NASA Eva Training

A billion years ago, our Moon was closer to Earth than it is now. As a result, it appeared to be a much larger object in the sky. During that ancient era, if human beings had been around to witness such a sight, it would have been possible to see the entire Moon--not merely the one near side face that we see now. A billion years ago, it took our Moon only twenty days to orbit our planet, and Earth's own day was considerably shorter--only eighteen hours long. Stupendous, almost unimaginably enormous tides, that were more than a kilometer in height, would ebb and flow every few hours. However, things changed, as the lunar orbit around our primordial planet grew ever wider and wider. Annually, Earth's Moon moves about 1.6 inches farther out into space. Currently, the lunar rate of rotation, as well as the time it takes to circle our planet, are the same. The team of scientists used data gathered by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, composed of a duo of twin spacecraft that circled Earth's Moon throughout 2012, each measuring the push and pull of the other as an indicator of lunar gravity. Astronomers are still debating Titan's origin. However, its intriguing atmosphere does provide a hint. Several instruments aboard the Huygens spacecraft measured the isotopes nitrogen-14 and nitrogen-15 in Titan's atmosphere. The instruments revealed that Titan's nitrogen isotope ratio most closely resembles that seen in comets that exist in the remote Oort Cloud--which is a sphere composed of hundreds of billions of icy comet nuclei that circle our Star at the amazing distance of between 5,000 and 100,000 AU. This shell of icy objects extends half way to the nearest star beyond our own Sun.