High wind above Cape Canaveral, Florida forced the U.S. space agency on Thursday to delay the launch of two spacecraft destined for the moon, the first mission dedicated to measuring lunar gravity. The countdown made it to the four-minute mark, but was stopped because of strong winds high in the atmosphere. The National Aeronautics and Space Administration (NASA) will attempt a launch Friday morning, despite another poor weather forecast. The space agency has only two extremely brief launch windows each day. The $496 million Gravity Recovery and Interior Laboratory (Grail) mission is the first in more than 50 years of lunar exploration that is dedicated to measuring the moon\'s uneven gravity-about one-sixth the gravity of Earth. None of the more than 100 previous moon missions-dating back to 1959, including the six manned Apollo landings from 1969 to 1972-focused on investigating what lies beneath the lunar surface. The two spacecraft of the mission, named Grail-A and Grail-B, will go into orbit around the moon, chasing each other. By measuring the varying gap between the two satellites, scientists will be able to create a precise map of the moon\'s gravitational field, which will provide information on the composition of the moon\'s interior as well as its evolution over the past 4 billion years. Unmanned rockets are the only launches at Cape Canaveral until private companies are able to launch their own vehicles with astronauts, expected in three to five years. NASA is relying on Russia to launch U.S. astronauts to the International Space Station in the interim, but Russian Soyuz rockets have been grounded following a launch accident two weeks ago involving an unmanned supply craft.