Earth-like planet would require about 10 hours of observation in total, spread out over several months. A 2002 design which would have used 1.5 metre mirrors was expected to take about 100 hours to get a spectrum of a possibly Earth-like planet. Were the Darwin spacecraft to detect a suitable planet, a more detailed study of its atmosphere would have been made by taking an infrared spectrum of the planet. By analyzing this spectrum, the chemistry of the atmosphere could be determined, and this could provide evidence for life on the planet. The presence of oxygen and water vapour in the atmosphere could be evidence for life. Oxygen is very reactive so if large amounts of oxygen exist in a planet's atmosphere some process such as photosynthesis must be continuously producing it. The presence of oxygen alone, however, is not conclusive evidence for life. Jupiter's moon Europa, for example, has a tenuous oxygen atmosphere thought to be produced by radiolysis of water molecules. Numerical simulations have shown that under proper conditions it is possible to build up an oxygen atmosphere via photolysis of carbon dioxide. Photolysis of water vapor and carbon dioxide produces hydroxyl ions and atomic oxygen, respectively, and these in turn produce oxygen in small concentrations, with hydrogen escaping into space. When O2 is produced by H2O photolysis at high altitude, hydrogenous compounds like H+, OH− and H2O are produced which attack very efficiently O3 and prevent its accumulation. The only known way to have a significant amount of O3 in the atmosphere is that O2 be produced at low altitude, e.g. by biological photosynthesis, and that little H2O gets to high altitudes where UV is present. For terrestrial planets, the simultaneous presence of O3, H2O and CO2 in the atmosphere appears to be a reliable biosignature, and the Darwin spacecraft would have been capable of detecting these atmospheric components. Candidate planets Planet Gliese 581 d, discovered in 2007, was considered a good candidate for the Darwin project. It orbits within the theoretical habitable zone of its star, and scientists surmise that conditions on the planet might be conducive to supporting life. Similar initiatives The interferometric version of NASA's Terrestrial Planet Finder mission is similar in concept to Darwin and also has very similar scientific aims. According to NASA's 2007 budget documentation, released on February 6, 2006, the project was deferred indefinitely, and in June 2011 the project