US space agency (Nasa) has postponed its next space shuttle mission until July at the earliest because of a faulty fuel tank sensor.
Faulty ECO sensors could shut down the engines early
Nasa said replacing the sensor would take three weeks, meaning it could not meet its scheduled May launch window.
The US space fleet has been grounded since July last year, when insulation foam broke off the fuel tank of the shuttle Discovery as it took off.
The same problem led to the loss of shuttle Columbia and its crew in 2003.
"We wish it had worked out differently, but it's first and foremost that we fly safely," shuttle programme manager Wayne Hale said.
"It was prudent to change these sensors out."
Problematic engine cutoff (ECO) sensors also dogged Discovery's launch efforts last year.
The ECO sensors warn the shuttle's computers if giant external fuel tank is about to empty, allowing the three main engines on the orbiter to be shut down in a timely fashion.
This is essential as the shuttle's engine pumps can force half a tonne of propellant per second out of the tank; and if these suddenly run dry, they can overspeed and disintegrate.
What Nasa engineers have to ensure is that faulty ECO sensors do not shut down the engines prematurely, denying the shuttle sufficient thrust to get to orbit.
Nasa has been working to eliminate critical foam loss
During testing, one of the tank's four ECO sensors had a slightly different reading than was expected. This has prompted shuttle officials to order the removal and replacement of all four sensors.
"We've been saying for months that our engineering work would determine when we fly our next mission. Targeting July is the right choice in order to make smart decisions," said Bill Gerstenmaier, Nasa associate administrator for space operations.
The delay will also give engineers time to assess modifications to the foam covering of the external tank, and to repair some accidental damage to the shuttle's robotic arm.
The new window for the launch runs from 1 to 19 July.