The Tool Desk
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1. Orbit screening finds possible close approaches
Operators begin with predicted trajectories for their spacecraft and the catalogued objects around them. A screening service checks whether an object is predicted to enter a protected volume around the spacecraft. Entry into that volume creates a conjunction alert: a warning that two objects may pass close to one another, not a declaration that they will collide.
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NASA says the U.S. Space Force 18th Space Defense Squadron conducts on-orbit conjunction screenings three times a day. The cadence applies to that operational screening description, not automatically to every operator or service. NASA’s conjunction-assessment overview explains the process at NASA Conjunction Assessment.
ESA describes a complementary workflow. It receives Conjunction Data Messages through a data-sharing arrangement with the United States and combines those messages with ESA’s own orbit information. Its operational predictions are updated several times daily and typically look seven days ahead. The timing can change as tracking data improves, so an alert is a living assessment rather than a fixed forecast.
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| Operational element | What the published service says | What it means for operators |
|---|---|---|
| NASA-described screening | Three screenings per day by the 18th Space Defense Squadron | Potential encounters are found repeatedly as catalog data changes; this is not a universal rate for all services. |
| ESA close-approach analysis | Several updates per day, generally seven days ahead | Mission teams may receive revised geometry, uncertainty and risk as new observations arrive. |
| Screening volume | A protected volume around the spacecraft | An object crossing the volume is a candidate threat for analysis, not automatically a dangerous collision. |
2. Analysts turn proximity into a risk estimate
A predicted miss distance alone cannot tell a team whether a maneuver is justified. Analysts combine the relative trajectories with uncertainty in each orbit and ask how often the actual miss distance could be smaller than the combined sizes of the two objects. NASA calls the resulting metric probability of collision, or Pc. Its explanation of this calculation is in Close Approach Risk Assessment.
The measurements that matter
- Probability of collision: the statistical chance implied by the current trajectory estimates and their uncertainties.
- Miss distance: the predicted separation at closest approach.
- Orbit uncertainty: how widely the real position and velocity could differ from the prediction.
- Encounter geometry: the direction and speed at which the objects pass, which affects the uncertainty calculation and maneuver options.
- Object characteristics: estimated size, mass and whether the object can maneuver or is an uncontrolled debris body.
- Consequence: the value and survivability of the protected spacecraft if a collision occurred.
ESA event records expose these same categories, including collision probability, miss distance, geometry, orbit uncertainties and information about the approaching object. A large uncertainty ellipse can make a seemingly comfortable nominal miss distance less reassuring; a well-known orbit can make a close nominal pass easier to evaluate.
Thresholds are mission policies, not a universal traffic law
Every mission’s risk authority sets tolerable levels in light of spacecraft design, mission value, fuel and operating constraints. NASA’s Conjunction Assessment Risk Analysis (CARA) guidance says Pc values above 1E-07 merit operational attention and values above 1E-04 require mitigation actions. Those are NASA CARA thresholds; they must not be treated as a rule binding every government, commercial or international operator.
| NASA CARA value | Published implication | Important qualification |
|---|---|---|
| Pc greater than 1E-07 | Operational attention | NASA CARA guidance, not a universal industry threshold. |
| Pc greater than 1E-04 | Mitigation actions required | NASA CARA guidance; another mission may use different criteria or additional consequence tests. |
Consequences can change the decision even when two events have similar Pc values. Losing a unique science spacecraft, for example, may justify a different tolerance than interrupting a lower-value demonstration mission. Analysts therefore evaluate probability, uncertainty and consequence together instead of sorting alerts by one number.
3. A candidate maneuver must pass several checks
When an event exceeds a mission’s criteria, flight-dynamics specialists develop an avoidance option. The objective is not simply to move the spacecraft in any direction; it is to create an acceptable future orbit without causing a new problem.
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- Confirm the event. Analysts review the latest tracking data, orbit solutions, covariance or uncertainty information, object identity and encounter geometry. They check whether the alert persists after an updated prediction.
- Define mission constraints. The team records available propellant, allowable thrusting times, spacecraft attitude limits, communications windows, payload activities, power and thermal restrictions, and any orbit requirements that cannot be violated.
