What time is it on the moon? Since the dawn of the space age, the answer has been: It depends. For decades, lunar missions have operated on the time of the country that launched them. But with several lunar explorations heading for the launchpad , the European Space Agency has deemed the current system unsustainable. The solution, the agency said last week, is a lunar time zone. “ESA is not taking the lead on this discussion, we’re just putting a finger on a problem we need to tackle,” said Brice Dellandrea, an engineer with the ESA. “But this is the kind of topic that needs international coordination and consensus.” The main objective of establishing a universal timekeeping system for the moon, the ESA said, is to streamline contact among the various countries and entities, public and private, that are coordinating trips to and around the moon. The discussion about how to do that is happening as things are starting to get busy on and above the lunar surface. The M1 lunar lander built by the Japanese company Ispace is set to arrive on the moon in April, when it will try to deploy a rover built by the United Arab Emirates; a robot built by Japan’s space agency, JAXA; and other payloads. A six-legged cylindrical robot called the Nova-C lander, built by the Houston-based company Intuitive Machines, is expected to launch on SpaceX’s Falcon 9 and land on the South Pole of the moon in June. Additional uncrewed missions will land by the end of the year, according to Jack Burns, director of the Network for Exploration and Space Science at the University of Colorado, Boulder. Those missions, among other possible lunar landings, are happening as NASA prepares to send four astronauts into orbit around the moon next year. That mission will pave the way for the first crewed moon landing since Apollo 17 in December 1972, currently planned for 2025. The European Space Agency, meanwhile, is contributing to NASA’s effort to build the Gateway lunar station, which will serve as a way station fo r future crews on their way to the lunar surface. Last year, China completed construction of its own space station and previously hinted that Chinese astronauts would be on the moon by 2030. South Korea launched its own lunar spacecraft, Danuri, on a SpaceX Falcon rocket from Florida in August. It joined India’s Chandrayaan-2 mission, as well as spacecraft from NASA and China, in its orbit of the moon. With increased exploration comes the potential for miscommunication.”These missions will not only be on or around the Moon at the same time, but they will often be interacting as well — potentially relaying communications for one another, performing joint observations or carrying out rendezvous operations,” the ESA said in a statement. For all those interactions to happen smoothly, the missions will need to operate on a standardized time, the agency said. “This idea of timekeeping on the moon is important because it shows the international development of the moon,” Dr. Burns said. “Precisi on timekeeping was key to navigation on the Earth, and it’s key to navigation between the Earth and the moon.” The ESA said that a universal timekeeping system for the moon is needed, but that many details remain to be worked out. One of the Questions that has yet to be settled, the agency said, was whether lunar time should be set on the moon or synchronized with Earth. Time on Earth is precisely tracked by atomic clocks, but synchronizing time on the moon is tricky because clocks run faster there , gaining around 56 microseconds, or millions of a second, per day. Once a new lunar time zone is established, the methods used to create it will be useful for future space exploration, Dr. Burns said. Astronauts could go to Mars in the next two to three decades, he said, and will face similar logistical hurdles that a Martian time zone could address. “We’re going to be an exploration civilization in which we’re going to be exploring beyond Earth’s orbit,” Dr. Burns said. “We’re going to be goi ng to the moon and then, after that, to Mars.” Kenneth Chang contributed reporting.