OCE_51.pdf

NASA Spacecraft Conjunction Assessment and Collision Avoidance Best Practices Handbook ii

February 2023

NASA/SP-20230002470 Rev 1

This is the first revision by the Office of the Chief Engineer of the 2020 Handbook updated to reflect the release of NPR 8079.1, NASA Spacecraft Conjunction Analysis and Collision

Avoidance for Space Environment Protection.

The cover photo is an image from the Hubble Space Telescope showing galaxies NGC 4038 and NGC 4039, also known as the Antennae Galaxies, locked in a deadly embrace. Once normal spiral galaxies, the pair have spent the past few hundred million years sparring with one another.

Image can be found at:

https://images.nasa.gov/details-GSFC 20171208 Archive e001327 iii

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Table of Contents

Preface

1.0 Introduction

2.0 Roles and Responsibilities

3.0 Conjunction Assessment: Past, Present, and Future

Origins…………………

USSPACECOM Conjunction Assessment Process

NASA Partnership with USSPACECOM

Conjunction Assessment in Cislunar Space

4.0 Spacecraft and Constellation Design

Ascent to/Disposal from the Constellation’s Operational Orbit

General Orbit Selection: Debris Object Density

Vehicle- and Constellation-Specific Orbit Selection: Spacecraft Colocation

Launch-Related Conjunction Assessment

Spacecraft Trackability

Spacecraft Reliability

Development of Capabilities for Ephemeris Generation and Conjunction Risk Assessment and Mitigation

5.0 Pre-Launch Preparation and Early Launch Activities

CONOPS Discussions and Arrangements with USSPACECOM Pertaining to Launch Phase

CONOPS Discussions and Arrangements with USSPACECOM and NASA Pertaining to On-Orbit Mission Phase

CONOPS Discussions and Arrangements with USSPACECOM and NASA Pertaining to Conjunction Assessment

In situ Launch Products and Processes

6.0 On-Orbit Collision Avoidance

Spacecraft Information and Orbital Data Needed for Conjunction Assessments v

Conjunction Assessment Screenings

Conjunction Risk Assessment

Conjunction Risk Mitigation

Automated Trajectory Guidance and Maneuvering

Other Considerations

7.0 Contact Information

Appendix A. Acronyms

Appendix B. Glossary

Appendix C. Best Practices List

Appendix D. Best Practices for NASA Missions

Appendix E. Use of Analytic Theory Orbital Data in Conjunction Assessment .. 81

Appendix F. Expected Conjunction Event Rates

Appendix G. Orbital Debris Density

Appendix H. Long-Term Collision Risk and Satellite Colocation Analysis

Appendix I. Satellite Covariance Realism Assessment Procedures

Appendix J. CARA Conjunction Risk Assessment Tools Validation

Appendix K. R-15 Message

Appendix L. Commercial Data in NASA Conjunction Assessment

Appendix M. Use of the Probability of Collision (Pc) as the Risk Assessment Metric for Conjunction Assessment

Appendix N. Pc Calculation Approaches

Appendix O. Collision Consequence

Appendix P. Event Actionability

Appendix Q. Notional Display Flow for Conjunction Event Processing

Appendix R. List of Works Cited vi

List of Figures

Figure 3-1 Locations and Types of SSN Sensors and Nodes

Figure E-1 Miss-Distance Distributions for Critical (Pc > 1E-04) and Non-critical (Pc < 1E-04) Events

Figure E-2 eGP and Regular SGP4 Errors: LEO > 500 km (log scale)

Figure E-3 Expansion of Figure E-1 to Include CDFs for Pc Thresholds of 1E-03 and 1E-02

Figure F-1 Conjunction Event Frequencies as a Function of Orbit Regime and Event Severity

Figure G-1 Debris Flux as a Function of Orbit Inclination, Orbit Altitude, and Object Size

Figure H-1 Kessler Collision Rates between NewSat and HST

Figure H-2 Total NewSat Collision Rates Among All Cataloged Satellites

Figure H-3 NewSat Collision Rates Excluding the Starlink Constellation

Figure H-4 Predicted Rates for NewSat and HST Satellite Conjunctions

Figure H-5 Predicted Rates for NewSat and FGST Satellite Conjunctions

Figure H-6 Predicted Rates for NewSat and Swift Satellite Conjunctions

Figure N-1 Relative Positions and Uncertainties of Two Objects

Figure N-2 Combining the Sizes of the Primary and Secondary Objects

Figure N-3 Geometry at TCA

Figure N-4 Two Equivalent Representations of the Conjunction Plane

Figure N-5 Pc Rate and Cumulative Pc for a Non-Rectilinear Conjunction

Figure N-6 Pc Rate and Cumulative Pc for a Low Velocity Conjunction

Figure N-7 Comparison of Monte Carlo Collision Probabilities with the Two-

Dimensional 𝑷𝒄 Method (left), the Three-Dimensional 𝑵𝒄 Method (center), and the Two-Dimensional 𝑵𝒄 Method (right) for a Large Set of CARA Conjunctions

Figure N-8 Cumulative Three-Dimensional 𝑵𝒄 Risk Over Four Repeating

Conjunctions

Figure N-9 Behavior of Relative Density Error by Perigee Height and Solar Activity

Figure N-10 Two-phase Application of Consider Parameters vii

Figure N-11 Profiles of 250 Conjunctions with Primary and Secondary Satellites of Non-negligible Drag

