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Malbet F.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Leger A.,University Paris - Sud | Shao M.,Jet Propulsion Laboratory | Goullioud R.,Jet Propulsion Laboratory | And 52 more authors.
Experimental Astronomy | Year: 2012

A complete census of planetary systems around a volume-limited sample of solar-type stars (FGK dwarfs) in the Solar neighborhood (d ≤ 15 pc) with uniform sensitivity down to Earth-mass planets within their Habitable Zones out to several AUs would be a major milestone in extrasolar planets astrophysics. This fundamental goal can be achieved with a mission concept such as NEAT-the Nearby Earth Astrometric Telescope. NEAT is designed to carry out space-borne extremely-high-precision astrometric measurements at the 0. 05 μas (1 σ) accuracy level, sufficient to detect dynamical effects due to orbiting planets of mass even lower than Earth's around the nearest stars. Such a survey mission would provide the actual planetary masses and the full orbital geometry for all the components of the detected planetary systems down to the Earth-mass limit. The NEAT performance limits can be achieved by carrying out differential astrometry between the targets and a set of suitable reference stars in the field. The NEAT instrument design consists of an off-axis parabola single-mirror telescope (D = 1 m), a detector with a large field of view located 40 m away from the telescope and made of 8 small movable CCDs located around a fixed central CCD, and an interferometric calibration system monitoring dynamical Young's fringes originating from metrology fibers located at the primary mirror. The mission profile is driven by the fact that the two main modules of the payload, the telescope and the focal plane, must be located 40 m away leading to the choice of a formation flying option as the reference mission, and of a deployable boom option as an alternative choice. The proposed mission architecture relies on the use of two satellites, of about 700 kg each, operating at L2 for 5 years, flying in formation and offering a capability of more than 20,000 reconfigurations. The two satellites will be launched in a stacked configuration using a Soyuz ST launch vehicle. The NEAT primary science program will encompass an astrometric survey of our 200 closest F-, G- and K-type stellar neighbors, with an average of 50 visits each distributed over the nominal mission duration. The main survey operation will use approximately 70% of the mission lifetime. The remaining 30% of NEAT observing time might be allocated, for example, to improve the characterization of the architecture of selected planetary systems around nearby targets of specific interest (low-mass stars, young stars, etc.) discovered by Gaia, ground-based high-precision radial-velocity surveys, and other programs. With its exquisite, surgical astrometric precision, NEAT holds the promise to provide the first thorough census for Earth-mass planets around stars in the immediate vicinity of our Sun. © 2011 Springer Science+Business Media B.V.

D'Amico S.,German Aerospace Center | Ardaens J.-S.,German Aerospace Center | Larsson R.,Swedish Space Corporation SSC
AIAA Guidance, Navigation, and Control Conference 2011 | Year: 2011

The Prototype Research Instruments and Space Mission technology Advancement (PRISMA) represents the first European technology demonstration of formation-flying and on-orbit-servicing techniques. Several hardware and software experiments, either at subsystem or system level, have been successfully conducted since the launch of the dual-satellite mission in June 2010. This paper describes the guidance, navigation, and control functionalities and presents key flight results from the so called Spaceborne Autonomous Formation Flying Experiment (SAFE) executed in September 2010 and March 2011 as one of the primary PRISMA mission objectives. SAFE is intended to demonstrate autonomous acquisition, keeping and reconfiguration of passive relative orbits for advanced remote sensing as well as rendezvous applications. As shown in the paper, the onboard GPS navigation system provides relative orbit information in real-time with an accuracy better than 10 cm and 1 mm/s (3D, rms) in position and velocity respectively. The impulsive formation control achieves accuracies of better than 10 m (3D, rms) for separations below 2 km with minimum usage of thrusters, ensuring high predictability for simplified mission operations and minimum collision risk for increased safety. © 2011 by German Aerospace Center (DLR).

