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Space traffic coordination is an operational loop, not a single warning or a central authority telling every satellite what to do. Operators supply orbit predictions and maneuver plans; screening services identify predicted close approaches; analysts assess risk; and the satellite’s owner or operator decides whether to maneuver and carries out that decision. The updated trajectory is then screened again. A close-approach alert calls for assessment—it does not by itself mean a collision is likely or that a maneuver is required.
What is conjunction assessment?
Conjunction assessment is the process of predicting and evaluating a possible close approach between space objects. A screening service compares predicted trajectories and flags events for further review. The prediction is an estimate based on available tracking data and operator-provided information, not a declaration that two objects will collide.
Risk assessment goes beyond the predicted distance between objects. Analysts and operators consider uncertainty in the trajectories, probability-based risk measures, the quality and age of the data, and how much time remains before closest approach. As tracking and orbit information are updated, the estimated miss distance and risk can change. There is no single maneuver threshold established for every spacecraft; mission constraints and the applicable service process matter.
What happens when my satellite has a close approach?
The response generally follows an iterative sequence. Exact interfaces, schedules, and responsibilities depend on the mission and the services it uses.
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- Establish the operating interface. Before launch, the mission and its service providers determine how orbit data, maneuver information, alerts, and decisions will be exchanged. NASA’s Conjunction Assessment and Risk Analysis (CARA) team describes pre-launch support that can help missions weigh conjunction-assessment considerations while selecting an orbit. For NASA missions, CARA documents support through an operations implementation agreement; that NASA arrangement should not be assumed to apply to every operator.
- Send current trajectory information. The operator provides predicted orbit data (ephemerides), information about uncertainty in that prediction (covariance), and planned maneuvers that fall within the prediction period, using the agreed channel. NASA CARA’s process incorporates planned maneuvers into the operator trajectory it screens. Stale or incomplete inputs can make screening less useful, so timely updates and clear maneuver communication are operationally important.
- Screen for predicted close approaches. In NASA’s described workflow, the 18th Space Defense Squadron (18th SDS) compares protected-asset trajectories with its high-accuracy catalog and identifies predicted close approaches. Screening finds events for follow-up; it is distinct from the subsequent risk analysis and from the operator’s maneuver decision.
- Assess the event. Risk analysis considers more than the closest predicted distance. NASA CARA describes automated processing of event data and trends followed by analyst review of events considered high risk. An event’s significance can depend on uncertainty, probability-based measures, data quality, time to closest approach, and whether later updates change the estimate.
- Coordinate and decide. The mission owner or operator evaluates the event against mission constraints and decides whether to take action. CARA supports mitigation planning for the NASA missions it serves; the owner/operator retains control of its spacecraft and executes its operational decisions. Coordination through the relevant service and agreed process helps account for another operator that may also be considering a maneuver. One operator does not thereby gain authority to command another operator’s satellite.
- Update and screen again. When a maneuver is planned or executed, the operator updates trajectory and maneuver information as required by the mission’s interface. The resulting orbit must be assessed for new close approaches. Screening is therefore a loop: a maneuver can change the prediction and create a need for another assessment.
What information do I need to provide?
The screening and assessment process depends on usable trajectory information and a reliable path for communicating changes. NASA CARA describes receiving ephemeris and covariance data from missions, including planned maneuvers within the prediction span.
- Ephemeris: the predicted position and motion of the spacecraft over the relevant period.
- Covariance: information describing uncertainty in the predicted state. It helps characterize how confident the assessment can be in the predicted paths.
- Planned maneuver details: information about intended maneuvers that affect the prediction period, sent through the agreed mission or service interface.
- Timely updates and coordination: revised trajectory information when operations change, plus a defined way to receive alerts and communicate maneuver intent.
The specific format, delivery schedule, and required fields are service- and mission-dependent. Operators should follow their own service agreement and current official procedures rather than infer a universal data interface from another program’s workflow.
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Who does what in the U.S. context?
