After the National Aeronautics and Space Administration encouraged former astronauts to tone down their criticism of their Russian colleagues, retired NASA astronaut Scott Kelly ended his dispute with the director of Russia's space program.
The Twitter war began when Roscosmos Director General Dmitry Rogozin and other Russian government institutions released a series of social-media messages.
Some of those include a video depicting Russian cosmonauts evacuating the International Space Station and leaving American astronaut Mark Vande Hei behind.
Kelly, who spent almost a year aboard the ISS in 2015 and 2016, got into a Twitter spat with Russia's space leader over postings last week.
IN SPACE - JULY 12: In this handout photo provided by NASA, Expedition 44 flight engineer and NASA astronaut Scott Kelly is seen inside the Cupola, a special module which provides a 360-degree viewing of the Earth and the International Space Station July 12, 2015 in space. Kelly is one of two crew members spending an entire year in space.
NASA Officials Tell Ex-Astronauts To Avoid Attacking Russian Colleagues
Several reports mentioned that a high-ranking NASA official wrote an email this week encouraging former astronauts to exercise caution before "attacking our Russian colleagues." However, they did not reveal the name of the official who authored it.
An email obtained by CNN reads: "As Americans, each of us enjoys freedom of speech and you are all empowered to speak your mind," the email reads. "However, please know that as former NASA astronauts, your words carry additional weight and attacking our Russian partners is damaging to our current mission."
Kelly - the twin brother of Sen. Mark Kelly, also a former astronaut - admitted receiving the email and said he would follow NASA's advice.
Kelly told CNN that he felt obliged to speak out when Rogozin published a Roscosmos-produced video of two Russian cosmonauts floating inside the space station and waving farewell to NASA astronaut Mark Vande Hei.
Kelly Chooses Not To Engage in Another Twitter Fight
Kelly told CNN that after receiving the email from the NASA official, he has chosen not to engage in another Twitter spat with Rogozin, as he implied in an op-ed on Tuesday.
"In the early 1990s, our countries' space agencies were willing to work together to ease Cold War tensions, and so the United States and Russia agreed to embark on a shared space station," Kelly wrote in an op-ed published in The Washington Post.
"To me," he added, "it has always been one of the great achievements of our nations that we came together to build and operate an orbiting station as a peaceful cooperation, and I was privileged to serve there."
But, as Kelly highlighted in the op-ed, subsequent remarks by Rogozin and Roscosmos have jeopardized the collaboration.
Rogozin "threatened to de-orbit the space station and smash into the United States," Kelly wrote. He emphasized that the rocket reboosts that sustain the station at its appropriate height are Russia's responsibility, while NASA ships may take over if required.
Worse, the former NASA astronaut highlighted this week that Rogozin tweeted a weird video depicting Russian cosmonauts disconnecting the Russian component from the space station and flying away after waving farewell to American astronaut Mark Vande Hei.
Vande Hei will return to Earth on March 30 with two cosmonauts on a Russian Soyuz spacecraft. Kelly said the video seems to threaten to abandon his fellow astronaut. For the retired astronaut, that means an unimaginable breach of the trust created between the two countries in space over decades.
Kelly's op-ed concluded with a call for peace.
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