Serendipitous detection of orbital debris by the International Liquid Mirror Telescope

Abstract

The International Liquid Mirror Telescope is a 4-m zenith-pointing optical telescope that employs a rotating liquid primary mirror. Located in the Indian Himalayas, it began operations in October 2022. The telescope is equipped with a CCD camera that has a 22 x 22 arcmin field of view and employs time-delay integration readout to compensate for the Earth’s rotation. While its primary purpose is to conduct astronomical survey observations using broad-band filters, the telescope is also sensitive to objects in Earth orbit that pass through its field of view, leaving detectable streaks. We have examined all images obtained during the first year of observations and determined the transit times and position angles of all detected objects. These were compared with publicly available two-line elements, propagated to the time of observation, in order to identify cataloged objects. A total of 301 streaks were found in 1838 images. Of these, 64% were identified with cataloged objects. Most of the identified objects are in low-Earth orbit, in the altitude range of 400–1600 km. The apparent magnitudes of the identified objects range from 3.6 to 15.1 in the V band. It was also possible to infer angular rates, apparent magnitudes and altitudes for 29% of the unidentified objects. The V-band magnitudes range from 6.4 to 19.5 and the estimated altitudes range from 285 to over 300,000 km.