEX is meant to fill a gap among surveys already underway or planned. The Nancy Grace Roman Space Telescope will run three surveys in infrared. The Vera Rubin Observatory will image the sky repeatedly over a decade. Europe's Euclid is conducting three deep-field surveys in visible light and has captured over 10 billion sources, which are galaxies. NASA's SPHEREx is mapping the whole sky in near-infrared. UVEX will add ultraviolet to that picture, and it will also take focused observations of individual targets.

The mission's authors describe a two-year baseline plan: a repeated, multi-cadence all-sky survey 50 times deeper than GALEX in the near-ultraviolet and 100 times deeper in the far-ultraviolet; cadenced surveys of the Large and Small Magellanic Clouds; rapid follow-up of sudden events; and spectroscopic follow-up of samples of stars and galaxies. GALEX, a NASA ultraviolet telescope, ended its mission in 2013, and UVEX is expected to be up to 100 times more sensitive.

The science rests on three pillars. The first is low-mass, low-metallicity galaxies in the local universe, where imaging and spectroscopy should show how star formation and stellar evolution at low metallicity shape the growth of such galaxies. UVEX will observe nearby low-mass galaxies that are bright in UV, complementing JWST and Roman. The second is the dynamic universe: time-domain surveys and prompt spectroscopy to probe the early aftermath of compact-object mergers found through gravitational waves, to discover hot, fast UV transients, and to diagnose the early stages of stellar explosions. The third is a large public archive of UV data for researchers studying a wide range of objects, much as the European Space Agency's recently completed Gaia mission has done and is expected to keep doing.

The gravitational-wave link may be the most concrete. When extremely dense objects such as neutron stars or black holes merge, they send ripples through space-time, and detections trigger alerts to telescopes that observe in wavelengths including radio and gamma rays. UVEX will add ultraviolet. Mansi Kasliwal, a Caltech astronomy professor, Palomar Observatory director and UVEX team member, said the mission will let astronomers follow up gravitational-wave events in ultraviolet light, with detailed spectra revealing more than before. Spectra also show which chemical elements are present.

Every mission launches with firm goals, but telescopes keep delivering surprises, as JWST did with early supermassive galaxies. Caltech's Kareem El-Badry, an associate professor of astronomy on the UVEX team, said that each new wavelength brings something new and unexpected. The first data are still years away, with launch set for 2031.