Orbital debris poses a significant threat to spacecraft health and safety. The current estimate of the number of “tracked” objects that are larger than 10 cm is greater than 11,000, with the number of objects increasing by several hundred per year. Most of these tracked objects are characterized as orbital debris. Satellites are routinely hit by small particles that cause little or no damage. However, if a large particle were to hit an operational satellite, the impact could result in the end of the mission. A large part of the orbital debris population resides in low earth orbit (LEO), where the density distribution of cataloged objects is concentrated near mean equatorial altitudes of 700–1100 km. Recent events such as China’s anti-satellite (ASAT) test and break-ups such as the Breeze-M rocket explosion have led to an even greater concern in the satellite community, thus demonstrating the necessity of an operations concept that includes monitoring, computing, and mitigating collision risks. Because of this threat posed by orbiting objects, National Aeronautics & Space Administration (NASA) has established a risk assessment and risk mitigation strategy for their operational spacecraft, driven by requirement NPR 8715.6a.

NASA has two conjunction assessment processes, one for protection of its manned assets, the other for its robotic assets. The NASA Robotic collision risk assessment process is performed by the GSFC Space Systems Protection Mission Support Office (Code 590.1) and Navigation and Mission Design Branch personnel and has been operational since January 2005. GSFC-managed missions are required to use this service to meet NPR 8715.6a requirements. The GSFC process consists of:

1. Close approach predictions are generated between the asset mission and other objects in the High Accuracy Space Object Catalog maintained by the Joint Function Component Command for Space (JFCC-Space).

2. The collision risk for each predicted close approach event is probabilistically assessed.

3. Any necessary risk-mitigating action is planned and potentially executed.

The collision risk assessment process begins with the generation of ephemeris files for each mission based on the latest orbit determination solution. Next, close approach predictions are generated for each spacecraft at the Joint Space Operations Center (JSpOC) by GFSC-dedicated Orbital Safety Analysts (OSAs). The nominal prediction frequency is once per day Monday through Friday. Predictions are made 7 to 10 days into the future.

Once the catalog screening function is completed, close approach prediction results are sent to GSFC. The GSFC CA Team examines the results and provides event trends to the mission stakeholders. Additionally, the GSFC CA Team performs collision risk assessment analysis and will work with the mission stakeholders to help quantify the risk and plan risk reduction maneuvers as required.