Figure N-12 Three-Dimensional Nc Temporal Risk Plot with Monte Carlo from Epoch Result Overlay (in Pink)

Figure N-13 Mismatch between Elongated In-track Covariances and Forced Cartesian Ellipsoidal Representation

Figure N-14 Comparative Results of 373 High-Pc Conjunctions

Figure N-15 Comparative Results of 63,603 Conjunction Events

Figure O-1 Range of Debris Production Possibilities

Figure O-2 CDF of the Ratio of Calibrated HBR Values to Known HBR Values for 586 Satellites

Figure O-3 CDF of the Ratio of Calibrated RCS+BC Mass Estimates to Known Masses for 554 Satellites

Figure O-4 CDF of the Ratio of Calibrated RCS+SRPC Mass Estimates to Known Masses for 303 Satellites

Figure O-5 Collision and Fragmentation Probabilities for Known-on-Unknown Conjunctions

Figure P-1 SWTS Plot for Invariant Situation

Figure P-2 SWTS Plot for “Ridge” Situation

Figure P-3 SWTS Plot for Variation Situation

Figure P-4 LUPI Upper Bound for Certain EDR Values

Figure Q-1 Ground Trace Plot

Figure Q-2 Conjunction Plane Plot

Figure Q-3 Probability of Collision Time-History Plot

Figure Q-4 Total Miss Distance Plot

Figure Q-5 Componentized Miss Distance Plot

Figure Q-6 State Comparison Consistency Plot

Figure Q-7 Solar Storm Prediction

Figure Q-8 Space Weather Trade-Space Plot

Figure Q-9 Maneuver Trade-Space Plot viii

List of Tables

Table 7-1 NASA Contact and Reference Information

Table 7-2 Contact and Reference Information for Non-NASA Personnel

Table C-1 Best Practices Summary

Table D-1 Best Practices for NASA Projects

Table P-1 Geopotential, Atmospheric Drag, and Solar Radiation Pressure

Table P-2 Ballistic Coefficient and Solar Radiation Pressure Coefficient Reasonability

Table P-3 LUPI Minimum and Maximum Values

Table Q-1 Primary Satellite Vital Statistics

Table Q-2 Secondary Satellite Vital Statistics

Preface 1

Preface

Since Explorer 1 was launched on January 31, 1958, the United States (U.S.) has reaped the benefits of space exploration. New markets and new technologies have spurred the economy and changed lives in many ways across the national security, civil, and commercial sectors. Space technologies and space-based capabilities now provide global communications, navigation and timing, weather forecasting, and more.

Space exploration also presents challenges that impact not only the U.S. but also its allies and other partners. A significant increase in the volume and diversity of activity in space means that it is becoming increasingly congested. Emerging commercial ventures such as satellite servicing, in-space manufacturing, and tourism as well as new technologies enabling small satellites and large constellations of satellites present serious challenges for safely and responsibly using space in a stable, sustainable manner.

To meet these challenges, the U.S. seeks to improve global awareness of activity in space by publicly sharing flight safety-related information and by coordinating its own on-orbit activity in a safer, more responsible manner. It seeks to bolster stability and reduce current and future operational on-orbit risks so that space is sustained for future generations. To this end, new and better Space Situational Awareness (SSA) capabilities are needed to keep pace with the increased congestion, and the U.S. seeks to create a dynamic environment that encourages and rewards commercial providers who improve these capabilities.

The overall purpose of the National Aeronautics and Space Administration (NASA) Spacecraft

Conjunction Assessment and Collision Avoidance Best Practices Handbook is to equip owners/operators (O/Os) with the best practices they need to conduct their operations safely. The

Handbook

• Reflects the goal of Space Policy Directive-3, the National Space Traffic Management

Policy (SPD-3), to develop safety standards and best practices that consider

“maneuverability, tracking, reliability, and disposal.”

• Reflects how NASA currently operates in space, which evolves over time.

• Is organized around a spacecraft’s life cycle. Consideration is given to important topics such as spacecraft and constellation design; spacecraft “trackability;” pre-launch preparation and early launch activities; on-orbit collision avoidance; and automated trajectory guidance and maneuvering.

• Encourages the use of commercially available SSA data and information, and for large constellations, it encourages mitigating light pollution to ground-based astronomy.

• Focuses on current NASA best practices. It will be updated as new approaches for conjunction assessment and on-orbit operations are developed.

• Provides guidance for the requirements in NASA Procedural Requirements (NPR)

8079.1, NASA Spacecraft Conjunction Analysis and Collision Avoidance for Space

Environment Protection.

Preface 2

A parallel document may be developed to highlight additional practices as the U.S. Department of Commerce develops the Open Architecture Data Repository (OADR) mandated by

SPD-3. Other commercial services may be offered to supplement or replace the government services currently offered.

The approaches outlined in this document are offered to spacecraft Owners/Operators (O/Os) as an example of responsible practices to consider for lowering collision risks and operating safely in space in a stable and sustainable manner. Entities offering, or intending to offer, SSA or conjunction assessment services should consider the information in this handbook from the perspective of augmenting or improving upon existing capabilities as the entire space industry benefits from advancing these capabilities. In the near term, raw observation data can be used to improve close approach predictions. Longer-term improvements might be found in enhancing notifications and data sharing, developing new models, and enabling increased automation.

