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​.you​tube​.com/​w​a​t​c​h​?​v​=​7​G​Z​3​e​Q​VjnH4