How to Get to the Nearest Star before We Die — Photonic Propulsion
Since its creation 4.6 billion years ago, Earth has become home to the only known living organisms in the universe. It took 4.6 billion years to create humans and we are the only intelligent species known. We can’t just let an asteroid kill everything on Earth or let bacteria create too much gas by creating too much gas. If you spent billions if not trillions of dollars looking for extraterrestrial life and trying to find out the beginnings of the universe, would you not want to explore it?
Until the twentieth century, humans have not been able to have the technology capable of bringing someone to space. Alan Shepard was the first man to orbit the Earth in 1961, Neil Armstrong was the first man on the Moon in 1969, however, almost 60 years later we haven’t made much progress in space travel. Are we even thinking about exploring the universe?
Why have We Not Returned?
Cost
Going to space is extremely expensive. Private companies such as SpaceX, Blue Origin and RocketLab are trying to bring the cost down by making rockets reusable but it still cost just less than $2500 to take 1 pound of stuff into space and $44 million to take one person into space.
Chemical propulsion is extremely slow
Chemical propulsion is a great way to launch into space but its not the best way to travel through interstellar space. Chemical propulsion requires large supply of fuel, adding mass to the spacecraft and decreasing speed.
Space is not ideal for life
Even if humanity goes back to the Moon or reaches Mars, it is extremely difficult to establish a base there because basic human necessities are not readily available. Other factors such as coronal mass ejections, radiation and micrometeors are also concerns. Many health concerns are also tied to being in space for prolonged periods such as loss of bone density.
In this article, we will focus on the first problem.
Propulsion Methods
Aside from chemical propulsion, there are several other techniques proposed that could be feasible in interstellar travel.
Photonic
Photonic propulsion harnesses photons from the Sun and other nearby stars to propel a spacecraft. It is currently not widely used and only used for unmanned missions.
Antimatter
Antimatter is the most energy-dense material known. When matter and anti-matter come into contact, they annihilate each other and generate a lot of energy. Antimatter is difficult and expensive to produce at the moment.
Ionic
Ionic propulsion are quite common in small spacecrafts such as CubeSats. Ionic propulsion can work in a variety of ways. Most commonly, its when Xenon is ionized and formed into plasma which is expelled and produces thrust to propel the spacecraft.
Fusion
Fusion propulsion is a theoretical concept at the moment but it holds a lot of potential. This technique will harness energy generated by fusing two atomic nuclei. This will most likely be done with isotopes of hydrogen called deuterium and tritium.
Blackhole
The most out-of-this-world concept I have ever heard. Blackhole drive uses something called Scharzschild Kugelbrlitz. Essentially, we are making artificial gravity by focusing a very powerful light into a small region, this will bend space-time enough to produce a singluarity. The singularity will release Hawking radiation like crazy and be sufficient to power a spacecraft.
All of these propulsion techniques are viable candidates for interstellar travel but in this article we will focus on photonic propulsion.
Photonic Propulsion
As mentioned above, photonic propulsion harnesses the power of the photons from the Sun or other nearby stars to propel a spacecraft. You might be wondering, how can something so small propel a spacecraft? Photons possess a lot of kinetic energy and they are abundant, so a lot of them will be able to propel a spacecraft. In fact, spacecraft propelled by photonic propulsion could travel at speeds up to 10%-20% the speed of light (30 000 km/s)!
To give you an idea of how much faster photonic propulsion is compared to chemical propulsion, it currently takes 5–10 months to get to Mars. With photonic propulsion it could take less than 2 months.
Photonic propulsion also does not require any fuel to be brought on the spacecraft and this will drastically decrease the mass, therefore, increasing the potential velocity it will travel.
How does it work?
Getting into how spacecrafts and photonic propulsion works will require some calculations (oops, sorry) but bear with me.
Photons propel spacecraft by transferring its momentum to it. Since photons are point particles and have no mass, using this equation wouldn’t make any sense (if mass is 0, momentum would be 0):
P = m x v
P = momentum
m= mass
v= velocity
Relativistic momentum
You might have seen this equation: E = mc². This equation actually derives a more complex equation which models the momentum of light more accurately.
E² = p²c² + (mc²)²
E = hc/λ
(hc/λ)² = p²c²
hc/λ = pc
p = h/λ
Now that we know a photon can behave like a particle, it makes sense that photons can propel spacecraft to 10%–20% the speed of light.
