ESA Clean Space: Probing satellites' mysterious death tumbling

On 15 April, the French space agency CNES rotated the Pleiades Earth observation satellite to capture this image of Envisat. 

At a distance of about 100 km, Envisat's main body, solar panel and radar antenna were visible. Image courtesy CNES. 

Down on the ground, death equals stillness - but not in space.

Derelict satellites can tumble in unpredictable ways and ESA's team tasked with developing a space salvage mission want to find out why.

In recent years, satellites beginning uncontrolled reentries have been tracked, such as Russia's Phobos-Grunt and Germany's Rosat.

In a few cases, satellites suffering unexpected failures in orbit have also been followed, including ESA's Envisat and Japan's ADEOS-II. In every case, the satellite has been seen to be tumbling - but the reason why remains a mystery.

Similarly, when control of a satellite is temporarily lost, ESA's Operations Centre team in Darmstadt, Germany, are accustomed to fixing the satellite's attitude as a prelude to recovery - helping to better understand the satellite's status.

ESA's Clean Space initiative - tasked with reducing the space industry's environmental impact on Earth and space - is seeking to transform our understanding of how large, dead objects behave in space, encompassing launcher upper stages as well as satellites.

The aim of a new study is to combine detailed computer analysis with a range of ground-based observations, some which have only rarely been tried.

Optical telescopes and ground radar are today's favoured monitoring methods, but the study will also investigate the potential of optical and radar satellites in nearby orbits for space-to-space observations.

Highly accurate laser ranging will also be attempted. A global network of ground stations would bounce lasers off a satellite's retroreflectors - like 'cat's eyes' built into a motorway.

Laser ranging can pin down a satellite's position to within centimetres, but has seldom been attempted on out-of-control objects.

The hope is that sustained observation of particular objects over time will give new insights into the kind of factors influencing attitude changes, and how this motion is likely to change over time.

Meanwhile, specialised simulations will seek to pin down these drivers and develop reliable forecasts of how derelict satellites behave.

The long list of potential perturbations include changes in the satellite's centre of gravity as parts break off, atmospheric drag, the faint but steady push of sunlight, micrometeoroid and debris impacts, internal magnetic fields, outgassing and fuel leaks, exploding batteries and even the sloshing of leftover fuel.

For Clean Space, this study is of more than academic interest. The team is planning a dedicated satellite salvage mission called e.DeOrbit and improving our knowledge of a target's condition will help to fine-tune the design.

Bidders are welcome on the study contract. For more information, check the invitation package, accessible here.

ESA Clean Space: Reducing debris threat from satellite batteries

So far, about 200 explosions and at least 5 collisions in space have occured. 

Further explosions and collisions are very likely. 

The explosions are mainly caused by onboard energy sources, either due to pressure build-up in propellant tanks, battery explosions, or the ignition of hypergolic fuels. 

Each explosion creates thousands of small debris objects. 

The most prominent collision event and the first known one between two catalogued objects was in 1996 between the French Cerise Satellite and a fragment of an Ariane upper stage explosion. 

Credit: ESA

Across a satellite's working life, batteries keep the craft's heart beating whenever it leaves sunlight. But after its mission ends, those same batteries may threaten catastrophe.

Space debris mitigation rules require the complete deactivation of electrical power sources aboard a satellite on retirement, to guard against explosive accidents that might produce fresh debris dangerous to other satellites.

Now a new study by ESA's Clean Space initiative – tasked with reducing the space industry's environmental impacts on both Earth and space – aims to evaluate battery behaviour after a satellite shuts down, assessing the risk of breakup and ensuring full 'passivation'.

Batteries are among a satellite's bulkier items of equipment. Typically, they feed their host with power during launch.

Once in orbit, it switches to power from its solar arrays, but the battery is an important backup to store power for eclipses and emergencies.

To reach the high reliability and performance a satellite demands, extending across many months or years, batteries are carefully designed and extensively tested in advance of launch.

By contrast, their behaviour after their parent mission has been shut down remains a relative blind spot.

As a satellite drifts freely, could batteries endure the harsh environment of orbit – including wild temperature swings, degradation of thermal control and components as well as radiation exposure – without leakage or bursting?

Some past satellite breakups have been triggered by battery malfunctions, although mostly before the 1990s and involving older non-lithium designs.

This multidisciplinary study aims to ensure a fully inert power system at the end of a mission, perhaps through physical disconnection, while also preventing accidental premature passivation.

ESA’s Clean Space targets orbital debris and greener environment

70% of all catalogued objects are in low-Earth orbit (LEO), which extends to 2000 km above the Earth's surface. 

To observe the Earth, spacecraft must orbit at such a low altitude. 

The spatial density of objects increases at high latitudes.

Note: The debris field shown in the image is an artist's impression based on actual data. 

However, the debris objects are shown at an exaggerated size to make them visible at the scale shown.

Credits: ESA

Next year’s Hollywood film Gravity features George Clooney stranded in orbit by cascading space junk. The threat is genuine, with debris levels rising steadily.

ESA’s new Clean Space initiative is developing methods of preserving near-Earth space – and the terrestrial environment, too.

Responding to public environmental concerns, Clean Space aims to reduce the environmental effect of Europe’s space activities, cutting waste and pollution on Earth and in orbit.

Industry is contributing to ESA’s draft plans for developing Clean Space technologies: new tools to assess environmental effects, more eco-friendly replacements for materials and techniques, and ways to halt the production of more space debris and bring down existing debris levels.