Adaptive optics to enhance tracking of space debris

Abstract

Since the first artificial satellite was launched in 1957 we have been sending more and more objects into orbit every year. With each launch there are items left behind in space after the functional satellite is delivered. These objects are collectively known as space debris, and range from old rocket motors and fuel tanks to fragments of metal from sacrificial assemblies holding rocket stages together. The space above Earth is full of this debris, which is starting to pose a real hazard to operational satellites.1 An object in orbit has a velocity on the order of kilometers per second, therefore even something the size of a small bolt could be fatal to the delicate hardware of an operational satellite. We can track large pieces of space debris easily from the ground either optically or using radar.2 Derelict satellites and old rocket bodies are objects that we know, catalogue, and track regularly. If there is any danger of one of these colliding with a functional satellite, tracking stations will receive that information and pass it to the satellite operator, who can judge whether to alter their system’s orbit to avoid a collision. Radar tracking works very well in this situation, providing orbital accuracy to within a few kilometers. However, there are many more objects of less than 1m in size that are difficult to track with radar. To address this problem we use Lidar, an active optical tracking technique in which we propagate a laser through a ground-based telescope onto the sky.3 The telescope tracks an object along its trajectory and measures the laser pulse’s round trip time to determine the target range. With this data we can determine precisely the orbit of each object, enabling us to assemble a catalogue and regularly survey the objects. This system is currently limited to tracking items down to approximately 10cm in size in the range 400–1500km, and larger objects at around 2500km. The return signal follows an A=r4 relationship, where the object cross section area is A, Figure 1. A ground-based telescope launches a IR laser ahead of the target (a) such that the object (b) intersects the beam. A laser guide star illuminates a layer of sodium in the atmosphere, producing a reference beacon to measure the atmospheric turbulence.