Laser-powered spacecrafts, interstellar travel and reaching Mars in 3 days
Tuesday, May 01, 2018
Solar sails and laser propulsion concepts have fed our imagination for decades. Reaching Alpha Centaury within a few years? Mars in just 3 days? That definitely ignites curiosity and it is not confined to the realm of science fiction anymore.
In addition to the fantastic science and industrial opportunities that such laser technology is about to unlock, we are at the dawn of a new interplanetary and interstellar exploration era.
Not afraid to flirt with the speed of light? Well, get onboard! It’s time to learn about laser-powered spacecraft.
Time for a change
Good old fuel burning rockets may have gotten some public interest back lately but they are incredibly slow considering the huge distances there are between stars in our portion of the galaxy. If we are to go visit habitable exoplanets outside of our solar system, I personally don’t want to wait until humanity goes extinct for the images to come back!
Rockets provide large payload capacities and proven safety records but even the latest technology will fly us to Mars in five months at best. For your crew to reach the closest star to our sun, it would take tens of thousands of years…
Using continuous photon pressure from lasers to accelerate a reflective material attached to a spacecraft carrying a payload is not a new idea. However, new programs have emerged during the last few years and they give hope to the restless space traveller enthusiasts. And to businesses as well.
Star shot roadmap
Breakthrough Starshot is a $100 million privately-funded program to demonstrate the feasibility of sending numerous lightweight probes in space at 20% of the speed of light using Earth-based, visible lasers combined together to reach a 100-gigawatt power level.
The key is to provide enough acceleration to light sails during a short amount of time for a fleet of nanocrafts to aim at interstellar targets with miniaturized electronics such as communication hardware and measurement devices. The longer the lasers are fired and the lighter the nanocrafts are, the faster they get.
On his side, Philip Lubin from University of California Santa Barbara received public funding from NASA to elaborate a roadmap to interstellar flight. He claims that 15 to 100 gigawatts of combined laser power could propel a 100 kg spacecraft to Mars in just a few days and a manned mission within a month. The amount of energy spent here is actually similar to what a traditional rocket spends during a few minutes to reach low-Earth orbit.
One can easily understand why keeping gigawatt ground-based lasers fired during the shortest amount of time will be a strong requirement considering civil aviation regulations, environmental concerns and general safety.
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How can lasers provide such an acceleration? What is the physics behind all this? Before running to your favorite laser power detectors manufacturer for quality control on your latest light device, how do we make sure about the amount of laser power to expect for interplanetary and interstellar flights to become a reality?
Photon thrust
When he postulated that the speed of light in vacuum was the absolute speed limit that nothing is able to exceed, Albert Einstein paved the way for the most accurate model of universe we have so far. That would eventually explain gravity in a way sharp enough to make our communication satellites work.
Explain atomic interactions in a way powerful enough to make electricity plants, war-ending weapons and computers work. Precise enough to predict the existence of gravitational waves one century before it was actually observed by laser interferometers. Yes, lasers are everywhere!
Photons are the elementary particles (the quanta) of light, they have no mass, travel at roughly 300,000 km/s in vacuum and carry both energy and momentum. When bouncing off the surface of a mirror, Newton’s third law of dynamics tells us that the total momentum is conserved and a photon's momentum is transferred to the mirror so that it is pushed in the same direction as the incoming photon beam.
Energy is also conserved and transferred from the photons to the mirror. The total energy transferred is equal to the total incoming photon energy when the light is 100% reflected by a perfect mirror.
Material absorption characterization and other laser wavelength considerations make one serious engineering challenge already, so let’s focus on hypothetic perfectly reflecting mirrors for the sake of this article. Physicists love good approximations!
Make it special
Considering a theoretical laser that would target 100% of its photons on the light sail surface without any loss of energy during light propagation in space, we can say that the total amount of photon energy received by the sail is transferred to it in the form of kinetic energy, that is motion.
This kinetic energy can then be expressed as the product of the average power of the laser by the amount of time during when the laser is turned on providing energy, let’s called this exposure time.
The sail will accelerate and accumulate kinetic energy equal to its total energy minus its energy at rest (the famous E = mc^2 with m the total mass of spacecraft). Here we need Einstein’s special relativity frame of work where the total energy is given by:
y is the Lorentz factor defined by:
v is the speed acquired by the spacecraft and c the speed of light in vacuum. y tells how relativistic we are. Simply try v=c in there, you’ll get an impossible operation, meaning that nothing can travel faster than the speed of light in vacuum in the frame of Einstein’s theory.
Therefore:
D is the distance of the object we want to visit from Earth and T(travel) the time we want to spend travelling in space to get there.
Using:
We get:
We have confirmed that a perfectly focused, dozen gigawatts-laser fired during a few minutes towards a hundred-ton spacecraft equipped with 100% reflective mirrors can carry humans to Mars in only 30 days! Confirming the aforementioned Philip Lubin’s claim.
This is just enough time before you need to get some space from your flight roommates, right?
Limitations and opportunities
The longer the push, the faster you will get. Hours of exposure on lightweight nanocrafts theoretically allows to reach Alpha Centaury in a few years, harnessing a significant fraction of the speed of light without any fuel loaded onboard your ship.
But how exactly do you manage to slow down? If not only for flybys but actually for entering an orbit around a planet or a star and stay there for a while, how the heck do we make a brake pedal?
What materials can withstand such accelerations without tearing apart? Do we also plan on having huge lasers powered on the surface of Mars for the return trip? How to combine laser beams for higher power like this while limiting divergence and power loss in space? How do we maintain targeting?
It took 35 years for the 800-kg Voyager 1 probe to reach interstellar space at the border of our solar system, only one light year away. The idea to reach and observe other potentially inhabited worlds with fully-equipped lightweight probes in just a few years is awe-inspiring.
Asteroid mining, Earth-encountering asteroids deviation and wireless energy transfer are some examples of applications where R&D will probably get funded shortly in the wake of the proof of concepts for these interstellar trips to come.
Space travel and observation has been one of the most powerful engines of innovation for companies and people to benefit from through the ages. So we better get to work, develop new materials and test those future spaceship-propelling lasers with available super high power detectors from Gentec-EO!
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