WASHINGTON — Limited budgets will force NASA to delay implementation of one major recommendation from an Earth science decadal survey for a smaller class of missions.

The latest Earth science decadal survey, published in early 2018, recommended a series of missions to study the planet from space. One on those programs was a new line of medium-class missions called Earth System Explorer, which would be competed missions with a cost cap of $350 million focused on studies in seven different areas of Earth science.

But at a meeting of the Earth Science Advisory Committee earlier this month, Sandra Cauffman, acting director of NASA’s Earth science division, said implementation of that line of missions was on hold given the funding available this year and the agency’s request for fiscal year 2021. That 2021 budget proposal requested $1.77 billion for Earth science, $200 million less than what Congress appropriated for fiscal year 2020.

“We really were hoping that we could release the first solicitation this year,” she said of that program. “But given the appropriations we got in ’20 and the look out that we have for ’21, it doesn’t look promising that we can do it next year or even ’22.”

She said she didn’t expect NASA to be able to start the Explorers program until fiscal year 2023 “unless something dramatic happens” in the upcoming debate about NASA’s 2021 budget. “We’ll just have to keep a watch on the budget developments.”

Once implemented, Cauffman said that the Earth System Explorers will be similar to small competed missions in both astrophysics and heliophysics also called Explorers. It will feature a two-step proposal process, with NASA selecting an initial set of proposals for further study before choosing one for full-scale development.

The agency is moving ahead with other elements of the decadal survey’s recommendations. Cauffman said NASA has studies ongoing for the five “targeted observables” recommended by the survey for larger-scale missions: aerosols; clouds, convection and precipitation; mass change; surface biology and geology; and surface deformation and change.

The designated observable mission most likely to be first to go forward is the surface biology and geology one, she said, which would fly a hyperspectral sensor. That would be followed by one that combines the aerosol and clouds, convection and precipitation observables, then the mass change mission, which would build upon the GRACE and GRACE Follow-On missions. The surface deformation and change mission, with a synthetic aperture radar, would likely be last since it will follow up on the NASA-ISRO Synthetic Aperture Radar mission currently in development.

Cauffman said NASA will compete specific mission concepts for each of those designated observables. “There’s going to be a lot of competition in those,” she said.

NASA has already made a selection regarding another recommendation from the decadal for a series of low-cost venture-class missions to maintain continuity of observations. The agency announced Feb. 26 it will fly an instrument called Libera to measure the balance between radiation from the sun arriving at Earth and radiation emitted by the Earth. Such measurements, which have been carried out for decades, are an essential element of climate studies.

Libera, which will fly as a hosted payload on the third Joint Polar Satellite System satellite launching in late 2027, will be a replacement for the canceled Radiation Budget Instrument (RBI), which suffered cost overruns. Cauffman said they used the first competition for an Earth Venture Continuity mission specifically to look for instruments that could replace RBI.