CAPE CANAVERAL, Fla. — The privately bankrolled Dragon capsule approached the International Space Station today for a historic docking after sailing through a practice rendezvous the day before.

The unmanned SpaceX Dragon was on track most of the morning to deliver a half-ton of supplies and become the first commercial vessel to visit the space station. But as the capsule drew within 100 feet, flight controllers commanded it to retreat.

The capsule backed off to 230 feet as the SpaceX company worked to resolve a problem with the on-board tracking sensors. Stray reflections from the Japanese part of the space station were interfering with the Dragon’s laser-based sensors, officials said. SpaceX mission controllers quickly fixed the trouble and resumed the docking operation.

On Thursday, the capsule came within 1½ miles of the space station in a practice fly-by. It returned to the neighborhood early today so Kuipers and U.S. astronaut Donald Pettit could capture it with a robot arm. First, the capsule went through a series of stop-and-go demonstrations to prove it was under good operating control.

NASA ordered extra checks of the Dragon’s imagers as the capsule drew ever closer to the space station, putting the entire operation slightly behind schedule. Given that the Dragon is a brand new type of vehicle and this is a test flight, the space agency said it wanted to proceed cautiously.