The massive mega constellation of satellites will have a significant impact on global space safety. With Starlink as an example, this paper is aimed at assessing the risk of in-orbit collision, analyzing the probability of collision in orbit in its natural operating state, and forecasting the probability of secondary collision between the collision-generated short-term debris cloud and satellites in the same orbit. The mass, size, velocity, and direction of space debris in a particular orbit of Starlink satellite are calculated based on the MASTER-8 model, and the shape characteristics of the Starlink satellite are added to the model to determine the probability of a Starlink satellite colliding with space debris in that orbit. A modified spacecraft impact disintegration model then is used to calculate the collision results and estimate the collision threat level of the short-term debris cloud formed by the Starlink satellite after its destruction to satellites in the same orbit. The results indicate that the collision probability of Starlink satellite in orbit natural operation exceeds the red warning threshold 10^-4 that the satellite disintegration after the first collision will generate 14088 pieces of debris over 1cm, of which 4092 debris are potentially dangerous to other spacecraft, and that the collision probability to a satellite in the same orbit exceeds the red warning threshold of 10^-4 within 30 minutes, implying that collision avoidance needs to be improved.

The process of dielectric barrier discharge (DBD) plasma flow-separation control on screw propeller is numerically simulated based on the unsteady compressible Navier-Stokes equations loosely with DBD-plasma body force aerodynamic actuation mechanism. The effect and the mechanism of plasma flow control to enhance the propeller aerodynamic characteristics are discussed. The results show that the propeller aerodynamic characteristics will decrease as the altitude increase without plasma flow control, but it will first increase and then decrease as the altitude increases with plasma flow control. The effect of DBD-plasma flow control on the distribution of pressure is small at ground condition, and the difference of pressure distribution between general propeller and plasma efficiency-enhance propeller will be increased as the altitude increases. The effect of DBD-plasma flow-separation control will increase as the altitude increases. Mainly, the mechanism of the plasma flow control to enhance the propeller aerodynamic is obtained as follows. The flow separation on the propeller blade surface could be effectually controlled by DBD-plasma aerodynamic actuation, thereby increasing the efficiency of the propeller. The pressure of the windward surface of the blade will decrease by the plasma body force, which will increase the thrust of the propeller.

The extent of the impact of mega constellations on the low-orbiting geospace environment, which has not yet been assessed in more concrete quantitative terms, is an extremely important issue to consider as mega constellations are built. Satellite safety and lifetime can clearly represent the situation of space targets, and thus can reflect the impact of mega constellations on geospace security. Three target satellites with different characteristics were selected and the Accepted Collision Probability Level (ACPL) was calculated to obtain the impact of Starlink on satellite mission lifetime. Upon considering Starlink without early avoidance control, the lifetimes of the three target satellites were shortened by 56.21%, 99.09%, and 99.82%, respectively. After 10 revolutions of early avoidance control, two were shortened to 92.166% and 91.99%, while the lifetime of JILIN-01 was extended by 155.44%. After joining Starlink, the total risk became larger; even if the target satellite avoided control far more frequently than before joining Starlink, it will face a worse geospace environment. Adopting the most aggressive orbit avoidance control cannot avoid the deterioration of the geospace environment from the perspective of satellite lifetime, which is an irreversible and deteriorating process.