Why historic SpaceX mission to space station will be so difficult

The reason: NASA is requiring that Dragon perform several kinds of maneuvers, including a rendezvous abort, in the station's general vicinity. The agency and its international partners have to be satisfied that the craft can maneuver itself precisely enough to avoid colliding with the station as it approaches, while also showing that it can reach and hold a position within a few tens of yards of the outpost while station crew members snag it with the station's robotic arm for final docking. 

The few-seconds-long launch window puts the Dragon on the most fuel-miserly trajectory to the station, leaving enough thruster propellant available for on-orbit do-overs if needed.

SpaceX's package is designed with a capability NASA says it needs and which the current automated resupply ships from Russian, Europe, and Japan do not have. These other cargo carriers burn up on reentry, serving as a high-tech incinerator for space trash. Dragon is designed to bring cargo back from the station.

Although this mission is a test flight, SpaceX has loaded some 1,000 pounds of cargo into Dragon in anticipation of a successful docking. It's cargo NASA and the station crew can use, although the world won't end if it doesn't arrive – some laptops, empty cargo bags for future return flights, food, clothing, a lab rack, blocks of ice for cooling experiment samples, and small items such as batteries.

“We didn't put anything on the vehicle we didn't think we could stand not to get home,” says Michael Suffredini, manager of NASA's International Space Station program.

And the craft is slated to bring nearly 1,500 pounds of cargo back with it.

While SpaceX's initial tests of its Falcon 9, and later its Falcon 9-Dragon combination in 2010 were successful, this mission “is pretty tricky,” said the SpaceX's founder and chief designer Elon Musk, during a preflight briefing last month.

The space station is traveling at speeds about 12 times faster than a bullet from an assault rifle. Dragon controllers have to track the station and their own craft to within inches. Moreover, Dragon includes an extra section it didn't have during its successful orbital flight in December 2010. Several systems are flying for the first time – from the craft's solar panels and the pressurized module's cooling and air circulation systems to the instrument package that will allow the craft to approach the station on its own.

With no space station on the ground and under space-like conditions, simulations approximating the conditions Dragon will encounter are the only tools for telling engineers whether they are on the right track, Mr. Musk says. “A lot can go wrong on a mission like this.”

The first of the two demonstration flights blended into this one mission involved the near-station maneuvers. NASA and SpaceX officials say that if the mission successfully passes most or all of these tests, but ultimately fails to dock, the company has two other opportunities this year to demonstrate Dragon's docking ability – the second of the two demo flights consolidated into this mission.

“Achieving any one of those incremental steps” that make up the objectives for this mission “would be considered a success,” says Alan Lyndenmoyer, manager of NASA's commercial crew and cargo program.

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