Space traffic coordination involves distinct tracking, screening, risk-analysis, and spacecraft-operation functions. The roles below describe the programs and organizations identified in official U.S. sources; they do not amount to a single command chain for all satellites.
| Organization or service | Role described in official material | Important scope |
|---|---|---|
| 18th Space Defense Squadron and Space Force tracking system | Tracking and catalog functions support screening of protected-asset trajectories against tracked objects. | NASA describes 18th SDS as providing screening support. Screening is not the same function as mission risk analysis or maneuver execution. |
| NASA Conjunction Assessment and Risk Analysis (CARA) | Receives mission data, assesses conjunction results, communicates trends and risk analysis, and supports mitigation planning for the NASA missions it serves. | NASA NPR 8079.1 requires spacecraft outside NASA’s Human Spaceflight enterprise to use CARA. This is a NASA-specific requirement, not a rule for all satellite operators. |
| NOAA Office of Space Commerce (OSC), TraCSS | TraCSS is intended to provide basic space situational awareness (SSA) data and services to civil and private space operators in support of spaceflight safety. | NOAA reports that operator registrations began in early 2026. As of August 2026, NOAA reported 82 pilot user organizations representing over 11,370 satellites, as well as 10 national government accounts. These are dated pilot figures, not a guarantee of permanent coverage or service levels. |
| Commercial providers | May offer supplemental tracking, SSA, or conjunction-assessment services. | Suitability depends on mission needs and service characteristics. A commercial offering is not, by itself, evidence of verified quality, regulatory approval, or replacement of a mission-specific workflow. |
NASA’s CARA FAQ and services material describe the NASA workflow and its relationship with 18th SDS. NASA also notes that some commercial catalogs may differ from the government catalog in both accuracy and the objects included. An operator should understand which data sources underpin any result it uses.
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How often are trajectories screened?
Screening cadence depends on the service and the spacecraft category; a figure from one program is not a universal promise. NASA’s general CARA overview says the on-orbit screening operation is conducted three times daily. CARA’s detailed services description specifies three screenings per day for low Earth orbit (LEO) spacecraft and two per day for other spacecraft. These are NASA’s stated schedules for those workflows, not a schedule guaranteed to every operator or service. The time between screenings is one reason operators need to understand the service’s update and alert arrangements.
How should operators compare screening or SSA services?
When evaluating a primary or supplemental service, compare the data and operational process behind the alert—not just whether the service produces one. NASA cautions that commercial and government catalogs can differ in accuracy and object coverage. Other points below are practical questions to ask; they are not claims that every provider offers each capability.
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- Object coverage and provenance: Which objects are tracked or cataloged, and where does the underlying tracking data come from?
- Accuracy and uncertainty: How are trajectory accuracy and covariance handled, and what limitations are explained to the operator?
- Observation and update cadence: How often are observations, orbit estimates, and screening results refreshed?
- Operator inputs: Can the service use the operator’s ephemerides and planned maneuver information, and how are updates submitted?
- Alert timing and scope: What event types are screened, and how quickly are relevant results communicated?
- Risk analysis: Does the offering provide screening only, or also probability-based assessment, analyst review, or mission support?
- Coordination and access: What interfaces support maneuver coordination and data sharing, and what eligibility, security, geographic, or service-term limits apply?
Commercial and nonprofit data or services can supplement other sources, but the right combination depends on the mission. A service’s existence or marketing description does not establish its accuracy or suitability; evaluate its documented inputs, methods, coverage, and operational support.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which operator guidance is useful?
NASA’s Spacecraft Conjunction Assessment and Collision Avoidance Best Practices Handbook and its Spaceflight Safety Handbook for Satellite Operators provide operator-oriented reference material. The Spaceflight Safety Handbook is Version 1.5, dated August 2020, and describes 18th Space Control Squadron (18th SPCS) processes. Because procedures and services can change, operators should check the current official NASA resources and the applicable service instructions before relying on a specific operational procedure. NASA CARA’s current page links to handbook and FAQ resources, including an appendix update dated August 2026.
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