WASHINGTON — The European Space Agency has restored operations of four space science missions it placed on hiatus because of the coronavirus pandemic, a move that illustrated how one agency’s actions can affect others.

ESA announced March 24 it had suspended operations of the Cluster, ExoMars Trace Gas Orbiter, Mars Express and Solar Orbiter missions, putting them into safe modes for indefinite durations. The agency said at the time staffing restrictions at the European Space Operations Centre (ESOC) in Darmstadt, Germany, prompted by the coronavirus pandemic, including one person there testing positive for COVID-19, forced ESA to put those missions on hold to devote staff to other missions.

However, on April 2, ESA announced it was resuming operations of all four missions that had been suspended, going into new detail about the COVID-19 case that led to those missions being suspended.

In the statement, Paolo Ferri, head of mission operations at ESOC, said an employee came into contact with about 20 people at the center in the two days before his diagnosis. Those 20 people, he said, primarily worked on the four missions whose operations were suspended, and were asked to stay home as a precautionary measure. Building at the center were also “thoroughly cleaned and disinfected” to eliminate any traces of the virus.

“We decided to preventatively suspend operations on these missions until the risk of a potential cascade of follow-on infections and quarantines disappeared,” Ferri said. None of the employees placed in self-quarantine showed symptoms of the disease, and Ferri said that person who was diagnosed with COVID-19 “is thankfully fine, and recovering well.”

At the time ESA announced the suspended operations, the agency didn’t link it to controllers working on specific missions being potentially exposed to the disease. Instead, it said it was primarily targeting interplanetary missions—of the four, only Cluster is orbiting the Earth—because they require more personnel on site.

Ferri said ESA was taking steps to minimize contact among those personnel who are still working at the center, while many others telework. “There are very few people. They work completely isolated. I think ESOC now, from a COVID-19 infection case, is one of the safest places you can find in Germany,” he said.

He added that if another employee does come down with the disease, the physical separation among personnel means “there’s very little chance we need to quarantine other people.”

ESA started restarting the missions with Solar Orbiter, launched Feb. 9 and still undergoing commissioning of its instruments. “It takes some time,” Ferri said. “We started with Solar Orbiter, because we’re still in a manual mode of operations. It’s easier to restart.”

Temporarily halting Solar Orbiter operations had an effect that went beyond ESA, since the mission is a joint one with NASA. At a March 31 meeting of the National Academies’ Committee on Solar and Space Physics, Nicola Fox, director of NASA’s heliophysics division, said that at time ESA put the spacecraft into safe mode, 9 of its 10 instruments had gone through initial commissioning.

At that meeting, she said it appeared ESA would soon resume spacecraft operations. “They worked out a way to do it remotely, so there will only be one or two people having to go into the mission ops center,” she said. “They are going to try and continue to do some low-level commissioning as we move forward.”

Ferri said that, after Solar Orbiter, ESA would move to restart the four Cluster satellites and the two Mars orbiters. The ExoMars Trace Gas Orbiter (TGO) also serves as a communications relay for spacecraft on the surface. Having it offline was a concern for Mars scientists because of NASA’s plans, in its fiscal year 2021 budget proposal, to end operations of the Mars Odyssey orbiter, which also serves as a communications relay for the InSight lander and Curiosity rover.

“I think the COVID situation we’re in right now highlights the fragility of that, because TGO is in safe mode right now and not taking passes because of how ESA is shut down,” said Bethany Ehlmann, a professor of planetary science at Caltech and member of the National Academies’ Committee on Astrobiology and Planetary Science, at a March 31 committee meeting. “If we didn’t have Odyssey, we would definitely be seeing impacts on InSight operations.”

Ferri said it’s not unusual for missions, in particular Mars orbiters, to be out of contact with Earth for extended periods. Outages lasting a few weeks at a time occur when Mars goes behind the sun as seen from Earth. “This time stopping it for issues related to health of the ground people of course is unique,” he said, “but, to be honest, you feel even more compelled to do that rather than saving a machine.”