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WASHINGTON — NASA defended the selection of two concepts for study as the agency’s first astrophysics probe-class mission amid community complaints that the two missions are not on an equal footing.
NASA announced Oct. 3 that it picked two proposals for further study for its new Astrophysics Probe Explorer, or APEX, line of missions. The two proposals each received $5 million for one-year studies to refine their designs. NASA plans to pick one of the proposals in 2026 as the first probe-class mission for launch in 2032.
The APEX program, based on a recommendation from the Astro2020 decadal survey, is designed to fill a gap between smaller Explorer-class astrophysics missions and larger flagship missions like the James Webb Space Telescope. NASA set a cost cap of $1 billion, excluding launch and any international contributions, and required proposals for this first competition to carry out far-infrared or X-ray astronomy.
One of the selected proposals, the Advanced X-ray Imaging Satellite, or AXIS, will carry out a wide range of astrophysics studies at X-ray wavelengths with a large field of view and high spatial resolution. The other, Probe far-Infrared Mission for Astrophysics, or PRIMA, would perform far-infrared imaging and spectroscopy using a 1.8-meter telescope.
In the weeks following NASA’s selection of AXIS and PRIMA, there had been speculation in the astrophysics community that the two were not necessarily the highest rated proposals that were submitted and evaluated by NASA. At an online town hall session Oct. 23, Mark Clampin, NASA astrophysics division director, declined to comment on questions submitted about the selection process because the agency was still debriefing teams whose proposals were not chosen.
He addressed the issue, though, at a Nov. 7 meeting of the agency’s Astrophysics Advisory Committee (APAC), defending the selection of the two proposals. “In selecting the two missions that we picked, PRIMA and AXIS, we really aimed to balance the astrophysics portfolio and align with the decadal priorities set out by the science community,” he said, arguing they could carry out science not possible with smaller Explorer missions.
“Both probes were chosen because they promised to yield insights that complement and extend our existing capabilities and expand community involvement in the use of these missions,” he continued. “The bottom line: they were the best science investigations, so that’s why they were picked.”
Committee members later raised questions about that selection process. That included the impression that the two proposals “are not on the same footing” in terms of technology readiness.
Clampin rejected that claim. “Our perspective is that those missions are on a similar footing and we selected them based on the quality of the science programs,” he said. “We are not starting with an uneven or unequal playing field.”
Neither the committee members nor Clampin discussed specifics about the two proposals and perceptions that they are not on an equal playing field. However, during a later public comment period at the APAC meeting, committee members went through questions submitted online and upvoted, with many of the top questions addressing the APEX selection.
Those questions included claims that one of the missions ranked lower than some that were not selected. NASA used a one-to-four scale to categorize proposals after they were evaluated on scientific merit; scientific implementation merit and feasibility; and technical, management and cost.
One of the selected proposals, the questions claimed, was rated as Category 1, the highest level. However, they claimed the other was placed in Category 3, which NASA defines as “meritorious investigations that require further development” and which are normally not considered for selection. The lower-ranked mission was reportedly picked over other proposals rated in both Categories 1 and 2.
Other questions indicated that the lower-rated proposal that was selected was AXIS, something that Clampin or others at the meeting did not confirm. Clampin stated that he would not discuss specific ratings on the proposals.
In response to one public comment, though, he said NASA was not bound to pick only Category 1 missions. “We opted to select two proposals that we felt were the ones we wanted to go forward with,” he said. “We are not required to select a Category 1 proposal and NASA has not always selected Category 1 proposals.”
“I would strongly urge the community to get behind these studies and stop trying to pick them apart,” he said earlier in the meeting, “because it’s not fair to the PIs [principal investigators] of these programs either.”
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