This year, two 41 year old American U-2 reconnaissance aircraft each achieved a record 25,000 hours in the air. One of these aircraft made three belly (landing gear up) landings, requiring extensive rebuilding after each incident. With a range of over 11,000 kilometers, the 18 ton U-2s typically fly missions 12-18 hours long. All U-2s have been upgraded to the Block 20 standard, so they can be kept in service for up to another decade. Or at least until the robotic RQ-4 Global Hawk is completely debugged, and available in sufficient quantity to replace all the U-2s. The U-2 has been in service since 1955, in small numbers. Only about 850 pilots have qualified to fly the U-2 in that time.
All this is something of a comeback for the U-2. Three years ago, the U.S. Air Force wanted to retire its 33 U-2 reconnaissance aircraft, and replace them with UAVs like Global Hawk. But Congress refused to allow it, partly for political reasons (jobs would be lost, which is always a live political issue), and some in Congress (and the air force) did not believe that Global Hawk was ready to completely replace the U-2.
Ever resourceful, the air force has decided to make the most of it by trying to turn the U-2s into UAVs. This is not cutting edge technology, the air force has been turning warplanes into UAVs for over half a century, mainly for use as target aircraft (usually for missile tests, which require very realistic targets if you want to be really sure the missile works.) Replacing the U-2 pilot with software and automated controls also solves several other problems. Since the U-2 only carries a single pilot, the aircraft cannot safely stay in the air as long as it could, because the pilot would become too fatigued. Currently, the max endurance for a U-2 is twelve hours. But without a pilot, and all the gear required by a pilot, you could carry more fuel, and keep a U-2 UAV in the air for up to 18 hours. Moreover, the U-2 can fly higher than the Global Hawk, and carry more sensors. So, in theory, a U-2 UAV is superior to Global Hawk.
The key unanswered question was how much will it cost to develop the software for flying the U-2 remotely, and how long would it take. The answers were too much and too long, so the pilots stayed, and the U-2 continues to get the job done. New Global Hawks continue to appear, but there is so much demand for the kind of recon work the two aircraft can do, that both pilots and robots will coexist for a while.