SpaceX cargo ship cleared for station capture
Updated at 6:47 p.m. ET
With its propulsion system working flawlessly, a SpaceX cargo ship loaded with supplies and equipment bound for the International Space Station was cleared by NASA Saturday to press ahead for a day-late capture by the lab complex early Sunday.
NASA space station managers and their SpaceX counterparts met Saturday to assess the Dragon capsule's performance following launch Friday and concluded all systems were "go" for a replanned rendezvous and capture by the station's robot arm Sunday at 6:31 a.m. EST.
"The station's Mission Management Team unanimously agreed that Dragon's propulsion system is operating normally along with its other systems and ready to support the rendezvous," NASA said in a web update.
The original flight plan called for capture and berthing Saturday morning, but mission managers were forced to replan the approach after SpaceX flight controllers ran into problems with the capsule's maneuvering thrusters.
Just after the Dragon capsule separated from its Falcon 9 rocket Friday, telemetry showed that three of four sets of thrusters were off line due to problems pressurizing oxidizer propellant tanks. As a result, the spacecraft could not be properly oriented for normal communications and battery charging once its two solar arrays were deployed.
After reviewing telemetry, SpaceX flight controllers concluded the problem likely was the result of blockage in a pressurization line somewhere in the system. By cycling other valves and "pressure hammering" the lines, whatever was causing the problem eventually went away and all four oxidizer tanks were successfully pressurized.
A series of rocket firings verified the thrusters were healthy and commands were uplinked to carry out a critical maneuver to raise the low point of the Dragon's orbit.
Since then, the Dragon, flying six miles below the space station, passed ahead and looped up to a point well above the lab complex. At the higher altitude, the Dragon's velocity was slightly less than the station's, causing it to drop back behind the outpost.
Rocket firings later this evening were planned to drop the Dragon behind and below the station where it will start its final approach early Sunday.
"SpaceX officials reported to the multinational management team that all of Dragon's systems are operating as planned," NASA reported in its update. "SpaceX said it has high confidence there will be no repeat of the thruster problem during rendezvous, including its capability to perform an abort, should that be required."
If all goes well, the cargo ship will reach a point about 30 feet from the lab shortly after 6 a.m. At that point, Expedition 34 commander Kevin Ford, operating the station's robot arm from a robotics work station in the multi-window Cupola compartment, will lock on to a grapple fixture around 6:31 a.m. to complete the extended rendezvous.
During the first two dockings of Dragon capsules last May and October, the station crew manually operated the arm to maneuver the supply craft to berthing at the Earth-facing port of the forward Harmony module.
But this time around, ground controllers at the Johnson Space Center in Houston will take over, sending commands to remotely operate the arm through berthing to demonstrate their ability to carry out complex arm procedures and to give the astronauts a bit of a break during a very busy day.
The Dragon capsule is packed with some 2,300 pounds of equipment and provisions, including 178 pounds of crew supplies; 300 pounds of space station hardware, including replacement components for the lab's carbon dioxide removal system; and more than 700 pounds of science gear, including a pair of Glacier freezers and experiment components.
A spacewalk equipment handling fixture called a grapple bar is mounted in the Dragon capsule's unpressurized trunk section. The station's robot arm, again operated by flight controllers in Houston, will be used a few days after berthing to extract the grapple bar assembly and stow it on the station's exterior for future use.
After unloading the capsule, Ford and his crewmates will re-pack it with 1.5 tons of no-longer needed gear, components that need refurbishment or failure analysis and experiment samples that are needed by scientists back on Earth.
The return manifest includes 209 pounds of crew equipment; 1,455 pounds of science gear, including a Glacier freezer and cold bags loaded with experiment samples; and 884 pounds of space station hardware.
The Dragon will be detached from the station March 25, setting up a fiery re-entry and splashdown in the Pacific Ocean southwest of Los Angeles.
The SpaceX Dragon capsule is the only space station cargo craft designed to bring cargo back to Earth, a critical capability that was lost when NASA's space shuttle fleet was retired in 2011.
The manned Russian Soyuz spacecraft that carry three-person crews to and from the space station can only bring back a few hundred pounds of cargo. All other station vehicles -- unmanned Russian Progress supply ships and European and Japanese cargo craft -- burn up during re-entry.
