This post has been written by Erwan Matton, stagiaire within the Clean Space team from 1st of May 2016 to 31st of August 2016.
Erwan is a 24 year-old student at ESTACA, a French engineering school for space and automotive techniques. He should graduate end of 2017. Among other tasks such as updating Clean Space roadmaps, we asked him to give an overview on space debris in a comprehensible way.
This is his article and we hope that you will not only enjoy the reading but learn a few things.
The Clean Space team
In the Clean Space initiative, we talk a lot about space debris: how many they are, where they are, why they are so pernicious, how to avoid the creation of new ones, how to actively remove some of them… 29000 items, both dead and operational, larger than 10 cm are orbiting the Earth , mostly located in low-earth orbit (up to 2000 km) and geostationary (36000km) orbit. But do you really know what these items are?
The experts of this issue have given a definition:
Space debris are all man-made objects, including their fragments and parts, whether their owners can be identified or not, in Earth orbit or re-entering the dense layers of the atmosphere that are non-functional with no reasonable expectation of their being able to assume or resume their intended functions or any other functions for which they are or can be authorized”. (Technical Report on Space Debris in 1999 by UN/COPUOS/STSC)
If you have to remember two things only, make sure they are the following ones: “all man-made objects” and “non-functional”! Hence, this means that a space debris can be anything, from a little screw to an entire derelict satellite. To make things clear, debris can be: non-functional satellites, launcher upper stages, fragments or pieces of satellite.
I mentioned satellites. Then you might wonder: “why does a satellite become a debris?” This is a very good question. The answer is pretty simple. Until now, nobody really cared about the destiny of the satellite after the end of the mission. Well, actually this used to be true, but this has been changing for the past few years. Anyway, most of the satellites are used until they are non-functional. This means until they don’t have any more fuel to follow their nominal trajectory, or that a failure occurred. From this moment, the satellite becomes a space debris, orbiting around the Earth, and no more action can be performed. Not even to avoid another debris! Today there are 3600 satellites in orbit, less than a third are operational. This means that 2500 debris are dead ones.
And what about the other debris, all the fragments? First, as mentioned before, there can be collisions. In LEO (low-earth orbit), the orbital speed is around 7.8km/s. The collision speed is more than 100 times higher than the one of two cars crashing into each other. In 2009, the collision between Iridium, an American operational satellite, and Cosmos, a Russian derelict one, created 2200 new debris. Another thing that can happen is the explosion of the satellite. The leftover fuel or batteries inside the old satellite could explode with the exposition to the intense orbital sunlight. The very same thing can also happen to upper stages, but now they have to passivate (it means to deplete themselves from their energy sources). One figure shows how important this is: 700,000 items of debris larger than 1cm are estimated to be in orbit
Another important fact is that reactivate a debris is not possible. It is like for humans, you don’t raise from the dead. Anyway, it is still possible to reduce the population of debris. Scientists estimate that 5 large objects per year can stabilise the environment once full mitigation is in place (For further information about mitigation measures, read about the CleanSat programme). The ESA’s e.Deorbit mission will be a first step with the removal of one of the biggest ESA’s debris.