James Blake
Satellites have played an instrumental role in most aspects of modern life since the formative years of the Space Age. One region of particular importance is the so-called ‘Clarke Belt’ of geostationary orbits, a name paying homage to Arthur C. Clarke, who popularised the concept of such an orbit in the 1940s. Situated roughly 36000km above the Equator, geostationary satellites benefit from the unique property of having an orbital period that matches that of the Earth’s rotation. This means that they appear fixed in the sky to an observer on Earth, so ground-based antennas do not need to move in order to track them. Operators in the telecommunications industry have exploited the localised nature of geostationary ground tracks for decades.
In the past few years, a number of anomalies and break ups of satellites and rocket bodies have been observed to take place in high-altitude orbits, producing fragments of debris that potentially pose a threat to active geostationary satellites. Simulations have found that relative velocities in the vicinity of these satellites could reach as high as a few km/s, so even small fragments can have sufficient energy to cause significant damage to a satellite. High-altitude objects smaller than around 1m are not routinely tracked, and comparatively few surveys have attempted to probe the faint (small) end of the debris population with large telescopes on Earth. We present results from DebrisWatch-I, a survey of high-altitude debris undertaken with the Isaac Newton Telescope in La Palma, Canary Islands. With standard assumptions in place, we detect objects around 10cm in size, though many exhibit large variations in brightness across the observation window.
Observations were taken with the 2.5m Isaac Newton Telescope and a 36cm robotic astrograph, both situated on the Canary Island of La Palma. To optimise our search for objects within the geosynchronous regime, we acquired images with the telescopes ‘stopped’, pointing at a fixed hour angle and declination. In this mode of operation, stars streak across the images at the sidereal rate (due to the rotation of the Earth), while candidate objects of interest manifest as point sources or short trails.
At the start of each night, we selected a field that was proximate to, yet outside of, the projection of the Earth’s shadow cone at the geosynchronous altitude, in order to maximise the apparent brightness of the objects residing there. We followed the chosen field throughout the night, scanning strips of varying hour angle and fixed declination, as shown by the pointing map on slide 2 of our poster. For each pointing of the telescope, we took several exposures to reveal the paths (arcs) mapped out by the objects and confirm them to be real.
Using a custom analysis pipeline, we extracted positional and brightness information for candidate geosynchronous objects uncovered by the survey. We cross-matched our detections against the publicly available US Strategic Command catalogue, which sources information from a global array of sensors known as the Space Surveillance Network. Over 75% of the orbital arcs detected failed to match with a known object. A mere 1% of objects fainter than 15th visual magnitude successfully matched, a feature of the roughly 1m size cut-off for catalogued objects in high-altitude orbits.
By placing apertures along trailed detections, we were able to extract high resolution light curves. On the final slide of our poster, we show the light curve for SBS-3, a retired communications satellite that now resides in a so-called ‘graveyard’ orbit. Operators are advised to move their satellites away from the operational zone of the geosynchronous region, usually to graveyard orbits in excess of 200km above the geostationary belt, where the majority of active geosynchronous satellites reside. With no stability control in place, the decommissioned satellite has started to tumble. We find strong signatures of this in its light curve, uncovering a 2.7s periodic signal.
Intriguingly, we see signs of similar behaviour in the light curves of fainter objects, examples of which are provided in slide 4 of our poster. Indeed, nearly half of the orbital tracks that failed to match with known objects exhibit brightness variations in excess of 4 magnitudes. It seems that, left to their own devices, pieces of geosynchronous debris typically develop some form of tumbling motion over time. Observations of these objects will be key to developing our understanding of the various perturbations at play in the geosynchronous regime.
Recent anomalies and break ups involving satellites and rocket bodies at high altitudes have added to the population of faint debris within the vicinity of the geosynchronous region. It is essential that we continue to probe these objects with large telescopes to better understand the risk they pose to active satellites as the debris environment evolves over time.