Using a single spacecraft with four telescopes in Earth orbit, Eddington was to examine different regions of the sky for intervals of about two months each. Observing more than 200,000 stars, it would have measured changes in light of one part of one million, and thus have allowed astronomers to learn more about what stars are like inside.
The mission was then planned to search for Earth-like planets orbiting other stars, pointing continuously at one region of the sky for three years. It would measure light from more than 100,000 stars and detect the tiny decrease in light as a planet passes in front of a star. This so-called transit method is also employed by NASA's Kepler.
Eddington was advocated as the culmination of an international attempt to perform asteroseismology from space. Two small precursor space missions are currently under way. The French COROT mission is currently searching for other planets. Microvariability and Oscillations of STars (MOST) is a Canadian mission using a 15 cm telescope that was launched in 2003.
Eddington was to be a European counterpart to Kepler, expecting to detect thousands of planets of any size and a few tens of terrestrial planets that are potentially habitable. Budget overruns with other ESA missions led to the cancellation of the mission in November 2003, despite strong protests from the scientific community.
A new ESA mission, PLATO, is expected to perform a mission similar to the one Eddington was to pursue.