The plasma environment of comet 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, is explored over a range of heliocentric distances throughout the mission: 3.25 AU (Rosetta instruments on), 2.7 AU (Lander down), 2.0 AU, and 1.3 AU (perihelion). Because of the large range of gas production rates, we have used both a fluid-based magnetohydrodynamic (MHD) model as well as a semi-kinetic hybrid particle model to study the plasma distribution. We describe the variation in plasma environs over the mission as well as the differences between the two modeling approaches under different conditions. In addition, we present results from a field aligned, two-stream transport electron model of the suprathermal electron flux when the comet is near perihelion.

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Three-dimensional Helios plasma and field data are used to investigate the relative changes in direction of the velocity and magnetic field vectors across tangential discontinuities, (TDs) in the solar wind at solar distances of 0.29-0.50 AU. It is found for tangential discontinuities with both Av and AB/B large that Av and AB are closely aligned with each other, in agreement with the unexpected results of previous studies of tangential discontinuities observed at 1 AU and beyond. It is shown that this effect probably results from the destruction by the Kelvin-Helmholtz instability of TDs for which Av and AB are not aligned. The observed decrease in the number of interplanetary discontinuities with increasing solar distance may be associated with the growth of the Kelvin-Helmholtz instability with decreasing Alfv6n speed.Tsurutani, B. T., and E. J. Smith, Interplanetary discontinuities: Temporal variations and the radial gradient from 1 to 8.5 AU, J. Geophys. Res., 84, 2773, 1979.

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today.This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.

The location of the Venus bow shock is found to be sensitive to both the angle of the interplanetary magnetic field (IMF) to the solar wind flow and the phase of the solar cycle. We attribute the former dependence to the effect of planetary ion pickup by the magnetosheath convection electric field analogous to cometary ion pickup by the solar wind electric field. We attribute the latter dependence to the solarcycle variation in the density of exospheric neutrals in the magnetosheath analogous to the cometary response to distance from the sun. We also find both a north-south and a pole-equator asymmetry in the location of the bow shock, controlled by the direction of the IMF in the plane perpendicular to the solar wind flow, for those data where the perpendicular component dominates. These observations suggest how the comet-solar wind interaction should also respond to the combination of the IMF orientation and the intensity of the ionizing solar radiation. 1984 Season 3.5-ß E ß • 3.0-ß ß • ß .• ß ß ß ß ß ß ß ß . . ....,.;.'3.." ß ß ß % e• ee e ß / .,. 2.0-ee ß ß ß ß --• ß ß ß ß ß • .

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O n 12 November 2014, Philae landed on the surface of comet 67P/Churyumov-Gerasimenko (67P), making an almost 30-year dream a reality. The pioneering flybys of 1P/Halley in 1986 revealed that despite being made primarily of ice, it was covered in highly absorbing carbonrich molecules. What is their composition? When did they form, and through which chemical routes? Might they have constituted prebiotic molecules necessary for life? At a larger scale, what can one learn from comets that has relevance to the evolution of the solar system and planets?To address such questions, the Rosetta mission sought to perform a broad range of in-depth structural, physical, and chemical measurements from remote, in situ, and landed vantages. The candidate payload opened for a competitive selection included an instrumented Surface Science Platform (SSP). The initial two that were selected later merged into what is known as Philae, instrumented by 10 principal investigators selected by the SSP providers. The Philae platform and payloads were developed and operated by a highly integrated consortium of institutes, agencies, and industries.Philae's scientific objectives were to provide ground-truth information and complement remote measurements performed from the Rosetta orbiter (Science 347, 23 January 2015) and to offer a self-standing suite of in situ measurements never before performed on a comet. This issue presents a first set of results acquired aboard Philae in the first 63 hours after it separated from Rosetta, descended, initially touched down on the comet at the site known as Agilkia, and finally came to rest at the site known as Abydos.The release and descent happened as planned, precisely documented by imaging (Mottola et al.), ranging (Kofman et al.), thermal mapping (Spohn et al.), and the evolution of the magnetic properties (Auster et al.). The prospect of landing on such an alien body, at 515 million km from Earth and 3 astronomical units (AU) from the Sun, was far more challenging than imagined. The unexpected bounce at touchdown required a major reshuffling and adaptation of the first sequence of science operations. It also provided the opportunity for additional measurements, whereas the bouncing and traversing constrained the mechanical (Biele et al.) and magnetic properties of the surface.ROLIS imagery at its highest resolution (1 cm per pixel) showed the surface of the comet near Agilkia to be dominated by the presence of granular material free of any dust deposits (Mottola et al.). Regolith mobilization processes appear to be involved with the formation of these features. Once Philae came to rest at Abydos, the revised first science sequence began. CIVA panoramic images characterized the surrounding cometary material down to the millimeter scale and the attitude of Philae at rest (Bibring et al.). The MUPUS package measured and constrained the thermal and mechanical properties of the near-surface material of the comet surface at Abydos (Spohn et al.), indicating that the near-surface layers consist of a har...