Comets are the best sample of primitive solar nebula material presently available to us, dating back 4.57 billion years to the origin of our planetary system. Past missions to comets have all been “fast flybys”: They provided only a snapshot view of the dust and ice nucleus, the nebulous coma surrounding it, and how the solar wind interacts with both of these components. Such space-based investigations of comets began in the 1980s with a flotilla of spacecraft: the European Space Agency's (ESA's) first deep space mission, Giotto, which pursued comet 1P/Halley; Deep Space 1 at 19P/Borrelly; Stardust at 81P/Wild 2; Deep Impact and Stardust NeXT at 9P/Tempel; and EPOXI at 103P/Hartley 2.
Rosetta is now taking a more prolonged look. The spacecraft is an ESA mission, with contributions from member states and from NASA, and it currently orbits the Jupiter family comet 67P/Churyumov-Gerasimenko (67P). Rosetta met the comet nucleus on 6 August 2014, at 3.7 astronomical units (AU) from the Sun, and delivered the Philae lander to the nucleus surface on 12 November 2014, when the comet was 3.0 AU from the Sun.
Rosetta is uniquely positioned to further the understanding of these primitive bodies, having revealed an unusual and fascinating object. After rendezvous, the Rosetta spacecraft moved from 100 km above the comet to a bound orbit only ~10 km away. This early period of the mission has revealed previously unseen details of a comet nucleus, as Rosetta's instruments recorded measurements that were once impossible. This issue of Science contains the first published scientific results from Rosetta at comet 67P.
The surface of the comet shows evidence of many active processes and is highly complex. The solid nucleus is an object for which neither horizontal nor vertical variations are modest (Thomas et al., this issue). The current comet shape model suggests that the mass is 1013 kg (about 100 million times the mass of the international space station), with a bulk density of ~470 kg/m3 (similar to cork, wood, or aerogel). The low mass and density values strongly constrain the composition and internal structure of the nucleus, implying a relatively fluffy nature, with a porosity of 70 to 80% (Sierks et al., this issue). The nucleus surface itself appears rich in organic materials, with little sign of water ice (Capaccioni et al., this issue).
The coma produced by ices sublimating from the nucleus is highly variable, displaying large diurnal and possibly seasonal changes. For example, both atomic H and O have been detected close to the nucleus and vary with time, probably stemming from electron impact dissociation of venting H2O vapor. The total H2O gas production rates varied from 1 × 1025 molecules per second in early June 2014 to 4 × 1025 molecules per second in early August, broadly consistent with predictions. In August, water outflow from the surface varied by a factor of at least 5, owing to the effects of terrain, comet shape, and daily illumination changes and possibly other factors (Gulkis et al., this issue). The science team reports the detection of several molecules, including H2 17O, H2 18O, CO, and CO2, and assessed their time variability and heterogeneous distribution (Hässig et al., this issue). A high D/H ratio in water, 5.3 × 10−4, was measured, which precludes the idea that Jupiter family comets contain solely Earth ocean–like water (Altwegg et al., this issue). As observed at 3.6 AU from the Sun, a cloud of about 105 grains (larger than 5 cm) surrounds the nucleus in bound orbits, likely from the previous perihelion passage. The nucleus currently emits dust grains up to 2 cm in size, giving a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface (Rotundi et al., this issue). This is higher than generally accepted for comets. In a progressive series of observations, Rosetta observed the emergence of an energetic ion environment from a low-activity comet nucleus under the influence of the solar wind (Nilsson et al., this issue).
The data presented here allow us to build a detailed portrait of comet 67P. These initial observations provide a reference description of the global shape, the surface morphology and composition, and the bulk physical properties of the nucleus. Subsequent measurements with the orbiter and with the Philae lander will further describe the comet over time. Rosetta will follow the comet at close range through its closest approach to the Sun, perihelion, in August 2015, and then as the comet moves away from the Sun. The spacecraft will perform many flybys that will allow the onboard instruments to measure the evolution of the nucleus and coma with respect to the comet's initial state, defined by the data presented here.
The Rosetta mission has begun to explore our origins, thanks to the efforts of thousands of people at ESA, NASA, industrial partners, and space agencies and to engineers and scientists from around the world. For more than 25 years, they dreamed of these moments when they designed, developed, and launched the Rosetta spacecraft and then followed its interplanetary journey, watched over its long sleep, and woke it from hibernation. These first papers are dedicated to all of them.