NASA is concerned about aliens. No, not undocumented immigrants or the scary space invaders loved by Hollywood. The real science questions are related to the search for microbial life in our solar system: on Mars, Europa, Titan, or Enceladus. Each of these has a possibility of supporting life: Mars has a geological and climate history most like Earth; Europa and Enceladus both have extensive oceans of warm liquid water under an ice crust; and Titan has a complex organic chemistry but is too cold for liquid water.
If life exists on any of these worlds, it is likely to have begun there, providing evidence of a second (or third or fourth) genesis within our solar system. Finding any lifeforms, or even life precursors or fossils, would be tremendously exciting to scientists and the public alike.