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This answer to How low is VLEO? (FCC's newest approval for SpaceX) suggests VLEO begins (or ends I guess) at 350 km.

The two bad things I know about in VLEO are an enhanced rate of altitude loss due to atmospheric drag and the effects of atomic oxygen (O atoms instead of O2 molecules) which are very reactive with many standard spaceflight materials that work well at higher altitudes. Satellites that use VLEO usually have one or more characteristics:

  1. They are in elliptical orbits with a much higher apoapsis. Drag will generally first circularize an orbit before bringing it in to burn up in the atmosphere, so energy stored by having a high apoapsis can buy you time by stabilizing the periapsis even though the semimajor axis steadily drops. (e.g. some spy satellites)
  2. They are expendable, their missions are short lived. (e.g. most cubesats)
  3. They have (electric) propulsion so they can fight the drag for the duration of their mission.

Question: What technologies enable or at least help satellite operation in Very Low Earth Orbit (VLEO)? Obviously electric propulsion is one category, but are there specific types that are more VLEO-enabling than the "run of the mill" COTS xenon thruster? Are there new materials that are VLEO-enabling because of their resistance to atomic oxygen? Special components or alternative designs that deviate from those used in regular LEO that reduce drag?


Background:

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Yes, there are a number of different technologies that I'm aware of that are being developed specifically to enable and assist satellites in operating sustainably in lower altitude orbits. The DISCOVERER project (European Horizon 2020) has the aim of developing some of these foundational technologies and Skeyeon, an emerging US company, also has some patents in this area.

Materials for Orbital Aerodynamics

The atmosphere in VLEO is highly rarefied and aerodynamics in VLEO are principally governed by the interactions that happen directly between the incident gas particles and spacecraft surfaces. These gas-surface interactions (GSI) are dependent on the surface and gas-particle properties. If significant energy or momentum exchange occurs at the surface the drag experienced will be generally high. However, if the energy or momentum exchange at the surface can be reduced, then drag can be reduced increasing the orbital lifetime and reducing the requirements for propulsive drag compensation.

The composition of the atmosphere in VLEO is also characterised by a relatively high concentration of atomic oxygen, a reactive gas species that can adsorb onto surfaces causing contamination and also cause surface erosion. Both of these processes cause typical materials in the VLEO environment to generate the experienced drag.

Research into new materials is currently being performed to identify those that are ideally resistant to surface adsorption and erosion by atomic oxygen, and that have GSI properties that can reduce the drag. Some interesting candidates include self-passivating polymers, thin-oxide coatings, nanocomposites, and other 2D materials. However, as of yet, these materials have not been tested in orbit or fully characterised under similar environmental conditions. See:

To achieve the reduction in drag, these materials also have to be combined with appropriate designs or concepts for satellites that have surfaces with shallow incidence angles with respect to the oncoming flow.

Atmosphere-Breathing Electric Propulsion (ABEP)

Whilst any improvement in electric propulsion will assist drag compensation in VLEO, the volume/mass of propellant launched with the satellite will still limit the lifetime unless additional propellant can be delivered whilst in orbit or the satellite is re-boosted (like the ISS]7). Furthermore, at very low altitudes the thrust required to compensate for drag increases significantly and the trade between thrust, specific impulse, and power becomes even more problematic.

ABEP systems propose to collect the particles from the residual atmospheric flow and use it as propellant in an electric thruster, eliminating the need to carry on-board propellant. Key technological issues include the design of the atmospheric intakes that can efficiently collect and trap the rarefied atmospheric particles for use as propellant and the design of electric thrusters that can accommodate varying mass-flow rates. The thrusters must also have sufficient lifetimes whilst using the collected atmosphere propellants, which is a challenge due to the erosion of electrodes that can occur.

Some interesting recent developments include the first on-ground demonstration of the RAM-EP prototype by ESA and the ignition of the Helicon-based Inductive Plasma Thruster of IRS at Stuttgart University that is contactless (i.e. no acceleration grids, electrodes, neutralisers) and does not suffer the issues of erosion.

Aerodynamic Attitude and Orbit Control

The aerodynamic forces and torques on satellites are often considered a disturbance. However, if utilised correctly, they could help to perform attitude or orbit control manoeuvres that reduce the requirements on conventional attitude and orbit control systems. Examples of orbit control include constellation maintenance, formation flying, rendezvous, and atmospheric re-entry interface targeting. Attitude control concepts include pointing, momentum management, and trim. Presently, these control methods are generally limited by the low aerodynamic lift-to-drag ratio of conventional materials. However, there is an extensive range of literature on these different concepts.

Differential drag based manoeuvres have been employed in orbit before, for example for deployment of elements of the Planet Labs constellation and in demonstration of relative formation maintenance by the AeroCube-4 mission. MagSat also demonstrated use of aerodynamic control to perform some trim and momentum control of reaction wheels.

Atmospheric Sensors

The implementation of both ABEP and aerodynamic control would benefit from new sensors that can provide information on the in-situ atmospheric density, composition, and velocity of the oncoming flow. Such sensors for rarefied flows do not currently have the combination of sensitivity and response to be used "in-the-loop" for these applications.

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