SpaceX cargo delivery gets green light ahead of ISS repair spacewalk

SpaceX's latest delivery to the International Space Station was nearly put on hold for a third time this weekend, when NASA discussed plans for an unscheduled spacewalk to repair a downed backup computer on the station. However, rather than causing another delay, the situation has made today's launch a priority, since the cargo on board includes equipment crucial for the spacewalk to succeed.

The launch of SpaceX's Dragon spacecraft occurs at 4:58 p.m. EDT today, with the NASA webcast of the event starting at 3:45 p.m. EDT. Click here if you'd like to stream the launch live.

This is the third attempt for this launch already, after a delay on March 13 due to a possible contamination issue with the unpressurized section of the Dragon spacecraft, then another rescheduling on March 26 because the Air Force radar station used to track rocket launches on the east coast was knocked offline due to a fire.

Normally, a situation on the ISS that requires a repair spacewalk, like the cooling system problem the station suffered back in mid-Dec., puts a hold on any scheduled launches. However, in this case, while the crew has the necessary components to swap out the backup computer that failed on Friday, the Dragon cargo vessel is carrying equipment that are need before the crew can perform the spacewalk. After the close-call with Italian astronaut Luca Parmitano back in July, when a water leak inside his helmet caused a near-drowning experience, the crew rigged up snorkels and extra absorbent padding to prevent this from happening again. However, the new cargo delivery has components on board for a permanent fix for the spacesuits, rather than going with that temporary solution again, so in this instance, the launch is putting a hold on the spacewalk.

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Some other interesting cargo being carried up to the station today includes new legs for one of the station's crew (Robonaut), a new laser communications system and a miniature space farm, but there's something extraordinary about the launch itself.

Remember SpaceX's incredibly cool Grasshopper test flights from last year? This launch is the next stage of their testing, and the first one to go into space. The rocket's first stage has four 7.6-metre-long landing legs folded up against the hull, and once it separates during the launch, the SpaceX team will attempt to bring that part of the rocket back down to Earth for a water landing. The odds of success are apparently fairly low, but the test itself will give them a great indication of how quickly they can get the system working.

Update (April 14, 4:15 p.m. ET): SpaceX announced that today's launch has been scrubbed due to a helium leak in the Falcon 9 rocket's first stage:

The next opportunity for a launch will be at 3:29 p.m. ET on Friday, April 18, although the company says that the weather forecast for that day is not as favourable. More updates on the way.

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