Satellite Safety


Conjunction Assessment Risk Analysis


The Conjunction Assessment and Risk Analysis (CARA) program is an Agency-level program required for all NASA operational assets by NPR 8715.6. NASA has established operational interfaces with the 18th Space Control Squadron to receive close approach information in support of NASA mission teams. The CARA Operations Team monitors and assesses potential collision threats and advises the mission team (Owner/Operator [O/O]) on potential avoidance maneuvers. To reach the CARA program, please contact


In early 2005, a formal CA process was established for the Earth Observing System (EOS) satellites Aqua, Aura, and Terra. This process was adapted from an existing process used to support the Human Spaceflight program at NASA JSC that was developed in the 1980's in support of the Space Shuttle. As the visibility of the conjunction problem grew following the collision between Iridium 33 and COSMOS 2251 on February 10, 2009, NASA reflected this awareness and need for an Agency-wide CARA process in its NASA Procedural Requirement for Limiting Orbital Debris (NPR 8715.6). Since then, the CARA program has grown to support all operational NASA non-Human Spaceflight missions, presently approximately 70 missions in all Earth-orbiting regimes, including the Earth Science Constellation in 705-km sun-synchronous orbits and the Tracking Data and Relay Satellite (TDRS) System (TDRSS) in geosynchronous orbit.

Basic Definitions:

  • A conjunction is defined here to mean a local minimum in the difference between the position components of two trajectories - the closest point of approach
  • Conjunction Assessment (CA) is the process of predicting the conjunction event by screening the ephemeris of the protected asset against the space object catalog
  • Conjunction Assessment Risk Analysis (CARA) is the process of analyzing the conjunction event to determine the associated threat to the asset
  • Collision Avoidance is the process by which the risk associated with a conjunction event is mitigated -- by a maneuver or other action.
  • COLA stands for Collision on Launch Assessments and is the screening of a powered flight trajectory against the catalog pre-launch for the purpose of closing the launch window for launch times that are predicted to result in a close approach during flight. This service is performed by the launch range and not by CARA. It is not required by NASA except for Human Spaceflight missions.

Close Approach Description

Figure 1

Figure 1 : Primary and Secondary Close Approach

In 3D space, the covariance P1 of the primary spacecraft, and covariance P2 of the secondary spacecraft can be illustrated using an uncertainty ellipse (usually 2-ïƒ). Conjunction analysis that assumes a hyperkinetic close approach, can be realized in a 2-D encounter plane. This involved projecting the total covariance into the plane that is normal to the relative velocity vector. The Hard Body Radius (HBR) is given as a circle of radius equal to the sum of both spacecraft circumscribing radii.

Figure 2

Figure 2 : Combined error ellipsoid on the encounter plane with circular cross-sectional area A with radius, HBR

The probability of collision (Pc) is calculated as shown in the equation

The probability of collision (Pc) is calculated as shown in the equation above, where Area is the cross-sectional with the defined HBR and C is the total covariance (P1 and P2 )..

CARA Process overview

The CARA operations process is a three-step process involving the CARA Orbital Safety Analysts (OSAs) resident at the 18th Space Control Squadron (18 SPCS) operations facility at Vandenberg Air Force Base (VAFB), the NASA CARA team, and the mission Owner/Operator (O/O).

The first step, Conjunction Assessment (CA), is the prediction of close approaches by "screening" the trajectory of the protected asset against the High Accuracy Catalog (HAC). This step is performed by the CARA OSAs using two asset trajectories - one provided by the owner/operator (O/O) that contains any planned maneuvers during the prediction span, and the other based on non-cooperative tracking data from the DoD Space Surveillance Network (SSN) that does not have any information about upcoming maneuvers. Results of both screening runs are transferred to CARA via SFTP 3x/day for LEO spacecraft and 2x/day for all others.

Step two is risk assessment. CARA has an automated tool, the Conjunction Assessment System (CAS), which processes the data and sends updated trending information to the mission customer. The CARA team then manually analyzes any conjunctions that they consider to be high risk to determine appropriate actions to be taken (i.e. Conjunction Assessment Risk Analysis).

Step three is Collision Avoidance, in which CARA works with the mission to assist with planning and execution of any risk mitigation strategies. A summary of the process is shown in Figure 3.

Figure 3

Figure 3: Overview of the NASA CARA Process

Sample Daily Summary Report

The sample daily summary report contains the Pc from both the ASW- and the Owner/Operator (O/O)-provided ephemeris and covariance information. It also contains a detailed analysis output showing Pc history, event flags, maneuver information, as well as miss-distance components in RIC for the ASW and the O/O solutions.

Sample High Interest Events (HIE) Report

The sample HIE report is a detailed analysis for a particular conjunction of an O/O spacecraft at TCA. Information includes information on both the primary and secondary objects, space weather, sensor coverage, conjunction geometry, maneuver planning, and CARA's recommendation.

