A team of scientists from ESA has developed a method of accurately tracking space debris using lasers, even during daylight hours. Using a combination of special telescopes, detectors and filters adjusted to a specific wavelength, the contrast between the sky and the laser-irradiated object can now be increased, making the latter visible.
At any one time, there are 2,000 active satellites and 3,000 inactive satellites operating in Earth orbit, and about 1 million pieces of debris more than a centimetre in diameter. Although the chances of a collision are slim, every piece of space junk is flying around the Earth at supersonic speeds, and a paint-speckled fragment can still cause as much damage as a powerful rifle bullet.
For these reasons, space agencies around the world are keen to map space debris in the hope that one day they will clean up the mess and avoid damaging spacecraft. One way is to use satellite laser ranging (SLR) to accurately locate and track debris by measuring the time it takes for a light pulse to hit a satellite and return it back, so that it can predict its trajectory.
When the Beacon-B satellite was launched in 1964, the technology was first demonstrated, and a year later it was positioned within a few meters. Since then, the technology has advanced so much that laser pulses lasting only seconds can track satellites to less than millimeters.
Unfortunately, the SLR needs a special reflector to bounce the laser back to the Earth station, and most of the debris does not have such a thing, so the tracker must wait a specific time to fire a laser at the suspect editing the debris. This is the very few minutes when debris surrounds the Earth in the sun, while the SLR station is in the dark. If an object enters the earth’s shadow, or during the day, it is impossible to track it.
However, new ESA technologies may change that. For daylight tracking, telescopes equipped with special filters and photon detectors can change the contrast in the daytime sky, so that satellites and even dim stars become visible. When the suspected debris is at an elevation of more than 15 degrees, it is received by a 20 cm (7.9-inch) tracking telescope mounted on the SLR telescope. Using special software, the SLR telescope can compensate for various deviations and track the object through its reflected laser features, collecting enough data to accurately predict the orbit.
So far, 40 debris objects and one-in-10 thinness of a star visible to the naked eye have been observed at noon.
“We’re used to seeing stars only at night, and the same is true of looking at debris with telescopes, but the time window for looking at low-orbit objects is much smaller.” Tim Flohrer, head of ESA’s space debris office, said. “With this new technology, it will be possible to track previously ‘invisible’ objects that have been lurking in the blue sky, which means we can use laser ranging to support collision avoidance throughout the day.”
The new technology is published in Nature-Communications.