Ultra-low orbit remote sensing, waiting for a breathing satellite.

The remote sensing satellite is approaching the edge of the atmosphere.

Recently, TelePIX, a South Korean optical payload developer, and Bellatrix Aerospace, an Indian propulsion enterprise, announced a collaboration, planning to conduct a remote sensing demonstration in a very low Earth orbit in 2028.

According to the plan, TelePIX's optical sensors designed for VLEO will be carried on Bellatrix's air-breathing electric propulsion satellite platform and operate at an altitude of 150 to 250 kilometers.

This altitude is very low. It's so low that the satellite can observe the Earth from a closer distance, and also so low that the residual atmosphere will continuously drag it downward.

This is precisely the allure of VLEO remote sensing, as well as its biggest headache.

In the past, high-resolution remote sensing often relied on larger optical apertures, heavier platforms, and higher system costs. VLEO offers an alternative path: getting the satellite closer to the ground and trading orbital altitude for better imaging conditions.

However, the lower the orbit, the stronger the atmospheric drag. While the satellite can see more clearly, it's also more difficult to stay in orbit.

What air-breathing electric propulsion aims to solve is precisely this problem. It attempts to capture the rarefied atmospheric particles in the very low orbit, feed them into the electric propulsion system, ionize, accelerate, and eject them to counteract the atmospheric drag.

In other words, the satellite is being dragged by the atmosphere while seeking the power to maintain its orbit from the atmosphere.

This is not a brand-new technology that suddenly emerged.

The European Space Agency completed a ground ignition test of an air-breathing electric thruster as early as 2018;

Even earlier, ESA's GOCE gravity-measuring satellite also relied on electric propulsion to continuously compensate for atmospheric drag at an altitude of about 255 kilometers. However, GOCE used its own xenon propellant, and its lifespan was still limited by the propellant reserve.

The highlight of the collaboration between Bellatrix and TelePIX lies in the attempt to integrate optical payloads, low-drag satellite platforms, air-breathing electric propulsion, attitude control, and orbit maintenance into the same VLEO remote sensing mission.

If this system works, the competition among remote sensing satellites may gain a new dimension: who can operate stably in a lower and more challenging orbit for a long time.

1. Why do satellites fly to lower orbits?

VLEO is the abbreviation of Very Low Earth Orbit, usually translated as Very Low Earth Orbit.

There is no completely unified boundary for its altitude range in the industry, but generally, low Earth orbits below the traditional low-orbit altitude, especially those below about 450 kilometers, are included in the VLEO discussion scope.

The altitude planned for demonstration by Bellatrix and TelePIX this time is even lower, concentrated in the range of 150 to 250 kilometers.

This area is quite special. It still belongs to orbital flight, not the activity range of airplanes or near-space vehicles; but it's not as far from the atmosphere as the traditional low orbit.

When a satellite operates there, it will be continuously affected by the rarefied atmosphere. Drag, disturbances, and atomic oxygen erosion will become more prominent.

The main reason why remote sensing satellites are eyeing VLEO is resolution.

Remote sensing imaging is essentially observing the Earth from space.

For an optical system of the same size, the closer it is to the ground, the easier it is to obtain a higher ground resolution. To improve the imaging ability of traditional high-resolution remote sensing satellites, larger optical apertures, more complex opto-mechanical systems, and more stable platforms are often required, which will increase the satellite's weight, cost, and development difficulty.

VLEO provides an alternative engineering approach. It doesn't solely rely on making the payload larger but instead reduces the orbital altitude to enable the same payload to obtain better imaging conditions.

For commercial remote sensing companies, there's a cost-benefit analysis behind this. How high a resolution a satellite can provide, how many areas it can image per day, how many customers it can serve, the manufacturing and launch costs, how long it can operate in orbit, and how often the constellation needs to be replenished will all affect the final business model.

If VLEO allows a smaller payload to achieve a higher resolution, there's room to reduce the manufacturing cost of a single satellite; if a lower orbit results in a shorter communication link, the data downlink and response speed may also improve; if the satellite can re-enter the atmosphere naturally more quickly after failure, it can also reduce the risk of long-term retention of space debris.

These advantages have made VLEO regain the attention of commercial space companies, defense agencies, and research teams in recent years.

It's not just a concept of a "lower orbit" but rather a search for a new balance point among high-resolution remote sensing, low-cost platforms, rapid network replenishment, and space safety.

However, this approach can't only focus on the benefits.

The allure and trouble of VLEO stem from the same point: the orbital altitude is low enough, which improves the imaging conditions but also increases the difficulty of platform survival.

At the traditional low-orbit altitude, satellites are also affected by atmospheric drag, but the drag is relatively weak, and most missions can maintain their lifespan through orbit design and limited propellant.

At an altitude of 150 to 250 kilometers, the density of the residual atmosphere increases significantly, and the satellite will be continuously decelerated during each orbit. Without sufficient thrust compensation, the orbital altitude will continuously decrease, and eventually, it will re-enter the atmosphere.

