This paper explores the uniqueness of ESA Rosetta mission operations from the Alice instrument point of view, documents lessons learned, and suggests operations ideas for future missions. The Alice instrument mounted on the Rosetta orbiter is an imaging spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy with the scientific objectives of measuring properties of the escaping gas and dust, and studying the surface properties, including searching for exposed ices. We describe the operations processes during the comet encounter period, the many interfaces to contend with, the constraints that impacted Alice, and how the Alice science goals of measuring the cometary gas characteristics and their evolution were achieved. We provide details that are relevant to the use and interpretation of Alice data and published results. All these flight experiences and lessons learned will be useful for future cometary missions that include ultraviolet spectrographs in particular, and multi-instrument international payloads in general.

The Alice ultraviolet spectrograph on the European Space Agency Rosetta spacecraft observed comet 67P/Churyumov–Gerasimenko in its orbit around the Sun for just over two years. Alice observations taken in 2015 October, two months after perihelion, show large increases in the comet’s Ly β , O i 1304, O i 1356, and C i 1657 Å atomic emission that initially appeared to indicate gaseous outbursts. However, the Rosetta Plasma Consortium instruments showed a coronal mass ejection (CME) impact at the comet coincident with the emission increases, suggesting that the CME impact may have been the cause of the increased emission. The presence of the semi-forbidden O i 1356 Å emission multiplet is indicative of a substantial increase in dissociative electron impact emission from the coma, suggesting a change in the electron population during the CME impact. The increase in dissociative electron impact could be a result of the interaction between the CME and the coma of 67P or an outburst coincident with the arrival of the CME. The observed dissociative electron impact emission during this period is used to characterize the O 2 content of the coma at two peaks during the CME arrival. The mechanism that could cause the relationship between the CME and UV emission brightness is not well constrained, but we present several hypotheses to explain the correlation.

The Alice instrument on NASA's New Horizons spacecraft observed an ultraviolet solar occultation by Pluto's atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N-2, CH4, C2H2, C2H4, C2H6, and haze. We derived line-of-sight abundances and local number densities for the 5 molecular species, and line-of-sight optical depth and extinction coefficients for the haze. We found the following major conclusions: (1) We confirmed temperatures in Pluto's upper atmosphere that were colder than expected before the New Horizons flyby, with upper atmospheric temperatures near 65-68 K. The inferred enhanced Jeans escape rates were (3-7) x 10(22) N-2 s(-1) and (4-8) x 10(25) CH4 s(-1) at the exobase (at a radius of similar to 2900 km, or an altitude of similar to 1710 km). (2) We measured CH4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH4 and Alice and REX N-2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm(2) s(-1). Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH4 surface mixing ratio was similar to 0.28-0.35%. (3) The abundance profiles of the "C2Hx hydrocarbons" (C2H2, C2H4 C2H6) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C2H4, near 320 km for C2H2, and an inflection point or the suggestion of a local maximum at 260 km for C2H6. We also detected local minima near 200 km altitude for C2H4, near 170 km for C2H2, and an inflection point or minimum near 170-200 km for C2H6. These compared favorably with models for hydrocarbon production near 300-400 km and haze condensation near 200 km, especially for C2H2 and C2H4 (Wong et al., 2017). (4) We found haze that had an extinction coefficient approximately proportional to N-2 density. (C) 2017 Elsevier Inc. All rights reserved.

The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed emissions from atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko during the entire escort phase of the mission from 2014 August to 2016 September. The initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of H2O. Following delivery of the lander, Philae, on 2014 November 12, the trajectory of Rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of H2O, CO2, and O-2. CO emissions were found to be due to both electron and photoexcitation processes. Thus, we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. Our results show asymmetric outgassing of H2O and CO2 about perihelion, H2O dominant before and CO2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on Rosetta.

Active hazard detection and avoidance will be required for landing on Europa due to a lack of a priori knowledge of surface features similar in size to the lander. A light detection and ranging (lidar) instrument can provide both long distance (8 km) ranging and close-range (500 m) imaging to enable real-time hazard detection during landing operations. An example space-qualified lidar instrument is the Vision Navigation Sensor (VNS) on the Sensor Test for Orion Relative-navigation Risk Mitigation (STORRM) mission in 2011 on STS-134. The VNS consists of a single box housing a laser, transmit and receive optics, focal plane assembly, electronic assemblies, and mechanical components. The instrument operates in dual mode to change the field of illumination for near or far targets. On STS-134 the VNS successfully acquired range and intensity images as the Shuttle docked with ISS. The VNS was subsequently installed on ISS in February, 2017, as part of NASA's Raven technology demonstration and is operating with more integrated processing algorithms. Raven's flash lidar observes vehicles as they approach and depart ISS, performing calculations onboard to test autonomous rendezvous capability. We present potential modifications to the VNS system that could address the unique challenges posed by the Europa environment and landing operations, while reducing payload size, weight and power. We present these different systems within the context of the landing Concept of Operations as laid out in the Europa Lander Science Definition Team Report.

