Detecting faint space debris with optical alternating exposures

Space debris orbiting around the Earth poses a significant risk to active satellites and operational spacecraft. The growing population makes the near-Earth orbital space dramatically crowded and poses a threat to the sustainability of the space environment. Optical surveys utilizing wide-field telescopes are an important way to observe and detect space debris. Considering there is a relative motion between space debris and background stars, both streak-like images and point-like images appear in the frames. To detect faint space debris, it is better to make the images of space debris appear as points. However, under these circumstances, challenges arise in extracting the object images from the large number of long streak background images. Furthermore, it is also difficult to estimate the centroids and flux intensity of these long streak images, reducing the astrometric accuracy of the space debris. To handle these challenges, a new strategy employing the alternating exposure is developed. In detail, based on the object-tracking mode, the short exposure and long exposure are performed alternately. The short-exposure frames are used to estimate the astrometric parameters while the long-exposure frames are used to extract the object images. The astrometric solutions are interpolated to the observation epochs of the long exposures to calculate the equatorial coordinates of the object. A test was performed utilizing a 70-cm wide-field optical telescope, and 145 fields were surveyed using alternating 1-second exposure and 20-second exposures. The results demonstrate that 1633 tracklets from 547 objects were detected, with an apparent magnitude down to 18, and the position detection accuracy of space objects was better than 0.3 arcseconds. Notably, our proposed method can make great contributions to the survey and discovery of faint celestial moving objects.