WASHINGTON — The head of NOAA’s space weather office used a recent hearing to caution that a failure of an aging spacecraft in the next few years could leave the agency “hurting a little bit” in its ability to monitor solar activity.

At a Feb. 12 hearing of the Senate Commerce Committee on “space missions of global importance,” Sen. Maria Cantwell (D-Wash.), ranking member of the committee, asked if NOAA should accelerate plans for its Solar Weather Follow-On mission, a spacecraft scheduled for launch in 2024 to collect solar wind data and take images of the sun’s corona from the Earth-sun L-1 Lagrange point.

NOAA currently uses the Deep Space Climate Observatory (DSCOVR) and NASA’s Advanced Composition Explorer (ACE) spacecraft to collect solar wind data, and uses the ESA/NASA Solar and Heliospheric Observatory (SOHO) spacecraft to observe the solar corona, using those data to forecast solar storms that can affect satellites and terrestrial infrastructure such as power grids.

However, SOHO, launched in December 1995, is well past its design life. In addition, DSCOVR has been offline since June 2019 because of technical problems, forcing NOAA to depend solely on ACE, which launched in 1997.

William Murtagh, program coordinator for NOAA’s Space Weather Prediction Center, said at the hearing that DSCOVR should return to service soon. “We had an issue with it, but fortunately it will be back in operations by the beginning of next month,” he said. NOAA said in September it expected DSCOVR to resume operations in the first quarter of 2020 after implementing a “software fix” to the spacecraft.

He added that NASA has assured NOAA that ACE should remain operational until about 2025. “So we’ve got two spacecraft up there now providing us this key information” on the solar wind, he said.

A bigger issue, though, is SOHO, for which there is no immediate replacement should it fail. “Our biggest concern is the SOHO spacecraft,” he said, citing its age. “It is a single-point failure to some degree.”

Murtagh said that NASA’s twin Solar Terrestrial Relations Observatory, or STEREO, spacecraft could eventually serve as a backup. The two were launched in 2006 into orbit around the sun, going in different directions, and equipped with an instrument suite that includes a coronagraph. In a few years, the spacecraft will drift back towards the vicinity of the Earth. While one of the two spacecraft, STEREO-B, has been out of service since 2014, STEREO-A is still functioning and could fill in for SOHO if needed.

“We won’t be blind, but we’ll be hurting a little bit,” he said of a failure of SOHO.

He reiterated that point later in the hearing. “The observations that we rely on to provide alerts and warnings are critical,” he said. “Should we lose some of those key spacecraft that we talked about, I won’t say we’re blind, but we’re darn close. It will impact our ability to support this nation’s need for space weather services.”

Eventually, all those satellites will be replaced by the Space Weather Follow-On L-1 mission, which will have a coronagraph and solar-wind instruments. NOAA also plans to place a coronagraph on the GOES-U geostationary orbit weather satellite, scheduled for launch in 2024.

Congress has pushed to speed up work on that mission, despite NOAA’s assurances about the availability of data from other spacecraft. NOAA sought about $25 million for the mission in its fiscal year 2020 budget request, but Congress appropriated $64 million. NOAA has yet to release its fiscal year 2021 budget request, more than a week after the White House published the overall federal government budget proposal.

Cantwell did not sound convinced by Murtagh’s argument that the Solar Weather Follow-On mission did not need to be accelerated. “Mark me down as somebody who wants to be more aggressive,” she said. “We should be as aggressive as we can possibly be.”