NASA Procedural Requirements

NPR 8079.1 Effective Date: June 27, 2023 Expiration Date: June 27, 2028

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

Responsible Office: Office of the Chief Engineer

Appendix A. Definitions

Ascent plan. An ephemeris file or set of files and associated maneuver plan that represent the predicted flight trajectory (including all modeled maneuvers) that a spacecraft will execute to move from its injection orbit to its final spacecraft orbit. The ascent plan should be updated in real time as flown to incorporate any changes to the plan.

Baseline. In the context of this NPR, baseline indicates putting the product under configuration control so that changes can be tracked, approved, and communicated to stakeholders and the team. Updates to baselined documents require the same formal approval process as the original baseline.

Collision avoidance. The planning and execution of risk mitigation strategies to avoid a collision between two space objects. (See also conjunction mitigation.)

Conjunction. A close approach between two objects that is predicted to occur because the secondary object passes within a chosen geometric or statistical safety volume about the protected asset (also called "primary object").

Conjunction analysis. The process of predicting a close-approach event by screening the ephemeris of the protected asset against the space object catalog (i.e., conjunction assessment) and then analyzing the event to determine the associated threat to the asset (risk assessment).

Conjunction assessment. The identification of close approaches using ephemeris screening against a catalog of resident space objects.

Conjunction Assessment Operations Implementation Agreement. An agreement between the project manager and CARA that documents the implementation of the operational requirements specified in this NPR for each non-HSF spacecraft under the project manager's authority.

Conjunction mitigation. An action taken to remediate conjunction risk, including a propulsive maneuver, an attitude adjustment (e.g., for differential drag or to minimize frontal area), or providing ephemeris data to secondary owner/operators so that they can perform an avoidance maneuver.

Conjunction risk assessment. The process of assessing a conjunction (predicted close approach) to determine the likelihood of two space objects colliding and to determine the expected consequence if they collide in terms of spacecraft inoperability and expected debris production.

Colocation. Being in the vicinity of another operational spacecraft close enough that systematic conjunctions occur.

Covariance. Characterization of uncertainty components and their interactions surrounding a space object's estimated state at a given time.

Crewed asset. Spacecraft that hosts or transports humans.

Demise. When a spacecraft is no longer in space such as a complete burn up on reentry. Spacecraft passivation and migration to disposal orbits occur prior to demise.

Disposal. An end-of-mission process involving a spacecraft's passivation and reentry into the atmosphere for its ultimate demise or its movement (if necessary) to an orbit or trajectory considered acceptable for orbital debris limitation. For purposes of this NPR, "disposal" includes the reorbiting or deorbiting of a spacecraft at the end of mission. Disposal occurs prior to spacecraft demise.

Disposal plan. An ephemeris file or set of files and associated maneuver plan that represent the predicted flight trajectory (including all modeled maneuvers) that a spacecraft will execute to move from its operating orbit to its disposal orbit. The disposal plan should be updated in real time as flown to incorporate any changes to the plan.

Ephemeris. (plural: ephemerides) A file containing a time-ordered set of position and velocity measurements describing an object's predicted trajectory.

Hard-body radius. The radius of a circle equal to the sum of the circumscribing radii of both the protected plus the secondary spacecraft.

Highly elliptic orbit. An orbit having an eccentricity greater than 0.25 and less than 1.00. Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; higher numbers indicate more elliptical orbits.

Low-Earth orbit. An orbit with an orbital period less than 225 minutes and an eccentricity less than 0.25.

Maneuver. Any action that changes a trajectory in a non-Keplerian fashion, i.e., in a way that cannot be modeled non-cooperatively.

Maneuver plan. The specific parameters that represent a planned spacecraft maneuver, including execution time, burn duration, and delta-v. The position and velocity of a single object at a specified epoch is specified in an Orbit Parameter Message (OPM). The industry message standard for communicating maneuver plans is the Consultative Committee for Space Data Systems (CCSDS) standard CCSDS 502.0-B-2, "Orbit Data Messages," Section 3 located at https://public.ccsds.org/Pubs/502x0b2c1e2.pdf.

Maneuverable spacecraft. A spacecraft that has capability permitting the manipulation of the spacecraft's trajectory in a non-Keplerian fashion.

