However, it is important to know that such an "open data" policy is not the norm for most ESA and NASA missions. Data from the Hubble Space Telescope, the Chandra X-Ray observatory, the MESSENGER mission to Mercury, or for that matter, some NASA Mars orbiters, are all subject to a so-called "proprietary period", as are the data from ESA's Mars Express, XMM-Newton, and Rosetta, for example.
This period, typically 6-12 months, gives exclusive access to the data to the scientists who built the instruments or to scientists who made a winning proposal to make certain observations. In ESA's case, the length of the period is decided by our Member States when a mission is selected, although in some cases, the period is made shorter when a mission has been in operation for some time.
...
With Rosetta, all data from its 21 instruments (11 on the orbiter, 10 on the Philae lander) are subject to a 6 month proprietary period. Thus any release of images and scientific results that we are making now, as we approach rendezvous with 67P/C-G on 6 August, is being done with direct involvement of the instrument science teams, who are agreeing to waive the proprietary period for those items.