The SpaceX Dragon capsule rendezvoused with the International Space Station (ISS) May 25, overcoming some late tracking issues to become the first U.S. commercial resupply craft to dock with the six-person orbital science laboratory.

Astronauts Don Pettit, Andre Kuipers and Joe Acaba grappled the unpiloted spacecraft with the station’s 17.6-meter (58-ft.) Canadarm2 at 9:56 a.m. EDT, as Dragon flew in formation 10 meters below. The freighter was berthed at the station’s U.S. segment Harmony module at 12:02 p.m. EDT.

The docking marks a significant milestone for NASA’s six-year-old Commercial Orbital Transportation Services (COTS) program as well as for the Hawthorne, Calif.-based SpaceX. NASA plans to initiate regular commercial cargo deliveries to the space station with SpaceX later this year. Orbital Sciences Corp., NASA’s second COTS participant, could be close behind with its first station mission.

“Looks like we got us a Dragon by the tail,” Pettit, the Canadarm2 operator, informed Mission Control when capture occurred while the two spacecraft orbited 250 mi. over Australia at more than 17,500 mph.

The final stages of the robot arm capture took longer than anticipated, as the NASA Mission Control team in Houston and SpaceX flight control team in Hawthorne sorted through some tracking discrepancies between Dragon’s thermal imager and lidar guidance systems that surfaced with 250 meters separating the two spacecraft.

As the freighter closed to nearly 30 meters, the lidar locked up on a reflector on the station’s Japanese Kibo module. That prompted a momentary retreat of the capsule to 70 meters, while SpaceX adjusted the lidar to ignore the uninvolved reflector. Subsequently, one of two Dragon lidar trackers failed—leaving the capsule primed to abort the capture operation if there was a second failure.

The nine-day mission was launched early May 22 from Cape Canaveral.

The NASA-led ISS mission management team (MMT) formally approved Dragon’s berthing late May 24. The action followed a review of the day’s “fly under,” the activities earlier in the day that propelled the capsule 1.5 mi. below the station for checkouts of the relative GPS navigation data exchange between the two spacecraft and the ability of the astronauts to slow, stop and command a retreat of the freighter if necessary.

Following the May 24 test, Dragon maneuvered out in front, above and then behind the station in a racetrack pattern while NASA and SpaceX flight control teams and ultimately the MMT reviewed the capsule’s performance. A series of nine major maneuvers and propulsive adjustments brought Dragon to a point 250 meters below the station at 5:22 a.m.

Dragon advanced to 235 meters for a series of “retreat” and “hold” command tests before approaching to 200 meters, then 150 meters to refine the tracking data from the capsule’s infrared imagers and converge them with Lidar readings.

Each of Dragon’s final advances to the 10-meter “capture point” followed a Go/No-Go decision by the control teams.

The astronauts plan to open Dragon early May 26 to unload slightly more than 1,000 lb. of food, clothing, research equipment, empty cargo containers and computer equipment.

Dragon will be reloaded with nearly 1,400 lb. of crew equipment, station hardware, spacesuit gear and experiment samples for return to Earth.

Currently, Dragon is scheduled to unberth with the help of Canadarm2 and the astronauts on May 31. The capsule is scheduled to reenter the Earth’s atmosphere and aim for a splashdown under parachute in the Pacific Ocean off Southern California at about mid-day.