NASA-owned or -operated spacecraft. A spacecraft owned, developed, or operated by NASA or operated principally for NASA.

Near-Earth object. A comet or asteroid whose orbit enters Earth's neighborhood, where it may present a hazard.

Non-cooperative tracking data. Data obtained describing the location of space objects that do not themselves actively provide location data. For example, a piece of orbital debris that has no transponder could not provide ephemeris data. Other examples include natural objects such as asteroids, and spacecraft belonging to operators who do not choose to share an ephemeris for their spacecraft.

Non-human space flight mission. A NASA space flight mission that is not related to human space flight, i.e., non-HSF. These space flight missions are supported by the CARA Program.

Orbital Collision Avoidance Plan. The early life-cycle planning document demonstrating to CARA or JSC FOD that the design considerations and preparation for operations of the space flight mission meet the intent of this directive.

Orbital debris. In this NPR, orbital debris is defined as any object placed in space by humans that no longer serves any useful function. Objects range from spacecraft to spent launch vehicle stages to components and include materials, fragments, trash, or other objects that are intentionally or inadvertently cast off or generated. (Derived from NPR 8715.6.)

Payload. A specific complement of instruments, sensors, equipment, and support hardware carried into space to accomplish a mission or a discrete activity in outer space. Personnel are not considered a payload or a part of a payload. (See also "primary payload" and "secondary payload.")

Preliminary. The documentation of information as it stabilizes but before it goes under configuration control. It is the initial development leading to a baseline. Some products will remain in a preliminary state for multiple reviews. The initial preliminary version is likely to be updated at a subsequent review but remains preliminary until baselined. (From NPR 7120.5 Appendix I.)

Primary payload. A payload for which a launch vehicle is procured. A primary payload typically defines the orbital placement/trajectory, flight design, critical path of the mission integration including launch preparation process, and mission operations. The primary payload's organization funds the launch service. (See also "payload" and "secondary payload.")

Protected asset. The asset of focus (also called "primary object") for which risk assessments of potential collisions are being performed and collision avoidance mitigation activities are being considered and possibly performed.

Resident space object. An artificial object that orbits Earth.

Rideshare. Accommodation opportunities for secondary payloads on a launch vehicle. Sharing access to space is possible when a launch configuration has excess performance that can be shared between primary payloads and compatible secondary payloads.

Safety volume. A volume defined around a protected asset as a zone for predicting close approaches with a "secondary" space object entering that volume.

Secondary object. Any cataloged object residing in space that passes through the safety volume of a protected (i.e., primary) asset when the protected asset with its safety volume is flown along its trajectory. The resident space object is identified as a "secondary object" and is the conjuncting object in a predicted future close-approach event. The secondary object may be an operated spacecraft.

Secondary owner/operator. The owner/operator of a "secondary object" (i.e., a predicted future close-approach event) identified when a protected asset with its safety volume is flown along its trajectory. (See "secondary object" above.)

Secondary payload. A payload that is manifested subordinate to a primary or co-manifested payload and is, therefore, subordinate in launch date and orbit selection. (See also "payload," "primary payload," and "rideshare.")

Space situational awareness. The knowledge and characterization of space objects and their operational environment to support safe, stable, and sustainable space activities. (From Space Policy Directive-3.)

Space surveillance network. A network of radar and optical sensors used by DOD to track space objects. Tracking data are used to perform orbit determination and maintain the space object catalog.

Spacecraft and Planet Kernel. A file format that allows ephemerides for any collection of solar system bodies (spacecraft, planet, satellite, comet, or asteroid) to be combined under a common file format and accessed by a common set of functions. (Derived from SPK Required Reading at https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html.)

Systematic conjunction. A situation in which two space objects repeatedly experience close approaches with each other due to their similar orbits.

Tailoring. The process used to adjust or seek relief from a prescribed requirement to accommodate the needs of a specific task or activity (e.g., program or project).

Trajectory plan changes. Any adjustments to orbit parameters or the trajectory occurring between launch and end of mission.

Update. Applied to products that are expected to evolve as the formulation and implementation processes evolve. Only expected updates are indicated. However, any document may be updated as needed. Updates to baselined documents require the same formal approval process as the original baseline. (From NPR 7120.5, Appendix I.)

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