The big-picture plan starts with using laser propulsion in the coming decades on near-Earth space missions, journeys to the moon, and visits to near-Earth asteroids. Within 50 years, he hopes for phase two: Mars. After that comes the gas and ice giants in the outer solar system and their intriguing moons. And then, beyond: “We envision that humans can fly to other stars or other planets in other solar systems,” he says.
One of the most intriguing parts about using laser propulsion for deep space journeys: There’s the potential for in-flight gravity, or something like it. The system would create acceleration similar to 1g, meaning that astronauts would have their feet on the ground. “Once you accelerate, then that acceleration acts like gravity,” Bae says. “Your feet will be toward the laser because of the acceleration. That way, I think the Star Trek–type of travel is possible.”
What’s holding Bae back? For one, you’d need tremendous power to realize such a mission with current technology. For even near-Earth and lunar missions, Bae estimates a requirement of 1 gigawatt of power, requiring a large amount of power to be generated by solar or nuclear power to generate the thrust.
Then there are the lasers themselves. As the distance between the photonic source and the craft increases, the signal spreads wider and wider, decreasing the precision of the guidance and reducing overall thrust. Think of it like shining a flashlight. Up close, the light is easy to narrowly direct to a particular object. But shining it on something farther away spreads the light out more, covering a wider distance but with less luminosity. Now, for a laser that’s supposed to be shooting precise guidance lasers, tshis becomes a problem. So while near-Earth and Martian flights will be okay, problems will arise getting farther and farther away. One compensation is the idea of doing smaller platforms to create a “photonic railway,” each acting as a sort of refueling station in between to get the craft where it needs to be. But Bae wants to also control the problem of the lasers spreading themselves too thin. Bae has his eye on research into Bessel beams, which don’t diffract, and therefore could be fired at a spacecraft from farther away.
Bessel beams are lasers that behave very differently from ordinary lasers. Consider how the typical laser pointer behaves, creating a small red dot where you point it. Instead of a single point on a wall, Bessel beams create a bullseye: one dot surrounded by concentric rings. The number of rings is some indication of the strength of the beam. Many commercial Bessel beam devices create beams with about eleven rings. The ideal Bessel beam would have an infinite amount – because an ideal Bessel would use an infinite amount of energy.
Unlike a typical laser beam, a Bessel beam does not diffract and get larger as the beam gets farther from its point of origin. One of the most prized attributes of Bessel beams is the fact that the central core of the beam can be blocked, without interrupting the beam. The laser essentially self-heals by using the rings which were not blocked. It’s the optical terminator.
Proven but classified technology should enable photonic propulsion to operate out to 100 kilometers
The German company Rheinmetall Defense demonstrated a 1-km long laser resonator similar to the PLT optical resonator in 1994-1995 with the use of a telescopic arrangement in the optical cavity, and that such long laser resonators can be scalable to 100 km with the usage of optics in the diameter of 70 cm. These successful demonstrations promise that PLT can be operated beyond distances in the order of 100 km. Further studies should be performed whether PLT can be used for interstellar scales, but so far there is no show stopper on this issue.
Another key technological issue in implementing PLT is in the intracavity laser beam aiming, aligning, and tracking, which will be addressed more in depth in the discussion section. With the rapid advancement in DE technologies, the aiming, alignment, and tracking of laser beams on rapidly moving uncooperative targets over the distance greater than 100 km have become technologically feasible.
Although the technical details of such aiming, alignment, and tracking system is grossly classified, the nut-shell of the technology is available in open literatures. Especially, the technology developed for ABL will play crucial role in PLT systems. Based on open literature, in ABL, the aiming, alignment, and tracking of the main laser rely on the scattered beam of the beacon laser (also diode pumped lasers at power level of a few kW). Similar to this, a small laser (power level of a few watts) in the mission vehicle can be used as a beacon laser. It seems that the aiming, aligning, and tracking system can be scaled to interstellar distances.
Other important issues involve the hardware weight and optical quality of the HR mirrors. The size of the HR mirrors to have the proposed high reflectance over long intracavity distances should be considerably larger than the one obtained with 1st order diffraction law. Therefore, the method to reduce beam divergence against diffraction law may be required eventually for long range operations of PLT/DEMB systems. One interesting approach needed to be investigated is to use Bessel beams for reducing the divergence of laser propagation. Another interesting approach needed to be investigated is to use Bose-Einstein Condensation (BEC) of photons
Young Bae recently received a $500,000 NASA phase 2 NIAC grant for Propellant-less Spacecraft Formation-Flying and Maneuvering with Photonic Laser Thrusters
Photonic Laser Thruster (PLT)
A photonic laser thruster bounces a laser between mirrors to boost the momentum transfer by recycling the photons.
Dr. Y.K. Bae demonstrated a Photonic Laser Thruster (PLT) built from off-the-shelf optical components and a YAG gain medium, and the maximum amplified photon thrust achieved was 35 µN for a laser output of 1.7 W with the use of a HR mirror with a 0.99967 reflectance. This performance corresponds to an apparent photon thrust amplification factor of ~3,000. More importantly, in the experimental demonstration, the author accidentally discovered that the PLT cavity is highly stable against the mirror motion and misalignment unlike passive optical cavities. In fact, in the demonstration experiment by Dr. Bae, the full resonance mode of the PLT was maintained even when one of the HR mirror was held by a hand. In a more systematic experiment, the PLT cavity was demonstrated to be stable against tilting, vibration and motion of mirrors. Subsequent theoretical analysis by the author showed that PLT can indeed be used for propulsion applications, and proposed Photonic Laser Propulsion (PLP), the propulsion with PLT. The reason for the observed stability results from that in the active optical cavities for PLT and PLP the laser gain medium dynamically adapts to the changes in the cavity parameters, such as mirror motion, vibration and tilting, which does not exist in the passive optical cavities.
A four-phased evolutionary developmental pathway of the Photonic Railway towards interstellar manned roundtrip travel is proposed:
1) Development of PLTs for satellites and NEO manipulation,
The first phase in the developmental pathway towards interstellar roundtrip manned flight is maturing PLT technologies and systematic scaling up of its power and operation distance capabilities. PLT is predicted to meet the needs of the next generation of space industry market by enabling a wide range of innovative space applications near the earth. In this phase, which is predicted to evolve over 5–30 year time frame, PLT would be capable of providing thrusts in the range of 1 mN–1 kN, which requires the operation power of 100 W–100 MW. The solar panel based space power currently can provide electrical powers up to 100 kW, therefore, the PLT capable of providing thrusts up to 1 N can be readily implemented in the near future.
2) Interlunar Photonic Railway,
3) Interplanetary Photonic Railway, and
4) Interstellar Photonic Railway.