Hi Stephanie,
I agree with Terry - sometimes scientists compete with one another, and sometimes we work together. And sometimes, we do both at the same time!
NASA satellites and sub-orbital missions are developed by teams of scientists working together, but each team competes against other teams to win funding. For small satellites, rockets, or balloon missions, groups of scientists working together as a team will write proposals to NASA to build an entire spacecraft or a rocket/balloon payload and all of its instruments. The mission proposals from each team of scientists are submitted to NASA and reviewed by other scientists to determine which team wins the grant or contract. For large satellite missions, teams of scientists write proposals to build an individual instrument or set of instruments for the mission. NASA then reviews all of the instrument proposals and picks the winning proposals based upon which instruments best fit the mission's science objectives and other requirements.
Most of the time, these competitions are friendly. Space scientists are a relatively small community, so when NASA issues a call for proposals to develop new missions, people generally have a good idea of who they are competing against. Sometimes there can be bad feelings between scientists due to competition for funding. However, at other times, scientists can suddenly find themselves collaborating with someone who they have competed with before!
Kris