Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space

Top 10 Conjunctions by Maximum Probability

Top 10 Conjunctions by Minimum Range


  • Introduction
  • As a service to the satellite operator community, the Center for Space Standards & Innovation (CSSI) now offers SOCRATES—Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space. This service will provide regular information on pending conjunctions on orbit over the coming week. Because of the potentially catastrophic consequences of such conjunctions going unnoticed, it is hoped that this service will help satellite operators avoid undesired close approaches through advanced mission planning.

  • Methodology
  • Twice each day, CSSI runs a list of all satellite payloads on orbit against a list of all objects on orbit using the catalog of all unclassified NORAD two-line element sets (TLEs) releasable to the public to look for satellite conjunctions over the next seven days. The reason for limiting the search to conjunctions with payloads is to give satellite operators an opportunity to plan—or obtain planning services—to move their satellites out of harm's way (if their satellite is maneuverable), or to take other appropriate measures. Since there currently is no way of knowing which payloads are still active, all payloads are considered.

    The runs are made using, STK/CAT—STK's Conjunction Analysis Tools—and the SGP4 propagator in STK. STK/CAT is set to look for all conjunctions which are within 5 km at time of closest approach (TCA) and reports both minimum distance and maximum probability for the conjunction. Because the minimum distance method ignores position covariance information and can lead to an exaggerated assessment of the true risk, CSSI believes the maximum probability method provides a more reasonable (although still conservative) assessment of the true risk. For more details on the maximum probability method, see Dr. Sal Alfano's paper titled "Relating Position Uncertainty to Maximum Conjunction Probability" (2,493,912 bytes).

    For more information on SOCRATES, see the papers, briefings, and animations on "Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space (SOCRATES)" presented at the 15th AAS/AIAA Space Flight Mechanics Conference, the Fourth European Conference on Space Debris, and the Sixth US-Russian Space Surveillance Workshop.

  • Proposed Enhancements
    • Improved covariance modeling using historical TLEs.

    • [Now available] Ability to download complete, ready-to-run STK/CAT scenario for each conjunction to facilitate additional analysis. Update: CelesTrak received authorization from Air Force Space Command to provide TLEs in support of SOCRATES on 2005 March 24 (see the System Notice for details). The original capability to build full STK/CAT scenarios has now been restored.

    • [Now available] Ability to search interactively for a particular conjunction (e.g., all conjunctions for the ISS within 1 km). Searches can be ordered by maximum probability, minimum range, or time ordered.

    • AGI Viewer files for the Top 10 conjunctions (a sample AGI Viewer file from the 6th US-Russian Space Surveillance Workshop paper is now available (932,839 bytes) and the free AGI Viewer software can be obtained at

    • [New: 2007 Sep 19] Ability to bookmark searches for future use. A link to bookmark each search will appear at the top of the Search Results page. This feature should make it easy for satellite operators to repeat searches of interest or to build a custom page with those links.

  • Additional Resources
  • Value-Added Analysis Providers
  • STK/CAT allows users to examine conjunctions of interest in further detail. For those users seeking additional comprehensive, customized, and/or recurring conjunction analyses, CSSI has partnered with 1Earth Research, LLC to provide these services. Please contact for further details.

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Dr. T.S. Kelso []
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Last updated: 2013 September 25 23:24:46 UTC
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