The Nebula Awards annually recognize the best works of science fiction or fantasy published in the United States. The awards are organized and awarded by the Science Fiction and Fantasy Writers of America (SFWA), a nonprofit association of professional science fiction and fantasy writers. They were first given in 1966 at a ceremony created for the awards, and are given in four categories for different lengths of literary works. A fifth category for film and television episode scripts was given 1974–78 and 2000–09, and a sixth category for game writing was begun in 2018. The rules governing the Nebula Awards have changed several times during the awards’ history, most recently in 2010. The SFWA Nebula Conference, at which the awards are announced and presented, is held each spring in the United States. Locations vary from year to year.
Dr. Porco believes that the icy moon, with its underground liquid sea of water, organics, as well as an energy source, may potentially host life similar to that found in analogous environments on Earth. The March 2012 images of Cassini's "tiger stripes" revealed that these cracks widen and narrow, as was suspected from pictures taken previously. The fissures also change over time more frequently than was originally thought. The two opposite sides of the fissures move laterally relative to one another. This is analogous to the way two banks of the San Andreas Fault can move forward and back, as well as in opposite directions. The greatest slipping and sliding happens when Enceladus is closest to Saturn--as scientists expected. Simply put, resistance to the creation of a space frontier originates with the insecurities of Western leaders. First, it is clear that everything changes with the emergence of a frontier. Established power structures are usually shaken, not reinforced. (If this is not clear, try reading Walter Prescott Webb's The Great Frontier, particularly the last chapter, and Divided We Stand: The Crisis of a Frontierless Democracy, by the same author.) We live in a Cosmic "shooting gallery". Objects inhabiting our Solar System have been profusely and mercilessly blasted by showering asteroids and comets for billions and billions of years. However, planets and large moons have their way of smoothing away the scars--their strong gravity pulls them into a nice ball-like spherical shape. Furthermore, some of these larger spheres possess sufficient internal heat to cause flows of fiery lava and other volcanic features that can fill in the scars of impact craters. A few such large bodies are blasted by strong winds and pouring rains, which also erode away the pockmarks left on their surfaces by showering impactors.