People have gazed at the stars, given them names, and observed their changes for thousands of years. NASA joined the ancient pursuit of knowledge of our universe comparatively recently; nevertheless, in our first half-century of space science we contributed to several major advances in astronomy, including:

• Detecting the atmosphere of a planet outside our solar system
• Completing the first detailed full-sky map of the oldest light in the universe
  
• Finding that dark energy is accelerating the expansion of the universe
  

Even so, we still have perplexing and important puzzles to solve. To answer these questions, NASA is planning a series of missions linked by powerful new technologies and complementary approaches with shared science goals. In the first few decades of this new century astronomers will greatly advance the study of classical cosmology: the description of the universe on the largest scales and how it works. We will also begin to write the final chapter of the story of galaxies, witnessing the actual birth of these vast collections of stars.

We will investigate the interaction of matter and energy that govern the universe, and come to understand how the universe relentlessly expands: even while gravity pulls pockets of its dark matter and other constituents together, the energy of collapse and resulting nucleosynthesis later flinging them apart once again.

In years to come, we will determine the properties of dark energy -- whose presence is inferred by astronomers but has not been seen directly. Probes will detect the imprints left by quantum effects and gravitational waves at the beginning of the Big Bang; later, we could detect these phenomena directly. We will count how many black holes populate our local universe, and one day actually take a picture of the area near the edge of a black hole.

We will peer one-by-one at hundreds of our nearest neighbor stars and inventory their planets, searching for solar systems resembling our own. More ambitious telescopes could study such worlds in greater detail, gathering enough light to find the signatures of life in the atmospheres of planets. We cannot yet know whether the worlds we seek are common or exceedingly rare, so our journey may eventually involve great flotillas of large telescopes that can extend our search to thousands or tens of thousands of stars.