“We’re ready to fly,” Kathy Lueders, the NASA manager of the program, said at a press conference last week.

Russia, on the other hand, wasn’t so sure.

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The United States and Russia work together on the ISS, along with 13 other countries. They share information on station operations, including the particulars of the SpaceX mission. Last week, NASA officials said that, while they had cleared SpaceX for launch, their counterparts at Roscosmos, the Russian space agency, had a “dissenting opinion.”

The disagreement centered on the SpaceX capsule’s approach to the ISS. Spacecraft that connect with the station are equipped with flight software that can prevent dangerous accidents. European, Japanese, and other spacecraft that previously have rendezvoused with the ISS carry independent systems designed to kick in if their primary computers fail. The backups would take over and maneuver the spacecraft away from the ISS to avoid a collision.

The SpaceX capsule, known as Crew Dragon, doesn’t have this configuration. Instead, it relies on redundant systems in its main computer system.

NASA officials said this configuration is sufficient, while Russian officials weren’t sold. The apprehension is understandable. A bungled approach could put the capsule, the ISS, or both at risk.

“The ISS still has three people on board, and so this vehicle coming up to the ISS for the first time has to work,” Kirk Shireman, the manager of the International Space Station program, said last week. “It has to work. This time up here and the people who worked around the country to make this successful are very much aware of that.”

Russia took a few days to consider NASA’s reasoning. On Wednesday, just days before blastoff, Roscosmos formally agreed to proceed with the launch and docking, Shireman tells The Atlantic.

The docking maneuver is new for SpaceX. The company regularly delivers cargo to the ISS on another spacecraft, Dragon 1. As the capsule approaches, a powerful robotic arm on the station reaches out and grabs it. Success is not guaranteed. In 2017, an approaching Dragon detected an error and automatically aborted the attempt just three-quarters of a mile away from the ISS. The capsule docked successfully in a second attempt the following day.

Unlike its counterpart, Crew Dragon will greet the station by locking up to a brand-new adapter—a metallic ring more than five feet across—that SpaceX itself actually delivered in 2016. (Boeing’s uncrewed capsule, Starliner, is expected to complete the same mission in April.) If Crew Dragon sticks the docking, it will become the first commercially built spacecraft designed to carry humans to join with the ISS.

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Bill Gerstenmaier, the associate administrator for NASA’s human-exploration division, said Roscosmos broached the subject in December, but admitted he “wasn’t diligent enough to stay in touch with them” over the 35-day government shutdown that kept 95 percent of NASA’s workforce from their jobs. As the historic launch approached, Gerstenmaier renewed discussions.