The space science enterprise encompasses the traditional scientific disciplines of astronomy and astrophysics, space and solar physics, and planetary science. Through investigations in these areas, NASA seeks to answer fundamental scientific questions, some of which are as old as the human race (Box 3.1). These questions have significance well beyond the scientific community; they lie at the heart of humanity's attempt to understand its place in the universe. OSS also investigates fundamental physical and biological laws using space environments as natural laboratories that cannot be duplicated on Earth.

The tools of the space science enterprise are extraordinarily broad and varied. They include deep-space probes; Earth-orbiting astrophysical observatories; aircraft-, balloon-, rocket-, and ground-based observatories; and laboratory and theoretical investigations. These tools, used in concert, have facilitated understanding of complex questions about the universe and the solar system.

The Earth science enterprise consists of one program—Earth science—whose primary objective is to understand the interactions among Earth's land, oceans, and atmosphere that influence weather, climate, Earth's ecosystems, agriculture, and hazards to populations. This cross-cutting approach to Earth studies has come to be known as Earth system science. Critical science questions from NASA's earth science strategic plan are listed in Box 3.2.

A sense of urgency emerged in the Earth sciences as investigators began to identify the strong linkages between phenomena as diverse as the depletion of stratospheric ozone and the terrestrial use of chlorofluorocarbons (CFCs); severe storms and the El Niño-Southern Oscillation; and the growth of infrared-active gases in the atmosphere and potential climate changes that will affect human health, economic decisions, fishery yields, and agricultural productivity. Progress in Earth system science is sensitively tied to the research strategy selected to attack these problems.³

Unlike the space and Earth science enterprises, the human exploration and development of space (HEDS) enterprise encompasses much that lies beyond concerns usually attributable to science. These range from issues as grand as the expansion of human life beyond Earth to issues as practical as the commercialization of access to space. OLMSA manages the HEDS research program, which spans biology, medicine, materials science, fluid and combustion physics, and biotechnology.

Critical questions within the life sciences largely concern the effects of gravity (or the absence of gravity) on plant, animal, and human physiological systems. For example, a primary question is, How do humans adapt to the space environment and readapt on return to Earth?