WASHINGTON — NASA has selected four finalists for the agency’s next small astrophysics mission, including both spacecraft and International Space Station experiments to study a wide range of astronomical phenomena.

NASA announced March 16 the selection of two Small Explorer, or SMEX, missions and two smaller Missions of Opportunity for further study. The full-scale SMEX mission proposals will receive $2 million each for nine-month concept studies, and the Missions of Opportunity $500,000 each for similar studies.

“Each of these missions would take the next steps in some of the hottest areas of astrophysics today,” Paul Hertz, director of NASA’s astrophysics division, said in a statement. “With the high science rewards for low dollar amounts, Explorers missions successfully fill the scientific gaps in our current fleet of space observatories.”

One of the SMEX missions is the Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution (ESCAPE). It will study nearby stars for ultraviolet flares that could strip the atmospheres of any planets orbiting them.

The other SMEX mission is the Compton Spectrometer and Imager (COSI). It will observe gamma rays from radioactive elements in the Milky Way to better understand the history of dying stars in the galaxy, and also study the production of gamma rays from more distant cosmic explosions.

Of the Missions of Opportunity, one is the Gravitational-wave Ultraviolet Counterpart Imager Mission. It would fly two small satellites to scan the sky at two different ultraviolet bands, looking for signatures from gravitational-wave events caused by neutron stars merging with each other or with a black hole.

The other Mission of Opportunity is the Large Area Burst Polarimeter, or LEAP, an experiment that would be mounted on the ISS. It would study polarization of gamma rays in jets created by stellar explosions or neutron star mergers. Which could help astrophysicists determine the best model for their formation.

NASA expects to select up to two missions by the fall of 2021 for launch no later than May 2025, according to a schedule in the announcement of opportunity for this competition last year. Full-scale SMEX missions will have a cost cap of $145 million and Missions of Opportunity $75 million.

The SMEX program is part of the Explorers program for small, competed astrophysics missions that can be traced back to the early years of the space agency. The most recent SMEX mission selected for development is the Imaging X-ray Polarimetry Explorer (IXPE), selected by NASA in early 2017. It is scheduled for launch on a Falcon 9 in 2021.