Selected Contribution 08 | Simulation of Aerodynamic Characteristics in Extremely Low Earth Orbit: The “Fifth Airspace” in the Aerospace Field Is Being Opened Up

Editor’s Note

In the place closest to Earth, space is not silent.

From the resistance dilemma of “molecular bumper cars” to the innovative concept of “eating air while flying,” simulation technology is becoming the digital compass for unlocking the uncharted domain of extremely low Earth orbit.

As the unknown gradually becomes foreseeable, the boundaries of aerospace are quietly extending closer to Earth.

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At an altitude of approximately 100 kilometers above the Earth’s surface, there exists a unique region of space.

It is closer to Earth than traditional low Earth orbit, yet almost no satellites can remain there for extended periods. The reason is not complicated—

Air.

Although its density is only one hundred-billionth of that at sea level, the gas molecules here continuously collide with spacecraft at a relative velocity of 8 km/s. Without sustained propulsion compensation, a spacecraft will rapidly descend at a rate of several to tens of kilometers per day, eventually plunging into the atmosphere and burning up.

This region is known as Extremely Low Earth Orbit (ULEO, 100–250 km).

It is the closest orbital region to Earth, yet it has long maintained a rare “silence”, and is considered one of the last uncharted domains in near-Earth space.

“The Fifth Airspace”: A Near-Earth Blue Ocean with Immense Potential

Figure 1: Artistic conception of an ultra-low orbit space vehicle. AI-generated image. Source: Official website of the Chinese Academy of Sciences.

In recent years, with the rapid development of aerospace technology, humanity has been continuously advancing into deeper and more distant regions of space. However, in the space closest to the Earth’s surface, there remains a vast “blue ocean” waiting to be explored.

This is the extremely low Earth orbit (ULEO), with an orbital altitude range of 100–250 km.

Within this orbital domain, satellites can achieve 0.1-meter resolution remote sensing imaging, millisecond-latency communication transmission, and ultra-high-precision scientific detection, all with ultra-low payload technology requirements and economic costs.

Due to these characteristics, the extremely low Earth orbit(ULEO) is known as the “fifth airspace,” following the airspace of aviation, outer space, near space, and low-altitude airspace.

However, unlike the traditional low Earth orbit space, which has become increasingly crowded in recent years due to the accelerated deployment of mega-constellations such as Starlink, the ULEO—this “golden fertile land”—remains remarkably silent. So, what is the “behind-the-scenes culprit” hindering human exploration?

A Large Amount of Air Continuously “Collides” with Satellites

If we zoom in to the microscopic level, we will find that the seemingly silent extremely low Earth orbit is not calm.

Although the air here is extremely thin compared to sea level, its density is more than 1,000 times that of the low Earth orbit atmosphere. A large number of gas molecules continuously strike the satellite’s surface at a relative velocity of approximately 8 km/s, much like a relentless game of “molecular bumper cars.”

Figure 2: Schematic diagram of aerodynamic drag generated by high-speed impact of rarefied atmospheric molecules on a spacecraft. Source: Acta Aeronautica et Astronautica Sinica (Chinese Journal of Aeronautics).

For comparison, the density of water is about 800 times that of air at sea level.. Satellites that can freely “run” in low Earth orbit, once in extremely low Earth orbit, are like drowning, rapidly sinking at a rate of several to tens of kilometers per day.

The chaotic incoming gas flow also continuously disrupts satellite attitude and raises its surface temperature, further accelerating the decay of orbital altitude. Without propulsion, satellites operating in the extremely low orbital domain will fall into the atmosphere and burn up within a few days.

The turbulent sea of gas molecules has created the silence of the extremely low orbital domain. The design principles of traditional spacecraft and aircraft are no longer applicable here, and there is currently no precedent for long-term residence in extremely low Earth orbit.

Building a “Digital Wind Tunnel” in the Computer

In the expedition to conquer this ocean, simulation technology has become the most reliable vessel. Unlike common continuous flow fields, the simulation of rarefied flow field characteristics in extremely low Earth orbit requires the use of particle-based simulation methods such as Direct Simulation Monte Carlo (DSMC) and Test Particle Monte Carlo (TPMC), constructing a “digital wind tunnel” within the virtual world.

This “wind tunnel” tracks and records the motion and collision trajectories of millions or even hundreds of millions of gas particles, simulating the high-speed, neutral, rarefied flow environment that is extremely difficult to reproduce in ground-based experimental facilities, and characterizing the effects of collisions between gas molecules and satellites.

This “digital wind tunnel” does not operate in isolation. It interfaces with high-precision space atmospheric environment prediction models based on long-term observations of solar and geomagnetic activity indices, and incorporates gas–material surface interaction models to accurately describe the collision process between gas molecules and satellite surfaces. This enables researchers to systematically investigate the aerodynamic characteristics of satellites in this uncharted domain.

Through cross-regime, multi-scale simulations of rarefied flow fields, researchers can determine the aerodynamic drag experienced by satellites under different material, configuration, and layout schemes, thereby carrying out the design and optimization of low-drag configurations suitable for the special flow field of extremely low Earth orbit.

Combined with simulation analyses of satellite thermal characteristics and attitude dynamics, it will also be possible in the future to accurately predict the orbital environment and aerodynamic characteristics throughout the full life cycle of extremely low Earth orbit satellites, promptly forecast potential risks, and propose solutions.

Simulation technology is becoming the most reliable “digital compass” for exploring this unknown space.

Figure 3: Schematic diagram of an air-breathing electric propulsion vehicle and its geometrically feasible region. Source: Official website of the Institute of Mechanics, Chinese Academy of Sciences.

Equip Satellites with a “Mouth”: Turn Drag into Thrust.

As understanding of the flow field environment in this orbital domain gradually deepens, an innovative concept has attracted increasing attention: “air-breathing electric propulsion (ABEP).”

The basic idea is to equip the satellite with a “mouth”—a specialized intake designed through flow field simulation analysis and configuration design.

A satellite equipped with this device and its supporting systems no longer passively withstands aerodynamic drag. Instead, it actively inhales rarefied gas molecules through its “mouth,” supplies them as propellant to the electric propulsion system, and generates thrust through ionization, acceleration, and ejection. By using the very gas molecules that produce drag to counteract that drag, the satellite can maintain its orbit.

This “eating air while flying” approach turns a liability into an asset, offering the potential for long-term residence in extremely low Earth orbit at low cost.

Advancing into the Last Uncharted Domain of Near-Earth Space

In this journey to conquer the last uncharted domain of near-Earth space, simulation technology is no longer merely an auxiliary tool but a “pilot” charting new routes.

From aerodynamic characteristic simulation to in-orbit environment prediction, from the proposal of innovative concepts to their engineering realization, simulation technology is leading humanity into a new era of exploring, developing, and utilizing extremely low Earth orbit.

Building on the exploration and practical achievements of satellite series such as Lixing-1, Qiankun-1, and Chutian-1, and guided by the “digital compass” of simulation technology, China will precisely navigate toward the future in the blue ocean of extremely low Earth orbit—a domain full of unknowns and opportunities.

About the Author

Li Jinkuan 

Ph.D. Candidate, DFH Satellite Co., Ltd., China Academy of Space Technology

Main Research Direction: design of extremely low Earth orbit vehicles