As mentioned before, photons propel spacecraft by transferring momentum to the object. To get a better idea of what this means, imagine two balls placed directly in front of the other, one meter apart. If you push one ball so that it hit the second one, the second ball starts moving. The first ball has transferred its momentum to the second ball.
Specific Impulse
Photons are the propellant which posses the largest specific impulse (Isp = 3.06 x 10⁷ s). This simply means that it generates the most thrust (force) per mass of propellant. This makes photonic propulsion an extremely efficient method of propulsion.
Specific impulse equation:
Isp = F/ m dot
Isp = specific impulse
F = thrust (force)
m dot = mass per time (ex. kg/s)
Applications
Solar Sails
Photons seem to be a pretty good propellant, but to optimize the force generated on a spacecraft using photons, one can increase the surface area of the spacecraft. This is the concept of solar sails. Solar sails are composed of aluminized Mylar, a thin polyester film. They appear as a large, thin sheet with payload, such as a CubeSat in the middle.
Currently, there are only a few functioning solar sails because they have such low function. The LightSail1 and LightSail2 are created by the Planetary Society and propel CubeSats in Low Earth Orbit (LEO). IKAROS is a solar sail created by the Japanese Aerospace Exploration Agency (JAXA) which is headed for Jupiter.
Photonic Laser Propulsion (PLP)
This concept uses the idea that if one shoots an incredibly high amount of photons at an object, the momentum transferred will be incredibly large. The thrust to power ratio that a photon provides a spacecraft is dramatically low. Therefore, there needs to be a concentrated amount of photons that continuously hits the solar sail in order for the spacecraft to accelerate constantly at efficient velocities.
One approach to this problem is through a laser. Bae Institute, a space research company, is currently looking into this field and their method includes photons that can be trapped between two high reflective mirrors (fundamental aspect of every laser) and directed toward the solar sail to give it thrust. There are two methods this can be implemented: passive resonant optical cavities and non resonant optical cavities.
In passive resonant optical cavities, a high intensity laser beam is injected into an optical cavity. This form of laser requires a high-power single frequency laser and this is inefficient. In addition, this method is also highly sensitive to small changes in cavity length and to deterioration in mirror quality. As a result, it is often not used in photonic propulsion.
Non resonant optical capacity requires tightly focused laser beam spots on each mirror to avoid beam interference. This type of laser can easily become a passive resonant optical cavity, which is impractical for photonic propulsion.
New ways are still being sought out. The Bae Institute is doing groundbreaking research in this area. Check it out:
Photon Tether Formation Flight (PTFF)
This concept revolves around the idea of sending laser probes into space and joining the probes with tethers to create a very powerful laser. This laser is powerful propel spacecraft into interstellar space. This will definitely require a lot of infrastructure and development in technology before it can be viable. It will also be extremely expensive to maintain.
This network of space probes could also be used as a spaceborne observation platform by attaching modules on the space probes.
Is it possible?
Within the space community, the mention of photonic propulsion is not always as exciting as it seems. This is because, to be able to get any useful result from photonic propulsion, a lot of infrastructure is required. However, we are seeing some development in this area not only from the Bae Institute.
NASA launched a program called Breakthrough Starshot. In 2016, this idea gained traction and a $100 million dollar grant led by billionaire, Yuri Milner, was allocated to the program. The mission being to develop probes that accelerate at 20% the speed of light to our nearest star neighbor, Alpha Centauri.
“I think fundamentally the future is vastly more exciting and interesting if we’re a spacefaring civilization and a multiplanet species than if we’re or not. You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. And that’s what being a spacefaring civilization is all about.” — Elon Musk, SpaceX
TL:DR
- Humans are the only intelligent species known, in order to preserve this, we must colonize different areas in space
- We are exploring space because it is expensive, chemical rockets are slow and heavy, establishing a base is difficult, space conditions are not ideal for humans
- Propulsion methods can include: photonic, ionic, antimatter, fusion, blackhole
- Photonic propulsion is a good candidate for interstellar travel because it is light and efficient
- Solar sails, photonic laser propulsion and photon tether formation flight are concepts which photonic propulsion can be applied to
- Progress in this area is slow but we’re seeing progress
Hi. My name is Rosa and recently I’ve been really passionate about space technologies, specifically photonic propulsion. If you want to contact me you can:
Email: rosali.tks@gmail.com
LinkedIn: https://www.linkedin.com/in/rosa-li-7ba119194/
If you liked the article please leave some claps!
See you next time!
