“Rosetta at Comet Nucleus 67P/Churyumov-Gerasimenko” with Jessica Agarwal (TU Braunschweig, Germany)

The ESA Cornerstone mission Rosetta was off to a difficult start after the launch had to be postponed and the target comet 46P/Wirtanen replaced in 2003 by 67P/Churyumov-Gerasimenko. Finally launched in April 2004 and after a ten years journey, the Rosetta space craft went into orbit around the nucleus in August 2014, carrying a suite of instruments and the Philae lander. Philae landed on November 12th – not quite at the foreseen sunlit location but on its side and in the shadow of a cliff. It transmitted data for roughly 60 hours until the power of its primary battery had been spent. The lander was located and its flight above the nucleus surface reconstructed using data from its magnetometer and it was finally “found” in images taken by the OSIRIS camera on board the orbiter. The mission ended in 2016 after the Rosetta orbiter spacecraft was crash-landed on the nucleus taking images and other data up to the very end. 

Rosetta had been named after the Rosetta stone because the data would be used to decipher the formation of the solar system just as the Rosetta stone was the clue to decipher the Egyptian hieroglyphs. The Philae lander – in turn – was named after the Philae obelisk that has a bilingual inscription in Greek and Egyptian hieroglyphs that complemented the information from the Rosetta stone.

Rosetta carried a substantial suite of instruments on the orbiter and the lander, many complementary and some with elements on both such as the CONSERT radar system that allowed part of the nucleus interior to be screened. Cameras and spectrometers covered the electromagnetic spectrum from ultraviolet to mm-wavelengths and mass spectrometers explored the composition of the cometary dust and ice. A radio science experiment helped determine the mass and the porosity of the nucleus. Rosetta was also equipped with several dust detectors and magnetometers.

The mission proved to be a masterpiece in space technology and operations – the latter in particular because of the sophisticated maneuvers to accelerate the spacecraft through a number of gravity assists to its target and because of the approach to a largely unknown, outgassing small body with an irregular gravity field, orbit insertion and finally landing. Its results put many new constraints to models of the origin and evolution of comets as well as models of the formation of the solar system. For instance, it was shown that the chemistry of the nucleus was highly complex with manifolds of organic compounds. Amongst the most surprising findings was the existence of molecular oxygen that suggested that the nucleus formed very early and was kept at very low temperatures for much of its existence until its orbit was disturbed and ended in the Jupiter family. Other findings concerned the extremely high porosity of the nucleus of roughly 70% and its variability of strengths at various scales, and the diversity of processes connected to the erosion of the surface. Although a large number of publications appeared in the first years after the mission ended – including an ISSI book also published online in Space Science Reviews – the scientific harvest will likely continue for decades. 

Prof. Jessica Agarwal is since May 2020 Lichtenberg professor at the TU Braunschweig in Germany. She was at the Max-Planck Institute for Solar System Research before where she was a member of the OSIRIS team. Jessica Agarwal specializes in the physics of active bodies, both comets and asteroids and has discovered the first active binary asteroid in 2016 using the Hubble Space telescope. She is a highly cited and renowned expert of active small bodies in the solar system. 

Seminar was recorded on September 24, 2020.