Mars has long beckoned to humankind from its travels high in the night sky. The ancients assumed this rust-red wanderer was the god of war and christened it with the name we still use today. Early explorers armed with newly invented telescopes discovered that this planet exhibited seasonal changes in color, was subjected to dust storms that encircled the globe, and may have even had channels that crisscrossed its surface. Recent explorers, using robotic surrogates to extend their reach, have discovered that Mars is even more complex and fascinating-a planet peppered with craters, cut by canyons deep enough to swallow the Earth's Grand Canyon, and shouldering the largest known volcano in the solar system. They found intriguing evidence that water played an important role on Mars with channels that bear a striking resemblance to stream beds and clouds of crystalline ice that still traverse its red sky. But they also found that Mars was cold and dry, and believed to be devoid of life. Now present day explorers have announced that pieces of Mars have arrived on Earth as meteorites, and that these bits of the red planet contain evidence pointing to the possible existence of life early in Mars history. This has resulted in renewed public interest in this fellow traveler of the solar system, adding impetus for exploration. Over the past several years studies have been conducted on various approaches to exploring Earth's sister planet Mars. Much has been learned, and each study brings us closer to realizing the goal of sending humans to conduct science on the Red Planet and explore its mysteries. The approach described in this publication represents a culmination of these efforts but should not be considered the final solution. It is our intent that this document serve as a reference from which we can continuously compare and contrast other new innovative approaches to achieve our long-term goal. A key element of future improvements to this document will be the incorporation of an integrated robotic/human exploration strategy currently under development. We will continue to develop alternative approaches, technologies, precursor missions, and flight demonstrations that collectively move us forward. Inputs have been, and will always be, encouraged from all sources-NASA centers, industry, research organizations, entrepreneurs, government agencies, international partners, and the public at large-which will improve our understanding and current planning. We plan to use the results of these assessments to shape our investments in technology, and to look for high leverage, innovative, breakthrough approaches to the most cost effective exploration. These data will also help us understand the required infrastructure, as well as provide important insights into how we can use the International Space Station to validate key assumptions and technologies. To achieve our goal, we must fundamentally change the way in which we explore with both humans and robots. We must search for alternatives to substantially reduce the cost of exploration, while increasing the inherent value to humankind. This Reference Mission provides a viable starting point for NASA's continuing efforts to develop the technologies and systems, as well as the international partnerships, needed for the grand adventure of sending humans to explore another planet in our solar system-one that may have once, and may yet again, harbor life.