NASA continuously examines and actively updates its best practices for conjunction assessment as the industry undergoes rapid evolution. Large constellations of satellites, for example, comprise a new and evolving paradigm for which NASA is developing in-house expertise.

NASA seeks input from the community to improve the content presented in this document.

Comments or suggestions may be submitted to ca-handbook-feedback@nasa.onmicrosoft.com.

For space operations regulated by other U.S. agencies such as the Department of Commerce, the

U.S. Federal Aviation Administration (FAA), and the U.S. Federal Communications

Commission (FCC), NASA defers to those agencies. As part of interagency consultations and to contribute to safe and sustainable space operations, NASA partners such as the FAA and FCC request NASA review of license, payload, and/or policy applications made by commercial space operators to U.S. Government regulatory agencies. In addition to the information required by those regulatory agencies, NASA has prepared examples of information for various types of missions that is valuable in expediting NASA’s review. Current examples can be found at https://www.nasa.gov/recommendations-commercial-space-operators. Commercial space operators are welcome to contact NASA with questions about these examples.

This document was developed in close collaboration with the U.S. Space Command

(USSPACECOM), one of NASA’s closest interagency partners in ensuring safe operations in space, as part of the NASA/U.S. Department of Defense (DOD) Interagency Working Group for

Large Constellation Conjunction Assessment Best Practices. Special thanks are due to working group members Mr. Jeff Braxton of USSPACECOM’s Strategy, Plans, and Policy Directorate, and Ms. Diana McKissock and Ms. Cynthia Wilson of the U.S. Space Force’s 18th Space

Defense Squadron (18 SDS).

1.0 Introduction 3

1.0 Introduction

This document is intended to provide NASA spacecraft program and project managers, generically referred to in this handbook as O/Os, with more details on implementing requirements in NPR 8079.1 and to provide non-NASA O/Os with a reference document describing existing NASA conjunction assessment and collision avoidance practices.

Within NASA, space flight missions are typically implemented through a program/project office structure. For example, a human exploration program might involve multiple missions (i.e., discrete launches) to a common goal. In this document, the term “mission” means the end-to-end activity that results in the creation, launch, operation, and eventual disposal of a spacecraft.

This document provides detailed information on spacecraft conjunction assessment and collision avoidance topics. Best practices for each topic are described and justified throughout this document by mission phase. A summary list of these best practices without the supporting explanatory text is provided in appendices C and D. Appendix C is a complete listing of all the best practices so that non-NASA mission managers can review them to determine which are useful for their mission. The mission manager would then need to identify the person or group responsible for performing the best practice and arrange for that effort. For NASA missions, Appendix D lists the best practices and the party responsible for performing them.

Different organizations use the term “conjunction assessment” in different ways, but NASA defines a 3-step process:

1. Conjunction assessment (otherwise referred to as “screening”) – The process of comparing trajectory data from the “primary object” (the “protected asset” in NPR

8079.1) against the trajectories of the objects in the applicable database1 and predicts when a close approach will occur within a chosen protective volume placed about the asset.

2. Conjunction risk assessment – The process of determining the likelihood of two space objects colliding and the expected consequence if they collide in terms of lost spacecraft and expected debris production.

3. 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 provision of ephemeris data to the secondary O/O to enable that spacecraft to plan and execute an avoidance maneuver.

1 For Earth-orbiting objects, screening is performed against the space object catalog maintained by the United States

Space Command (USSPACECOM) to predict when a close approach will occur within a volume of space called a

“safety volume” placed about the asset. This catalog includes information about international and commercial operational spacecraft as well as all trackable debris. For non-Earth-orbiting objects, the ephemeris of the primary

(protected) asset is only screened against other provided ephemerides.

1.0 Introduction 4

This document compiles best practices based on the U.S. Government process that exists today where conjunction assessment screening is performed by USSPACECOM/18 SDS and 19 SDS.

The risk assessment process is necessary because the orbit solutions of the catalog objects have varying accuracies depending on factors described in this document. Currently, O/Os need to perform this function for themselves (or hire a third party) because 18 SDS and 19 SDS are not tasked to perform this necessary risk assessment function. In the future, the Department of

Commerce may offer risk assessment services as it takes on a space traffic management role, and broader commercial services will be available.

Some entities use Two-Line Elements (TLEs) to perform conjunction assessment. This practice is not recommended because the TLE accuracy is not sufficient to perform the necessary conjunction assessment calculations. (See Appendix E for more information on TLEs.)

The terms “satellite” and “spacecraft” are interchangeable in this document. The word “object” means any discretely identifiable debris or other cataloged item in addition to satellites and spacecraft.

The term “large constellation” is defined loosely in this document. The U.S. Government Orbital

Debris Mitigation Standard Practices (ODMSP) defines “large constellation” as containing 100 or more spacecraft. However, because constellations having as few as 10-20 spacecraft can experience greater conjunction risk, the O/O of any constellation of spacecraft is asked to consider the intent of the best practices in this document and implement all of them to the degree possible.

2.0 Roles and Responsibilities 5

2.0 Roles and Responsibilities

This section provides a brief introduction to the organizations referenced in this document.

