Gotcha! Robot arm plugs SpaceX Dragon into the ISS

Paul Marks, senior technology?correspondent"Looks like we caught a dragon by the tail." The relief in the words of ISS astronaut Don Pettit was audible as he grabbed the uncrewed SpaceX Dragon spacecraft using a robotic arm today - marking the first time a civilian-designed spacecraft has shown docked with the space station.

Once held fast, the arm - steered by ISS crew - orientated the Dragon correctly and moved it in to dock with the standard non-Russian docking hatch called a Common Berthing Mechanism. The historic docking followed a morning in which the spacecraft underwent a series of control tests - such as approaching the ISS and then retreating a precise distance to order - to prove that it is controllable in an emergency. The idea was to prevent a repeat of the 1997 accident when a Russian Progress cargo freighter collided with the Mir space station?(video) and caused a dangerous depressurisation and power loss (luckily all on board survived)."Dragon started backing away as it was meant to do, reaching a 250-metre hold position," said NASA on its online TV channel. It successfully completed follow up manoeuvres to 36 metres, then 30 metres - using thermal imaging and laser radar (LIDAR) data to double check it's range. For a time the thermal and LIDAR range data were not in agreement - but after the numbers "converged" later Dragon was finally given a 'go' to berth with the ISS.?
Effectively, Dragon's docking repeated a spectacular cosmic manoeuvre pioneered by Japan's unmanned cargo supply ship, the H-II Transfer Vehicle (HTV), on its first two successful missions. This involves a precision flight alongside the ISS that allows the station's 18-metre-long robotic arm, Canadarm2, to grasp the spacecraft and - literally - plug it into a docking hatch.A standard ISS robotic handhold unit,?called a power data grapple fixture, allows the arm's grippers to safely and firmly grasp units to which they are attached. Indeed, a series of such fixtures dotted around the station allows Canadarm2 to self-relocate, moving end-over-end like an inch worm. A grapple fixture attached to Dragon allowed the Canadarm2 to grasp it.On Saturday, the hatch will be opened to the pressurised capsule so the ISS crew can unload the cargo - which as you can see from the flight manifest includes astronaut rations, clothes, laptops and a bunch of zero-g experiments.

Unlike all other ISS resupply cargo vessels - the Russian Progress, European ATV and Japan's HTV - Dragon's overarching advantage if that it is designed to be recoverable via an Apollo-style ocean splashdown. Re-entry, and a splashdown with a return cargo intact, will be SpaceX's next challenge in two weeks time.

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