The Ultraviolet Explorer (UVEX) is a Medium-class Explorer mission that will conduct an all-sky ultraviolet survey. Credit: Caltech

WASHINGTON — NASA has selected four mission concepts for additional study in the latest round of an astrophysics program, but selected missions could face delays because of budget constraints.

NASA announced Aug. 18 it selected two mission proposals for its next astrophysics Medium Explorer, or MIDEX, mission, and two others as missions of opportunity. The MIDEX mission proposals will each get $3 million for nine-month concept studies, while the missions of opportunity will get $750,000 each for nine-month studies.

One MIDEX proposal, called the Survey and Time-domain Astrophysical Research Explorer or STAR-X, would fly a wide-field X-ray telescope and an ultraviolet telescope to study transient phenomena like supernova explosions. STAR-X would also perform a deep X-ray survey to map hot gas in galaxy clusters to help astronomers understansd how those clusters developed over time.

The other MIDEX proposal, called Ultraviolet Explorer or UVEX, would survey the whole sky in two ultraviolet bands to study the evolution of stars and galaxies. UVEX would also be able to detect ultraviolet emissions linked to gravitational wave events.

Of the two missions of opportunity, one, the Large Area Burst Polarimeter or LEAP, would be installed on the International Space Station. LEAP would study gamma-ray bursts linked to supernova explosions or mergers of compact objects. The other, the Moon Burst Energetics All-sky Monitor or MoonBEAM, would fly in cislunar space and detect gamma-ray bursts, working in cooperation with telescopes on Earth and in low Earth orbit to triangulate the source of the bursts.

NASA expects to select one MIDEX mission and one mission of opportunity in 2024. The MIDEX mission would have a cost cap of $300 million and launch by 2028, while the mission of opportunity would have an $80 million cost cap and launch by 2027.

That schedule could face delays, though, based on reduced projections in the agency’s fiscal year 2023 budget proposal for astrophysics. “There’s hundreds of millions of dollars’ worth of things we were planning to do in the ’22 budget request that don’t fit in the ’23 budget request any more,” said Paul Hertz at a meeting of the NASA Astrophysics Advisory Committee July 20. Hertz, at the time director of NASA’s astrophysics division, stepped down from that role earlier this month to become a senior adviser for the overall Science Mission Directorate.

One of the effects of those reduced budgets, he said, was on the upcoming MIDEX selection. “The impact of the lowered out-year budget will start in Phase B,” he said, after a mission is selected for development, “where we’ll have to flatten out the ramp-up.” The selected mission will spend more time in Phase B than planned, delaying its launch by an unspecified period.

Hertz said that the proposals selected for the mission concept studies will receive instructions from NASA on how to incorporate that revised funding profile and delay into their studies. The selected mission of opportunity will also see a similar delay, he added.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...