Photonic propulsion: visit the Solar System by sail

Des­pite the extraordin­ary tech­no­lo­gic­al advances in the field of space, it is clear that the prin­ciple of space­craft propul­sion has not changed much. This prin­ciple is called action-reac­tion: the faster we throw a large amount of mat­ter in one dir­ec­tion, the more force we cre­ate in return to move for­ward in the oth­er direction.

This is how a small steam engine was powered back in the 1^st Cen­tury AD, and it is the same prin­ciple that gov­erns the lift-off of the Ariane 5 launch­er or the move­ment of the Japan­ese Hay­abusa 2 probe around its aster­oid Ruy­gu. So, as long as we are tak­ing inspir­a­tion from ances­tral tech­niques to move around in space, why not use the same means that has long allowed man­kind to pilot and pro­pel its boats to dis­cov­er new con­tin­ents: the sail?

This, now fam­ous, pop-cul­ture phrase was the tagline for the “Ali­en” series of sci­ence-fic­tion films and reveals a fun­da­ment­al aspect of space: there is no air. For sound to propag­ate, a mater­i­al medi­um (gas, liquid or sol­id) is needed, so it is impossible to call for help in the vacu­um of space. And there­fore, if there is no air, there is no wind either. As such, even our finest sail­ing ships would be ill-advised to try to move out­side the Earth’s atmo­sphere. For­tu­nately, how­ever, this does not spell the end of our plans for space sail­ing ships. Indeed, what is improp­erly called the “vacu­um of space” is not entirely empty.

While the volume of mat­ter is reduced to a few atoms per cubic cen­ti­metre, space is filled with photons: in oth­er words, particles of light. At the level of the Earth’s orbit, each square metre of sur­face receives about 1²¹ (1,000 bil­lion bil­lion) photons from the Sun every second. When a photon bounces off a sur­face, it trans­fers a tiny amount of energy to the sur­face in the form of recoil. This effect of the pres­sure of light on mat­ter has been observed for centuries.

In 1619, the great astro­nomer Johannes Kepler sug­ges­ted that the ori­ent­a­tion of a comet’s tail was due to the ‘blow­ing’ effect of the Sun’s light. The first math­em­at­ic­al explan­a­tions came later, from the fam­ous phys­i­cist James Max­well in 1873, and the first meas­ure­ments of this tiny effect of light reced­ing from a wall were made a few years later, at the very begin­ning of the 20th century.

So, in the­ory, it is pos­sible to build a sail pushed by light. But is this thrust suf­fi­cient? How big would a space sail have to be to cap­ture enough photons to move under the sole effect of sunlight?

Cal­cu­la­tions show that in order to accel­er­ate 1kg of mat­ter and increase its speed by 1 m/s every second, a sail of about 100,000 m² is needed at the level of the Earth’s orbit, i.e. a square sail of about 330 metres on each side. Hence, you need a very large sur­face area for still a very small accel­er­a­tion. At this stage, it would be tempt­ing to declare this meth­od inef­fect­ive, throw the paper on which these cal­cu­la­tions were made in the bin and move on to anoth­er research top­ic. But this would be to miss a cru­cial piece of inform­a­tion that has not been con­sidered so far. This energy source is free and never-ending!

Indeed, the Sun has been burn­ing for almost 5 bil­lion years and will con­tin­ue to do so for just as long. Unlike oth­er space propul­sion meth­ods, which always require a fuel tank to be heated and ejec­ted back­wards to push the space­craft for­ward, no tank is needed here. There is no need to worry about break­ing down!

And if the start of our sol­ar sail’s jour­ney was quite mod­est, let’s not for­get that the accel­er­a­tion would be con­stant, and could be main­tained for years, even decades!