- Generate maneuver candidates. Small changes in timing, altitude, along-track position or another permitted orbital direction are modeled. A candidate must create useful separation at the encounter time while remaining within the spacecraft’s capabilities.
- Screen the resulting orbit. The proposed post-maneuver orbit is screened against the original threatening object and all other catalogued objects over the relevant planning period. ESA says its experts screen orbit files for candidate maneuvers to verify that the target risk falls and other conjunction risks do not rise above acceptable levels.
- Check mission effects. Controllers determine whether the maneuver changes pointing, communications, imaging, science collection, formation geometry, station-keeping or later orbit operations. Fuel consumption is compared with the remaining mission budget and expected future avoidance demand.
- Obtain authority to act. A designated mission authority accepts the residual risk and approves, rejects or defers the maneuver as newer data arrives.
NASA’s Spacecraft Conjunction Assessment and Collision Avoidance Best Practices Handbook describes this as a risk-management activity that must account for the spacecraft’s mission and operational limits, not just the geometry of one encounter.
4. Operators coordinate when both objects can maneuver
If the approaching object is an active spacecraft, an uncoordinated response can make matters worse: both vehicles might maneuver into the same region, or each might assume the other will move. Teams therefore exchange event data, intended actions and timing through established contact channels whenever possible.
NASA and SpaceX illustrated one bilateral arrangement in their 2020 safety agreement. SpaceX agreed that Starlink satellites could maneuver autonomously or manually to protect NASA science missions; NASA agreed not to maneuver unless SpaceX informed it otherwise. The arrangement reduced the chance of incompatible simultaneous maneuvers. NASA summarized the principle this way: “Effective mitigation relies on inter-operator coordination, accurate data, a sound technical basis for risk analysis, as well as proactive processes for appropriate actions to mitigate risks.” The announcement is at NASA, SpaceX Sign Joint Spaceflight Safety Agreement.
That agreement is an example, not a universal binding rule. Each operator may have different contacts, response times, autonomy policies and legal responsibilities. When the object is a dead satellite, a rocket body or a fragment, there is no counterpart that can respond; the protected spacecraft must plan around the predicted path and its uncertainty.
5. The spacecraft executes, then the loop continues
After approval, controllers upload or transmit the command during a suitable communications and attitude window. Thrusters or electric propulsion alter the orbit by a deliberately calculated amount. The team then verifies spacecraft health, reconstructs the achieved orbit from tracking data and reruns conjunction screening.
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A maneuver that lowers one event’s Pc can create a different close approach days later. For that reason, operators examine follow-on conjunctions, routine maneuvers and updated catalog solutions rather than declaring success at engine cutoff. ESA says its operational process tracks analysis history for each event and screens planned routine maneuvers as part of continuing assessment. Its collision-avoidance service is described at ESA Reentry and collision avoidance.
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Catalogs and orbit predictions are extremely useful but not perfect. Atmospheric drag changes low-orbit trajectories, tracking observations have measurement errors, and an object’s identity or physical characteristics may be uncertain. As observations arrive, the predicted time, miss distance, geometry and Pc can move up or down.
- A conjunction is a predicted close passage, not a confirmed collision.
- A nominal miss distance is not the same as a guaranteed separation because both orbits have uncertainty.
- A high Pc does not automatically identify which spacecraft should move; coordination and mission authority still matter.
- A low Pc does not make every maneuver wise if fuel, mission disruption or a newly created conjunction would cost more than the modeled risk.
These distinctions explain why operators may monitor several alerts without firing thrusters, while a single event can trigger urgent planning if uncertainty, consequence and mission policy combine to exceed the response threshold.
7. Spacecraft design determines how many options exist
Avoidance starts during mission design, long before the first alert. NASA’s best-practices handbook advises examining debris density when selecting an orbit and estimating the expected frequency of high-interest conjunctions for candidate altitudes and inclinations. Planners should estimate how many lifetime risk-mitigation maneuvers may be needed, reserve enough fuel for the intended mission duration and account for effects on data collection or other mission activities.
