Space situational awareness (SSA) is the information and analysis used to understand activity in orbit and protect spacecraft. To prevent collisions, operators use conjunction assessment: they compare predicted trajectories, evaluate close-approach risk, and decide whether to maneuver or take another mitigating action. A screening alert is a warning to assess—not proof that a collision will occur.
SSA and conjunction assessment are related, but not the same
SSA is the broader picture of activity in the orbital environment. Conjunction assessment is the specific collision-risk workflow for a spacecraft: it identifies predicted close approaches between that spacecraft and tracked objects, then supports decisions about what to do.
A conjunction is a predicted closest approach. It does not mean that the objects will collide. Screening finds encounters worth examining; risk analysis helps determine whether a particular encounter warrants action.
How operators assess and mitigate a potential collision
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Prepare trajectory and operations data
The spacecraft owner or operator supplies trajectory information, including planned maneuvers. NASA’s Conjunction Assessment Risk Analysis (CARA) process also uses a separate trajectory based on non-cooperative tracking data; that trajectory does not know about upcoming maneuvers. Keeping trajectory and maneuver information timely helps analysts interpret a predicted encounter.
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Screen for predicted close approaches
A screening service compares the protected spacecraft’s predicted path with tracked objects in a catalog. Cadence varies by service and orbital regime; NASA’s descriptions give these examples:
Service description Screening cadence stated by NASA Scope stated NASA overview of the 18th Space Defense Squadron Three times a day NASA says the squadron conducts screenings; the overview does not establish that every operator worldwide receives this exact cadence. NASA CARA service description Three times daily LEO spacecraft NASA CARA service description Twice daily Other spacecraft in the described service These are descriptions of the cited NASA services, not a universal schedule for all operators or SSA providers.
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Evaluate whether the encounter is risky
Not every screened close approach presents the same risk. NASA CARA describes automated trend information and analyst review for conjunctions considered high risk. A screening result is therefore a prompt for assessment, not a collision forecast by itself.
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Coordinate and choose a mitigation
The spacecraft owner or operator weighs the assessment against mission constraints and decides whether to maneuver or take another mitigating action. NASA CARA describes supporting risk-mitigation planning and execution. U.S. Space Force doctrine, dated November 2023, assigns maneuver decisions to spacecraft owners and operators and describes U.S. Space Command as supporting avoidance efforts through predictive-ephemeris screening.
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Who supplies information, and who decides?
Government screening and NASA support inform collision-avoidance decisions; they do not automatically command a spacecraft to maneuver. The owner or operator provides operational trajectory information and retains responsibility for deciding what action best fits the mission. This distinction matters because an upcoming maneuver known to the operator may not appear in a tracking-based trajectory that lacks maneuver information.
What the available numbers do—and do not—tell you
NASA CARA reports using more than 450,000 Conjunction Data Messages (CDMs) from 2015–2018 to evaluate whether machine-learning capabilities could enhance or augment collision-avoidance decision-making. That is a historical research dataset, not a current count of active conjunctions or objects, and it does not establish a global annual collision-warning total.
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The cited material also does not establish one collision-probability threshold that applies to every spacecraft. Whether to act depends on mission-specific procedures; operators should consult their own applicable guidance rather than assume a universal maneuver trigger.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where commercial SSA services fit
In a NASA release dated December 15, 2020, NASA Administrator at the time Jim Bridenstine urged operators to leverage commercially available SSA data, information, and analysis to mitigate potential in-orbit collisions and sustain key orbital regimes. Commercial services are therefore one possible resource alongside government screening and agency support. The NASA statement does not establish the current availability or capabilities of any particular provider.
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When comparing workflows or services, the useful distinctions are screening cadence and orbital regime, the trajectory and maneuver data an operator must provide, whether risk analysis and analyst support accompany screening, and who retains authority over mitigation decisions. NASA CARA’s process is an example, not a universal standard.
What to consult for operational guidance
NASA publishes a best-practices handbook on spacecraft conjunction assessment and collision avoidance. It can provide more detailed operational guidance than a general overview, but mission-specific procedures remain essential where no universal action threshold has been established.
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