Humanity has left a significant mark on the space beyond Earth. In more than half a century of space activities, more than 4800 launches have placed some 6000 satellites into orbit, of which less than a thousand are still operational today.
More than 12 000 orbiting items in total are regularly tracked by the US Space Surveillance Network and maintained in their catalogue, which covers objects larger than approximately 5 to 10cm in low Earth orbit (LEO) and 30cm to 1m at geostationary altitudes (GEO).
Only 6% of their catalogued orbital population represent operational satellites, while 38% can be attributed to decommissioned satellites, spent upper stages and mission-related objects (launch adaptors, lens covers, etc.). The remaining 56% originates from more than 200 in-orbit fragmentations which have been recorded since 1961.
Except for a few collisions (less than 10 accidental and intentional events), the majority of the 200 break-ups were explosions of spacecraft and upper stages – typically due to leftover fuel, material fatigue or pressure increase in batteries.
Space debris is recognised as a major risk to space missions – , an object of just 1cm size can expend the energy of an exploding hand-grenade when impacting a satellite. ESA is playing a major role in the establishment of requirements to mitigate the production of fresh space debris.
While ESA has a limited contribution to the current space debris environment it and European industry is playing a major role in the establishment of requirements to mitigate the production of fresh space debris and their implementation and verification in missions.
Reducing mass from high density debris regions
The most effective means of stabilising the space debris environment is simply the reduction of mass within regions with high densities of space debris.
Corresponding requirements therefore mandate the avoidance of injection of mission related objects into densely populated regions such as low-Earth orbit (LEO) and geostationary orbit (GEO). They also request the removal of space systems that interfere with the LEO region not later than 25 years after the end of the mission. In practise this is implemented by either launching into an orbit altitude on which the natural orbital lifetime is short, to reduce the orbital height to such altitudes after the mission, or to re-orbit in a way that no part of the orbit interferes with the LEO region anymore.
In GEO satellites are to be disposed of in adjacent ‘graveyard orbits’ to keep the geostationary ring in use.
Mitigation requirements also ban space systems from undergoing uncontrolled re-entry if the associated ground casualty expectancy exceeds 0.0001 per event. For such cases a controlled re-entry over unpopulated areas is mandated instead.
Clean Space will investigate technologies that enable, simplify and make the compliance of missions with mitigation requirements more efficient and will oversee efforts to comply with these mitigation, seeking to plug current technological gaps in this area.
Small micro-satellites in LEO do not always have full orbital control capability while larger satellites require extra propellant and a high-thrust engine to ensure a controlled re-entry, increasing their mass and cost.
End-of-life re-entry technologies
Hence, the development of compact, robust and autonomous systems for de-orbiting of space craft in EIO and the re-orbiting of space craft in GEO is necessary.
Passive de-orbiting systems such as drag augmentation devices and tethers can be used for de-orbiting and re-entry (uncontrolled) of small satellites in LEO. direct and controlled re-entry are required, A number of candidate technologies can be applied to accelerate orbit decay, de-orbiting of space craft in LEO and re-orbiting of space craft in GEO, such as what are called ‘terminator sails’, with deployable or inflatable booms or electro-dynamic tethers using producing force from Earth’s magnetic field to increase the drag and speed up the spacecraft decay to speed up the dragging of satellites back to Earth by the atmosphere.
Even estimating how close a satellite has come to its end-of-life remains difficult: reliably measuring the amount of propellant in fuel tanks is challenging, because standard fuel gauges do not work in weightless environments. An accurate propellant gauging system is essential for end-of-life operations.
In order to avert the risk of spacecraft break-up which could result in debris scattering, the passivation of propulsive systems and power systems could be employed. This involves the venting of remaining propellant and pressurant from the tanks at the end of missions and the discharge of batteries.
'Design for Demise'
Mission controllers need to know that space systems fulfil the on-ground safety requirement by design when undergoing an uncontrolled re-entry, a concept know as 'Design for Demise'. This engineering process was established for the intentional design, assembly, integration and testing of spacecraft so that a spacecraft will break-up once it enters the Earth’s atmosphere to such an extent so that it will not cause a threat to people or property on Earth.
Space debris environmental modelling
Research will also be conducted into modelling the behaviour of the dynamic ever-changing space debris environment, as well as the development of technical means to collect measurements on man-made objects between 1mm and a few centimetres in diameter – which remain invisible to current detection methods – within the most critical orbital altitudes between 800 to 1000 km.