Hi Veronica!
Space is a very harsh environment, and spacecraft can experience lots of extremes in temperature and radiation over their lifetimes. The instruments on spacecraft eventually stop working or become obsolete and spacecraft batteries can wear down over time and not provide enough power to keep instruments running. Spacecraft can also run out of fuel for orbital maneuvers too. NASA plans ahead for these things happening, so NASA is always developing and planning new missions to study the Sun, Earth, and the other planets in our Solar System.
The Magnetospheric Multiscale (MMS) Mission, which will study the interaction between the solar wind and Earth's magnetosphere, will probably be launched sometime within the next year. My co-workers and I at the University of Iowa are involved with developing an instrument for MMS. Solar Probe Plus is a mission under development to study the Sun's corona, but it won't be launched until 2018. Solar Orbiter is a collaboration between NASA and the European Space Agency that will be launched in 2017.
Developing new missions is very important, so NASA is constantly seeking input from the scientific community on what the priorities for upcoming missions should be. Every 10 years, the National Research Council (NRC) puts together a Decadal Survey of Solar and Space Physics to guide NASA on the key scientific targets for the next decade and recommends new missions for development. About every 3 years or so, NASA also asks scientists for input into a new Heliophysics Roadmap for the next 20 years of research in solar and space physics. And every 1-3 years, NASA does what we call a Senior Review of the currently operating missions to determine which missions are still capable are returning great science data, and which ones should be retired to make way for development of new missions.
Kris