Lasers to shift the trajectory of satellites in the sky

Why do we talk about space debris so much? 

Christophe Bon­nal. Because it is a very impor­tant issue today. The num­ber of space debris – mean­ing non-func­tion­al orbital objects, such as derelict rock­et stages or satel­lites –, has increased expo­nen­tial­ly since the launch of Sput­nik 1 in 1957. 

Today, more than 24,000 objects are found in space. They mea­sure more than 10 cm in low orbit, and more than 1 m in geo­sta­tion­ary orbit. To these are added more than 900,000 objects mea­sur­ing more than 1 cm and some 130 mil­lion objects mea­sur­ing more than 1 mm. All these objects are liable to neu­tralise a satel­lite, depend­ing on the posi­tion of their impact. As a mat­ter of fact, we have reached a crit­i­cal den­si­ty of objects at the alti­tude rang­ing between 700 and 1,100 km: the num­ber of new objects is high­er than the atmosphere’s nat­ur­al abil­i­ty to burn them off.

How­ev­er, even if we were to stop launch­ing objects in space today, the num­ber of debris would con­tin­ue to rise, because each col­li­sion gen­er­ates new space junk: it is called the Kessler syn­drome. The most famous exam­ple is the col­li­sion in 2009 of two satel­lites weigh­ing more than 750 kg: the Russ­ian satel­lite Kos­mos-2251 and the active Amer­i­can telecom­mu­ni­ca­tions satel­lite Irid­i­um-33. This col­li­sion gen­er­at­ed 4,000 new debris.

Don’t these debris fall into the atmos­phere at some point? 

Indeed, every­thing even­tu­al­ly falls into the resid­ual atmos­phere, but it will take 200 years for an object to descend 800 km…and 1,000 years for it to descend 1,000 km. Mean­while, they are very like­ly to col­lide with oth­er objects, espe­cial­ly in low orbit. The Toulouse space cen­tre thus received no less than 3 mil­lion con­junc­tion warn­ings in only one year! We can avoid col­li­sions between objects that can be manoeu­vred, but not between inert objects.

That being said, the sit­u­a­tion has been evolv­ing since the end of the ‘90s. The big space agen­cies have adopt­ed reg­u­la­tions to reduce the num­ber of debris, espe­cial­ly by striv­ing not to cre­ate new ones, or by avoid­ing col­li­sions as much as pos­si­ble. The prob­lem is that these rules, which have become Euro­pean and inter­na­tion­al stan­dards, are still sel­dom respect­ed: there are still a dozen of explo­sions of space objects per year. Only France has trans­formed inter­na­tion­al stan­dards into law, with its Law on Space Oper­a­tions (Loi sur les Oper­a­tions Spa­tiales – LOS). 

What solu­tions could deal with these col­li­sion risks and reduce their consequences? 

There are two types of solu­tions. Up to now, the main approach was to clean space of its biggest debris. It is called Active Debris Removal (ADR). It applies to debris as big as a stage of the Zen­it Russ­ian launch­er. It mea­sured 9 m and weighed 9 tons and 45 of these units are float­ing around in space. The Euro­pean Space Agency thus assigned the Clear­Space Swiss start-up with the pro­duc­tion of a clean-up demon­stra­tor to retrieve a stage of the Vega launch­er locat­ed in low orbit since 2013. These strate­gic solu­tions are effi­cient in the long run. If we could man­age to remove a dozen of these big debris per year, the growth would no longer be expo­nen­tial, and in 20 years, we could sta­bilise the gen­er­al situation.

How­ev­er today, we have anoth­er prob­lem: how can we avoid col­li­sion between non-func­tion­al space objects? Nowa­days, radars can antic­i­pate col­li­sions, but not avoid them. In this case, we need a tac­ti­cal solu­tion. It is named Just-in-time Col­li­sion Avoid­ance (JCA). It con­sists in mod­i­fy­ing the orbit of one of the two objects to avoid col­li­sion. The speed of a debris being 8 km per sec­ond, you only need to mod­i­fy the tra­jec­to­ry by a hun­dredth of a sec­ond only 24h before the col­li­sion to get a 1 km margin.

How can you reduce the speed of an orbit­ing non-func­tion­al debris?

There are cur­rent­ly three meth­ods. The first con­sists in send­ing a swarm of lit­tle minia­turised satel­lites, like the Cube­Sat mod­ules, in the mid­dle of a big debris cloud. They will cling to them before becom­ing dor­mant. In the case of a col­li­sion risk, they can be “awak­ened” in order to move the debris. But there are sev­er­al aspects to devel­op, like the cling­ing to the debris, com­mu­ni­ca­tions with Earth, or the cost.

The sec­ond solu­tion, a very ele­gant one at that, con­sists in mod­i­fy­ing the orbit using lasers. Very small puls­es deposit a very high ener­gy at the sur­face of the debris to vapor­ize the sur­face. The laser caus­es a very slight abla­tion which gen­er­ates a small gas plume when it is eject­ed, and thus acts like a rock­et engine. More than the tra­jec­to­ry of the debris, the aim is to mod­i­fy its time peri­od. A vari­a­tion of one sec­ond on an orbit of 90 min­utes means that the debris will lose 14 sec­onds in 24 hours. This is enough to avoid collision.

The third solu­tion, named Space Blow­er, con­sists in cre­at­ing an arti­fi­cial atmos­phere before the debris by spray­ing a cloud of micropar­ti­cles to slow it down.

Do all these solu­tions require an inter­ven­tion from space? 

Not nec­es­sar­i­ly, but it cer­tain­ly makes these inter­ven­tions eas­i­er. There already are sev­er­al projects of “in-orbit ser­vices”, ser­vice sta­tions orbit­ing in space. Much like util­i­ty trucks, these sta­tions could per­form dif­fer­ent mis­sions like, for exam­ple, to deor­bit or refu­el a satel­lite. It could also low­er the cost of inter­ven­tions and help to fund the dif­fer­ent solu­tions to avoid collisions.

For fur­ther reading

• C. Bon­nal, La pol­lu­tion de l’espace: L’état d’urgence (2016), Belin
• Video of Space Blow­er: https://​www​.youtube​.com/​w​a​t​c​h​?​v​=​7​G​Z​3​e​Q​VjnH4