Therefore, VLEO remote sensing isn't simply "lowering the satellite a bit".

Once the orbital altitude is reduced, the entire system needs to be redesigned.

The satellite's shape needs to reduce drag, the propulsion system needs to be able to operate continuously, the attitude control needs to handle stronger disturbances, the materials need to withstand atomic oxygen erosion, the power system needs to support long-term propulsion, and the payload needs to maintain stable imaging in a more complex thermal and force environment.

What Bellatrix and TelePIX need to verify is precisely this boundary.

If a satellite is simply sent to a very low orbit for short-term imaging, it doesn't count as a real breakthrough.

What truly matters is whether the satellite can continuously maintain its orbit at this altitude, stably control its attitude, ensure the imaging quality of the payload, and transform these capabilities into replicable platform and constellation solutions.

For the remote sensing industry, the core issue of VLEO has never been "whether it can fly there".

The real question is whether it can operate there stably and economically for a long time.

2. What exactly is an air-breathing remote sensing satellite "breathing"?

The term "air-breathing remote sensing satellite" may easily make people think of an aircraft engine.

However, it's not the same as an aero-engine.

An aircraft engine takes in air within the atmosphere and burns it with fuel to generate thrust.

An air-breathing electric propulsion satellite still flies in space orbit. It's just that the very low orbit altitude it's in is already close to the edge of the upper atmosphere.

There's no air in the ordinary sense there, and it can't rely on oxygen combustion like an airplane, but there are still extremely rarefied residual atmospheric particles.

What it needs to do is something counterintuitive: send the rarefied atmosphere that originally slows down the satellite back into the propulsion system.

When the satellite flies in a very low orbit, an air intake device is installed at the front end or the windward side to capture the residual atmospheric particles in the orbit.

After the captured particles enter the propulsion system, they are ionized and accelerated by the electric propulsion device and then ejected at high speed from the tail to generate thrust. This thrust is used to counteract the atmospheric drag and help the satellite maintain its orbital altitude.

The satellite doesn't inhale ordinary air or oxygen fuel but rather the rarefied molecules and atoms in the upper atmosphere that are almost invisible. It's not for combustion but to provide something for the electric propulsion system to accelerate and eject.

Traditional electric propulsion satellites usually need to carry their own propellants, such as xenon, krypton, iodine, etc. Once the propellant is consumed, it's difficult for the satellite to continue maintaining its orbit, adjusting its attitude, or extending its mission lifespan.

Air-breathing electric propulsion aims to change this. Since there's residual atmosphere in the very low orbit itself, can the satellite carry less or even no traditional propellant and instead "source materials" continuously during flight?

For remote sensing satellites, the significance of this technology is obvious.

The allure of VLEO remote sensing is that the satellite is closer to the ground. Closer means better imaging conditions and may also mean a smaller optical payload and lower platform cost.

But closer also means stronger atmospheric drag. Without continuous propulsion, it's difficult for the satellite to operate at this altitude for a long time.

The traditional method is to carry more propellant. However, more propellant means that the storage tank, pipelines, and related systems will all increase in weight, occupy satellite space, and also squeeze the payload, power supply, and other platform equipment.

Especially for small satellites and commercial remote sensing constellations, every kilogram of mass is related to the manufacturing cost, launch cost, and platform design.

The allure of air-breathing electric propulsion lies in its attempt to turn the disadvantages of VLEO into advantages.

A lower orbit brings stronger drag but also more capturable residual atmospheric particles. As long as the air intake efficiency, propulsion efficiency, and system power can be matched, the satellite has a chance to maintain its orbit for a longer time in a very low orbit.

However, this is only an ideal state.

In reality, there are many difficulties.

The air intake device must capture enough particles without significantly increasing drag; the propulsion system needs to generate a "net gain" and not cause the satellite to fall faster due to the increased drag from the air intake duct and platform shape; the density of the upper atmosphere is also affected by solar activity, geomagnetic disturbances, day-night changes, and orbital position, and the propulsion system must adapt to the constantly fluctuating environment.

In addition, the composition of the inhaled "air" is complex and different from the xenon commonly used in traditional electric propulsion. The thruster needs to adapt to atmospheric components such as oxygen and nitrogen and also deal with the material erosion problem caused by atomic oxygen.

Therefore, the real difficulty of an air-breathing remote sensing satellite lies in whether it can integrate air intake, propulsion, low-drag configuration, and orbit control into a reliable system.

For Bellatrix and TelePIX, if the 2028 mission is successfully implemented, what it needs to prove is whether a VLEO satellite equipped with a remote sensing payload can fly at the edge of the atmosphere, maintain its orbit with the rarefied particles in the environment, and continuously obtain commercially valuable image data.

This is what the industry truly cares about.

3. An old concept waiting for a commercial mission verification

Air-breathing electric propulsion sounds new, but strictly speaking, it's not a newly emerged technical concept.

What's truly new is that it's moving from concept research, ground verification, and government projects to the systematic practice of commercial remote sensing missions.

