Traditionally, spacecraft maneuvering is performed by onboard particle- based thrusters, such as ion thrusters, with a limited amount of fuel that restricts lifetime and V capability. In addition, the traditional “all-in-one” single-vehicle architecture requires that mission-critical, expensive components accompany the less critical, inexpensive or consumable components, such as fuel and main power supplies, which otherwise can be readily replaced.

Figure 1. In a Photonic Laser Thruster (PLT), photons are trapped in an intracavity between two spacecraft vehicles to amplify photon thrust by repetitive bouncing between high reflectance mirrors. Inserting a gain medium within the optical cavity permits straightforward power scaling and extraordinary stability against the mirror or space-platform movements. In this scenario, PLT is used for momentum beaming from the resource vehicle to the mission vehicle for advanced spacecraft maneuverings, such as station-keeping, orbit changing, and drag compensation.

In principle, physically separating the highly valuable mission vehicle from a lower-cost, replaceable resource vehicle can significantly lower the life-cycle cost in a wide range of missions. Furthermore, if thrust can be provided or beamed from the resource vehicle to the mission vehicle, the fuel-limited lifetime of the mission vehicle can be drastically lengthened similar to aerial refueling of jetfighters. Photonic Laser Thruster (PLT) technology is an enabler of such a multi-vehicle approach in which pairs of spacecraft are connected through propellantless thrust beaming or exchange via trapped or recycled photons between vehicles.

The chief innovative aspect of the PLT spacecraft maneuvering is in its exploitation of non-contacting inter-body thrust exchange, which consists of beaming thrust between space vehicles via PLT as illustrated in Figure 1. In a way that’s analogous to the fact that the inter-body interactions between atoms open the door to the formation of vastly rich varieties of molecules, the inter-body interactions between space vehicles are predicted to open the door to extremely rich missions well beyond what can be achieved by solo-acting spacecraft alone.

PLT spacecraft maneuvering incorporates a means to react forces between two space vehicles independently actuated by PLT and it is operable for distances up to several hundred kilometers with existing laser and optics technologies. The thrust exchange can be either unidirectional or bidirectional depending on the mission. Through such thrust exchange, the PLT spacecraft system will be able to drastically reduce fuel consumption, or separate the highly valuable mission vehicle from a lowercost, replaceable resource vehicle to lower the life-cycle cost significantly in a wide range of missions. Examples include precision formation flying, orbit-raising or escape, drag compensation, and rendezvous and docking.

Photonic Laser Thruster (PLT)

Under the auspices of the former NIAC program, PLT was developed for propellant-free nanometer accuracy formation flying, and photon propulsion was chosen to be a means of tensioning the tethers without propellant and with ultra-high thrust precision. However, we discovered that PLT has a much broader scope in in-space applications. Since photons have extremely small momenta, thus thrust per power, the thrust amplification by recycling photons between two high reflectance mirrors located separately in two pairing satellites was proposed to amplify the photon thrust. Initially, we proposed using a passive cavity that consisted of two mirrors with a vacuum between them; however, we found difficulties in injecting sufficient laser power in such a high-Q optical cavity over a long distance. Furthermore, we found another difficulty in maintaining the stability of the laser power in the passive cavity against small perturbation on the two mirrors. In further studies after numerous failures, we discovered that an active cavity with an amplifying medium in the cavity could overcome such difficulties.

Subsequently, we successfully demonstrated the proof-of-concept of PLT. In this demonstration, a PLT was 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, we discovered that the PLT cavity is highly stable against the mirror motion and misalignment, unlike passive optical cavities. In fact, in the demonstration experiment, the full resonance mode of the PLT was discovered to maintain even when one of the HR mirrors was held, moved, and tilted by hand.

Figure 2. An artist’s rendition of a persistent annulus formation with an aperture diameter exceeding 100m in LEO with the use of PLT for persistent “out-of-plane” spacecraft maneuvering without the expenditure of propellant. In this structure, pushing-out photon thrust is counter balanced by gravity gradient.

In a more systematic experiment, the PLT cavity was demonstrated to be highly stable against tilting, vibration and motion of the mirrors. The reason for the observed stability for PLT is that in the active optical cavities for PLT, the laser gain medium dynamically adapts to the changes in the cavity parameters, such as mirror motion, vibration and tilting. The surprising discovery opens the door for much wider NASA mission applications, such as precision formation without tethers and unprecedented spacecraft maneuvering without any traditional fuels. Recently, we have proposed that PLT can also be used for main propulsion, propelling spacecraft to a fraction of the light velocity and may enable even interstellar manned flight in the far future.

One of the most crucial technologies for developing and implementing the proposed PLT formation flying and spacecraft maneuvering is Directed Energy (DE) technology that has evolved and matured the long-range delivery of high-power laser beams. State-of-the-art DE technology is capable of delivering powers of multi-megawatts over distances of hundreds of kilometers, with matured precision pointing and focusing capability. Various solid state lasers, which are considered to be most ideal for PLT, are capable of delivering powers over 150 kW and according to the DE community, multimega- watt powers will be achievable by solid state lasers in a decade. Recently, Dr. Latham at AFRL demonstrated ~650 kW (with a 99% output coupler) of intracavity laser power with the use of a 6.5 kW ceramic Thin Disk Laser, which now is considered to be an ideal laser system for PLT. Although, the AFRL TDL is not optimized for PLT, its intracavity power translates into ~4.3 mN photon thrust in an active cavity for PLT. That’s more than 100 times more than our previous NIAC demonstration of 35 μN. In principle, photon thrusts in the range of 1 mN – 1 N from an operational power source of 100 W – 100 kW can be powered by currently available space-based solar panels.