The SpaceX Dragon, working flawlessly, sailed 1.5 mi. below the International Space Station (ISS) early May 24, clearing the way for the first U.S. commercial delivery of cargo to the orbiting laboratory.

Following a pair of crucial, close-range navigation and communications checks during the “fly under,” the unpiloted Dragon began to move out in front, above and behind the six-person station in a racetrack pattern. The trajectory will swing the freighter back to the orbital outpost overnight and, if all goes well, to a Canadarm2 capture tomorrow morning.

The linkup marks a significant step in the relationship between NASA and the U.S. commercial space industry that started six years ago when the space agency established its Commercial Orbital Transportation Services initiative to transfer orbital cargo and crew transport responsibilities from the space shuttle to the private sector.

Struggling to survive in Congress, the crew transport portion of the NASA initiative is scheduled for 2017, if the agency acquires adequate funding and a commitment from its commercial partners. If successful, the strategy will free NASA to pursue a decades-long goal of sending human explorers on deep space missions, first to an asteroid, and then Mars.

Dragon’s mission also marks the first visit of a U.S. spacecraft to the space station since the final shuttle mission in July 2011.

The significance of Dragon’s fight was underscored late May 23, when President Barack Obama called SpaceX founder and CEO Elon Musk to offer his congratulations on the successful launch of the company’s Falcon 9 rocket with Dragon early the previous day and the subsequent series of power, navigation and communications checkouts of the capsule.

Demonstrations on the eve of the May 25 “track and capture” of Dragon using the space station’s 58-ft.-long Canadarm2 established a successful exchange of relative GPS navigation data between the ISS and the capsule as well as a UHF communications link that permitted station astronauts Don Pettit, Andre Kuipers and Joe Acaba to command the freighter to turn on a strobe light.

The demonstrations confirmed that Dragon can move precisely in relation to the station and that the astronauts can command the capsule to pause its advance and even move away if there was a collision threat. But NASA and SpaceX officials still sounded a cautious note.

“Although today was successful, this is still a demonstration mission,” Holly Ridings, NASA’s lead flight director for the exercise, told a Johnson Space Center news briefing. “To get through the first piece obviously makes you feel positive. But in terms of the activities tomorrow, there’s still a lot of really new things the teams and the vehicles need to perform.”

“We retired a lot of risk,” added John Couluris, SpaceX mission manager at company headquarters in Hawthorne, Calif. “But there is more fine maneuvering and Dragon has a lot of automatic systems on board to protect station and Dragon itself if we see a malfunction. We have not seen anything like that, but it is the first time we are using them. We are being cautiously optimistic.”

The May 24 operations began to unfold shortly before 4 a.m. EDT, as Dragon fired thrusters to start an approach from about 60 mi. behind and 6 mi. below the station. A second maneuver sent Dragon 1.5 mi. directly underneath the station at 7:28 a.m. and into the racetrack pattern.

The timeline was disrupted temporarily when one of three monitors in the station’s Cupola observation deck faltered. The monitors are used by Pettit and his colleagues to follow Dragon’s status with external cameras and operate the robot arm. The system responded to a reboot, and there is a backup Canadarm2 control post in the station’s U.S. lab.

As Dragon returns to the station on May 25, the capsule will approach to a point 820 ft. below the outpost. Ground control teams as well as the station crew will share a series of commands, directing Dragon to advance, hold, retreat and finally advance to a point within 35 ft. of the station. Each step will receive “go/no-go” scrutiny from NASA’s Mission Control.

Posted in the Cupola observation deck, Pettit and Kuipers will reach with the station’s 58-ft.-long robot arm to grapple Dragon shortly after 8 a.m. They are scheduled to conclude the berthing operation by hoisting Dragon to a port on the station’s U.S. segment Harmony module shortly after 11 a.m.

The ISS crew plans to open Dragon early May 26. The capsule is carrying just more than 1,000 lb. of food, computer gear and a Nanoracks student experiments package.

The freighter will be released on May 31. Following re-entry, the spacecraft will descend under parachute into the Pacific Ocean off the southern California coast, splashing down just before noon.

SpaceX teams will be prepared to recover the reusable spacecraft and nearly 1,400 lb. in return cargo consisting of research samples and equipment, station hardware and spacesuit gear.