Light Push

What Light Push Actually Is

Light Push is the practice of using photon momentum to generate force without any propellant. A photon has zero rest mass, but it carries momentum equal to its energy divided by the speed of light. When you reflect or absorb photons on a surface, that momentum transfer creates a tiny push. It sounds like science fiction until you remember that space is big, time is long, and even a billionth of a newton of acceleration adds up over years. Before you design anything around Light Push, you need to understand the basic equation. The force from a perfectly reflecting surface is F = 2P/c, where P is the optical power in watts and c is the speed of light. For a perfectly absorbing surface it's just P/c. That means a 100-watt laser hitting a mirror gives you roughly 0.67 microNewtons of force. Not a lot. But a 1-kilogram satellite with that force accelerates at about 0.7 nanometers per second squared. Over a year, that's a velocity change of roughly 22 meters per second. That number matters more than the force does. The practical path usually goes like this. You pick your photon source. Then you size your reflective or absorptive surface. Then you figure out the pointing requirements, because the beam spreads and the target moves. Then you simulate the trajectory over whatever timescale you're working with. Then you build a breadboard and find out how much your alignment shifts when the thermal environment changes.

I worked on a project where we used a compact laser diode array pushing against a small deployable film reflector. The math was fine on paper. The real problem showed up during thermal cycling. The laser housing expanded by about 0.1 millimeters, which shifted the beam pointing by roughly 2 milliradians. That caused the radiation pressure force vector to drift off the satellite's center of mass by enough to induce an unwanted torque. We ended up mounting the entire laser assembly on a thermally compensated Invar bracket and added a fine steering mirror with closed-loop feedback from a star tracker. The system worked, but the fix added about three weeks and another two thousand dollars in parts we hadn't budgeted for.

The Physics Behind It

Photon momentum comes from the electromagnetic field itself. The Poynting vector describes the energy flow, and the radiation pressure is just the momentum flux hitting a surface. For a collimated beam, the intensity at distance r from a point source drops as 1/r², but a well-collimated laser maintains its intensity much longer before diffraction takes over. The diffraction-limited spot size is roughly theta = lambda/D, where lambda is the wavelength and D is the aperture diameter. That angle determines how much your beam spreads and how much power lands back on your target kilometers away. There are two main categories people talk about. Ground-based Light Push uses a fixed or airborne laser platform to push a spacecraft. Space-based Light Push puts the laser on a chaser vehicle or a dedicated relay satellite. Each has different geometry constraints. Ground-based systems fight through atmospheric turbulence, which can degrade beam quality by an order of magnitude depending on seeing conditions. Space-based systems avoid that but need their own propellant or power budget to maintain position relative to the target. A counter-intuitive point that most people miss: a reflective surface actually gets twice the momentum transfer compared to an absorptive one, but the absorbed power turns into heat. On a small spacecraft, thermal management can be the real bottleneck, not the thrust level. I've seen designs where the radiation pressure calculation looked great on a whiteboard, and then someone forgot that the absorber side of a hybrid reflector would melt the underlying composite panel at power levels above about 50 watts per square centimeter. That should have been obvious. It wasn't obvious until we ran the thermal model.

Common Implementation Approaches

Solar sails are the most mature Light Push implementation. They use sunlight rather than an artificial source. IKAROS demonstrated this back in 2010, and the more recent LightSail 2 project showed controlled attitude and orbit raising using only solar radiation pressure. The advantage is that you don't need any power system or laser hardware. The disadvantage is that solar irradiance drops with the square of the distance from the Sun, so you're limited to roughly 9 microNewtons per square meter at Earth's orbit. Laser-powered light sails were popularized by the Breakthrough Starshot concept, which proposes using a ground-based megawatt-class phased array to accelerate gram-scale probes to 20 percent of the speed of light. The engineering challenges here are enormous. Beam steering over thousands of kilometers, maintaining coherence across a megawatt phased array, managing the thermal load on the sail material, and dealing with the fact that any small misalignment causes the sail to yaw and lose coupling with the beam. That last point is critical and often underappreciated. A phased array light sail system needs active beam-pointing control on milliradian timescales, or the sail simply rides the edge of the beam and never gets the acceleration you calculated. Optical tweezers are a completely different scale application of the same physics. They manipulate microscopic particles in the lab using focused laser beams. The gradient force pulls particles toward the highest intensity region, and the scattering force pushes them along the beam direction. If you're working at the microscale, Light Push becomes a precision tool rather than a propulsion method. The force regimes are similar in principle but different by about 30 orders of magnitude compared to spacecraft applications.

