A satellite conjunction alert means two tracked objects are predicted to pass near one another; it does not, by itself, mean a collision is imminent or likely. Confirm the latest event data, assess it under the spacecraft operator’s approved risk criteria, coordinate with the authorized decision-maker and any relevant owner/operator, and screen any planned mitigation before execution.
What does a conjunction alert mean?
A conjunction screening flags a predicted close approach between two tracked objects. It is an initial detection, not a final collision-risk assessment. NASA’s Conjunction Assessment Risk Analysis (CARA) explains that screening results “merely indicate that two objects will come into somewhat close proximity; they do not determine what the collision risk actually is.” NASA CARA, Step 2: Close Approach Risk Assessment.
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NASA describes its own on-orbit screening as driven by U.S. Space Force 18th Space Defense Squadron catalog screenings conducted three times a day. That cadence describes NASA’s setting; it should not be assumed for every screening provider. NASA CARA.
What information should you check first?
Use the current alert product and the mission’s established workflow. Capture the details needed by the conjunction-assessment team, and confirm whether newer data have changed the event:
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- Both object identifiers and which object is the spacecraft of concern.
- The time of closest approach (TCA) and the alert’s latest update time.
- The reported miss-distance estimate and probability of collision (Pc), if supplied.
- Available uncertainty or covariance information and the data product’s assumptions.
- The mission’s decision deadlines, escalation path, and applicable actionability criteria.
A single alert label or miss-distance estimate is not enough to determine whether a maneuver is warranted. The assessment must account for the available tracking uncertainty, the objects’ dimensions as represented by their hard-body radii, and the time remaining to make and execute a decision.
How should the operator assess collision risk?
Route the event to the mission’s designated conjunction-assessment team and use its approved tools and criteria. NASA CARA treats screening, risk assessment, and mitigation as separate stages because close proximity alone does not establish either collision likelihood or consequences. NASA identifies Pc as its principal risk metric and also uses miss distance relative to the combined hard-body radius in mitigation decisions. NASA CARA risk assessment.
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NASA’s figures are NASA-specific guidance, not universal thresholds for all spacecraft operators:
- Pc above 1E-07: NASA CARA says this level merits operational attention in its process. NASA CARA risk assessment.
- Pc above 1E-04, or miss distance below the combined hard-body radius: NASA’s 2023 procedural requirement identifies these as baseline mitigation criteria at the mitigation commitment point. The requirement applies within its NASA scope, not as a rule for every operator. NASA NPR 8079.1.
- Approximately 3E-06 post-mitigation Pc: NASA CARA describes this as an approximate reduction goal in its framework—1.5 orders of magnitude below its 1E-04 mitigation requirement. It is not a universal target. NASA CARA mitigation.
For operators outside NASA, the mission’s own approved criteria and authority govern. Do not substitute NASA thresholds for an operator’s established policy, or treat a threshold as a substitute for reviewing the event’s uncertainty and operational context.
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When should the mission decide whether to maneuver?
The decision is time-sensitive, but there is no single timeline that fits every event or spacecraft. In NASA CARA’s high-risk process, mitigation planning begins about three days before TCA; a final risk examination and decision are typically made at a maneuver commitment point 1 to 0.5 days before TCA. These are NASA operational reference points, not guaranteed deadlines for other operators or every conjunction. NASA CARA mitigation.
Use the mission’s actual operations plan to establish when the analysis must be refreshed, when approval is needed, and how much time remains for command preparation and maneuver execution. A changing event assessment or a late update may affect whether an action remains feasible.
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Who makes the decision and when should operators coordinate?
The mission’s authorized decision-maker decides whether to act; the assessment team provides the technical analysis. Under NASA NPR 8079.1, CARA recommends whether mitigation is appropriate from a space-environment-safety perspective, while the project manager makes the decision. This is the NASA governance model and should not be presumed to define another operator’s authority structure. NASA NPR 8079.1.
If the other object is an active, maneuverable spacecraft, NASA’s handbook recommends direct owner/operator coordination to agree on the response and which vehicle will maneuver. Its guidance calls for the other spacecraft’s planned trajectory to remain stable through TCA once the response is coordinated. Apply this within the relevant operational and coordination arrangements. NASA Conjunction Assessment Handbook.
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What mitigation options are available?
The appropriate response depends on spacecraft capability, mission constraints, assessed risk reduction, and time. NASA identifies several approaches; they are not interchangeable for every vehicle. NASA CARA mitigation.
| Option | When it may fit | Operational consideration |
|---|---|---|
| Propulsive trajectory change | NASA’s most common mitigation for high-risk events in its framework. | Changes the spacecraft’s path; the proposed trajectory must be screened for new conjunctions. |
| Differential drag | Available to some missions with suitable spacecraft capability. | Uses drag differences to alter relative motion; suitability depends on the vehicle and operational plan. |
| Attitude change to reduce projected area | May suit spacecraft without trajectory-modification capability. | Can reduce frontal area presented to the other object; mission attitude constraints still apply. |
Evaluate a proposed action against the same operator-approved criteria used for the original event, including the resulting Pc and miss distance. The maneuver should meaningfully reduce the assessed risk without creating an unacceptable new event.
How should a planned maneuver be screened and communicated?
- Prepare the proposed action and trajectory data. Use the mission’s approved maneuver-planning process and produce the ephemeris needed for screening.
- Screen the planned trajectory. NASA’s handbook recommends screening the maneuver ephemeris against the full catalog and checking that the action does not introduce another conjunction above the applicable mitigation threshold. NASA Conjunction Assessment Handbook.
- Coordinate and distribute maneuver intent. Send the planned action and updated trajectory information through the mission’s required channels to the screening authority and any coordinating operator, early enough to allow screening.
- Confirm the executed result. If the maneuver differs from the plan, submit updated ephemeris according to the operator’s procedures so the post-maneuver trajectory can be assessed.
Exact submission channels and deadlines vary by operator, so use the mission’s documented procedures rather than assuming a universal reporting method.
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