Nov 2 2018
According to experimental cosmologist Philip Lubin, new directed energy propulsion systems may facilitate the first interstellar missions, with small, robotic spacecraft discovering neighboring solar systems. He will present these and other developments at The Optical Society’s (OSA) Laser Congress, Light the Future Speaker Series, scheduled between 4th and 8th November, in Boston.
Visualize a wafer-thin spacecraft powered by laser light with speeds touching more than one quarter the speed of light—fast enough to reach the closest neighboring star to Earth’s solar system within 20 years, or something closer to the Earth, like transporting people to Mars in a month. By exploiting photonics-driven propulsion, scientists are well on their way to making this apparently impossible science-fiction feat a reality, said Lubin, who is a professor of physics at the University of California, Santa Barbara.
The research results Lubin will present stem from NASA’s Starlight and Breakthrough Starshot programs, both of which support innovative research in photonics. Lubin is director of the Starlight program.
“Photonics, the production, and manipulation of light, is already a part of our daily lives—from cellphones to computers to light-emitting-diode (LED) light bulbs to fiber optics that carry your data all over the place—even though you may not see it,” said Lubin. “You can point to practical examples of photonics in everyday life and it appears to have nothing to do with interstellar flight, but in fact it does, because it’s synergistic with the technology you need to achieve interstellar flight.”
One of the ultimate challenges in confirming this photonics concept as it relates to propulsion is the demonstration of the laser power necessary to speed up the planned/hypothetical spacecraft, according to Lubin.
Synthesized optics for directed energy propulsion systems
Large directed energy systems are not constructed using a single enormous laser, but instead depend on beam combining, which involves the use of a number of very basic power laser amplifiers.
“Our system leverages an established typology called ‘Master Oscillator Power Amplifier’ design,” said Lubin. “It’s a distributed system so each laser amplifier ‘building block’ is between 10 and 1000 Watts. You can hold it in your hand. Instead of building a gigantic laser, you combine a lot of small little laser amplifiers that, when combined, form an extremely powerful and revolutionary system.”
Lubin puts forward a comparison with supercomputers, which are designed using a large number of central processing units (CPUs). “By coherently combining billions of low poser laser power amplifiers—similar to the same power of a typical modern household LED—you suddenly have this amazingly capable directed energy system,” he said.
Interstellar probes powered via laser light
Directed energy systems may support interstellar probes as part of human exploration in the not-too-distant future, and they are at the core of the NASA Starlight program and the Breakthrough Starshot Initiative to enable the first-ever interstellar missions. The same central technology has a number of other applications, such as fast interplanetary travel for high mass missions, including those carrying people; the search for extraterrestrial intelligence (SETI); and planetary defense.
“Our primary focus currently is on very small robotic spacecraft. They won’t carry humans onboard—it’s not the goal for the interstellar portion of our program,” said Lubin. “If humanity wants to explore other worlds outside our solar system, there are no other physically obtainable propulsion options for doing this—with two exceptions.”
“One way would be if we could master a technological approach known as antimatter annihilation engines, which are theoretical propulsion systems that generate thrust based on energy liberated by interactions at the level of subatomic particles. But we don’t currently have a way to do that,” Lubin said, “and it involves a number of complexities we do not have a current path to solving.”
“The other option is directed energy or photonic propulsion, which is the one we’re focusing on because it appears to be feasible,” Lubin said. In one variant, directed energy propulsion is like using the force of water from a garden hose to propel a ball forward. Minuscule interstellar spacecraft (basically less than a kilogram and some that are spacecraft on a wafer) can be driven and steered via laser light, he said.
“Miniaturizing spacecraft isn’t required for all of the mission scenarios we’re considering, but the lower the mass of the spacecraft the faster you can go,” Lubin said. “This system scales in different ways than ordinary mass ejection propulsion.”
Thus far, all of the rockets that have taken off from Earth are based on chemical propulsion systems whose standard designs date back to World War II. They are just scarcely able to make it off the Earth’s surface and into orbit. Building a bigger rocket does not make it go faster, it just allows the rocket to take on more mass. Photonic propulsion works contrarily because the less dense the payload, the faster one gets propelled. Therefore, the aim is to lower the mass to travel faster.
