Low Earth Orbit (LEO) is no longer an empty space. Orbital Mobility has become a critical capability for satellite constellations operating in an increasingly crowded environment. The number of satellites and space debris has grown exponentially. Therefore, for satellite constellations, the ability to quickly change course and avoid potentially dangerous close encounters is becoming increasingly important. It is especially important for nanosatellites. Hence, the decision to manoeuvre often has to be made almost immediately. Because there is usually not enough time available between the warning and a potential collision.
A 6U CubeSat typically weighs around 12 kg. Avoiding a dangerous close approach depends on orbital calculations and on how quickly it can execute a manoeuvre command, which directly defines its orbital mobility capabilities. Slow reaction or low thrust leaves almost no time. Thus, when selecting a propulsion system, it is important to consider its practical effectiveness:
- startup speed
- thrust reserve
- safety
- compliance with mission requirements
For this reason, water-propulsion systems are particularly appealing. They provide small satellites and CubeSats with manoeuvrability without unnecessary risks or complications regarding launch clearance. Further, we examine the key characteristics of a water-propulsion system and show why it could be a practical solution for orbital mobility.
Space Sustainability Standards, CAM Distance and Orbital Mobility Timeline
Satellite operators activate pre-established safety protocols when the risk of a dangerous close encounter in LEO exceeds the acceptable threshold Pc > 10-4. They assess the situation according to strict criteria:
- Miss Distance: to avoid collision, the operator has to determine in advance how close the two objects will pass each other. Also, whether the satellite will have time to change its trajectory. At the Time of Closest Approach (TCA), the safety radius between the centres of mass of the two encountering objects must be at least 1–2 km.
- Trajectory Displacement: the satellite doesn’t need to change the orbit; the small manoeuvre is usually enough. Operators must alter the satellite’s path by 200 to 500 meters relative to its original trajectory.
- The Planning Window: these situations shouldn’t be resolved at the last moment. The first warnings of a possible collision usually appear a few days in advance. Approximately 3 to 7 days before TCA. Operators make the final decision on the manoeuvre closer to the event, 24–36 hours in advance. The thruster itself is usually activated 12–24 hours before a dangerous close approach, so that the satellite has time to gradually move to a safe distance.
Propulsion Comparison: Finding the Best Thruster for Orbital Mobility in Small Satellites
For a 12 kg 6U CubeSat, the time it takes for the thrust to provide the nominal velocity change for a quick evasion manoeuvre (Delta V = 0.2 m/s) depends on the type of propulsion system used. The effectiveness of each propulsion technology directly impacts orbital mobility and the ability of a spacecraft to perform timely collision avoidance maneuvers.
Propulsion Performance Overview
1. Chemical thrusters deliver the highest thrust level (~1000 mN). This thrust level enables an extremely fast response — a Δv of 0.2 m/s is achieved in just ~2.4 seconds. These systems are highly responsive in emergency windows. They are capable of reacting up to 1–2 hours before TCA. However, the trade-off is complex integration: the propellants are hazardous and highly volatile, which complicates their use in a 6U satellite.
2. Cold gas systems offer low thrust (~10–50 mN). They require 48 seconds to 4 minutes to reach a Δv of 0.2 m/s. These thrusters remain robust in emergency scenarios, responding up to 3–4 hours before TCA. Safety is their main advantage. However, they are highly inefficient and consume a large amount of internal payload volume. It is a significant drawback for compact CubeSats.
3. The SteamJet TunaCan produces ~20 mN of thrust. Our thruster delivers a Δv of 0.2 m/s in ~2 minutes. It is dependable in emergency windows, responding up to 4–5 hours before TCA. Its standout benefit is the 0U design footprint. Zero internal volume impact makes it ideal for space-constrained missions.
4. Electric propulsion (FEEP/Ion) provides the lowest thrust (~0.4–1.2 mN). This kind of thruster needs 33 minutes to 1.6 hours to reach a Δv of 0.2 m/s. As a result, FEEP/Ion systems are ineffective for late-notice or urgent threats. On top of that, it imposes a high electrical power drain, forcing slow, multi-day tracking rather than rapid maneuvers.
Chemical thrusters respond quickly to imminent collision threats. Whereas electric engines (FEEP/Ion) are too slow for emergency maneuvers. The SteamJet TunaCan system offers the best balance of safety, response speed, and internal space efficiency. And it does not take up any usable space on the satellite.
Emergency vs. Nominal Manoeuvres: The Reality of Orbital Risks
There are two categories of risk collision based on the analysis of manoeuvres in low Earth orbit. These scenarios demonstrate why orbital mobility is becoming an essential requirement for modern satellite operations.
1. Nominal/Planned Manoeuvres, 95%–97% of cases
In most of the cases, the operator receives a warning in advance. Therefore, the satellite has enough time to avoid collision. Under these conditions, high-thrust systems and water-based electrothermal thrusters produce nearly identical practical results.
2. Emergency Manoeuvres, ~3%–5% of cases
Situations like this happen when there is almost no time left to react. Several reasons may lead to these situations. For example, radar blind spots, solar activity spikes expanding the upper atmosphere, or sudden data revisions in Conjunction Data Messages (CDMs) less than 24 hours before TCA.
The TunaTank Advantage: Next-Generation Water Propulsion Technology
Although electric propulsion (FEEP) has a high specific impulse, its thrust is in the micro-newton range. Hence, it is not well-suited for rapid manoeuvres. The corrections have to be planned days in advance. On top of that, the system consumes valuable power from the onboard batteries in the process. Conversely, chemical thrusters present additional challenges. Primarily in terms of safety during storage, launch, and operation.
The TunaTank Thruster is our new water-based propulsion system. We developed it based on our experience with the Steam TunaCan. The system supports small satellites and gives them more capabilities without unnecessary risks. In fact, TunaTank offers a perfect balance between effectiveness and safety.
- Clean Propulsion Technology: runs on water, a non-toxic, “green” propellant stored at low pressure.
- Space Qualified Reliability: designed according to ESA ECSS and NASA GSFC standards.
- Flight-Proven 3D Printing: custom printed manifolds allow the system to be adapted to the layout of a specific CubeSat.
- Uncompromised Volume: The external scalable architecture preserves the satellite’s internal volume for the payload.
About SteamJet Space Systems
SteamJet Space Systems is a leading UK-based provider of high-performance satellite propulsion solutions. We specialise in water-based propulsion solutions designed specifically for CubeSats and Small Satellites (SmallSats), prioritising operational safety and rapid launch integration.
By pioneering the use of green propellants and intelligent thermal engineering, SteamJet enables complex LEO (Low Earth Orbit) manoeuvres — including orbital maintenance, collision avoidance, and de-orbiting — without the risks associated with toxic hydrazine or high-pressure cold gas systems, advancing green propulsion for space missions.
Steamjet Propulsion Technology
Our modular systems are engineered for seamless integration and maximum safety compliance:
Steam TunaCan Thruster: A compact, high-efficiency solution for 1U-3U CubeSats.
Steam TunaTank Thruster: A safe, high-performance electrothermal propulsion system.
Steam Thruster One: Scalable propulsion for larger SmallSat constellations.
Discover how SteamJet’s sustainable space propulsion innovations are providing the safety and reliability required for the next generation of crewed and robotic missions. Contact our engineering team for technical specifications and ICDs.