CARA Services

Beyond the basic operations service described above, CARA provides many other services to missions though all lifecycle phases. Examples of CARA support include, but are not limited to:

  • Pre-launch support: Sims, ICD/CONOPS development, mission education on process and reports, MOR/FOR support, interface set-up/system configurations, schedule staff support of maneuvers and launch phases, perform pre-launch screening
  • Critical event support: launch & early orbit, maneuvers, decommissioning / end-of-mission
  • Space Situational Awareness: support for breakups, 18 SPCS outages, and receiving other classified notifications and helping missions with any impacts; EMI event reporting
  • Metrics: maintain CA statistics, historical data exploitation & statistical analysis, hold periodic Users' Forum meetings
  • Orbital Data Request (ODR) form submission: review, submission, and tracking of Orbital Data Request (ODR) forms on behalf of all NASA entities. CARA is the sole entity with authority to submit ODR forms requesting SSA-related support for non-Human Spaceflight NASA missions. ODR submissions should be sent to

Research and Development

CARA directs R&D efforts in all CA-related areas, but the principal focus is on collision risk assessment (RA), meaning determination of which events are truly risky and thus require mitigation. In this area, CARA has established a number of important findings over the last five years:

  • TLEs are not sufficient for CA mitigation decisions (CATAC statement, 2016)
  • Maximum Pc not sufficient as a stand-alone RA parameter (Hejduk, 2016)
  • Calculated Pc is a durable RA parameter (Hejduk and Snow, 2019)
  • Determined tests for proper use of 2-D Pc calculation approach (Hall, 2019a)
  • Developed fast, appropriate Monte Carlo techniques for situations in which 2-D Pc calculation is not reliable (Hall et al., 2018)
  • Evaluated approaches for eliminating correlated error between primary and secondary object covariances (Casali, 2018)
  • Developed approaches to establishing Pc confidence intervals through covariance error histories (Hejduk and Johnson 2016; Elrod 2019)
    • Established collision consequence as an important and useful component of CA risk (Hejduk et al. 2017, Lechtenberg 2019)

      Other research areas are adjusted to fluctuate with this rapidly evolving field of study, but include such topics as:

    • OD product adequacy (g., Degenerate and unrealistic covariances; commercial data
    • CA screenings (g., Proper screening volume sizing; screening filtering techniques; autonomous spacecraft CA)
    • Pc calculation (g., 2-D Pc Limitations; non-Gaussian error volumes; Monte Carlo techniques)
    • Special studies (g., Effects of SF radar, mega-constellation deployments)

Archived Technical Papers

The CARA program has published numerous conference papers. These technical papers summarize the analysis and research and development work done by the CARA team in the technical subjects surrounding CA and operations. Access to the archived technical papers, organized by year of publication, is found here: .

Policy Documents

There are two policies that pertain to the CARA process. The most recent versions of these documents are available on NODIS.

NPR 8715.6b:

NPR 8715.6 is the NASA Procedural Requirement for Limiting Orbital Debris. Although its purview extends from mission design to mission disposal, the requirements levied within seek to limit debris generated from putting a mission into space. Examples include a lifetime requirement for end-of-life disposal orbits. NPR 8715.6 requires all operational non-human spaceflight-related NASA missions to utilize the CARA service.

NID 7120.132

NID 7120.132 is the Collision Avoidance for Space Environment Protection published in Dec 2020 to codify existing practice and implementation in policy form at a lower level than what is covered by NPR 8715.6. Various roles and responsibilities, largely implemented yet not documented in policy, are now defined. In order to ensure NASA is proactively managing collision risk, additional planning requirements are established to focus attention on conceptual and pre-implementation actions that will manage operational impacts. For contracts and partnerships, the NID provides additional guidance to help ensure NASA continues to lead in protecting the space environment through managing collision risk. Eventually, the NID will be replaced by an NPR, and any duplication with NPR 8715.6 will be resolved at that time.

Setting up CARA for a new mission

CARA support is an Agency service provided via HQ Mission Directorate funding for NASA missions. The funding level is reviewed and approved yearly by SMD, and missions will receive the services agreed-to and paid for by SMD. These services are defined in the NASA Interim Directive (NID # TBD) "Collision Avoidance for Space Environment Protection". Missions desiring support in addition to those provided as part of the baseline HQ-funded service can request additional services for a fee assuming available CARA resources to perform the services.


