Jason DavisMay 24, 2012

Dragon completes initial ISS fly-under

SpaceX’s Dragon spacecraft completed a successful fly-under of the International Space Station this morning at 7:24AM EDT (11:24 UTC). From a distance of 2.5 kilometers beneath the ISS, the commercial capsule could be seen as a dark shape against the blue-white backdrop of Earth, its two solar arrays occasionally glinting in the sunlight.

Dragon fly-under in progress
Dragon fly-under in progress In this screencap from NASA TV, the Dragon spacecraft and its solar arrays can be resolved from the International Space Station. The Dragon flew under the ISS at a distance of 2.5 kilometers.Image: NASA TV

Expedition 31 crew members aboard the ISS were able to use the CUCU (COTS UHF Communication Unit, pronounced “cuckoo” ) to activate a strobe light on the Dragon, verifying the vehicle’s ability to receive commands from the station. At a press conference this morning, SpaceX mission director John Couluris reported the CUCU maintained its link with the ISS out to a distance of 90 kilometers, far above design specifications. Couluris also said that the Dragon has 36 kilograms of surplus propellant in its tanks.

NASA posted a video of a portion of the fly-under. It marked the first time the Dragon was photographed in Earth orbit by another vehicle:

The Dragon will fly back above the ISS today, eventually positioning itself behind and beneath the station in preparation for tomorrow’s berthing attempt on flight day four. Coverage begins on NASA TV at 2:00AM EDT (6:00 UTC).

The activities begin with Dragon closing on the station and passing through a series of checkpoints:

  • Height adjust burn to close within 2.5 kilometers of station
  • Dragon receives and sends information from/to the CUCU unit on the ISS
  • Height adjust burn, Dragon closes to 1.2 kilometers
  • Height adjust burn, Dragon enters the station’s approach ellipsoid
  • LIDAR demo, shows Dragon’s LIDAR is processing the necessary information for proximity operations
  • Dragon holds at 250 meters for R-bar maneuvers

The station’s radial bar, or R-bar, is an imaginary line connecting the ISS to the center of the Earth. The Dragon will approach the station on this path and must pass another round of checkpoints before it is permitted to berth.

Dragon ISS approach, flight day four
Dragon ISS approach, flight day four After demonstrating its ability to retreat and hold, the Dragon will close to within ten meters of the station to be grappled by Canadarm 2.Image: NASA

The R-bar events are as follows (EDIT: I have seen varying timelines posted for the capture and berthing, so this may change):

  • Dragon closes to 220 meters
  • Dragon receives retreat command, pulls back to 250 meters
  • Dragon closes to 220 meters
  • Dragon enters 200-meter ‘keep-out sphere’
  • Dragon closes to 30 meters
  • Dragon closes to 10 meters and enters free drift
  • ISS crew captures Dragon with Canadarm2 (approximately 8:00AM EDT / 12:00 UTC)
  • ISS crew guides Dragon to Harmony node with Canadarm2
  • Berthing (approximately 11:30AM EDT / 15:30 UTC)

I will continue to post periodic updates on my blog, and via my Twitter account, as the mission progresses.

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