Because of its high ionization potential and weak interaction with hydrogen, Neutral Interstellar Helium is almost unaffected at the heliospheric interface with the interstellar medium and freely enters the solar system. This second most abundant species provides some of the best information on the characteristics of the interstellar gas in the Local Interstellar Cloud. The Interstellar Boundary Explorer (IBEX) is the second mission to directly detect NISHe. We present a comparison between recent IBEX NISHe observations and simulations carried out using a well-tested quantitative simulation code. Simulation and observation results compare well for times when measured fluxes are dominated by NISHe (and contributions from other species are small). Differences between simulations and observations indicate a previously undetected secondary population of neutral helium, likely produced by interaction of interstellar helium with plasma in the outer heliosheath. Interstellar neutral parameters are statistically different from previous in situ results obtained mostly from the GAS/Ulysses experiment, but they do agree with the local interstellar flow vector obtained from studies of interstellar absorption: the newlyestablished flow direction is ecliptic longitude 79.2 • , latitude −5.1 • , the velocity is ∼ 22.8 kms −1 , and the temperature is 6200 K. These new results imply a markedly lower absolute velocity of the gas and thus significantly lower dynamic pressure on the boundaries of the heliosphere and different orientation of the Hydrogen Deflection Plane compared to prior results from Ulysses. A different orientation of this plane also suggests a new geometry of the interstellar magnetic field and the lower dynamic pressure calls for a compensation by other components of the pressure balance, most likely a higher density of interstellar plasma and strength of interstellar magnetic field.

Context. A recent analysis of IBEX measurements of the neutral interstellar He flux showed that the inflow velocity vector is different from the results of an earlier analysis of observations from the GAS instrument onboard Ulysses. A recently published compilation of published results on the helium inflow direction from the past ∼40 years suggested that the inflow direction may be changing with time. Aims. We reanalyzed the old Ulysses data and reprocessed them to increase the accuracy of the instrument pointing to investigate whether the GAS/Ulysses observations, carried out during almost two solar cycles, support the hypothesis that the direction of the interstellar helium inflow is changing. Methods. We employed a similar analysis method as had been used in the analysis of the IBEX data. We sought a parameter set that minimizes the reduced χ 2 , using the Warsaw test-particle model for the interstellar He flux at Ulysses with a state-of-the-art model of neutral He ionization in the heliosphere that precisely reproduces the observation conditions. We also propose a supplementary method of constraining the parameters based on cross-correlations of parameters obtained from an analysis of carefully selected subsets of data. Results. We find that the ecliptic longitude and speed of interstellar He agree very well with the values reported in the original GAS analysis. We find, however, that the temperature is markedly higher. The three-seasons best-fit parameter set is λ = 255.3 • , β = 6 • (J2000), v = 26.0 km s −1 , and T = 7500 K. We do not find evidence that these parameters are varying with time, but their uncertainty range is wider than originally reported. Conclusions. The originally derived parameters of interstellar He from direct sampling on GAS/Ulysses agree well with those currently derived, except for the temperature, which seems to be appreciably higher; this agrees well with interstellar absorption line results. While the results of our analysis agree marginally with the previously reported results from IBEX, the most likely values from the two analyses differ for reasons that are still not understood.

MUPUS, the multi purpose sensor package onboard the Rosetta lander PHILAE, will measure the energy balance and the physical parameters in the near-surface layers -up to about 30 cm depth-of the nucleus of Rosetta's target comet Churyumov-Gerasimenko. Moreover it will monitor changes in these parameters over time as the comet approaches the sun. Among the parameters studied are the density, the porosity, cohesion, the thermal diffusivity and conductivity, and temperature. The data should increase our knowledge of how comets work, and how the coma gases form. The data may also be used to constrain the microstructure of the nucleus material. Changes with time of physical properties will reveal timescales and possibly the nature of processes that modify the material close to the surface. Thereby, the data will indicate how pristine cometary matter sampled and analysed by other experiments on PHILAE really is.

Post-launch boresight of the IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) is determined based on IBEX-Lo Star Sensor observations. Accurate information on the boresight of the neutral gas camera is essential for precise determination of interstellar gas flow parameters. Utilizing spin-phase information from the spacecraft attitude control system (ACS), positions of stars observed by the Star Sensor during two years of IBEX measurements were analyzed and compared with positions obtained from a star catalog. No statistically significant differences were observed beyond those expected from the pre-launch uncertainty in the Star Sensor mounting. Based on the star observations and their positions in the spacecraft reference system, pointing of the IBEX satellite spin axis was determined and compared with the pointing obtained from the ACS. Again, no statistically significant deviations were observed. We conclude that no systematic correction for boresight geometry is needed in the analysis of IBEX-Lo observations to determine neutral interstellar gas flow properties. A stack-up of uncertainties in attitude knowledge shows that the instantaneous IBEX-Lo pointing is determined to within ∼0. • 1 in both spin angle and elevation using either the Star Sensor or the ACS. Further, the Star Sensor can be used to independently determine the spacecraft spin axis. Thus, Star Sensor data can be used reliably to correct the spin phase when the Star Tracker (used by the ACS) is disabled by bright objects in its field of view. The Star Sensor can also determine the spin axis during most orbits and thus provides redundancy for the Star Tracker.