Simulation and Modeling Tools

For preliminary trajectory analysis, you can build a simple propagator in Python using the basic radiation pressure equations. The key variables are the optical power, the effective area of your collector or reflector, the reflectivity coefficient between 0 and 2, and the cosine of the angle between the beam and the surface normal. Most open-source orbital mechanics libraries like poliastro or Orekit don't have built-in Light Push models, so you'd add the perturbation as a custom acceleration term. A basic script with a runge-kutta integrator can give you trajectory estimates in minutes for low-fidelity work. For higher fidelity work, especially if you're dealing with articulated deployable structures or attitude coupling, you need a coupled orbital-attitude simulator. The tradeoff is complexity. A fully integrated model where the orbit affects the pointing, and the pointing affects the force, and the force affects both orbit and attitude, converges slowly and is sensitive to initial conditions. I found that decoupling the problems solved most of the convergence issues. Run the orbital propagation independently, feed the resulting geometry into the attitude simulation, and iterate. It's approximate but fast enough for design space exploration.

Pitfalls and Where This Approach Fails

Light Push doesn't work well when you need high thrust over short durations. The acceleration is inherently small, and scaling up the power introduces thermal and structural problems that grow faster than the benefit. If your mission requires an impulsive delta-V maneuver, use chemical or electric propulsion instead. Light Push is best suited for continuous low-thrust operations where the mission profile can absorb the gradual velocity change. Atmospheric propagation is another hard limitation for ground-based systems. Even with adaptive optics, the residual wavefront error limits the concentration of power on a moving target. The coherence length and isoplanatic angle impose hard constraints on what kind of telescope aperture and laser wavelength combination will work. You can't engineer past the physics here. If your target is above the atmosphere, these constraints disappear, and the system becomes viable again. Beam diffraction sets a fundamental limit on how much power you can concentrate at range. A 1-meter aperture laser at 1 micron wavelength produces a divergence angle of about 1.22 microradians. At a range of 100 kilometers, that beam is roughly 24 centimeters across. At 1,000 kilometers, it's about 2.4 meters. If your sail is smaller than the beam, you're wasting power. If it's larger, you're not using the full aperture efficiency. There's an optimal sail size for any given range and wavelength, and it's usually smaller than people expect.

Another issue worth mentioning is the recoil on your source. If you're bouncing a laser off a target and the reflection is specular, the momentum transfer to your laser platform matters. A 1-kilowatt laser emitting a beam and then reflecting it back from a distant sail transfers about 6.7 microNewtons of recoil force. For a small spacecraft hosting the laser, that's a non-negligible perturbation on its own orbit. It's small, but in a precision formation flying scenario, it can couple into attitude errors that feed back into the pointing accuracy.

Resources and Where to Look Next

The foundational papers on solar sail dynamics date back to the 1970s and 80s, but the modern treatments are more accessible. The NASA Technical Standards for solar sail systems and the ESA's documentation on the OpenSatelliteArchitecture projects both cover the modeling aspects. For laser-based approaches, the literature is less organized because it spans aerospace, optics, and plasma physics communities. ArXiv papers on phased array beam steering and laser communication links contain relevant technical detail, even if they're not published under the Light Push label. If you want to start building something, begin with a simulation. Don't buy hardware until you've verified that your assumed performance numbers match what a first-principles model produces. I've seen too many teams order custom optical components only to discover later that their initial assumptions about beam quality and pointing stability were off by a factor of five. The hardware is not cheap once you get into laser-grade optics and precision deployment mechanisms. The field is moving slowly but steadily. New lightweight polymer films with measured reflectivity above 0.95 across visible and near-IR wavelengths have made sail designs more practical. Laser diode arrays are becoming more powerful and more efficient. The component costs are dropping. But the core physics hasn't changed, and neither have the fundamental tradeoffs between power, mass, pointing accuracy, and achievable acceleration.