The U.S. Space Command (USSPACECOM), one of the combatant commands within DOD, is responsible for U.S. military space operations. For the purposes of this document, it is the entity responsible for establishing SSA sharing agreements with domestic and international entities, both governmental and private. It establishes guidance and direction governing execution of the congressionally mandated SSA sharing program on behalf of the U.S. Secretary of Defense. It also oversees execution of the SSA sharing program as well as day-to-day operations of the

USSPACECOM SSA and space flight safety support.

The U.S. Space Force's 18th and 19th Space Defense Squadron (18 SDS and 19 SDS) maintain the U.S. catalog of space objects and provide 24/7 space flight safety support on behalf of USSPACECOM. 18 SDS conducts advanced analysis, sensor optimization, human space flight support, reentry/break-up assessment, and launch analysis; 19 SDS conducts conjunction assessment and launch conjunction assessment.

The NASA Human Space Flight Operations Directorate (FOD) of the Johnson Space Center

(JSC), through the console positions Trajectory Operations Officer (TOPO) and Flight Dynamics

Officer (FDO), provides conjunction risk analysis support to the space flight missions that fall under NASA human space flight. The International Space Station (ISS) and vehicles visiting the

ISS receive conjunction risk assessment support from the TOPO,2 while Artemis space flight missions receive conjunction risk assessment support from their assigned FDO. The TOPO group is the sole liaison to USSPACECOM and the U.S. Space Force for matters related to trajectory maintenance and the orbital safety of human space flight assets.

The NASA Conjunction Assessment Risk Analysis (CARA) Program located at the Goddard

Space Flight Center (GSFC) provides conjunction analysis and risk assessment services for all

NASA spacecraft not affiliated with human space flight. CARA is responsible for protecting the orbital environment from collision between NASA non-human space flight missions and other tracked and cataloged on-orbit objects. CARA is responsible for routinely collecting predicted orbital information from NASA spacecraft operators, passing it to NASA Orbital Safety Analysts

(OSAs) for screening against the space object catalog, analyzing the screening results to determine the risk posed by predicted close approaches, and working with NASA spacecraft operators to determine an appropriate mitigation strategy for the risks posed by close approaches.

CARA is the sole entity with authority to submit Orbital Data Requests (ODRs) to DOD on behalf of NASA non-human space flight entities in accordance with NPR( 8715.6, NASA

Procedural Requirements for Limiting Orbital Debris and Evaluating the Meteoroid and Orbital

Debris Environments. CARA is the designated point of contact between NASA and

USSPACECOM and the U.S. Space Force for matters related to trajectory maintenance and orbital safety of non-human space flight assets.

2 However, for missions deployed from ISS or visiting vehicles, JSC FOD certifies that the mission has a conjunction assessment process as outlined in jettison policies but does not provide direct conjunction risk assessment support.

3.0 Conjunction Assessment: Past, Present, and Future 6

3.0 Conjunction Assessment: Past, Present, and Future

This section provides a brief history of the conjunction assessment function, including an overview of who the actors are and why the activities are arranged as they are. It explains why the conjunction assessment and related safety-of-flight services currently available exist in their present forms and how they are evolving to accommodate new requirements.

Origins

The present conjunction assessment process, which predicts close approaches between space objects, began in support of the NASA Space Shuttle Program return-to-flight effort after the

Challenger accident in 1986. Drawing on the DOD’s ability to track space objects for missile defense and SSA, the two U.S. Government entities partnered to expand the capability to offer a methodology that would protect humans in space.

What most people consider the “space catalog” is a database of TLE sets that permits a medium-fidelity propagation and orbit prediction of all tracked objects larger than approximately 10 cm in

Low Earth Orbit (LEO) using Simplified General Perturbation Theory #4 (SGP4).3 The satellite catalog was originally designed not for conjunction assessment but for generalized space safety, predicting the location of objects with sufficient accuracy for sensors in the network to reacquire those objects, thereby maintaining custody for purposes such as ensuring that intercontinental ballistic missile threats could be differentiated from satellites in orbit. The catalog was then, and is today, maintained using the radars, telescopes, and other sensors comprising the Space

Surveillance Network (SSN). In recent years, however, USSPACECOM has been working to augment the existing network by incorporating observations from international and commercial entities.

For ISS conjunction assessment to better protect humans in space, NASA collaborated with the

U.S. Air Force to develop an improvement to the TLE catalog that would be accurate enough to compute a Probability of Collision (Pc) between two objects. To compute Pc, orbit determination covariance data were needed. Since the general perturbations theory used to maintain the TLE catalog (SGP4) did not produce a usable covariance, the current Special Perturbations (SP) Space

Object Catalog was developed. 18 SDS, the U.S. Space Force unit that currently maintains the catalog at Vandenberg Space Force Base (VSFB) on behalf of USSPACECOM, 18 SDS at

VSFB, California and 19 SDS at Dahlgren Naval Base, Virginia have greatly expanded the process of screening protected assets since that time (especially since the Iridium 33/COSMOS

2251 collision in 2009) to include all operational assets currently on orbit.

However, since the process was developed 50 years ago and was grown piecemeal to meet existing needs, it contains certain oddities of evolution that may not be anticipated by O/Os who are accustomed to using current technology and methods. Understanding how the 18 and 19 SDS process works is key to using it properly to protect on-orbit assets and the space environment.

