Optimal ballistic coefficient control for Mars aerocapture
Taking advantage of the aerodynamic force to capture the vehicle from hyperbolic orbit to planetary orbit is a preferable approach because of the lower fuel consumption. This paper aims to research the Mars aerocapture process using the ballistic coefficient as control variable, and the inflatable deceleration device is used to change the ballistic coefficient. Firstly, the corridor is described in detail considering the different ballistic coefficients. Besides, the optimal aerocapture trajectories are analyzed, which show that they have bang-bang control structures. Additionally, the optimal results are showed under the different control structures, and the corresponding performances are presented. Finally, the optimal constant control problem is analyzed through the statistical results with the different initial conditions, and the change rules of the peak heating rate and the ballistic coefficient are given.
Approaches to autonomous aerobraking at Mars
Planetary atmospheric aerobraking will most likely be incorporated in every future Mars orbiting mission. Aerobraking requires an intensive workload during operations. To provide safe and efficient aerobraking, both navigation and spacecraft system teams must be extremely diligent in updating spacecraft sequences and performing periapsis raise or lower maneuvers to maintain the required orbital energy reduction without exceeding the design limits of the spacecraft. Automating the process with onboard measurements could significantly reduce the operational burden and, in addition, could reduce the potential for human error. Two levels of automation are presented and validated using part of the Mars Global Surveyor aerobraking sequence and a simulated Mars Odyssey sequence. The simplest method only provides the capability to update the onboard sequence. This method uses onboard accelerometer measurements to estimate the change in orbital period during an aerobraking pass and thereby estimates the beginning of the next aerobraking sequence. Evaluation of the method utilizing MGS accelerometer data showed that the time of the next periapsis can be estimated to within 25% 3σ of the change in the orbital period due to drag. The second approach provides complete onboard orbit propagation. A low-order gravity model is proposed that is sufficient to provide periapsis altitude predictions to within 100-200 meters over three orbits. Accelerometer measurements are used as part of the trajectory force model while the spacecraft is in the atmosphere.
A preliminary account of a new tyrannosauroid theropod from the Wessex Formation (Early Cretaceous) of southern England
Abstract A new genus and species of coelurosaurian theropod dinosaur from the Wessex Formation (Early Cretaceous, Barremian) of the Isle of Wight, England, is described and named. Distinctive characters of the premaxilla, its dentition, maxilla and nasals allow it to be diagnosed. The teeth in the premaxilla are D-shaped in cross-section and the nasals are fused. The hands are elongate and slender and the hindlimbs are gracile. Lack of element fusion elsewhere in the skeleton suggests that it is a subadult. Numerous character states are shared with tyrannosaurids but the new taxon appears to be excluded from the group that comprises aublysodontine and tyrannosaurine tyrannosaurids. We conclude that the taxon is a basal tyrannosauroid and as such it is one of the earliest and (with the exception of some teeth and an isolated ilium from Portugal) the first from Europe. Implications for tyrannosauroid biogeography and evolution are discussed. The animal was part of an unusual taphonomic assemblage in which some elements were partially articulated while others were scattered or broken.