Thirty years ago, the 12th of March 1986, the European Giotto spacecraft flew by Comet Halley, obtaining the conclusive evidence that comets indeed have a nucleus. The series of images obtained by the Halley Multicolour Camera was abruptly interrupted when a cometary dust grain blew away its external structure (plane mirror and baffle), thus preventing to obtain data on the other side of the comet. However, these images were sufficient to disprove the then prevailing Whipple’s model of dusty ice balls in favor of an icy dust body. Giotto was also a spectacular political success, with a continuous exchange of real time information between the European and the then Soviet Union Space Agencies. The last minute positions of the nucleus provided by the two Vega Spacecraft allowed Giotto to make a final course correction and pass at about 600 km distance from the nucleus.
Taking advantage of the experience gained with Giotto, in 1995 ESA decided a larger more ambitious mission, Rosetta, which had as primary scientific objective a year-long cruise around a cometary nucleus and as secondary objectives the fly byes of two asteroids. As the Rosetta stone and Philae obelisk were instrumental to decipher the ancient Egyptian hieroglyphs, so the Rosetta S/C and landing module Philae complement of instruments would insure the clarification of several questions about asteroids and comets.
Rosetta and Philae carry indeed a large set of instruments, among them two (Alice and Miro) provided by NASA. The mission has been up to now an outstanding success, to the point that the originally foreseen end date of Dec. 31, 2015, has been extended to Sept. 30, 2016, when conjunction with the Sun will prevent communications with the S/C.
After a review of some general aspects of Rosetta mission, my talk will concentrate on the images obtained by OSIRIS, the two-camera imaging system on board the S/C, of asteroids Steins and Lutetia, and comet 67/P. The data base of OSIRIS of comet 67/P is really impressive, with some 50.000 plus images obtained from March 2014 till now, and still growing. The images were taken over a broad range of distances from the nucleus, which was well resolved already at 3000 km, down to ten km or so, and have produced most detailed shape models, with accurate determinations of the gravity and slope field, of different geomorphological surface features and their spectrophotometric properties, of changes on the surface due to activity. The origin of the comet has been put on firmer basis. Thousands of dust grains have been individually resolved, and their kinematics and rotation measured. In addition, the wide angle images have produced unexpected results about the mechanisms of gas production.
Rosetta is heading towards a spectacular end, with the attempt of a slow touch down on the comet at the end of next September. Surely, the analysis of the data will take us busy for quite a few years more.