3 Hoots and Roehrich 1980.

3.0 Conjunction Assessment: Past, Present, and Future 7

USSPACECOM Conjunction Assessment Process

The full conjunction assessment screening process used by USSPACECOM with all accompanying details is documented in the Spaceflight Safety Handbook for Satellite Operators

(the Spaceflight Safety Handbook), which can be obtained along with other helpful information from the USSPACECOM website space-track.org. This website is the principal way

USSPACECOM communicates and exchanges conjunction assessment-related information with

O/Os.

• All non-NASA spacecraft O/Os should familiarize themselves with the Spaceflight

Safety Handbook as well as other website contents and offerings.

• NASA spacecraft O/Os do not need to be familiar with the Spaceflight Safety Handbook since CARA or JSC FOD will perform all actions on their behalf.

(See Section 7 in this document for USSPACECOM contact information including Uniform

Resource Locators (URLs) to obtain products. See Sections 5.1 through 5.3 for details of some topics from the Spaceflight Safety Handbook to provide end-to-end understanding of the conjunction assessment process.)

Commercial conjunction assessment providers, often using space catalogs assembled from private sector space tracking assets, are now an available resource for O/Os who wish to obtain conjunction assessment services. NASA encourages the use of validated commercial conjunction assessment services when they can augment the existing USSPACECOM capabilities. NASA recommends that all O/Os avail themselves of the USSPACECOM conjunction assessment service at a minimum as an adjunct to a commercially procured service. (See Section 6.1 for additional information about orbital data for conjunction assessment; Appendix J, which outlines conjunction risk assessment tools validation procedures; and Appendix L, which addresses the question of the use of commercial SSA data in conjunction risk assessment.)

To address this topic, the following specific practices are recommended:

A. Obtain a space-track.org account.

B. Become familiar with the Spaceflight Safety Handbook contents.

C. Use USSPACECOM conjunction assessment service as a minimum for screenings, even if additional commercial data or services are used.

SSN Tracking Functionality Description

The Space Surveillance Network (SSN), a global sensor network made up of optical telescopes and radars, is used to collect the tracking observations that are processed to maintain the High

Accuracy Catalog (HAC) of on-orbit space objects. Figure 3-1 shows the current locations of the member sensors.

3.0 Conjunction Assessment: Past, Present, and Future 9

The USSPACECOM process is different from a commercial process in which a vendor is compensated directly for the tracking of an object with the flexibility of being able to obtain additional data if more funds are available to support the request. Because of the time required to collect additional data on an object of concern using the USSPACECOM process, it is recommended that O/Os provide a significant length of ephemeris prediction (including planned maneuvers) to USSPACECOM for screening. This screening duration allows time for

USSPACECOM conjunction assessment operators to increase tasking for identified secondary objects with insufficient orbit determination solutions, and thus to enable the best Pc to be computed for use in conjunction risk assessment decisions. (See Section 6.1 for a more detailed discussion of these issues.)

To address this topic, the following specific practice is recommended:

A. Provide seven (7) days of predicted ephemeris (including maneuvers) to

USSPACECOM for screening for LEO spacecraft, and provide 14 days for other

Earth orbits; e.g., High Earth Orbit (HEO)/Geosynchronous Orbit (GEO).

USSPACECOM Conjunction Assessment Screening Timeline and Process

Personnel at the USSPACECOM facility at Dahlgren Naval Base currently perform screenings of protected assets against the entire HAC once per day at a minimum and up to three times per day every day. Screenings are performed using both the ephemeris data that O/Os share (which include planned maneuvers for the asset) as well as the SSN-derived orbit determination solution for the asset (which does not consider predicted maneuvers).

Results from the screenings, in the form of Conjunction Data Messages (CDMs), are posted to space-track.org for customers with accounts. Entities without accounts who have provided an email address to USSPACECOM may receive basic prediction information by email if an emergency is detected.

Special/off-cycle screenings can be requested per the guidance in the Spaceflight Safety

Handbook. It is important for safety of flight for all spacecraft operators to ensure that there are no large latencies or gaps in the process as uncertainties in the propagated orbit continue to grow over time. For instance, submitting an ephemeris for screening today at noon that has an epoch of today at 0 hours means that the data is already 12 hours old. If it then takes 8 hours to screen that data and another 2 hours to analyze the results, the data will be very old indeed. Ensure that the data screened is for a future time and is analyzed and acted upon before it becomes stale. (See

Section 6.2 for more information.)

3.0 Conjunction Assessment: Past, Present, and Future 10

To address this topic, the following specific practices are recommended:

A. Provide at least one ephemeris per day to be screened and three ephemerides per day in the lower-drag LEO regime (perigee height less than 500 km).

B. Determine whether the O/O’s process for obtaining screening results and performing conjunction risk assessment aligns with the timeline of the

USSPACECOM process. If the timelines do not align in such a way as to enable timely and efficient screening support, pursue a rearrangement of the O/O’s process to minimize data latency and optimize screening efficiency.

Data Sharing with Other O/Os

To facilitate communication between operators to mitigate a close approach, USSPACECOM maintains a list of contacts for each asset. Maintaining this list of contacts is critical so that someone can always be reached. Data sharing is further facilitated though the use of standard formats and coordinate frames. (See Section 6.4 for a more thorough treatment.)

