The SpaceX Dragon spacecraft successfully berthed with the International Space Station this morning after a long overnight approach including several unplanned maneuvers. The crew at SpaceX headquarters in Hawthorne, California, concluded a long night of flight demonstrations and troubleshooting by watching astronaut Don Pettit control the station’s robotic arm and grapple the Dragon at 6:56 a.m. PDT.
“Looks like we’ve got a Dragon by the tail,” Pettit said from the station’s Cupola module once the capture was made.
Pettit’s successful capture of the Dragon was greeted by cheers at both SpaceX’s Hawthorne headquarters and NASA’s mission control in Houston. For both SpaceX and NASA the capture moment marks the beginning of a shift in how cargo will be delivered to and from the space station, with the eventual goal of changing how manned flight itself is done to low Earth orbit.
But Dragon’s overnight approach was not without hiccups, demonstrating the true test-flight nature of the mission. A problem with the devices used to guide the Dragon as it approached the station forced an initial retreat. In the end there were a handful of changes made to the initial flight plan, but at 6:49 a.m. PDT, the Dragon sat just 10 meters (32 feet) from the ISS when NASA flight director Holly Ridings gave the command SpaceX had been waiting years to hear: “go for capture.”
Early Friday morning the SpaceX team in Hawthorne completed the approach initiation burn of the Dragon’s Draco thrusters to move the spacecraft roughly 1,000 meters to a point where it could change its alignment relative to the station before performing the first series of demonstration maneuvers close to the ISS. The Dragon spacecraft could be seen on Earth by its flashing strobe light against the night sky.
Once in place at 350 meters, Dragon completed a 180-degree yaw rotation to align itself, and then another short burn was performed to move to the 250-meter point where the demonstrations would begin.
At 2:29 a.m. PDT, the SpaceX team confirmed Dragon was holding at 250 meters (820 feet), but Andre Kuipers, the Dutch astronaut on board the station, noticed the spacecraft was slightly forward of where it was expected to be. NASA engineers in Houston said the position was acceptable.
Sketch of the demonstration maneuvers planned for Dragon near the ISS. Image: NASA
As the ISS and Dragon passed in and out of sunlight orbiting the Earth every 90 minutes, the teams in Houston and Hawthorne prepared for what has long been considered the most challenging and critical part of the mission, demonstrating Dragon can make several different maneuvers in close range to the ISS, with commands being sent from both the ground and from the astronauts on the station.
Just before 3:00 a.m. PDT, with a short burst of the thrusters, Dragon again began approaching the ISS. Minutes later, with the Dragon 220 meters from the station, astronaut Kuipers sent a command via the UHF communications link and Dragon aborted its approach as expected and returned to the 250 meter hold position. Test one was complete.
Kuipers planned to send a command for Dragon to hold at 235 meters, but problems with Dragon’s on-board thermal camera used for the rendezvous with the ISS kept it at the 250 meter point. After a few minutes the test resumed and Kuipers issued Dragon a hold command at 235 meters, but it happened a bit earlier than planned.
Over the next half hour or so, the teams in Hawthorne and Houston were busy evaluating the data from the on-board sensors to make sure both the station and Dragon agreed on their relative positions before moving any closer, particularly inside the simply named “Keep Out Sphere” that surrounds the ISS at 200 meters.
As time progressed, some questions were raised from the data being analyzed from Dragon’s thermal image sensors and the on-board LIDAR (light detection and ranging) sensors. The two different devices are used to independently measure the distance between the two spacecraft. This information is then used by Dragon’s guidance system as it approaches the ISS. The data provided by the thermal cameras was causing the engineers to further evaluate the sensor.
Dragon was sent to an unplanned hold position at 200 meters in hopes of giving the thermal sensors a chance to obtain better data on the position of Dragon relative to the ISS.
Dragon at 30 meters from the station. Photo: NASA
By 4:20 a.m. PDT Dragon was once again on the move, this time to a position 150 meters from the station. After checking the sensors again, Dragon was cleared to fly to the next hold point at 30 meters. But less than 20 minutes later Dragon was held at 78 meters as SpaceX made some changes to the spacecraft’s LIDAR equipment. At 5:21 a.m. PDT the approach was resumed but just four minutes later SpaceX issued a retreat command, moving Dragon away from the ISS.
Dragon returned to 78 meters while a problem with the LIDAR was analyzed. It turns out the laser used by the LIDAR was receiving stray reflections from the Japanese Kibo laboratory on the station. Over the course of the next half hour, SpaceX engineers analyzed how to resolve the problem with the stray LIDAR signals, eventually deciding to narrow the view of the LIDAR. Essentially they put blinders on the sensors so they could only see straight ahead, where the Dragon was set to be berthed with the station.
Eventually Dragon was given the go-ahead to proceed to the 30-meter point, and then to the 10-meter location where it would be captured by the station’s robotic arm. After rescheduling the planed grapple a few times during the morning, the go-ahead was given for a capture at 7:02 a.m., which would take place in the dark as the two spacecraft passed over Australia. Dragon took roughly 20 minutes to fly the final 20 meters to its final hold position.
The station’s robotic Canada arm approaches Dragon. Photo: NASA
With Dragon in place at 10 meters, NASA’s Holly Ridings sat at her flight director desk in Houston with a purple stuffed dragon toy on the console above her. She anxiously twirled her pen in her hand as she told astronaut Pettit that Dragon was operating on a single LIDAR and should that one fail, the spacecraft would abort.
But in the final minutes, everything went well as Pettit maneuvered the arm towards Dragon. As the end of the arm inched towards the capsule, lights from the ISS bathed Dragon in an orange glow. A few minutes ahead of schedule at 6:56 a.m. PDT, capture was confirmed by NASA, marking the first time a private spacecraft was attached to the International Space Station.
The capture occurred 3 days, 6 hours, 11 minutes and 23 seconds after the Falcon 9 had lifted off from Launch Complex 40 at Cape Canaveral Tuesday morning. After another hour and a few reconfigurations of Dragon the robotic arm slowly pulled the capsule towards the station and at 8:52 a.m. PDT NASA confirmed Dragon was firmly attached to the station itself and the robotic arm was no longer holding it in place. After tightening the 16 bolts attaching Dragon to the station’s Harmony module, the ISS officially had its first private spacecraft visitor.
Dragon berthed to the Harmony module on the ISS. Photo: NASA
There are several more steps before the hatch between the ISS and Dragon will be opened early Saturday. Once opened, the crew on board the station will spend several days unpacking the 1,014 pounds (460 kilograms) of cargo on board Dragon. Once empty, the crew will load up Dragon with 1,367 (620 kilograms) of cargo before the spacecraft is released from the station and returns to earth with a splashdown in the Pacific Ocean on May 31.