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NASA / JPL-Caltech
The launch of Nasa's Mars rover Perseverance has been pushed back again to 30 July at the earliest.
In an update, the US space agency said a technical issue needed to be investigated, prompting the delay.
The robot rover will search for signs of past life on the Red Planet and also carries a drone-like helicopter which will demonstrate powered flight in the Martian atmosphere.
It is scheduled to land in February 2021.
In its statement, Nasa said: "A liquid oxygen sensor line presented off-nominal data during the Wet Dress Rehearsal, and additional time is needed for the team to inspect and evaluate."
The mission's original launch window extended from 17 July to 11 August.
But the rover will now get more time to launch.
"Flight analysis teams have expanded the mission launch opportunities to August 15 and are examining if the launch period may be extended further into August," Nasa said.
The mission needs to be launched this year, or its next opportunity will come up in 2022.
The targeted launch date has been pushed back three times, first to 20 July, then to 22 July and now 30 July.
The Perseverance rover will land on Mars to search out signs of past microbial life, if it ever existed. It will be the first Nasa mission to hunt directly for these "biosignatures" since the Viking missions in the 1970s.
The rover will collect samples of rock and soil, encase them in tubes, and leave them on the planet's surface for return to Earth at a future date.
Perseverance will also study the Red Planet's geology and test how astronauts on future Mars missions could produce oxygen from CO2 in the atmosphere. This oxygen could be used for breathing and for fuel.
