Spacecraft powered by solar sails propelled by photons could explore interstellar space, redefining the term “traveling light.”
Image Credit: Jurik Peter/Shutterstock.com
Humanity has come on leaps and bounds in its knowledge and exploration of space since the Apollo moon missions. But even as we prepare to send crewed craft back to the lunar surface and eventually Mars, there exists a major barrier to our physical exploration of space.
With rocket propulsion technology as it exists today, space is simply too vast to feasibly reach beyond the limits of the solar system. A journey to even the nearest star system to ours, Alpha Centauri 4.4 light-years away, would be unfeasible with current technology.
Even the fastest craft ever created by humanity, The Parker Solar Probe which travels around the sun at 330,000 miles per hour, would take 880 years to reach the nearest of the three stars in Alpha Centauri. A rocket-propelled craft would take even longer, perhaps as much as tens or hundreds of millennia.
Revolutionary technology with its basis in physics discovered in the early years of the twentieth century could offer a solution to this problem and make interstellar travel more feasible.
Solar sails that collect the photons generated by the sun or even from laser-based propulsion could propel micro or nano craft at speeds as great as 100 million miles an hour — a significant fraction of the speed of light.
One such system is the Breakthrough Starshot craft¹, a microchip-sized craft, with a ten-foot sail 1,000 times thinner than a sheet of paper. The system could reach Alpha Centauri in a period as short as 20 years, its developers say.
How Do Solar Sails Work?
Light sails are powered by photons from the Sun a massless particle that can impart momentum.
According to Newtonian physics, simply put, light sails are impossible, nothing without mass should have momentum according to the principle that momentum is the product of a moving body’s mass and velocity.
Einstein’s theory of Special Relativity however suggests that momentum and energy can be represented as mathematical objects known as four tensors and transforming quantities between different reference frames in relativity is pretty straightforward.
Amongst these four-tensor quantities are four-position and the four-velocity, and as Newtonian mechanics suggest, we can obtain four-momentum by multiplying the four-velocity by mass.
Relativity depends on conversions known as ‘Lorentz transformations’. Because of the way the four-momentum transforms under these rules, it becomes entwined inseparably with energy. This means that in much the same way that Special Relativity unites space and time as a single entity — spacetime — it unites energy and momentum.
What appears as just energy to one observer, appears as energy and momentum to another. So regardless of the inertial reference frame of the observer, the momentum of a particle (p) is given by the relationship E=p²c² + m²c⁴.
This has an interesting consequence for massless particles, even if the mass is zero — as is the case with photons — momentum does not disappear as it does in Newtonian physics.
This leaves the momentum of photons defined as p=E/c, or as Planck’s constant (h) times the speed of light (c), divided by wavelength (λ), or E = hν = hc/λ.
Because Planck’s constant is so small it leaves photons carrying very little momentum. That is why your bedroom mirror does not move when you open the curtains or turn on the light.
This leads to the question, how many photons would it take to power a solar sail-propelled craft?
Powering a Solar Sail-Based Spacecraft
For a beam of monochromatic — single frequency — light with a frequency of 5.00 x 10¹⁴ HZ, the amount of momentum delivered by a single photon is 1.1x10-²⁷ kg m/s.
Fortunately, the Sun produces a lot of photons, so if spacecraft requires a push of 10 Newtons, it would take around 9.0 x 10²⁷ photons per second to power its flight. This is roughly 10 followed by 27 zeroes photons per second to impact a small amount of force.
This leaves the designers of light sail-based crafts with a clear dilemma — the sails need to be of a significant enough size to capture sufficient photons to impart momentum, but also light enough that demands for momentum are not too great. This requires working with some extraordinary materials.
The principles listed above have already been put in place by the Planetary Society's LightSail2 project², which launched from Kennedy Space Center in Florida in June 2019.
The LightSail 2’s sail is about 344 ft², roughly the size of a boxing ring, and is constructed of mylar with a thickness of just 4.5 microns, less than the width of a human hair — making it a feat of human engineering.
But whereas the LightSail 2 takes propulsion energy from photons from the sun, Starshot will use photons from a different source.
Laser-Based Space Travel
If all goes according to plan, Starshot³ will be powered photons by ground-based lasers here on Earth rather than those from the sun. This light will be millions of times more intense than sunlight alone, driving the craft to relativistic speeds.
Currently, many engineering challenges remain before solar sail or laser sail spacecraft can revolutionize our exploration of star systems beyond our own. One of these is ensuring that the solar sail can survive the rigors of international space travel without tearing or melting.
One solution suggested by a paper published in the journal NANO Letters⁴ is making the solar sail billow out rather than remaining flat. The authors argue that this could reduce their intrafilm mechanical stresses and avoid tears.
The promises these craft hold and the capability to return images of distant worlds such as Alpha Centauri's Proxima b is tantalizing enough to continue attempts to overcome these hurdles and reach further into the Universe than ever before.
References and Further Reading
¹ Breakthrough Starshot, [https://breakthroughinitiatives.org/initiative/3]
² LightSail2, The Planetary Society, [https://secure.planetary.org/site/SPageNavigator/mission_control.html]
³ Brewer. J., Campbell. M.F., Kumar. P., et al, , “Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation,” NANO Letters, [https://doi.org/10.1021/acs.nanolett.1c03273]
⁴ Brewer. J., Campbell. M.F., Kumar. P., et al, , “Relativistic Light Sails Need to Billow,” NANO Letters, [https://doi.org/10.1021/acs.nanolett.1c03272]