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Space situational awareness (SSA) data helps satellite operators spot potential close approaches early, estimate whether they pose a real collision risk, and decide whether to maneuver. A conjunction alert is the start of that process—not a prediction that a collision will happen.
How SSA data becomes a collision-avoidance decision
Collision avoidance is a chain of tracking, prediction, screening, risk analysis and coordination. Each stage adds context; no single observation or alert makes the decision on its own.
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- Estimate objects’ orbits. Space-surveillance observations support orbit estimates for catalogued objects. Operators can also supply predicted ephemerides for their active spacecraft: future positions and velocities, together with covariance information describing uncertainty. Planned maneuvers matter because they can change the predicted path. NASA’s conjunction-prediction explainer describes how these inputs support event prediction.
- Screen predicted paths for close approaches. A screening system projects a volume around a protected spacecraft along its predicted trajectory. If another catalogued object enters that volume, the event is flagged for attention. NASA says the U.S. Space Force’s 18th Space Defense Squadron conducts screenings three times daily for NASA’s on-orbit conjunction assessment. A flagged volume penetration is a screening result, not a confirmed collision.
- Send a conjunction message. For this screening service, a penetration generates a Conjunction Data Message (CDM). NASA says a CDM contains summary parameters for the conjunction, both objects’ states and covariances at the time of closest approach, and information about the orbit determination behind those estimates. The message gives analysts data to examine; it is not itself a completed risk analysis.
- Assess the probability of collision. Risk analysis considers predicted miss distance, uncertainty in the objects’ estimated positions and velocities, and the objects’ combined hard-body radius—the effective size relevant to whether their physical bodies could overlap. As NASA cautions, “CDMs are not collision warnings”: they are proximity alerts, and collision risk has not necessarily been assessed when they are generated. A small predicted separation therefore cannot be interpreted without the uncertainty and object-size information.
- Coordinate and act if warranted. Analysts and operators review the risk, share relevant orbit and maneuver information, compare options and arrange an avoidance maneuver when appropriate. The European Space Agency (ESA) describes support for mission teams in risk estimation and maneuver planning. Space-Track says collision avoidance is the owner/operator’s responsibility, with 18th Space Defense Squadron screening and CDMs supporting that work.
Why alerts need interpretation
Orbit predictions are estimates, not exact future positions. Covariance communicates uncertainty in those estimates, which is why risk assessment combines it with miss distance and object size. Two events with similar predicted closest-approach distances can warrant different responses if their uncertainties or object dimensions differ.
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Forecasts also change as observations and operator information are updated. ESA says its operational collision-avoidance predictions are updated several times daily and typically cover seven days ahead. NASA describes its current practice as looking roughly a week into the future. These are descriptions of particular services, not a universal update schedule or forecast horizon for every provider and spacecraft.
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Who receives the data and makes the decision?
SSA services do not necessarily provide the same data, analysis or recommendations to every spacecraft operator. Space-Track says its main purpose is to distribute SSA data to approved users, and access requires account credentials. It also describes emergency conjunction-assessment support and CDMs. NASA says its Conjunction Assessment and Risk Analysis (CARA) program performs risk analysis and supports maneuver planning for the operators it serves.
Screening systems and analysts inform decisions; the owner or operator remains responsible for deciding whether and how to maneuver. Sharing planned ephemerides and maneuver information helps keep predictions useful, while coordination between operators can reduce the chance that independently planned actions create new conflicts. NASA and SpaceX’s 2021 agreement highlighted inter-operator coordination, accurate data, sound risk analysis and proactive mitigation as parts of effective risk reduction.
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Why the workload is growing
More objects in orbit mean more opportunities for close approaches to be screened and assessed. ESA’s 2025 Space Environment Report estimates that more than 1.2 million debris objects larger than 1 cm were in orbit; that estimate uses data collected through the end of 2024 and does not mean every object is tracked in the public catalogue. ESA also reports that increasing traffic and debris are driving more events that trigger avoidance procedures.
What to compare when evaluating SSA services
NASA and ESA’s descriptions illustrate useful questions for assessing an SSA data or conjunction-assessment service. They do not constitute a head-to-head evaluation of commercial vendors.
Quick Recap
- Coverage and provenance: What objects are covered, and where do the tracking observations and orbit estimates come from?
- Estimate quality: Are state and covariance information available, and how are they used?
- Timeliness: How often are predictions refreshed, and how far ahead do they extend?
- Analysis scope: Does the service flag proximity only, or does it also provide collision-risk assessment?
- Operator inputs: Can operators submit predicted ephemerides and planned maneuvers?
- Operational support: Does the service assist with maneuver screening and coordination between operators?
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