# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 1 Introduction  
### Background and Motivation  
Space debris presents several significant risks to space operations and long-term sustainability of space activities [1]. Collision hazards are a primary concern, as debris can damage or destroy operational satellites, spacecraft, and the International Space Station (ISS). The risk of Kessler Syndrome, in which debris collisions generate more debris, threatens the viability of certain orbital regions, particularly the Low Earth Orbit (LEO) [2]. The presence of space debris also increases the operational costs for space agencies and satellite operators, necessitating investment in tracking systems, collision avoidance maneuvers, and protective shielding. Ultimately, the increasing quantity of debris threatens the long-term sustainability of space activities, presenting a complex challenge for the global space community. As part of Space Situational Awareness (SSA) [3], Space Surveillance and Tracking (SST) focuses on technologies such as radar, optical telescopes, and advanced data analysis algorithms to identify and forecast the paths of active satellites and space debris with precision [4]. A global network of surveillance sites is continuously being developed to ensure uninterrupted space monitoring. The United States maintains a comprehensive catalog through its Space Surveillance Network (SSN), utilizing both optical and radar sensors [5, 6]. Based on the data collected by these sensors, Two-Line Element (TLE) sets aregenerated and made freely available to promote international cooperation, enabling orbit propagation using the standard SGP4 algorithm [7].  
Radar has been employed in space surveillance studies since the early years of the Space Age because of its key benefit of operating efficiently under all weather conditions without the need for sunlight, which optical sensors do [8]. In addition to traditional radar systems, complementary techniques such as phased array radars and LIDAR (Light Detection and Ranging) have been employed for tracking smaller space debris due to their enhanced accuracy and rapid beam steering [9; 10]. However, the high cost of phased arrays and the technical challenges associated with high-power lasers in LIDAR systems limit their scalability [11; 12]. Thus, research is increasingly focusing on developing innovative and cost-effective solutions for space surveillance by leveraging existing infrastructure and passive sensing techniques.  
In particular, systems that employ a sensitive radio telescope as a bistatic receiver operating in beam-park mode have gained interest, due to their potential for wide-area coverage with reduced infrastructure requirements [13]. Radio telescopes, originally designed for observing faint cosmic signals [14], have demonstrated their capability as receivers for radar applications. Researchers have been investigating the use of radio telescopes as radar receivers for space surveillance applications, and several experiments have been performed with configurations of interest. A few of these experiments, with a special focus on the LEO region, were reported in [15]; however, an extensive review is still missing. This article comprehensively examines debris monitoring experiments conducted using radio telescopes in the past, provides a detailed analysis, and elucidates the progress made to date in this field. Figure 1 shows the locations of radio telescopes around the globe that have been utilized in past space monitoring campaigns. Several countries, including Australia and South Africa, have contributed to such efforts; however, Europe remains the primary contributor, particularly in advancing innovative radio-telescope-based approaches. Furthermore, the classification of systems into active and passive types, based on their transmitter configuration, is explained in the following section.