SpaceX has aborted its Falcon 9 rocket launch at the last minute, postponing a landmark bid to try the first-ever platform landing of the rocket's first stage.
"The issue developed fairly late in the terminal count," said NASA commentator George Diller.
The problem that led to the delay was not immediately clear, but involved an "anomaly" with the rocket, SpaceX said.
The earliest SpaceX can try again is on Friday.
Once the first stage of the rocket launches, it will try to head back to Earth for the landing - targeting a sea barge in the Atlantic.
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The California-based company headed by billionaire internet entrepreneur Elon Musk has given itself just a 50-50 chance of success.
