A call for proposals for a satellite mission. AO is generally used to solicit proposals for unique, high-cost research investigation opportunities that typically involve flying experimental hardware provided by the proposer on one of NASA's Earth-orbiting or free-flying space flight missions. Proposals selected through AOs can be long-term projects, involve budgets of many millions of dollars for the largest programmes, and are usually awarded through contracts, even for non-profit organizations, although occasionally grants are used.