Live coverage: Ariane 5 soars into space from Kourou

SpaceX’s launch director halted a countdown Tuesday to replace a sensor on the upper stage of a Falcon 9 rocket set to blast off with the GovSat 1 communications craft, the product of a joint venture between the commercial satellite operator SES and the government of Luxembourg.

Engineers working on the James Webb Space Telescope, which had its launch delayed to 2020 earlier this year, recently discovered another problem during testing at the observatory’s prime contractor Northrop Grumman in Southern California.

SpaceX called off the launch of a Falcon 9 rocket from Cape Canaveral on Wednesday to make sure the booster’s supply of super-chilled cryogenic liquid oxygen is cold enough to support the delivery of the 5.8-ton SES 9 commercial communications satellite to an orbit nearly 25,000 miles above Earth.