Very low orbit flight itself isn't unfamiliar.

ESA's GOCE gravity-measuring satellite once operated at an altitude of about 255 kilometers. This altitude is very low, and the satellite needed to continuously counteract atmospheric drag to maintain a stable orbit.

GOCE's approach was to use an electric propulsion system and continuously compensate for drag through the carried xenon propellant. It proved one thing: as long as the propulsion and control systems are reliable enough, a satellite can operate in a very low orbit for a long time.

However, GOCE also left another problem - traditional electric propulsion still can't avoid the propellant lifespan.

The amount of propellant a satellite carries determines how long it can compensate for drag. Once the propellant is consumed, the mission lifespan will be limited.

For scientific exploration missions, this is acceptable; but for commercial remote sensing constellations, if each satellite needs to rely on a large amount of propellant to maintain very low orbit operation, the cost calculation will be very difficult.

The concept of air-breathing electric propulsion emerged in this context.

The European Space Agency announced in 2018 that it had completed a ground ignition test of an air-breathing electric thruster. This test verified the possibility of using rarefied atmosphere as a propulsion working medium.

That is to say, a satellite doesn't necessarily have to rely entirely on its own propellant. It's also possible to capture environmental particles during very low orbit operation to provide a working medium for the electric propulsion system.

Since then, some European research projects, relevant programs of the US Defense Advanced Research Projects Agency, and many startup companies have started to layout around VLEO and air-breathing electric propulsion.

However, so far, this technology can't be said to be mature.

Because air-breathing electric propulsion isn't just a problem of a single engine; it must be designed together with the satellite platform.

Proving that the thruster can be ignited on the ground is still a long way from long-term in-orbit operation. Short-term flight in orbit also doesn't mean it's suitable for commercial constellations.

The truly valuable verification is whether the satellite can continuously capture atmospheric particles at a low altitude, stably generate thrust, counteract drag, and at the same time ensure the normal operation of payload imaging, attitude control, power thermal control, and data services.

This is also the special feature of the collaboration between Bellatrix and TelePIX.

Bellatrix plans to develop a VLEO satellite platform around air-breathing electric propulsion, and TelePIX provides optical sensors designed for the VLEO environment.

The combination of the two means that the mission goal has moved further from "verifying technical components" to "verifying remote sensing applications".

For commercial space, the value of a technology can't be judged solely by laboratory parameters. It ultimately needs to return to the mission scenario: whether it can operate stably, whether it can reduce costs, and whether it can deliver data products that customers are willing to buy.

Air-breathing electric propulsion shouldn't be written as a suddenly emerged black technology. It's more like an old proposition waiting for commercial mission verification: the principle has been proven feasible in the past, and now it needs to prove that the system is usable and the operation is cost-effective.

4. Global layout is divided into three routes

Air-breathing electric propulsion and VLEO remote sensing aren't only being watched by Bellatrix and TelePIX.

In the past few years, European space agencies, the US defense research system, commercial space companies, and some startup teams have all been exploring this route. However, each has a different entry point. Some are solving basic verification problems, some are working on platforms, and some are developing thrusters.

From a global perspective, this route hasn't reached the large-scale commercialization stage yet, but it has gradually moved from papers, laboratories, and conceptual schemes to the eve of in-orbit demonstration.

The first route is government and institutional verification.

ESA has long been concerned about the problem of long-term flight in very low orbits. The GOCE gravity-measuring satellite is an important reference. It once operated at an altitude of about 255 kilometers and continuously counteracted atmospheric drag through electric propulsion to complete the high-precision Earth gravity field measurement mission.

GOCE used traditional electric propulsion and its own xenon propellant and isn't an air-breathing electric propulsion satellite, but it proved a key premise: as long as the propulsion and control systems are reliable enough, a satellite can operate stably in a very low orbit.

Later, ESA promoted the ground verification of air-breathing electric propulsion. In 2018, ESA announced the completion of the ignition test of an air-breathing electric thruster, verifying the feasibility of using upper atmospheric molecules as a propulsion working medium.

This node is very important because it pushed the idea of "using orbital residual atmosphere to compensate for drag" from theory to engineering verification.

The US direction has more of a defense and space security background.

DARPA's Otter project focuses on long-term operation in VLEO, with goals including the development and demonstration of air-breathing electric propulsion technology.

Very low orbits have a natural allure for defense missions. They can provide closer observation conditions and also have the orbital characteristic of faster natural re-entry, which has special value in some missions that are sensitive to timeliness, concealment, rapid network replenishment, and space security.

The significance of such government and institutional projects lies in pushing the technical risks forward. They don't necessarily directly prove the business model but can first answer basic questions such as "whether the principle is feasible, whether the system can work, and whether the orbital environment can be controlled".

The second route is the platform company route.

Redwire is one of the representative companies on this route in the US. Its proposed SabreSat platform is designed for VLEO missions and emphasizes a low-drag shape and long-term operation ability