Earth’s lasers are used to measure the position of space debris high above, providing crucial information on how to avoid collisions in space. So far, this technique suffers from a fatal drawback.
For a while, lasers could only be used to measure the distance to space debris during the few hours of dusk, during which the Earth’s “laser distance” station is in darkness, but the debris remains still bathed in the last of the sun’s rays.
In the same way that the Moon is brightest when it shines in sunlight while it is at night on Earth, cosmic debris is easier to see when it reflects the light of the Sun, as seen from a dark point of view.
Because the debris objects are so close to Earth, however, there is only a small window in which they are illuminated, but there are no observers on Earth.
A recent study has shown that it is indeed possible to use lasers in daylight to determine the distance to debris. This new laser range method will help improve the prediction of the debris orbit, drastically increase the available observation time and preserve valuable spacecraft.
Distribution of space debris in orbit around the Earth.
Using a special combination of telescopes, detectors and light filters with a certain wavelength, the researchers found that it was actually possible to increase the contrast of objects in relation to the daytime sky by revealing objects previously hidden in view.
“We’re used to the idea that you can only see stars at night, and that’s true for observing debris with telescopes, in addition to a much smaller time window for observing low-orbit objects,” explains Tim Florer, head of ESA Space Debris Office.
“Using this new technique, it will be possible to track previously” invisible “objects lurking in the blue sky, which means we can work all day with laser radiation to support collision avoidance.”
Debris dancing in the dark
Our planet is shrouded in a veil of debris – millions of small but dangerous fragments left over from previous space launches and explosions and collisions in orbit.
They are joined by hundreds of whole but non-existent spaceships and rocket bodies that have failed or been abandoned orbiting uncontrollably in space.
Even millimeter-long fragments traveling at about seven kilometers per second can damage a satellite on impact, but a collision with a dead spacecraft or large fragments can destroy functioning missions.
It is therefore important to understand where the fragments of waste are so that we can avoid them – but obtaining this information is not easy.
Laser distancing is a very well-established technology that uses a laser on Earth to send light pulses to a satellite carrying a reflector.
By measuring how long it takes for a signal to return to a telescope on Earth, known as “two-way travel time,” the distance to the satellite can be determined exactly.
Unfortunately, few satellites carry a “retro-reflector” that would allow light to reflect easily and return to Earth. Determining the distance to such sites was demonstrated only a few years ago and the development of related technologies is advancing rapidly.
Detection of daylight debris
During recent tests, 40 different debris objects (and stars approximately 10 times paler than what can be seen with the naked eye) were observed using the new technique, protruding against a blue sky, first observed in the middle of the day – something that would not have been possible before.
“We expect these results to significantly increase the observation time of debris in the near future,” explains Michael Steindorfer of the Austrian Academy of Sciences.
“Ultimately, this means that we will get to know the population better than waste, which will allow us to better protect Europe’s space infrastructure.”
The further development of such technologies is a key objective of ESA’s space safety program, including the creation of a network of space-based laser waste disposal stations.
A new laser station next to ESA’s renowned Optical Ground Station in the Canary Islands is awaiting deployment, which will serve as a “test bed” for laser technology as well as for the development of network concepts.