To address this topic, the following specific practices are recommended:

A. Populate and maintain the point of contact section on space-track.org with your operations contact data. Be sure that the operations contact can be reached 24/7 due to time zone differences between operators and the immediate nature of certain conjunction assessment emergencies.

B. Use standard ephemeris, CDM, and maneuver notification formats defined by the

Consultative Committee for Space Data Systems (CCSDS).

SSA Sharing Agreements

USSPACECOM/J535, SSA Data Sharing Branch, negotiates SSA sharing agreements, which establish the parameters within which data will be exchanged by the signing parties to facilitate ongoing cooperation and advance space flight safety. These agreements are useful because they substantially expand the types of data products that can be received over the default products provided to entities without agreements.

Any member of the space community, including satellite operators, launching agencies, commercial service providers, and research/academic institutions, that wishes more and more frequent SSA products than are provided through a generic space-track.org account should contact USSPACECOM. (See Section 7 of this document for contact information.)

U.S. Government organizations and their contractors have implied agreements and do not need to pursue a separate formal arrangement with USSPACECOM.

NASA Partnership with USSPACECOM

Since the development of the HAC and its use in routine conjunction assessment screenings, NASA has partnered with USSPACECOM and its predecessor organizations. That partnership currently includes special, dedicated conjunction assessment screening support. The human

3.0 Conjunction Assessment: Past, Present, and Future 11

space flight program uses U.S. Air Force civilians for conjunction assessment screening support, while NASA’s non-human space flight missions are supported by NASA contractors called

Orbital Safety Analysts (OSAs) who work on the operations floor in the VSFB operations center.

While both groups do essentially the same work with the same input data, NASA OSAs focus specifically on NASA needs and can write scripts that are used to tailor the output data for use by

CARA.

Conjunction assessment is a 3-step process:

1. The first step is conjunction assessment “screening,” which involves computing the predicted close approaches between the protected asset and the catalog of space objects.

This is the step performed at the USSPACECOM facility for mission customers, and screening results are provided that describe predicted close approaches. Both the

USSPACECOM conjunction assessment cell and NASA OSAs perform this function.

2. The second step is conjunction “risk assessment,” in which the screening results are analyzed to determine the level of risk posed by each predicted close approach and to determine whether the predicted close approaches warrant additional investigation and, ultimately, mitigation. This step is critical because not all predicted close approaches require mitigation, and often close approaches require analysis to determine what action is warranted. This step is not performed by USSPACECOM as it is a responsibility allocated to O/Os or organizations that perform this activity on the O/O’s behalf. NASA

JSC FOD performs this function for human space flight program assets, and NASA

CARA performs this function for all other NASA spacecraft.

3. The third step, if required, is conjunction “mitigation,” in which the O/O plans, and perhaps executes, a collision avoidance maneuver or other mitigation solution to reduce the collision risk to an acceptable level. This activity is allocated to the individual O/Os although NASA CARA does provide some basic support tools to aid in the initial choice of mitigation actions for its mission customers.

Because risk assessment requires different products and yields different analyses than screening, it is important to consider the tools and processes required to assess risk properly and adequately.

For large constellations, the cadence of conjunctions and the consequence of a collision for the proper operation of the constellation are both heightened. (See Section 6 for more detail on risk assessment processes.)

3.0 Conjunction Assessment: Past, Present, and Future 12

A. Develop a robust safety-of-flight process that includes both conjunction assessment screening and risk assessment to inform close approach mitigation decisions.

B. Large constellation operators should work with USSPACECOM pre-launch to determine if variations from the standard approach are necessary and, if so, to define a tailored screening process.

C. Large constellation operators should consider working with NASA to define a risk assessment process. NASA is an experienced representative operator.

Including NASA in discussions regarding establishing a conjunction assessment process will ensure that the process will work with most operators for risk assessment purposes.

Conjunction Assessment in Cislunar Space

Significant investments by the U.S., China, other nations, and commercial companies in missions to cislunar space between Earth and the Moon have highlighted the need for proper measures to ensure the safety of the space environment in that regime. While cislunar space will remain “big” for some time yet, popular locations and transit trajectories are likely to experience rapid growth, requiring effective conjunction risk analysis to maintain safety. Implementing appropriate measures for spacecraft will ensure readiness to perform effective conjunction risk assessment as cislunar space traffic increases.

In November 2022, the White House National Science & Technology Policy Council released the National Cislunar Science & Technology Strategy, an interagency strategy to guide developments across a spectrum of emerging areas in the cislunar “ecosystem” (National

Cislunar Science & Technology Strategy, 2022). Several objectives espoused in this document pertain to Space Situational Awareness (SSA) and space safety, including the following:

• Develop technical foundations of best practices for safe cislunar space flight operations.

• Develop an integrated cislunar object catalog.

• Develop procedures for publicly sharing cislunar SSA data as well as navigation and space flight safety support in cislunar space.

• Ensure that capabilities for U.S. Government cislunar operations are scalable and interoperable with systems operated by private and international actors.

(For additional details and context, refer to the Strategy document.)

Key Goals

For the purpose of outlining best practices for conjunction risk assessment, key goals center on sharing data and ensuring interoperability. These goals can be achieved through the delivery of

3.0 Conjunction Assessment: Past, Present, and Future 13

standardized file formats and content, along with the use of consistent trajectory models and coordinate frame definitions.

