Satellite conjunction alerts predict that two tracked objects may pass close to one another; they do not mean a collision is certain. Spacecraft operators should monitor the official service available to their mission, review each current Conjunction Data Message (CDM), compare updated risk estimates and orbit uncertainties, and follow mission procedures before deciding whether to maneuver. The owner or operator—not the alert system—makes that decision.
How do you track satellite conjunction alerts?
Start with the official conjunction-assessment service that supports your spacecraft. For U.S. operators, Space-Track documents a basic emergency on-orbit service in which the 18th Space Defense Squadron (18 SDS) screens active satellites against its catalog several times per day. When an approach meets the service’s emergency reporting criteria, the owner or operator is notified by email and through the Space-Track Operator Panel. CDMs are available on Space-Track.org.
Space-Track describes this basic emergency service as no-cost and says it does not require an SSA Sharing Agreement or an Orbital Data Request (ODR). To request it, operators are asked to provide contact details, Space-Track account names, and their satellites’ catalog numbers and names. Eligibility, criteria, contacts, and procedures can change, so confirm the current requirements on Space-Track’s official “Basic Emergency Collision Avoidance (On-Orbit)” page.
Space-Track also documents advanced services for entities with an SSA Sharing Agreement, including additional screening and CDM or ephemeris support depending on the service. Access and data approval depend on the current process; the basic emergency service and advanced services are not interchangeable.
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Use the CDM and its revisions
A Conjunction Data Message is the standardized format used to exchange information about a predicted close approach between people and computer systems. Treat it as an event-specific data product, not as a standalone verdict: review the latest message and its revisions rather than relying on a copied alert that may no longer reflect the current orbit estimates.
Track the event through closest approach
Space-Track says operators can submit predictive ephemerides in approved formats and coordinate systems for screening. Follow its current file-format and maneuver-notification guidance. The Spaceflight Safety Handbook describes continued monitoring through time of closest approach (TCA), whether or not the operator proceeds with collision avoidance. Recheck the event as updates arrive and retain the event history used in the decision.
Which alert and assessment routes are available?
| Route | Audience and access | What it provides | Important limitation |
|---|---|---|---|
| Space-Track basic emergency on-orbit service | Satellite owners and operators who provide the requested account and spacecraft details; Space-Track describes the service as no-cost and says an SSA Sharing Agreement or ODR is not required. | Email and Operator Panel notifications for reportable close approaches, with CDMs available on Space-Track.org. | Emergency reporting criteria apply. The operator remains responsible for the maneuver decision. |
| Space-Track advanced services | Entities using the applicable SSA Sharing Agreement and, where relevant, ODR process. | Expanded screening and additional CDM or ephemeris support, depending on the service. | Eligibility and access to requested data depend on current approval procedures. |
| ESA operational conjunction-risk service | ESA and other supported agency missions, as described by ESA. | Analysis combining U.S.-provided CDMs with ESA orbit data; event histories and risk metrics in a web interface. | ESA does not describe it as a general-purpose consumer tracking service. |
| ESA CREAM automation project | Project and prototype context. | ESA has described alerting, maneuver-option generation, and coordination support as project capabilities. | The project description does not establish universal operational availability. |
ESA’s event view illustrates the information a professional operator dashboard may present: collision probability, miss distance, encounter geometry, orbit uncertainties, chaser-object details, and the history of analyses. It is an example of operational support, not a substitute for confirming which service your mission can access.
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How do you assess satellite collision risk?
Read the variables together and compare successive analyses. A predicted miss distance is the estimated separation at closest approach; by itself, it does not establish whether the objects will collide. ESA identifies object sizes, predicted miss distance, flyby geometry, and orbit uncertainties as relevant to an event assessment. Its 2025 standard also calls for probabilistic assessment that models uncertainty in position and velocity.
- Miss distance: Check the predicted separation alongside its uncertainty and the encounter geometry, rather than treating the distance as a guarantee.
- Collision probability: Read the estimate as model-dependent and consider the inputs, methods, and procedures behind it. ESA includes probability in its event analysis.
- Orbit uncertainty and data quality: Uncertainty in either tracked orbit affects how meaningful a predicted separation is. Review the available uncertainty information and how it changes between analyses.
- Geometry and object dimensions: Relative approach direction and the objects’ sizes matter to the assessment, not just the distance between their predicted positions.
- Updates and event history: Compare revisions to see how the prediction changes as observations and orbit data are updated. ESA’s 2019 explanation says most alert risks decline over the course of a week as more orbital information is gathered; this is a general observation, not a guarantee about an individual event.
- Maneuver consequences: Screen any proposed maneuver for its effect on the current conjunction and other objects. A maneuver may also consume limited fuel or disrupt mission operations.
Does a close-approach alert mean two satellites will collide?
No. A conjunction assessment predicts the point and time of closest approach between tracked orbital objects. It flags a possible close pass based on available tracking and orbit information; it does not prove that an impact will occur. The estimate can change as new observations and updated orbit data become available, which is why operators monitor the event rather than treating an initial alert as the final result.
ESA’s 2019 account gives a sense of the workload behind monitoring: it described hundreds of alerts per week for a typical low-Earth-orbit satellite, with about two alerts per week per satellite requiring detailed analyst follow-up for ESA’s then-current fleet of low-altitude spacecraft. These are historical figures from 2019, not current global totals or forecasts for every satellite. ESA also estimated in January 2019 that there were 34,000 objects larger than 10 cm, 900,000 between 1 and 10 cm, and 128 million between 1 mm and 10 cm in orbit. Those, too, are dated estimates, not present-day counts.
Who decides whether a satellite should maneuver?
The spacecraft owner or operator decides, using the mission’s safety case, procedures, available event data, and operational constraints. Space-Track’s documentation puts the responsibility plainly: “Collision avoidance is the responsibility of the satellite owner/operator, but 18 SDS will support their collision avoidance efforts by screening their predictive ephemeris and providing results in the Conjunction Data Message (CDM) format.” Screening and alerting support that decision; they do not make it for the operator.
A proposed avoidance maneuver should be assessed for whether it reduces the intended risk and whether it creates a new conjunction risk elsewhere. Continue monitoring through TCA whether a maneuver is made or not, and follow the service’s current notification and data-submission requirements.
Do not use a universal probability cutoff
There is no single collision-probability value that automatically requires every spacecraft to maneuver. ESA’s 2019 public explainer said that, for its fleet, a probability greater than “typically 1 in 10,000” prompted multiple teams to begin preparing a maneuver. That is a dated description of typical ESA practice, not a general rule for all operators.
ESA’s Issue 1 standard, with a revision date of 2025-10-23, makes the context dependence clearer: requirements distinguish normal operations and orbit conditions, and the 10-4 and 10-6 values used in one constellation assessment are not maneuver thresholds. Apply the safety requirements and decision criteria that govern the specific mission rather than converting a probability figure from another context into an automatic trigger.
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