For each mission that receives services from CARA, a CAOIA is formulated and signed between the CARA team and the specific mission. The purpose of the CAOIA is to define any data and information exchanges between to the two teams, the mechanism(s) for that exchange, and the cadence / frequency of exchange. Each CAOIA is tailored to the covered mission and includes any mission-specific requirements. The CAOIA should be signed 12 months prior to launch to ensure that any product exchanges with CARA are defined and agreed to prior to the mission ground software development completion. The CARA Operations Team maintains a template CAOIA containing all the relevant information about the process. That template should be obtained and used in preparing the mission CAOIA. In general, the CARA-Mission CAOIA includes, but is not limited to:

  • Mission Overview, including nominal orbit and orbit maintenance plans
  • Mission data provided to CARA (e.g. predicted ephemeris, covariance, maneuver notification)
  • CARA data and services provides to the mission (e.g. CARA Summary Report, High Interest Event notifications & information, etc)

The CAOIA review process follows the following steps:

  1. Mission obtains the latest template document from CARA
  2. Mission personnel fill in pertinent details specific to the mission
  3. Iterate draft CAOIA between missions and CARA until all information has been defined completely
  4. Final review by CARA
  5. Proceed to signatures
  6. Can update CAOIA after signed with new revision as needed


Each mission routinely provides an ephemeris, or file containing the spacecraft state and specific time increments, to CARA. This ephemeris is transferred to the CARA Orbital Safety Analysts at the 18th Space Control Center (18 SPCS) facility at Vandenberg AFB for screening, i.e. the identification of close approaches. The ephemeris format, duration, time-step, and delivery frequency are mission-specific and defined in the CAOIA. Typically, these files are delivered daily and span seven to ten days in duration. CARA can accept many formats and convert them to the format required by the 18 SPCS; however CCSDS OEM v2 or 18 SPCS native format is preferred. The routinely provided nominal mission ephemeris should include any planned maneuvers within the duration of the ephemeris. For some missions or during periods of high interest, multiple ephemerides can be provided for screening to evaluate multiple potential maneuvers, no-burn cases, or potential maneuver variations due to modeled maneuver performance variations (hot/cold maneuvers).

Similar to the ephemeris data product, CARA also requires missions to provide a predictive covariance file. The purpose of this product is to allow CARA to compute a collision probability based on the Owner/Operator ephemeris and covariance data. The format details are to be defined in the CAOIA, but it is necessary to have the predictive covariance file at the same time-step cadence as the ephemeris. Moreover, the covariance should be provided from the same orbit determination process that produces the epoch state and propagation to produce the ephemeris.

Although the CARA software has the capability to detect maneuvers in an ephemeris, with the diverse set of missions and maneuver capabilities, that capability is intended for quality assurance purposes. As such, CARA prefers that each mission notify the CARA Operations Team as soon as a maneuver time is known or, at minimum, when it is modeled in the ephemeris. For each maneuver notification, CARA would like the maneuver time, size, and type to be reported.

Data distribution restrictions

In order to receive any CARA data derived from 18 SPCS data, each individual must provide proof of US citizenship.

All reports containing 18 SPCS-provided data or analysis derived from 18 SPCS-provided data is not redistributable. Permission to publish or otherwise redistribute data or derived analysis should be requested via an ODR and sent to CARA for submission to 18 SPCS for adjudication.

Software Development Kits (SDK)

CARA has developed and released software development kits (SDKs) that are available for download and use. SDKs contain source code and benchmark test data for use in implementing the code within user tool suites. The list of SDKS available are listed below:

  1. Monte Carlo Pc Calculation from TCA (in equinoctial elements) (Open Source)
  2. Conjunction Consequence Assessment (Open Source)
  3. OD Quality Assessment (US Govt Only)
  4. Two-Dimension Probability of Collision (Pc) calculation (Open Source)
  5. Single Covariance Maximum Pc (Open Source)
  6. Maneuver Trade Space (US Govt Only)
  7. Sensor Coverage Tool (US Govt Only)

All NASA-users are able to download these tools for use from the following link. The open source tools are available on the NASA Software website by searching for "CARA".

Frequently Asked Questions

What is the Probability of Collision?

The Probability of Collision (Pc) represents the likelihood that the miss distance between two objects may become less than some protected radius during the encounter time interval. CARA uses the Hard Body Radius (HBR), which is the sum of the hard body radii (sizes, essentially) of both the primary and secondary object. Pc employs miss distance, event covariance, and hard body radius in its calculation.

What is an AnalystSat?

An Analyst, or sometimes AnalystSat, is simply an object for which the 18 SPCS has tracking observations and an elset but cannot determine from which launch it originated. Because the ownership of the object cannot be attributed, the object is not officially cataloged. These objects are given a SatID in the 80,000 object number range. These objects are screened in the CARA process, but no TLEs are available for them in the public catalog on

What is

Space-Track is a website ( hosted and maintained by the 18th Space Control Squadron. The site's primary purpose is to disseminate space situational awareness (SSA) data to approved users. Examples of SSA data available on Space-Track are Two-Line Elements (TLEs) of publicly-cataloged objects and satellite reentry predictions. Access to requires an account with login credentials. Entities desiring to request support and/or data from 18 SPCS can find a template Orbital Data Request form on For NASA, these ODRs are collected and submitted via CARA who ensures that data requests are correct, complete, and understood by both parties. ODR submissions should be sent to