NASA is working on photophoretic levitation to uase light-induced heating to create a force that lifts and propels specially designed structures in rarefied atmospheres, such as Earth's mesosphere (approximately 50-80 km altitude).
This involves metamaterial plates or 3D hollow geometries (e.g., cones, spheres, or rocket shapes) with porous sidewalls that act as Knudsen pumps: light (typically sunlight) is absorbed on one side, creating a temperature gradient that drives gas flow through microchannels, generating lift via a downward jet of air.
Arxiv- 3D photophoretic aircraft made from ultralight porous materials can carry kg-scale payloads in the mesosphere
Photophoretic aircraft would greatly benefit from a three-dimensional (3D) hollow geometry that pumps ambient air through sidewalls to create a high-speed jet. To identify optimal geometries, we developed a theoretical expression for the lift force based on both Stokes (low-Re) and momentum (high-Re) theory and validated it using finite-element fluid-dynamics simulations. Systematically varied geometric parameters, including Knudsen pump porosity, to minimize the operating altitude or maximize the payload. Assuming that large vehicles can be made from previously demonstrated nanocardboard material, the minimum altitude such vehicles can levitate at is approximately 55 km, while
the payload can reach approximately 1 kilogram at 80 km altitude for vehicles with 10-meter diameter. In all cases, the maximum areal density of the sidewalls cannot exceed a few grams per square meter, demonstrating the need for ultralight porous materials.
The system operates optimally at pressures of 0.1-1000 Pa, with peak performance in the mesosphere's 1-100 Pa range (50-80 kilometers of altitude), where traditional aircraft can't fly and satellites experience too much drag for stable orbits.
Planes are at 30000-70000 feet or 9 kilometers to 20 kilometers of altitude.
Low satellites can get to about 200-300 kilometers.
50-80 kilometers would be great for direct to cellphone communication networks. This would be for niche applications and enhancements for specific situations for low altitude satellites.
For Starlink-like applications (e.g., broadband internet relay):
Feasibility: Kg-scale payloads could accommodate compact communication equipment, such as antennas, transceivers, and batteries for nighttime operations (since lift requires continuous illumination).
A constellation of these could provide regional coverage at lower latencies than LEO satellites due to closer proximity to the ground. They might integrate with existing stratospheric balloon tech for trajectory control.
Challenges and Limitations: Operations are daylight-dependent (typically 12 hours/day at low/mid-latitudes, longer in polar regions), requiring energy storage or hybrid systems for 24/7 service.
Coverage would be limited by line-of-sight horizons (roughly 1,000 km radius per vehicle at 80 km), necessitating denser networks than Starlink's. Atmospheric variability (e.g., winds, turbulence) could affect stability, and deployment would rely on balloons or rockets.
This isn't true "satellite" tech but more akin to advanced drones or balloons, and regulatory hurdles (e.g., airspace management) would apply. Early prototypes are centimeter-scale with milligram payloads, but scaling to meter sizes is proposed for practical use.
They would be deployed via balloons/rockets and operational life is limited by UV degradation/material fatigue.
For a 100 kg payload: The diameter would need to be approximately 120-140 m (e.g., ~136 m for a sphere-like design). It seems like 5-10 kilogram payloads are the size of ultra large balloons that NASA has some experience. Getting up to 100 kilograms would be larger than the largest airship like vehicles and those vehicles can move 20-100 tons.