A team led by James Blake, a PhD Student in the Warwick Astronomy and Astrophysics Group, has recently published findings from a survey of the geosynchronous (GSO) region carried out with Isaac Newton Telescope (INT) in La Palma, Canary Islands. The GSO region is a collection of near-circular orbits located roughly 36,000 kilometres above the Equator, where satellites match the period of the Earth’s rotation. This unique property allows for GSO satellites to remain near-fixed in an observer’s sky, making them incredibly useful for telecommunications, navigation and weather monitoring.
The United States Strategic Command (USSTRATCOM) maintains what is widely regarded as the most complete catalogue of artificial space objects. The catalogue is fueled by the Space Surveillance Network (SSN), an array of sensors across the globe comprising over thirty ground-based radars and optical telescopes and six satellites in-orbit. Despite its impressive coverage, the SSN is only able to monitor objects larger than around 1 metre in the GSO region, owing to sensitivity limitations. Although some residents of the GSO region are often referred to as ‘stationary’, simulations have shown that collisions can still occur with relative velocities of kilometres per second! With this is mind, even small (sub-1 metre) debris could cause a lot of damage to active satellites.
Motivated by a number of recent anomalies exhibited by GSO satellites, the team aimed to carry out a deep survey of the GSO region. To help uncover objects as faint as possible, the wide-field camera of the INT acquired images with the telescope ‘stopped’, such that stars would trail through the images, while candidate objects of interest would integrate over fewer pixels. Sky fields were chosen in close proximity to the Earth’s shadow (without eclipsing), in order to maximize the apparent brightness of the objects. This observational strategy, in conjunction with the large collecting area of the 2.54 metre INT, helped the team uncover GSO objects down to roughly 10 centimetres in size.
It was found that over 75% of the debris tracks detected by the survey could not be matched to a known object within the USSTRATCOM catalogue. The majority of these uncatalogued tracks were fainter than 15th visual magnitude, corresponding to objects smaller than the 1 metre cut-off associated with the catalogue. The distribution of detected tracks continues to rise as the sensitivity limit of the INT is reached, suggesting that the modal brightness could be fainter still.
Caption: Examples of light curves extracted from the survey images for faint debris tracks.
For trailed detections, like those shown in the figure, high-resolution light curves could be extracted. Many of the faint, uncatalogued objects appear to be tumbling, showing significant brightness variation across the observation window. Light curves encode a great deal of information about the object itself (shape, reflectivity, orientation) and its in-orbit attitude (stable or tumbling). Disentangling this information from other factors (atmospheric interference, sensor characteristics) is a challenging problem that forms the basis of a very active area of research. It is important that we continue to probe this region of space with large telescopes to obtain more data and gain a better understanding of the behaviours exhibited by these potentially threatening pieces of debris.
The presented survey forms the first instalment of DebrisWatch, an ongoing collaboration between the University of Warwick and the Defence Science and Technology Laboratory (UK) aiming to provide a fresh take on the surveys of the geosynchronous region that have been conducted in the past. The results are reported in the journal Advances in Space Research and available via ResearchGate for open access.