Missions to probe exoplanets, galaxies, and cosmic inflation vie for $250 million NASA slot

By Daniel Clery

From exoplanet atmospheres to the dynamics of galaxies to the stretch marks left by the big bang, the three finalists in a $250 million astrophysics mission competition would tackle questions spanning all of space and time. Announced last week by NASA, the three missions—whittled down from nine proposals—will receive $2 million each to develop a more detailed concept over the coming 9 months, before NASA selects one in 2019 to be the next mid-sized Explorer. A launch would come after 2022.

Explorer missions aim to answer pressing scientific questions more cheaply and quickly than NASA’s multibillion-dollar flagships, such as the Hubble and James Webb (JWST) space telescopes, which can take decades to design and build. The missions are led by scientists, either from a NASA center or a university, and NASA has launched more than 90 of them since the 1950s. Some Explorers have had a big scientific impact, including the Wilkinson Microwave Anisotropy Probe, which last decade mapped irregularities in the cosmic microwave background (CMB), an echo of the universe as it was 380,000 years after the big bang; and Swift, which is helping unravel the mystery of gamma-ray bursts that come from the supernova collapse of massive stars.

One finalist, the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx), will map galaxies across a large volume of the universe to find out what drove inflation, a pulse of impossibly fast expansion just after the big bang. “The physics behind inflation is unclear,” says Principal Investigator Jamie Bock of the California Institute of Technology in Pasadena, and it happened at energy scales too high for earthbound particle accelerators to investigate. The prevailing theory is that a short-
lived quantum field, mediated by a hypothetical particle called an inflaton, pushed the universe’s rapid growth. But rival theories hold that multiple fields were involved. Those fields would have interfered with each other, leaving irregularities in the distribution of matter across the universe that would differ statistically from the distribution expected in conventional inflation.

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