The use of consistent trajectory models and coordinate frame definitions is best realized by conforming to the recommendations in the recently released NASA Technical Publication

“Astrodynamics Convention and Modeling Reference for Lunar, Cislunar, and Libration Point

Orbits” (Folta et al. 2022). This document details coordinate and time systems, numerical integration techniques, and trajectory modeling of three-body systems at different levels of fidelity appropriate for mission design and navigation of spacecraft well beyond geosynchronous orbit (GEO). A cislunar conjunction risk assessment best practice is to conform to the astrodynamics conventions and modeling recommendations of this NASA technical publication to enforce consistency across ephemerides from different sources, thereby enabling more accurate conjunction assessments.

The goal of standardizing file formats and content has been addressed by DOD, NASA, and academia through a Cis-Lunar SSA Technical Steering Group led by the National Geospatial-

Intelligence Agency (NGA). The Steering Group has produced a draft document outlining an enhanced message set that enables communication of space object catalog and observational data for the cislunar regime (Butt 2021).5 The recommended set covers object state and covariance information in the catalog content message and sensor observations in the tracking data message.

These formats overcome shortcomings of the current SSA message set that prohibits accurate expression of cislunar data and also update features of the current set that have not evolved with advances in SSA since their inception in the 1960s.

A cislunar conjunction risk assessment best practice is to conform to these recommendations when developing SSA architectures to support cislunar missions.

Conjunction Screening Process

Sharing of standardized ephemeris and covariance data is the intent of the current practice of

Earth orbiter owner/operators (O/Os) delivering files to conjunction screening providers such as the U.S. Space Force 18th Space Defense Squadron (18 SDS), 19th Space Defense Squadron (19

SDS), or CARA for use in computing a probability of collision (Pc) during encounters. However, cislunar conjunction assessment currently has no catalog of cislunar objects with independent trajectory and covariance solutions and so requires a different screening process.

NASA currently performs conjunction assessment around Mars, the Moon, and Sun/Earth libration points through the Multimission Automated Deepspace Conjunction Assessment

Process (MADCAP) based at the Jet Propulsion Laboratory (JPL). More information about

MADCAP and related processes is available on the organization’s website:

https://www.nasa.gov/cara/madcap and in the document Tarzi et al. 2022. Spacecraft operators, including for non-NASA and even non-partner international spacecraft, deliver ephemerides to the NASA Deep Space Network (DSN) Service Preparation Subsystem (SPS) portal, through

5 https://www.nasa.gov/sites/default/files/atoms/files/cislunar ssa proposed message set-nov2021 rev2 without emailspdf.pdf

3.0 Conjunction Assessment: Past, Present, and Future 14

which MADCAP can access them. MADCAP assesses conjunction risk based on Pc if a covariance is provided, or (if necessary) via typical radial and timing prediction uncertainties provided by the respective mission navigation teams. These uncertainties are fit to polynomials in time and mapped to the relative node of the two orbits to assess the risk of collision at the crossing. However, as described in Tarzi et al. 2022, MADCAP has implemented the capability to compute two-dimensional Pc (2D-Pc) when given ephemerides with covariance. When potentially risky conjunctions are discovered, notifications and conjunction event data are forwarded to both satellites’ operations teams, who then are expected to work with each other to determine if a mitigation action is necessary and, if so, execute it. JPL’s MADCAP Team will facilitate the decision process if the operations teams cannot agree on a course of action.

Regarding the lack of a centralized catalog of cislunar objects, that is currently in work by

19 SDS. In fact, many cislunar conjunction assessment capabilities are evolving with the potential for significant advances in deep space tracking, trajectory modeling, and ephemeris sharing interfaces. Because many of these advances are still in development, the baseline for conjunction risk assessment must rely on ephemeris and covariance produced by the O/O rather than catalog screening.

As such, the O/O should deliver a CCSDS Orbit Ephemeris Message (OEM) formatted ephemeris with covariance to both space-track.org and to the DSN SPS portal. This will allow access to the trajectory and its uncertainty for screening by both 19 SDS (through SpaceTrack) and MADCAP (through SPS).

The CCSDS format is the current industry standard, aligning with the standardization goal outlined above. Because a non-cooperative orbit solution for cislunar spacecraft is not currently available, inclusion of the covariance in the O/O ephemeris file is essential to allow computation of the probability of collision. Establishing the best practice of O/Os providing accurate trajectory and covariance for their spacecraft in cislunar space to current ephemeris interfaces sets the stage for effective conjunction screenings as operational advances continue to be made in the cislunar regime.

To address this topic, the following specific practices are recommended:

A. Owner/operators should conform to recommendations in the NASA Technical

Publication “Astrodynamics Convention and Modeling Reference for Lunar, Cislunar, and Libration Point Orbits.”

B. Cislunar mission support entities (sensors, catalog maintainers, etc.) should conform to the enhanced SSA message set (Butt 2021) when creating cislunar SSA architectures.

C. Owner/operators should maintain accurate orbit determination solutions and predicted covariance for objects in cislunar space.

D. Owner/operators should deliver predicted ephemeris with covariance in the CCSDS format to space-track.org and to the DSN SPS portal.

