Outer space -- Exploration 12 Astrophysics 8 Space sciences -- United States 8 United States. -- National Aeronautics and Space Administration 6 Mars (Planet) -- Exploration 5 Space sciences -- Research -- United States 5 United States. -- National Aeronautics and Space Administration -- Research 5 more ... Astronautics 4 Astrophysics -- Research -- United States 4 Mars surface samples 4 National Polar-orbiting Operational Environmental Satellite System 4 Outer space -- Exploration -- United States 4 Planets -- Exploration 4 Space sciences 4 Artificial satellites in earth sciences 3 Climatology -- Research -- United States 3 Life -- Origin 3 Life on other planets 3 National Research Council (U.S.). -- Space Studies Board 3 Planetary quarantine 3 Space microbiology 3 United States. -- National Aeronautics and Space Administration -- Appropriations and expenditures 3 United States. -- National Aeronautics and Space Administration -- Planning 3 Aeronautics 2 Aeronautics, Commercial -- United States 2 Artificial satellites in remote sensing 2 Astronautics -- Research -- United States 2 Astronauts -- Health risk assessment 2 Astronomy -- Research -- Forecasting 2 Astronomy -- Research -- United States 2 less ...