After 52 days, it’s “highly unlikely” that any debris will be found on the surface, Australian Prime Minister Tony Abbott told reporters in Canberra today. Malaysia and Australia will hire commercial companies to carry out an intensified underwater search that may take several months, he said. The hunt for the Malaysian Airline System Bhd. jet, which vanished March 8 with 239 people, is already the longest for a missing passenger plane in modern aviation history. An unmanned submarine has failed to find any wreckage on the ocean floor after scouring 400 square kilometres and the underwater search area will be expanded. “What we are looking to do is conduct as thorough an undersea search as is humanly possible, if necessary of the entire probable impact zone” measuring about 700 kilometres by 80 kilometres, Abbott said. Aircraft have spent 3,000 hours searching 4.5 million square kilometres of ocean, with no debris found, he said. Flight 370’s disappearance has baffled authorities because contact was lost less than an hour into a routine trip to Beijing from Kuala Lumpur. The widebody plane vanished from civilian radars while headed north over the Gulf of Thailand, then doubled back and flew over Peninsular Malaysia and on into some of the world’s most remote waters. The jet’s data recorders, designed to emit regular pings, had a battery life of only about 30 days. The Bluefin-21 submersible had focused on an area within a 10-kilometre radius of where acoustic pings were detected on April 8. The search zone is about 1,600 kilometres (1,000 miles) northwest of Perth, Australia. — Bloomberg