LOS ANGELES — Years of trying to do too many things with too little money have put NASA at risk of ceding its leadership in space exploration to other nations, according to a new report that calls on the space agency to make some wrenching decisions about its long-term strategy and future scope.

At a time when other countries — including some potential adversaries — are investing heavily in space, federal funding for NASA is essentially flat and is under constant threat of being cut. Without a clear vision, that fiscal uncertainty makes it all the more difficult for the agency to make progress on ambitious goals such as sending astronauts to an asteroid or Mars while executing big-ticket science missions, such as the $8.8 billion James Webb Space Telescope, says the analysis released Wednesday by the National Research Council.

“These problems are not primarily of NASA’s doing, but the agency could craft a better response to the uncertainty,” wrote the report’s authors, a group of 12 independent experts led by Albert Carnesale, former chancellor at the University of California, Los Angeles. “If the United States is to continue to maintain international leadership in space, it must have a steady, bold, scientifically justifiable space program in which other countries want to participate, and, moreover, it must behave as a reliable partner. Despite decades of U.S. leadership and technical accomplishment, many of these elements are missing today.”