Satellite Safety

Ensuring the safety of the international Earth observing constellation satellites.

The purpose of this site is to provide coordination between agencies, countries, and organizations to ensure the safety of the international Earth observing constellation satellites. If any satellite passes too near to other satellites or to orbital debris, the collision risk must be mitigated in advance. When the approaching satellites are both maneuverable, coordination is essential. If the satellites are operated by agencies from different countries, the coordination can be more complex due to language and time zone differences. This web site establishes the mechanism for the agencies to coordinate their actions.

Both constellations 2014 07 17

International Earth Observing Constellations (Morning and Afternoon)


This web site is primarily concerned with the safe operations of the International Earth Observing Constellations at the 705 km orbit. The satellites fly within seconds to minutes of each other to enable near-concurrent observations from a variety of active and passive sensors that collect data regarding the Earth's land, atmosphere, and oceans. The data is used by the science community worldwide to study climate change, air quality, vegetation, deforestation, sea salinity, etc. The data are also used operationally to assist in disaster management and monitoring including fires, volcano eruptions, hurricanes/tsunami, industrial disasters, etc.

At 705 km, the constellation satellites have to avoid collisions with both operational satellites and non-operational space objects.

Orbital Debris

Orbital Debris

Orbital Debris

There are thousands of pieces of orbital debris (fragments of old satellites, old rocket bodies, or similar non-active objects). The estimated population of particles between 1 and 10 cm in diameter is approximately 500,000. The number of particles smaller than 1 cm exceeds 100 million. Orbital debris are all man-made objects in orbit about the Earth which no longer serve a useful purpose. These cannot be maneuvered out of the way of a collision.

» Orbital Debris Program Office

Operational Satellites

Operational Satellites

Satellites near the 705 km Constellation orbit

Most operational satellites can and do change their orbit periodically. Typically this is done to counteract the effects of atmospheric drag, thereby meeting their altitude or ground track requirements. Sometimes, they will maneuver to avoid a close approach with either Orbital Debris or with another satellite. This type of maneuver is called a risk mitigation maneuver (RMM).

» Additional information on the types of orbital maneuvers

In contrast, some operational satellites (for example, Cubesats) do not have the capability to change their orbit to avoid a collision. Satellites that have run out of fuel also fit in this category. And sometimes, it is not known whether a satellite is maneuverable or not.

If two space objects collide, there is the potential for a debris cloud that would threaten operations at that orbital location for all future missions. If the collision involves one of the satellites in either the Morning Constellation (Terra, Landsat 7, and Landsat 8) or the Afternoon Constellation, also known as the A-Train (Aqua, Aura, CloudSat, CALIPSO, GCOM-W1, and OCO-2), the effect would be magnified since the debris cloud would render future operations at the 705 km altitude orbit risky for the foreseeable future.

Why Coordination Is Important

When a maneuverable satellite is predicted to pass very closely to a piece of orbital debris or a non-maneuverable satellite, there is only one course of action - the maneuverable satellite must change its orbit to mitigate the risk.

When a maneuverable satellite is predicted to pass near another maneuverable satellite, the two mission teams must coordinate their actions to ensure that the two satellites are not independently commanded to maneuver in ways that may actually increase (rather than decrease) the risk of collision. If the mission teams reside in different countries, the coordination may be hampered by time zone and language differences.

Case History : Landsat 5 and the A-Train

The Morning Constellation and the Afternoon Constellation satellites all follow similar 705 km orbits. Their orbit planes intersect near the poles. In February 2010, it was discovered that Landsat 5 was crossing through the orbit plane intersections at the poles between CloudSat and CALIPSO. In the next several weeks, Landsat 5 passed behind CALIPSO. Further investigation revealed that Landsat 5 had passed through the Afternoon Constellation over several weeks in late 2004, again in 2008, and then starting again in late 2009. There were several agencies involved - the United States Geological Survey (USGS), the Centre National d'Etudes Spatiales (CNES) from France, and NASA.

A 'Red Team' consisting of both Landsat 5 and the Afternoon Constellation stakeholders was formed in March 2010 to analyze the situation and determine the best courses of action to minimize risks while continuing to provide the most science return from all satellites. The final recommendation coming from the Red Team meeting held at NASA Goddard Space Flight Center (GSFC) in April 2010 was that a "managed crossing approach" should be adopted, taking advantage of the highly predictable nature of the orbits.

This approach was based on the following assumptions being true:

  • A minimum separation during close approaches of 400 meters,
  • At least one of the two missions having a close approach maintains positive control,
  • Any Afternoon Constellation mission or Landsat-5 that experiences an anomaly, failure, or need to exercise debris avoidance prior to a planned maneuver shall immediately provide such information to all other missions, and
  • Contingency plans, including triggers, would be documented within the crossing implementation plan.

The specific procedures to be used to manage the Landsat 5 crossings with the Afternoon Constellation were documented and agreed to by all parties. These procedures were executed over the next several months and resulted in safe passings by all affected spacecraft.

Future inter-constellation crossings may differ and will require consultation between members of both the Morning and Afternoon Constellation teams to devise the most appropriate strategy.

» Additional case histories

NASA's Approach to Satellite Safety

The Morning and Afternoon Constellation (A-Train) satellites maintain their locations in such a way as to satisfy their science requirements. The member satellites teams have agreed to monitor and maintain their orbital locations. This ensures that there are no close approaches between the satellites. For example, the current A-Train satellite spacing is shown in the figure below.

International Earth Observing Constellations

International Earth Observing Constellations (Morning and Afternoon)

In addition, all NASA satellites and a number of related constellation satellites are monitored by the Conjunction Assessment Risk Analysis (CARA) team at GSFC. The CARA team works closely with the United States Air Force's Joint Space Operations Center (JSpOC) to assess the risk of predicted potential close approaches between NASA and International Earth observing assets and other space objects.

If a potential threat from a secondary space object is detected, CARA notifies the affected mission team. The next step depends on the capabilities of the secondary object:

  • If the secondary object is non-maneuverable (for example, orbital debris), the mission team may decide to perform an avoidance maneuver (RMM) if the risk of a collision is too high.
  • If the secondary object can maneuver, the 2 affected mission teams and CARA work together to ensure that the risk is mitigated satisfactorily before the time of close approach (TCA). This is the highly recommended approach.

In either case the time to react to a predicted conjunction to ensure that the risk is mitigated is usually very short (less than 5 days). A small maneuver performed early may have the same net effect as a larger maneuver performed closer to the TCA.

» Additional details of the CARA process

Other space agencies use other services to monitor close approaches. For example, CNES uses the Conjunction Analysis and Evaluation, Assessment and Recommendations (CAESAR) system for risk evaluation and avoidance recommendations.