D'Amico S.,German Aerospace Center | Larsson R.,Swedish Space Corporation SSC
IFAC Proceedings Volumes (IFAC-PapersOnline) | Year: 2011

This paper presents flight results and lessons learned from the Spaceborne Autonomous Formation Flying Experiment (SAFE) conducted by the German Space Operations Center in the frame of the Swedish PRISMA mission. SAFE represents one of the first demonstrations in low Earth orbit of an advanced guidance, navigation and control system for dual-spacecraft formations. Innovative techniques based on carrier-phase differential GPS, relative eccentricity/inclination vectors and impulsive maneuvering are validated and tuned in orbit to achieve centimeter accurate real-time relative navigation, reliable formation keeping at the meter level and flexible formation reconfiguration capabilities. © 2011 IFAC.

Murphy A.G.,University College Dublin | Mathiesen R.H.,Norwegian University of Science and Technology | Houltz Y.,Swedish Space Corporation SSC | Li J.,Swedish Space Corporation SSC | And 5 more authors.
Journal of Crystal Growth | Year: 2016

A novel isothermal solidification furnace (XRMON-SOL) has been developed for operation under terrestrial and microgravity conditions, using aluminium-based alloys, in conjunction with real-time in situ X-radiography. The furnace is scheduled to fly on board the MASER 13 Sounding Rocket with a view to investigating isothermal equiaxed solidification under microgravity conditions. The present work provides details of the furnace design as well as detailed analysis of pre-flight terrestrial solidification experiments investigating nucleation and growth characteristics of a thin sample of grain refined Al-20 wt%Cu alloy isothermally solidified at a constant cooling rate of -0.05 K/s. Measurements of nucleation density, nucleation undercooling, grain orientation, grain envelope evolution, primary tip growth velocities, and equiaxed grain interactions are provided. Although the effects of gravity were minimised by choice of sample orientation, we show that such effects can only be eliminated in a microgravity environment. © 2016 Elsevier B.V.

Murphy A.G.,University College Dublin | Mathiesen R.H.,Norwegian University of Science and Technology | Houltz Y.,Swedish Space Corporation SSC | Li J.,Swedish Space Corporation SSC | And 4 more authors.
Journal of Crystal Growth | Year: 2016

For the first time, isothermal equiaxed solidification of a metallic alloy has been observed in situ in space, providing unique benchmark experimental data. The experiment was completed on board the MASER 13 sounding rocket, launched in December 2015, using a newly developed isothermal solidification furnace. A grain-refined Al–20 wt%Cu sample was fully melted and solidified during 360 s of microgravity and the solidification sequence was recorded using time-resolved X-radiography. Equiaxed nucleation, dendritic growth, solutal impingement, and eutectic transformation were thus observed in a gravity-free environment. Equiaxed nucleation was promoted through application of a controlled cooling rate of −0.05 K/s producing a 1D grain density of ~6.5 mm−1, uniformly distributed throughout the field of view (FOV). Primary growth slowed to a visually imperceptible level at an estimated undercooling of 7 K, after which the cooling rate was increased to −1.0 K/s for the remainder of solidification and eutectic transformation, ensuring the sample was fully solidified inside the microgravity time window. The eutectic transformation commenced at the centre of the FOV proceeding radially outwards covering the entire FOV in ~3 s Microgravity-based solidification is compared to an identical pre-flight ground-based experiment using the same sample and experiment timeline. The ground experiment was designed to minimise gravity effects, by choice of a horizontal orientation for the sample, so that any differences would be subtle. The first equiaxed nucleation occurred at an apparent undercooling of 0.6 K less than the equivalent event during microgravity. During primary equiaxed solidification, as expected, no buoyant grain motion was observed during microgravity, compared to modest grain rotation and reorientation observed during terrestrial-based solidification. However, when the cooling rate was increased from −0.05 K/s to −1.0 K/s during the latter stages of solidification, in both 1g and micro-g environments, some grain movement was apparent due to liquid feeding and mechanical impingement of neighbouring grains. © 2016 The Authors

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