At a moment when Russian forces are struggling to gain momentum in their full-scale onslaught into Ukraine, a war of words has broken out between Russian and U.S. space professionals over cooperation aboard the International Space Station, one of the few ongoing cooperative efforts between the two superpowers.

“Without continuing their cooperation on the ISS, the Russian space agency will have hard time continuing to exist,” says veteran NASA Astronaut Scott Kelly, who lives in the Denver area now after a move from Houston.

“They don’t have a big budget from the Russian government, and ISS is very important to them even though we don’t give as much to them as we used to.”

After crippling sanctions were placed on Russian banks and businesses following the Feb. 24 invasion, Russian space chief Dmitry Rogozin tweeted a video seeming to show cosmonauts abandoning the ISS and leaving its western crew members behind. An animation that was included in the video seemingly showed the Russian component being decoupled from the station.

Russia provides transport to and from the ISS for U.S. and other western astronauts, via its Soyuz capsules launched from the Baikonur Cosmodrome in Kazakhstan. The message was widely interpreted as a threat to the crew on the ISS, with two Russians, one German and four U.S. astronauts aboard, including Mark Vande Hei – on track to break Kelly’s own record of 340 days aboard ISS as the longest duration by a U.S. astronaut. Vande Hei is supposedly scheduled to return aboard the Russian Soyuz on March 30.

That tweet and another one by Rogozin on March 3 that showed a Russian crew at Baikonur painting over U.S., British and other western flags on the side of a Soyuz rocket drew a quick response from Kelly, the best known of the current generation of U.S. astronauts, with a huge Twitter following that includes many in Russia.

“Without those flags and the foreign exchange they bring in, your space program won’t be worth a damn,” Kelly tweeted Rogozin. “Maybe you can find a job at McDonald’s if McDonald’s still exists in Russia.”

“I was appalled with that posted video that showed the Russian segment floating away, and I felt compelled to call him out,” says Kelly, a Russian speaker who trained for months at the Cosmodrome before the most recent of his four trips into space.

Kelly’s tweet called Rogozin by the name ‘Dimon’ (a more personal form of his first name Dmitri) and referred to him using the personal ‘ty’ form of the pronoun you that’s typically used only when Russians are considered to be on a first-name basis. That drew a sharp rebuke from the Russian.

“I did it to get his attention, and to get the attention of average Russian speakers, to give weight to something that wasn’t the story they’re getting from their state-controlled media,” says Kelly.

Kelly adds that the Russian space chief’s threat is largely unrealistic. “The U.S. segment has been connected to the Russian segment since 1999, and we’re not even sure you can physically disconnect it.”

Kelly notes that any kind of separation, even a symbolic one, would not be good for either the Russians or Americans.

“Just as the ISS crew relies on each other, the hardware is interdependent,” Kelly adds. Russian components include the thrusters that keep the station from slowing in its orbit and reentering earth’s atmosphere, while U.S. components provide electricity and water to both sections.

Kelly has expressed strong support for Ukrainian President Volodymyr Zelensky and his people during their spirited defense of the nation over the past two weeks. But Kelly, who was station commander aboard the ISS during his last flight, says that even if the situation in Ukraine deteriorates into a drawn-out insurgency against Russian forces, U.S. and Russian crew members should be able to continue working together aboard the ISS.

“I was up with Russians during the (2015) Syria crisis,” Kelly notes. “We had discussed that as a crew. It was almost like talking about two different countries in an abstract way.

“When you’re off the planet, what’s important is that the crew members are supporting each other. Short of a shooting war, the situation on board would be workable, and I suspect the guys up there now are getting along fine.”

During the 2015-16 flight Kelly commanded, also aboard was Cosmonaut Anton Shkaplerov, the commander on the ISS. “Knowing him, I know he’s a professional and dedicated to the program,” Kelly says.

Kelly adds that he has a high regard for many of the Russians working on the ISS program, and that NASA and Roscosmos are each working to move the program ahead.

“The space station is an outpost for humanity,” Kelly says. “Certainly, for the U.S. and Russia, if we lose that we have very little left to keep working cooperatively. It’s critical to maintain that.”

Kelly’s 340-day ISS trip was the last of four space missions for NASA, starting with a 1999 mission aboard Space Shuttle Discovery to service the Hubble Space Telescope. During the yearlong ISS mission, Kelly’s twin brother Mark Kelly, now a U.S. senator from Arizona, was on Earth, giving researchers a rare chance to compare how the body changes over time in space compared to a near identical one back home – crucial to planning a future Mars mission.

Under any circumstances, NASA is readying a new space transportation system that will use either Boeing or SpaceX vehicles and avoid using Russian rockets – already in the works before the current conflict.

But Kelly notes that he has a sentimental attachment to Soyuz, first flown in the 1970s before the Space Shuttle flew and that grew into an essential component of the ISS program lasting for decades.

His best ride ever, Kelly says, was returning to Earth aboard Soyuz – a dependable craft but one that gives a wild reentry that he describes as like going over Niagara Falls in a barrel, on fire.

“I would love to do that last 20 minutes again,” he says.