No abstract

No abstract

No abstract

We present an autonomous visual landmark recognition and pose estimation algorithm designed for use in navigation of spacecraft around small asteroids. Landmarks are selected as generic points on the asteroid surface that produce strong Harris corners in an image under a wide range in viewing and illumination conditions; no particular type of morphological feature is required. The set of landmarks is triangulated to obtain a tightly fitting mesh representing an optimal low resolution model of the natural asteroid shape, which is used onboard to determine the visibility of each landmark and enables the algorithm to work with highly concave bodies. The shape model is also used to estimate the centre of brightness of the asteroid and eliminate large translation errors prior to the main landmark recognition stage. The algorithm works by refining an initial estimate of the spacecraft position and orientation. Tests with real and synthetic images show good performance under realistic noise conditions. Using simulated images, the median landmark recognition error is 2m, and the error on the spacecraft position in the asteroid body frame is reduced from 45m to 21m at a range of 2km from the surface. With real images the translation error at 8km to the surface increases from 107m to 119m, due mainly to the larger range and lack of sensitivity to translations along the camera boresight. The median number of landmarks detected in the simulated and real images is 59 and 44 respectively. This algorithm was partly developed and tested during industrial studies for the European Space Agency’s Marco Polo-R asteroid sample return mission.

Solar sails are currently being studied and developed as alternate propulsion vehicles that can provide high velocities. Their ability to reflect photons coming from the sun on a large lightweight reflective surface enables many unique space science missions. One such mission is the GeoSail mission, for which the aim is the study of Earth's magnetotail. Recent advances in solar sail technologies, satellite bus miniaturization, and attitude control motivate the present, study of an alternate systems design approach for GeoSail. This paper details a practical systems approach toward the design of a 40 x 40 m sail, focusing on the design and use of niche enabling technologies with applications to the proposed GeoSail mission. The study is based on mission kind system design requirements from ESA's technology reference studies, which focus on the development of strategically important technologies in preparation of future scientific missions: in this case, for the 2015-2025 time frame

Several techniques have been developed to obtain optimum trajectories with low-thrust propulsion. However, few low-thrust guidance schemes have been investigated to fly the reference optimum trajectories. The guidance algorithm successfully employed in the DeepSpace1 mission was the first approximation through the presented guidance schemes, valid for various interplanetary low-thrust trajectories, independently of the optimization technique they result from. A method is presented to transform any given thrust profile to a thrust law defined by a finite set of control variables. This law allows the definition of a control vector to be optimized for the guidance purposes. Simulations were carried out to compare the performances of the algorithms to very different missions, such as SMART-1 and BepiColombo. The good performance of the enhanced guidance schemes prove the generic applicability of the algorithm. Parametric analysis allows the assessment of stability and robustness of the schemes and the sensitivity to certain parameters. Table.

This paper presents the design of an autonomous GNC system for the rendezvous phase of low-cost missions to very small, faint Near Earth Object (NEO). The optical navigation problem presents a strong relation between the approach strategy and the best combination of sensors, algorithms and actuators. First of all the problem is globally analyzed and the rendezvous strategy defined, from detection of the asteroid to insertion into a desired bounded orbit, considering the observability of the full spacecraft state relative to the asteroid. Then, for each sub-phase the GNC algorithms, including the Image Processing, are described for different propulsion systems (impulsive and low-thrust), and including or not an altimeter. Monte Carlo simulations for different NEO (Apophis and 2003 SM84) are run to tune the algorithms and to asses the system performances. The simulation tool includes high-fidelity models of the optical sensors (navigation camera and star-tracker). The conclusions show the driving parameters and the feasibility of the proposed GNC system.

No abstract