Con­tinu­ing with the cal­cu­la­tions star­ted above, it is shown that after 100 days of oper­a­tion, under real­ist­ic con­di­tions, a sol­ar sail could reach 14,000 km/h. Three years later, its speed would reach 240,000 km/h. This would be enough to reach Pluto, one of the most dis­tant bod­ies in the Sol­ar Sys­tem, in just five years. (By way of com­par­is­on, it took the New Hori­zons probe almost 10 years to make the same journey)

It is this idea of a small but con­tinu­ous thrust that has led sev­er­al research groups to test increas­ingly soph­ist­ic­ated pro­to­types in recent years. The first sol­ar sail was developed by the non-profit organ­isa­tion The Plan­et­ary Soci­ety. The pay­load con­sisted of a cent­ral body weigh­ing 100 kg sur­roun­ded by eight small sol­ar sails of 30 metres each. In 2001, a first launch of the pro­to­type ended in fail­ure. The pro­to­type was rebuilt and launched in 2005 by an old inter­con­tin­ent­al bal­list­ic mis­sile from a Rus­si­an sub­mar­ine. Once again, com­mu­nic­a­tion was quickly cut off and no one heard any­thing more about the device. These dif­fi­cult begin­nings do not dis­cour­age research­ers and phys­i­cists. In 2010, the Japan Aerospace Explor­a­tion Agency (JAXA) sent the 14-metre IKAROS sol­ar sail into orbit. 

Light­Sail 1 and Light­Sail 2, built by The Plan­et­ary Soci­ety and sent into space in 2015 and 2019 respect­ively, will fol­low, con­firm­ing the pos­sib­il­ity of attach­ing a sol­ar sail to a satel­lite to modi­fy its orbit. This time it was suc­cess­ful. After one month, the speed of the 315 kg craft (includ­ing 15 kg of sail) has increased by about 10 m/s. The Japan­ese agency has thus val­id­ated the prin­ciple of the sol­ar sail and con­firmed that it is pos­sible to deploy and steer such a craft in space. 

One of the short­com­ings of early sol­ar sails was that they could only set small pay­loads in motion at the centre of the sail. The first pro­to­types were too heavy to be accel­er­ated sig­ni­fic­antly. Today, the devel­op­ment of new tech­no­lo­gies, the use of new mater­i­als and mini­atur­ised elec­tron­ics have made it pos­sible to build nanosatel­lites weigh­ing just a few kilo­grams, with per­form­ances that prom­ise to be as good as today’s huge satel­lites weigh­ing sev­er­al hun­dred kilo­grams, thanks, among oth­er things, to the use of on-board arti­fi­cial intel­li­gence algorithms.

The sails them­selves are the sub­ject of intense research to optim­ise the way they inter­act with light. NASA, for example, is devel­op­ing a so-called ‘dif­fract­ive’ sol­ar sail. This pro­ject, Dif­fract­ive Sol­ar Sail­ing, uses small optic­al grat­ings embed­ded in the sail’s thin films to make more effi­cient use of sun­light, without sac­ri­fi­cing the craft’s manoeuvrability.

Finally, it should be remembered that the space land­scape has under­gone intense and pro­found changes in recent years. The evol­u­tion of tech­niques and the drop in the cost of access to space now allow new start-ups to test and com­mer­cial­ise innov­at­ive space-related pro­cesses and ser­vices. This new dynam­ic is known as “New Space”.

One of these French star­tups, Gama Space, recently raised $2 mil­lion from the CNES (Centre Nation­al d’Études Spa­tiales) to devel­op a small 72 m² sol­ar sail, 50 times thin­ner than a human hair, which should ini­tially serve as a pro­pel­lant for a small 11 kg satel­lite launched in Octo­ber 2022. Of course, the aim is not to lim­it itself to Earth orbit, and Thibaud Elzière, its founder, is already think­ing of a means of dir­ect­ing future explor­at­ory probes through­out the Sol­ar System…

The final lim­it­a­tion inher­ent in the sol­ar sail tech­nique is, of course, the num­ber of photons that ‘push’ the craft. And the fur­ther away from the Sun the sail is, the faster this quant­ity decreases, until it has almost no effect on the craft.

To solve this prob­lem and envis­age space explor­a­tion out­side our Sol­ar Sys­tem, the StarShot pro­ject plans to build a thou­sand small sol­ar sails, each weigh­ing no more than one gram. In order to reach Prox­ima Cen­tauri, the closest star to the Sun, in a reas­on­able time, it is planned to build a 100-Gigawatt laser beam array on Earth, gradu­ally accel­er­at­ing these small sails to 20% of the speed of light, which would enable them to leave the Sol­ar Sys­tem by 2030 and fly past Prox­ima Cen­tauri around 2060.

Will we have detailed images of anoth­er sun and the plan­ets around it by the end of the cen­tury? All it takes is a few photons and a little human ingenuity…