We present the design for a far-UV integral field spectrograph for astronomy enabled by two-axis digital micromirror arrays. Techniques used for integral field spectroscopy in the Optical-IR bandpass either do not apply to the far-UV due to low material transmission, or have other UV-specific challenges. In order to circumvent this limitation, we have designed an all-reflective method of dynamically reformatting the focal plane of a telescope with micro-opto-electro-mechanical systems (MOEMS). The Adaptive Micromirror Array Demonstration Experiment for Ultraviolet Spectroscopy (AMADEUS) is a benchtop far-UV/Optical spectrograph designed to demonstrate that the stability, repeatability, and scattered light contamination are all sufficiently controlable to use these devices in a high sensitivity astronomical instrument. The use of MOEMS devices enables the focal plane mapping to be reconfigured at will, providing some field sampling and path length control advantages relative to conventional Optical/IR techniques for integral field spectroscopy. We report on the design of AMADEUS and present a spectrograph concept for a future sub-orbital mission.

We have detected H2O and O-2 absorption against the far-UV continuum of stars located on lines of sight near the nucleus of Comet 67P/Churyumov-Gerasimenko using the Alice imaging spectrograph on Rosetta. These stellar appulses occurred at impact parameters of rho = 4-20 km, and heliocentric distances ranging from R-h = -1.8 to 2.3 au (negative values indicate pre-perihelion observations). The measured H2O column densities agree well with nearly contemporaneous values measured by VIRTIS-H. The clear detection of O-2 independently confirms the initial detection by the ROSINA mass spectrometer; however, the relative abundance of O-2/H2O derived from the stellar spectra (11-68 per cent, with a median value of 25 per cent) is considerably larger than published values found by ROSINA. The cause of this difference is unclear, but potentially related to ROSINA measuring number density at the spacecraft position while Alice measures column density along a line of sight that passes near the nucleus.

Several UV emission lines of the coma of 67P/Churyumov-Gerasimenko have been observed by Alice/Rosetta before the 67P/CG perihelion. The H and O emissions are mainly produced by impact dissociation of water molecules by suprathermal electrons. In this paper, we explore further the electron dissociative excitation of H2O to produce the UV emissions by using simultaneous observations of water and H Lyman beta done by the VIRTIS-H and Alice instruments during four periods of time in 2014 December, 2015 March and 2015 May. We used simple theoretical considerations to link the UV brightness to the water vapour column density. Two cases are studied. In the first case, we assume the suprathermal electron density is decreasing radially as the thermal electron population; in the second case, we assume the suprathermal electron density does not vary radially. The second case seems more consistent with the Rosetta Plasma Consortium Ion and Electron Sensor measurements during 2015 March and May. The efficiency of the electron dissociative excitation of H2O is lower during the three last periods of time studied compared to the first period in 2014 December. The variability of the efficiency of the electron dissociative excitation between the four studied periods is not simply inversely proportional to the square of the comet-Sun distance but is most likely associated with the variability of the suprathermal electron distribution.

ABSTRACT Measurements of extinction curves toward young stars are essential for calculating the intrinsic stellar spectrophotometric radiation. This flux determines the chemical properties and evolution of the circumstellar region, including the environment in which planets form. We develop a new technique using H 2 emission lines pumped by stellar Ly α photons to characterize the extinction curve by comparing the measured far-ultraviolet H 2 line fluxes with model H 2 line fluxes. The difference between model and observed fluxes can be attributed to the dust attenuation along the line of sight through both the interstellar and circumstellar material. The extinction curves are fit by a Cardelli et al. (1989) model and the A V (H 2 ) for the 10 targets studied with good extinction fits range from 0.5 to 1.5 mag, with R V values ranging from 2.0 to 4.7. A V and R V are found to be highly degenerate, suggesting that one or the other needs to be calculated independently. Column densities and temperatures for the fluorescent H 2 populations are also determined, with averages of log 10 ( N (H 2 )) = 19.0 and T = 1500 K. This paper explores the strengths and limitations of the newly developed extinction curve technique in order to assess the reliability of the results and improve the method in the future.