Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. Aims. The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods. Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results. (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4–7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.

We present results derived from the first multi-chord stellar occultation by the trans-Neptunian object (229762) 2007 UK 126 , observed on 2014 November 15. The event was observed by the Research and Education Collaborative Occultation Network (RECON) project and International Occultation Timing Association (IOTA) collaborators throughout the United States. Use of two different data analysis methods obtain a satisfactory fit to seven chords, yelding an elliptical fit to the chords with an equatorial radius of R = 338 +15−10 km and equivalent radius of R eq = 319 +14 −7 km. A circular fit also gives a radius of R = 324 +30 −23 km. Assuming that the object is a Maclaurin spheroid with indeterminate aspect angle, and using two published absolute magnitudes for the body, we derive possible ranges for geometric albedo between p V = 0.159 +0.007 −0.013 and p R = 0.189 +0.009 −0.015 , and for the body oblateness between = 0.105 +0.050 −0.040 and = 0.118 +0.055 −0.048 . For a nominal rotational period of 11.05 h, an upper limit for density of ρ = 1740 kg m −3 is estimated for the body. Subject headings: Kuiper belt objects: individual (229762, 2007 UK126) -occultations -Planets and Satellites: fundamental parameters R eq = 319 +12 −6 km and albedo that may vary from p V = 0.159 +0.006 −0.011 to p R = 0.189 +0.008 −0.013 . The two solutions give comparable minimum χ 2 per degree of freedom (0.59 and 0.56, respectively). The equivalent radii we derive here are consistent with, but more accurate than the value based on Herschel observations, R eq = 299.5 ±38.5 km (Santos-Sanz et al. -20 -2012).A range for density ρ was estimated to be [318 , 1740] kg m −3 using a lowest apparent oblateness from the GBR solution and considering the rotation period of 11.05 hours. Those values are comparable to other TNOs densities found in literature (as presented in Brown, M. E. (2012), Santos-Sanz et al. (2016) and references). No other information could be derived since there is insufficient orbital information available. Obtaining an orbital solution for the satellite of 2007 UK 126 would be an importnat step foward, as it would firmly constrain the mass of the primary, and thus, its density from our size measurement.The occultation chords also shows that there is no evidence of a very close binary and no satellite could be detected from the shadow track direction.The authors want to thank

Typically we can deliver astrometric positions of natural satellites with errors in the 50-150 mas range. Apparent distances from mutual phenomena, have much smaller errors, less than 10 mas. However, this method can only be applied during the equinox of the planets. We developed a method that can provide accurate astrometric data for natural satellites -the mutual approximations. The method can be applied when any two satellites pass close by each other in the apparent sky plane. The fundamental parameter is the central instant t 0 of the passage when the distances reach a minimum.We applied the method for the Galilean moons. All observations were made with a 0.6 m telescope with a narrow-band filter centred at 889 nm with width of 15 nm which attenuated Jupiter's scattered light. We obtained central instants for 14 mutual approximations observed in 2014-2015. We determined t 0 with an average precision of 3.42 mas (10.43 km). For comparison, we also applied the method for 5 occultations in the 2009 mutual phenomena campaign and for 22 occultations in the 2014-2015 campaign. The comparisons of t 0 determined by our method with the results from mutual phenomena show an agreement by less than 1-sigma error in t 0 , typically less than 10 mas. This new method is particularly suitable for observations by small telescopes.