Subject: NASA Spacecraft Conjunction Analysis and Collision Avoidance for Space Environment Protection

Responsible Office: Office of the Chief Engineer

Chapter 4. Conjunction Analysis and Mitigation Process Implementation

4.1 Overview

4.1.1 A program or project transitioning from development to operations implements a conjunction analysis and mitigation process to maintain safe transit in space. Figure 4-1 illustrates activities and decision points throughout this regularly iterated process. These activities apply to the equivalent of the project manager in operations, which may be called the "spacecraft operations manager," "spacecraft owner," "spacecraft operator," or other titles.

4.1.2 The CAOIA, which is agreed to between the project and CARA, documents the specific implementation of the conjunction assessment and mitigation process.

4.2 Conjunction Assessment Step

4.2.1 The first step of the conjunction analysis and mitigation process entails routinely collecting ephemeris-with-covariance information from spacecraft O/Os. The O/O of the protected asset submits an ephemeris with covariance predicting its planned trajectory (including any maneuvers) and expected maneuver execution error to CARA or JSC FOD for screening to predict any close approaches.

4.2.2 Process for Earth-Orbiting Spacecraft

4.2.2.1 Orbital safety analysts (OSAs) screen the predicted ephemeris against the predicted positions of all the objects in the resident space object (RSO) catalog maintained by USSPACECOM. The protected asset with its chosen safety volume is then flown along its trajectory. Any RSO that passes through the safety volume is identified as a "secondary object" and is a predicted future close-approach event.

4.2.2.2 OSAs also have a predicted ephemeris with covariance for the protected asset created from "non-cooperative tracking data," that is, tracking data from independent sensors. That ephemeris is also screened against the catalog.

4.2.2.3 The results of both screenings are sent back to CARA or JSC FOD for risk assessment.

4.2.3 Process for Non-Earth-Orbiting Spacecraft

4.2.3.1 For non-Earth-orbiting spacecraft, collision avoidance relies on O/O ephemeris files for the spacecraft orbiting a body or point or on a non-orbital trajectory. Data that can be used to non-cooperatively predict conjunctions between spacecraft in orbit regimes outside of Earth orbit are not currently available.

Figure 4-1 Conjunction Analysis and Mitigation Process Flowchart

4.2.3.2 The predicted ephemeris is screened against all other ephemerides available to NASA for spacecraft in the same orbit regime.

4.2.3.3 The results of the screenings are sent to the project manager for decision making.

4.2.4 For both Earth-orbiting and non-Earth-orbiting spacecraft, ephemerides with covariance are produced and furnished to CARA or JSC FOD throughout the spacecraft's active orbital life in accordance with the specifications provided in Section 5.1.1 of this NPR. Ephemeris data for subsidiary deployed child payloads throughout their active life are also provided to CARA or JSC FOD. Spacecraft and intentionally generated orbital debris (e.g., jettisoned objects and trash) that are below normal detectability thresholds may need enhancements to enable ephemeris data to be generated using non-cooperative tracking. (See CA2 Handbook section on trackability.) These ephemerides model any planned maneuvers. For non-maneuverable spacecraft, ephemerides may not be necessary as determined during OCAP preparation, provided the spacecraft is trackable non-cooperatively.

4.2.4.1 Project managers shall provide launch-related information to CARA or JSC FOD as soon as it is available. This information facilitates rapid cataloging and identification of launch-related objects and includes injection vectors and initial ephemerides based on the launch vehicle separation vector for deployed objects under the purview of the project manager. (See the CA2 Handbook, Section 6.1.)

4.2.4.2 Project managers shall provide updated predicted ephemerides for each spacecraft under their authority as soon as communications are established and sufficient tracking data are received from the payload(s) to update the separation ephemerides.

4.2.5 For planned maneuvers included in the routine ephemerides described in Section 4.2.4, the project manager provides an updated ephemeris with covariance as soon as possible after the time of these planned maneuvers. This delivery is in addition to the routine delivery described in Section 4.2.1 and in Section 5.1.1 of this NPR. For Earth-orbiting missions, the routine ephemeris delivery is usually sufficient.

4.3 Conjunction Risk Assessment Step

4.3.1 CARA or JSC FOD processes the predicted close-approach data using a combination of automated and manual methods. To determine whether the predicted event poses a high risk to the protected assets, analysts compute appropriate collision likelihood and consequence parameters, examine the conjunction geometry, and for Earth-orbiting spacecraft, assess whether the orbit determination solution for the other object is sufficient to serve as a basis for conjunction risk assessment. CARA or JSC FOD then makes a recommendation on whether the event should be mitigated for flight safety.

4.3.1.1 The CARA program manager determines which spacecraft conjunctions constitute high collision risks to the space environment based on conjunction assessment-related criteria and advises the project manager on appropriate mitigation actions.

4.3.1.2 The CARA program manager determines whether the orbit determination data for a conjunction event allow sufficient fidelity for use by the project to plan and execute a mitigating action.

4.3.2 If adequate mitigating action cannot be taken for a close approach due to extenuating circumstances, the CARA program manager and JSC FOD communicate information regarding the residual risk to OCE.

4.4 Conjunction Mitigation Step

4.4.1 Project managers weigh CARA or JSC FOD's recommendation against any other constraints or risks to determine whether to take mitigating action, considering risks to their spacecraft and sustaining a safe space environment.

4.4.2 Project managers take mitigating action or notify CARA or JSC FOD that no mitigation will occur. If the decision is to mitigate, project managers plan mitigation for conjunctions at the mitigation threshold (i.e., baseline criteria for mitigating an identified conjunction) documented in the CAOIA.

4.4.3 Ephemerides for the planned conjunction mitigations are provided to CARA or JSC FOD for screening to ensure the action is safe; specifically, that it mitigates the conjunction and does not introduce new risk.

4.4.4 For systematic conjunction situations identified through the analysis described in Appendix D.4, Section 2.2.1, project managers develop and implement a process with the other operator(s) to routinely coordinate maneuver plans and optimize relative spacing during operations. (See the CA2 Handbook, Section 4.2.)

4.4.5 Prior to finalizing their ascent plan and disposal plan, project managers shall coordinate with JSC FOD to identify and mitigate persistent or problematic orbital crossings with HSF missions (including the ISS). For non-HSF projects, CARA can provide a JSC FOD point of contact. (See the CA2 Handbook, Section 4.3.)

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