Propulsion and autonomy
A maneuverable spacecraft has more ways to change encounter geometry, but every response consumes propellant, electrical energy, time or payload availability. Electric propulsion can provide efficient, gradual changes but may require longer thrusting periods; chemical propulsion can deliver larger impulses but uses a finite fuel supply. The appropriate choice depends on the spacecraft and mission, so no single propulsion type represents all satellites.
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NASA’s 2020 Starlink example described satellites equipped with GNSS receivers for orbital-parameter estimates, ion propulsion and autonomous maneuvering. Those are Starlink-specific capabilities, not baseline equipment on every satellite.
Orbit selection as prevention
Choosing a less crowded altitude or inclination can reduce expected conjunction workload. Designers balance that benefit against coverage, latency, launch energy, radiation, atmospheric drag and the mission’s required viewing geometry. A mission that begins with a lower expected encounter rate may need fewer disruptive avoidance maneuvers over its lifetime.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.8. The debris environment makes avoidance a continuing job
ESA’s CREAM project update dated 12 August 2025 stated that Earth orbit contained more than 11,000 active satellites and more than 1.2 million debris pieces larger than 1 cm. Those figures are the quantities in that dated ESA account and should not be read as a live, continuously updated count.
ESA also states an average of 12 collision-avoidance maneuvers per year on its operational page. The page does not specify the period behind that average, so it is best understood as ESA’s stated operational average rather than a current-year statistic or a prediction for every mission.
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Collision avoidance is only one part of debris mitigation. ESA’s Space Debris FAQ describes end-of-life measures such as removing spacecraft from crowded altitude regions and preventing on-orbit explosions. Those actions limit the creation of new debris; they do not eliminate the large objects already in orbit, which is why screening and maneuver planning remain necessary.
9. What automation can and cannot do yet
ESA’s Collision Risk Estimation and Automated Mitigation (CREAM) project targets automation of parts of conjunction evaluation, maneuver planning, decision support and operator coordination. In an update dated 12 August 2025, ESA described the prototype as ground-based: it could issue alerts and generate actionable maneuver plans for ground operators, while in-orbit demonstrations were being prepared.
ESA’s project update calls collision-risk assessment and maneuver design “two labour-intensive tasks.” Automation can reduce repetitive calculations and help teams compare options quickly, but the published status does not describe a universal autonomous traffic-control network. Human operators still need to validate data, apply mission constraints, coordinate responsibilities and authorize actions.
Project details are available from ESA CREAM: avoiding collisions in space through automation. Because the cited status is explicitly dated, its deployment state may have changed after August 2025.
10. How to compare two collision-avoidance programs
A meaningful comparison looks at the whole operating chain rather than ranking programs by one Pc cutoff.
| Comparison axis | Questions to ask |
|---|---|
| Screening data and cadence | Which catalog and tracking sources are used? How often are predictions refreshed, and how far ahead do they extend? |
| Risk metric and threshold | Is Pc combined with consequence and mission rules? Are thresholds published as attention levels, action levels or both? |
| Uncertainty and data sharing | How are covariance, object identity and updated observations handled? Can operators exchange information quickly? |
| Maneuver capability | What propulsion, attitude control, autonomy, fuel reserve and communications windows are available? |
| Mission impact | How do planned burns affect science, imaging, coverage, formation geometry, power or schedule? |
| Coordination policy | When both objects are active, is there a designated maneuvering party and a reliable notification process? |
| Post-maneuver validation | Are the new orbit and all other upcoming conjunctions screened before and after execution? |
The strongest programs are those that connect accurate tracking, transparent risk analysis, practical maneuver authority and continuous follow-up. A single headline number cannot capture that operational resilience.
What happens when two satellites are on a collision course?
In operational terms, “on a collision course” usually means their current predicted trajectories produce a conjunction worth investigating. Teams refine the orbit data, calculate Pc and consequences, identify who can maneuver, agree on responsibilities where possible, plan a constrained maneuver, screen that plan against every relevant object, execute only after approval and continue monitoring the result. The process is deliberately iterative because the prediction changes as the spacecraft and tracking network reveal more information.
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