Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page.

The Gaia all-sky astrometric survey is challenged by several issues affecting the spacecraft stability. Amongst them, we find the focus evolution, straylight and basic angle variationsContrary to pre-launch expectations, the image quality is continuously evolving, during commissioning and the nominal mission. Payload decontaminations and wavefront sensor assisted refocuses have been carried out to recover optimum performance. An ESA-Airbus DS working group analysed the straylight and basic angle issues and worked on a detailed root cause analysis. In parallel, the Gaia scientists have also analysed the data, most notably comparing the BAM signal to global astrometric solutions, with remarkable agreement.In this contribution, a status review of these issues will be provided, with emphasis on the mitigation schemes and the lessons learned for future space missions where extreme stability is a key requirement.

UV/Vis spectroscopy was used to study the photodecoloration of the water-soluble fraction of saffron. Based on the absorbance in the visible region, which peaks at 440 nm, the photodecomposition of the pigment crocin obeys a first-order kinetic law. The photodecoloration process was also examined as a function of various CIE-L*a*b* chromatic parameters (particularly Chroma) that confirmed the first-order nature of the kinetics. The Chroma of crocin can be related to the amount of pigment present in solution as its determination involves the use of absorbance data for the whole crocin band.

The photodegradation technique was used to record the visible spectra for carotenoids and chlorophylls in virgin olive oil. Principal component analysis of the visible spectra for 81 samples of this type of oil with widely variable composition revealed that only two components contribute more than 99.7% of the spectral information. The Varimax factors and the absorbances of the oil pigments are linearly related. The joint use of these spectra and the absorbances at 455 and 670 nm allows the accurate reproduction of the visible spectrum for any virgin olive oil sample. Therefore, one can question whether a commercially available product is actually virgin olive oil if a comparison of its experimental and simulated absorbance values gives R(2) < 0.995 and RMSD > 0.006. The simulated spectra provide CIE L *a*b* chromatic coordinates that describe the color of the samples with little difference from those provided by the spectra for real samples.

A method has been developed for the determination of acid-base equilibrium constants, which is applicable to compounds that absorb in the visible. The tristimulus (X, Y, Z) and the colorimetric coordinate values ( L, a, b) of the chromatic space CIELAB 1976 are calculated by means of the transmittances of the entire spectral range. The curves, in the a,b plane, of chromatic differences vs. pH intersect at a pH value agreeing with the p Ka; this configuration occurs because a chromatic equilibrium of both species takes place at this point, which corresponds to the chemical equilibrium. With this method, the acidity constants are found easily and quickly, and they yield very good results. The validity of the method has been tested with the acid-base indicator bromophenol blue, obtaining p Ka= 3.943 at 25 °C; this result is compared with those obtained through the usual spectrophotometric methods. Also, thermodynamic magnitudes obtained from p Ka values at different temperatures from 20 to 70 °C have been determined, resulting in Δ H0 = -(13.93 ± 0.88) kj mol−1 Δ S0 = -(122.6 ± 2.8) J K−1 mol−1.

The decolorizing effects of different quantities of activated carbon adsorbent on the color of sherry samples (Fine wine) were studied. The color change was measured through the use of chromatic parameters determined by the CIELAB 76 system, using transmittances taken at 1-nm intervals throughout the visible spectrum. The effect of the adsorbent on the color of the wine has been expressed by equations that relate the values of the chromatic parameters with the quality of the added adsorbent. These equations show that the discoloring process follows a Freundlich-type adsorption isotherm. Using these equations, it is possible to predict the amount of carbon adsorbent necessary to attain a desired color.

After a careful check of the response linearity of the spectrophotometer in absorbances and transmittances at wavelengths between 250 and 550 nm with potassium dichromate, the influence of the concentration of solutions on the tristimulus coordinates and the coordinates of the CIE 1931 (Commission Internationale de l'Éclairage) and CIELAB colorimetric systems was analyzed. How to use the value of only some chromatic coordinates to obtain a sample's concentration simultaneously is also shown.

Color characterization of olive oil may be of great importance to the industry. To determine the color of a solution, it is necessary to accurately measure a series of tristimulus coordinates for which several methods exist. This study analyzes the errors in the calculation of tristimulus values of olive oil color based on methods, by using several selected ordinates and an increasing number of weighted ordinates, and how these errors affect the values of the chromatic parameters defined in the various chromatic systems. The above analysis shows that the use of a large number of ordinates will lead to better results in the color definition of oils. For its determination, we have used the CIE 1931, CIELUV 1976 and CIELAB 1976 spaces; the latter yields the best results.

The IR (4,000 - 400 cm-1) and UV-VIS (850 - 200 nm) absorption spectra of 2- and 4- pyrimidinone in solid state have been studied in varying conditions of concentration and resolution. Vibrational assignment has been done assuming both molecules planar and with C(s) symmetry. The UV spectra have been analysed on the basis of spectral comparison with spectra of analogue compounds. The compounds present tautomeric lactama form (o-quinonoid for 4 pyrimidinone) in solid state, but a quasi-aromatic and polar canonical form is predominating. The broadening of the IR stretching bands and the low frequency values of some fundamental bands have led us to postulate the existence of a strong intermolecular association by hydrogen bonding, and an association model of these pyrimidinones in solid state is proposed. The 2-pyrimidinone appears to show the most aromatic, and most polar structure, as well as the strongest hydrogen bonding (in good agreement with m.p. and solubility).

Interaction energy values to a total of 136 solute-solvent hydrogen bonded pairs of purine and three of its oxoderivatives with water, methanol, ethanol and acetonitrile have been calculated using the "supermolecule approach". Quantum mechanical calculations were carried out by the CNDO/2 method. Hydrogen bond distances have been optimized and annular tautomerism was considered. The relative stability of different solute-solvent complexes is discussed. Hydrogen bonding energy values are in good agreement with those ab initio obtained for similar molecules. The highest hydrogen-bonding energy values are obtained for aggregates in which the purine molecules act as proton acceptors. Results obtained allow a correct interpretation of solubility experimental data.