The SpaceX Dragon capsule has commenced Thursday's "fly under" of the International Space Station, an exercise of the relative GPS navigation and communication links between the two spacecraft that is serving as a dress rehearsal for the activities on Friday that would lead to a berthing of the first U.S. commercial resupply mission.
Thursday's test activities began in earnest just before 4 a.m., EDT, as the unpiloted Dragon began its propulsive approach from a point 60 miles behind and six miles below the 250 mile high six person orbiting science lab.
The significance of Dragon's fight was underscored late Wednesday, as President Obama called SpaceX founder and CEO Elon Musk to offer his congratulations for the successful launch of the company's Falcon 9 rocket with Dragon early Tuesday and a successful series of solar power, navigation and communications check outs that followed.
"The president just called to say congrats," Musk said by Twitter, with a touch of humor. "Caller ID was blocked, so at first I thought it was a telemarketer :)"
Thursday's tests are scheduled to conclude with a second burn just before 5 a.m., EDT, that will send Dragon sweeping 1.5 miles below the station a little over 90 minutes later.
During the fly under, the two spacecraft will exchange relative GPS data for the first time. Station astronauts Don Pettit, Andre Kuipers and Joe Acaba will also use a command console in the station's cupola observation deck to switch on a Dragon strobe light.
Thursday's command test will confirm the ability of Pettit and his colleagues to control Dragon on Friday, should they need to abort efforts to grapple the freighter with the station's Canadarm2.
If the exercise goes well, Dragon will bring Thursday's proximity operations to a close and maneuver along a race track course that will take the spacecraft out in front, over the top and ultimately to a point about 150 miles behind and above the station.
Following a review by NASA flight control teams and the station MMT, Friday's scheduled re-rendezvous, grapple and berthing operations will begin. Dragon should reach the Canadarm 2 grapple point about 30 feet below the station shortly before 8 a.m., EDT.
Pettit, Kuipers and Acaba would reach out with the 58 foot long robot arm and grapple Dragon a few minutes later. Using the arm they will berth Dragon to the station's U. S. segment Harmony module shortly after 11 a.m., EDT.
Currently, the commercial freighter is scheduled to remain berthed until May 31, when it will undock with help from Canadarm2 for a release shortly after 6 a.m., EDT. The nine-day mission would conclude with a parachute descent and splashdown in the Pacific Ocean off the southern California coast before noon, EDT.