The UK-built Zephyr solar-powered plane has smashed the endurance record for an unmanned aerial vehicle (UAV).
The craft took off from the US Army's Yuma Proving Ground in Arizona at 1440 BST (0640 local time) last Friday and is still in the air.
Its non-stop operation, day and night, means it has now gone four times longer than the official mark recognised by the world air sports federation.
The plane has been developed by the defence and research company Qinetiq.
Its project manager, Jon Saltmarsh, said Zephyr would be brought down once it had flown non-stop for a fortnight.
"Zephyr is basically the first 'eternal aircraft'," he told BBC News.
The UAV has been under development for a number of years at Qinetiq.
Solar-powered high-altitude long-endurance (Hale) UAVs are expected to have a wide range of applications in the future.
The military will want to use them as reconnaissance and communications platforms. Civilian and scientific programmes will equip them with small payloads for Earth observation duties.
Their unique selling point is their persistence over a location. Low-Earth orbiting satellites come and go in a swift pass overhead, and the bigger drones now operated by the military still need to return to base at regular intervals for refuelling.
But as Zephyr has now proved, solar UAVs can be left in the sky.
Their solar cells drive propellers during the day and top up their batteries to maintain the craft through the dark hours of night. An autopilot keeps them circling over the same spot.
The latest version of Zephyr is now 50% bigger than its predecessors.
The updated vehicle has a wingspan of 22.5m, and features a new wingtip and tail design that dramatically improve aero performance.
It also has a wider configuration near the main body to accommodate more equipment. In addition, the team has upgraded the avionics and power management systems on board.
"The launch was absolutely beautiful; it was just so smooth," said Mr Saltmarsh. "We had five people lift it above their heads, start running and it just lifted away into the sky."
The current official world endurance record for a UAV is 30 hours, 24 minutes. This was set by the US robot Global Hawk. Zephyr itself has already recorded an 83-hour continuous flight but representatives from the Federation Aeronautique Internationale (FAI) were not present to witness proceedings.
However, they are at Yuma this time and so the latest flight will go down as an official world record provided the FAI is satisfied its rules have been followed.
"This is a huge milestone that puts us at the leading edge," Mr Saltmarsh told BBC News. "It's a practical system that can actually be used rather than simply a demonstration of small technical achievements."
The Zephyr flight is the second event of note this year in solar-powered aviation. Earlier this month, Andre Borschberg became the first person to pilot a manned solar plane through the night.