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…