Like driving in a rain storm—in space
One major challenge for relativistic spacecraft is radiation hardening, because “when we begin to achieve speeds close to the speed of light, the particles in interstellar space, protons in particular, that you plow into—ignore the dust grains for the moment—are the primary radiation source,” said Lubin. “Space isn’t empty; it has roughly one proton and one electron per cubic centimeter, as well as a smattering of helium and other atoms.”
Smashing into those particles can be intense at high speeds because while they may be moving slowly within their own frame of reference, for a spacecraft that’s moving fast, they make for high-speed impacts.
“When you hit them it’s like driving in a rainstorm. Even if the rain is coming down straight from the sky your windshield gets plastered because you’re going fast—and it’s quite a serious effect for us,” Lubin said. “We get enormous radiation loads on the leading edge as the front gets just absolutely clobbered, whereas the rest of the spacecraft that is not the forward edge and facing in different directions doesn’t get hit much at all. It’s an interesting and unique problem, and we’re working on what happens when you plow through them.”
With regards to a timeframe for putting directed energy propulsion technology into operation, “We’re producing laboratory demos of each part of the system,” said Lubin. “Full capability is more than 20 years away, although demonstration missions are feasible within a decade.”
Getting to Mars quickly
The same fundamental photonics technology in the NASA Starlight program also allows for very rapid interplanetary missions, including missions to Mars that could carry people on trips as short as one month. This would greatly minimize the dangers to humans on the long journey to the red planet and is presently being explored as one option.
Trillion Planet Survey
Photonics progresses also mean that man can currently leave a light on for extraterrestrial intelligence within the universe if one wants to be found—in case there is other intelligent life that is also keen to know the answer to the question, “are we alone?”
Lubin’s students investigate this concept in their “Trillion Planet Survey” experiment. This experiment is currently searching the nearby galaxy Andromeda vigorously, which has about a trillion planets, and other galaxies as well as Earth’s for signals of light.
Integrating Lubin’s research with his students’ experiment, there are openings for signaling life. When technological developments allow for the exhibition of lasers powerful enough to propel the tiny spacecraft, these lasers could also be used to shine a guiding light towards the Andromeda Galaxy in anticipation that any life form there could find and detect that source of the light in their sky.
The reverse case is more fascinating. Maybe another civilization is present with capability akin to what humans are currently developing in photonics. They may understand, as scientists do, that photonics is a very efficient means of being detected across cosmic distances far outside the galaxy. If there is an extraterrestrial society that is broadcasting their existence via optical beams, like those planned for photonic propulsion, they are contenders to be detected by a large-scale optical survey such as the Lubin team’s Trillion Planet Survey.
“If the transmission wavelength of an extraterrestrial beam is detectable, and has been on long enough, we should be able to detect the signal from a source anywhere within our galaxy or from nearby galaxies with relatively small telescopes on Earth even if neither ‘party’ knows the other exists and doesn’t know ‘where to point’,” Lubin said. This “blind-blind” state is paramount to the “Search for Directed Intelligence” as Lubin calls this strategy.
Planetary defense
Possibly one of the most fascinating uses for photonics—closer to Earth—is to exploit it to help protect Earth from external threats such as hits from comets and asteroids.
The same system the scientists are beginning to build for propulsion can be used for planetary defense by directing the beam onto the comet or asteroid. This causes surface destruction, and as portions of the surface are ejected during the reaction with the laser light, force would push the debris one way and the comet or asteroid in the opposite direction. Therefore, gradually, it will deflect the threat, Lubin said.
“The long-term implications for humanity are quite important,” he added. “While most asteroid threats are not existential threats, they can be quite dangerous as we saw in Chelyabinsk, Russia in 2013 and in Tunguska, Russia in 1908. Sadly, the dinosaurs lacked photonics to prevent their demise. Perhaps we will be wiser.”