4.0 Spacecraft and Constellation Design 15

4.0 Spacecraft and Constellation Design

Safety of flight is an integral aspect of satellite space operations and should be considered as part of design decisions to enable cost- and mission-effective solutions so that potential impacts to other operators in the space environment can be avoided. Specific design areas for which the consideration of on-orbit safety issues is appropriate include:

• orbit selection,

• spacecraft ascent and disposal activities,

• sensor trackability of the spacecraft,

• spacecraft reliability,

• capabilities for ephemeris generation, and

• capabilities for risk assessment and mitigation.

Each of these items is treated in more detail in this section.

Ascent to/Disposal from the Constellation’s Operational Orbit

Safety-of-flight issues may be considerable when related to the ascent from the launch injection point to the satellite’s on-station position or to the descent to an orbit from which disposal/re-entry can be accomplished directly. Typical ascent and descent trajectory design using now common electric propulsion or low-thrust chemical propulsion can take months to accomplish, potentially passing through highly populated regions of space along the way. For large constellations, the amount of nearly continuous transiting satellite traffic could be especially large as old satellites pass out of service and replacement satellites are added.

All missions should plan for orbit disposal to reduce long-term orbital debris. Reducing the number of inactive space objects minimizes the probability of generating debris through orbital collisions, and the fastest disposal option should be pursued; for example, existing large constellations are promptly deorbiting end-of life spacecraft. The U.S. Government has established Orbital Debris Mitigation Standard Practices (ODMSP) including for post-mission disposal.6 NASA missions are required by NPR 8715.6 to comply with NASA Standard 8719-

14B, Process for Limiting Orbital Debris, which defines how NASA implements the ODMSP.

During ascent and descent, a spacecraft will pass by other assets that are already on orbit. Some but not all of those may be maneuverable. The ascending/descending spacecraft that is equipped to maneuver needs to yield the right-of-way to existing on-orbit assets by performing risk mitigation maneuvers or ascent/descent trajectory alterations.

Special caution is needed to protect humans on orbit. If the ascent or descent trajectory will pass through the ISS altitude, operators should coordinate with the NASA JSC FOD to avoid perigee-lowering approaches that pose persistent and problematic orbital crossings with ISS and other human space flight assets.

6 U.S. Government Orbital Debris Mitigation Standard Practices. 2019 version https://orbitaldebris.jsc nasa.gov/library/usg orbital debris mitigation standard practices november 2019.pdf.

4.0 Spacecraft and Constellation Design 16

A. Perform a study to compute the number of expected close approaches anticipated during ascent and descent as well as the imputed additional satellite reliability that will be required to meet satellite disposal requirements at the chosen operational orbit.

B. If the results of the study show a large burden, consider choosing a different mission orbit with a lower burden.

C. All missions should review and follow the ODMSP guidance standards.

D. When practicable, pursue active disposal using the fastest disposal option available.

E. Recognize that transiting spacecraft should yield way to on-station spacecraft and thus take responsibility for any conjunction risk mitigation maneuvers or transient trajectory alterations that may be required.

F. When planning descent using thrusters, if not planning an approach of circular-orbit altitude reduction, coordinate with the NASA JSC TOPO to ensure that perigee-lowering approaches do not present persistent and problematic orbital crossings with the ISS and other human space flight assets.

General Orbit Selection: Debris Object Density

Space debris is not uniformly distributed about the Earth. Because most of the present space debris is generated from a relatively small number of satellite collisions or explosions, the orbital parameters of these collided or exploded satellites determine where the large debris fields reside.

For example, the debris object density is much greater in the 750-900 km altitude band than in other parts of LEO. If possible, this region is one to avoid as a destination orbit since a larger number of serious satellite conjunctions with debris objects can be expected. An orbit that results in a high number of close approaches can lead to the need to perform many maneuvers that may not fully mitigate the risk of close approach. Spacecraft that cannot maneuver will operate at higher risk of debris creation, effectively presenting additional risk to other operators.

Orbit selection should be informed by a study to determine expected conjunction rates; that is, the expected number of lifetime conjunction risk mitigation maneuvers and the amount of satellite fuel needed to permit the number of desired years on orbit. Published results by NASA

CARA are available and can be consulted to obtain first-order high-interest event density information. (See Appendix F for more information and examples.) Use this information to assess the mission impact for conjunction risk mitigation maneuvers, given the expected lifetime of each satellite (e.g., additional propellant required, impacts on data collection).

4.0 Spacecraft and Constellation Design 17

A. Include considerations of orbital debris density when selecting candidate mission orbits. Debris density diagrams, indexed by object size, are published by the

NASA Orbital Debris Program Office (ODPO). (See Appendix G for more information and examples.)

B. When choosing from the final set of candidate orbits, perform a conjunction frequency analysis to determine the number of conjunction high-interest events per spacecraft per year that can be expected in each of the proposed orbital areas.

Vehicle- and Constellation-Specific Orbit Selection: Spacecraft Colocation

While there is a process used by the International Telecommunication Union (ITU) to ensure that spacecraft locations are deconflicted from a radio frequency perspective, there is no similar centralized process for ensuring that spacecraft locations are deconflicted from a location perspective. This gap has resulted in spacecraft injecting into their planned mission orbit only to find another spacecraft already in or very near that location (colocated), which causes close approaches that then have to be mitigated, potentially very frequently, over the whole mission lifetime.

Another similar problem is that of systematic or repeating conjunctions in which a spacecraft has multiple repeated close approaches on successive orbits with another object over a long period of time; for example, when using electric propulsion to move slowly through a congested area.