It’s really getting crowded up there! The immediate area around Earth is cluttered with space debris, with recent estimates suggesting almost 4,000 man-made satellites in a near-Earth orbit, only one-third of which are currently operational. These non-operational units are subject to leakage, fragmentation and even explosions — further littering the immediate region around our planet. On top of this is a further population of near 20,000 known space debris objects.
If humanity is going to continue to exploit the space immediately surrounding the Earth measures need to be taken to avoid this space debris. Collisions between this space junk and operating satellites aren’t just costly and damaging, they also create more debris. Now researchers at the University of Bern have made a breakthrough that just might help satellites avoid just collisions.
The Bern team is the first in the world to successfully determine the distance from Earth to a piece of space junk in daylight. The researchers performed the feat on June 24th using a geodesic laser fired from Swiss Optical Ground Station and Geodynamics Observatory Zimmerwald. The achievement opens up the possibility of spotting space debris during the day, this means that possible collisions between satellites and space debris can be identified early and mitigation strategies such as evasive manoeuvres can be implemented earlier.
Being Evasive
Spotting space debris during the day should help prevent events such as the collision that occurred between the operational communications satellite Iridium 33 and the obsolete Cosmos 2251 communications satellite in 2009. Occurring at an altitude of 800 km over Siberia the impact at 11.7 km/s created a cloud of over 2000 pieces of debris — each larger than 10 cm in diameter. Within a matter of months, this cloud of debris had spread across a wide area, and it has been a threat to operational satellites ever since.
But one positive did come out of the event, it made both scientists and politicians wake-up to the fact that the problem of space debris can no longer be ignored.
In fact, the risk of collision with space junk in certain orbits around the Earth is so great, that evasive manoeuvres are commonplace. The ESA alone receives thousands of collision warnings for each satellite in its fleet per year! This leads to satellites performing dozens of evasive acts each year. But, it’s vitally important to accurately assess when evasive action is actually needed as they can be costly and time-consuming to perform.
“The problem of so-called space debris — disused artificial objects in space — took on a new dimension,” says Professor Thomas Schildknecht, head of the Zimmerwald Observatory and deputy director of the Astronomical Institute at the University of Bern. “Unfortunately, the orbits of these disused satellites, launcher upper stages or fragments of collisions and explosions are not known with sufficient accuracy.”
Thus, as well as reducing collision risk, daytime observations of space debris could mean that unnecessary evasive action is avoided. There could be another benefit to early debris detection too.
Many researchers are currently investigating the possibility of missions to clear space debris. One such example is the work of Antônio Delson Conceição de Jesus and Gabriel Luiz F. Santos, both from the State University of Feira de Santana, Bahia, Brazil, recently published in the journal EPJ Special Topics. The pair modelled the complex rendezvous manoeuvres that would be required to bring a ‘tug vehicle’ into contact with space junk. Better positioning debris clusters could assist these efforts considerably.
Fun with Lasers
Currently, the position of space debris can only be estimated with a precision of around a few hundred metres, but the team from Bern believe that using the satellite laser ranging method they employed to make their daylight measurement, this margin of error can be slashed down to just a few meters, a massive improvement in accuracy.
“We have been using the technology at the Zimmerwald Observatory for years to measure objects equipped with special laser retroreflector,” Schildknecht says, adding that these measurements were also previously only possible to make at night. “Only a few observatories worldwide have succeeded in determining distances to space debris using special, powerful lasers to date.”
Despite providing more accurate measurements, geodetic laser systems such as the one at the Zimmerwald observatory employed by the researchers are actually at least one order of magnitude less powerful than specialized space debris lasers. Additionally, detecting individual photons diffusely reflected by space debris amid the sea of daylight photons is no mean feat.
These problems were overcome by the use of highly sensitive scientific CMOS camera with real-time image processing to actively track the space junk, and a real-time digital filter to detect the photons reflected by the object.
“The possibility of observing during the day allows for the number of measures to be multiplied. There is a whole network of stations with geodetic lasers, which could in future help build up a highly precise space debris orbit catalogue,” Schildknecht concludes. “More accurate orbits will be essential in future to avoid collisions and improve safety and sustainability in space.”
