Satellite Safety

Case Histories

CloudSat Exit and Return
April 2011 — June 2012

On April 17, 2011, CloudSat went into safehold and temporarily lost maneuver capability. The CloudSat team determined that the spacecraft's battery had experienced an anomaly which significantly reduced its capacity. The battery degradation was permanent and uncorrectable.

The immediate concern was that CloudSat was drifting towards Aqua and would need to either exit the A-Train constellation or regain full maneuvering capability by June. It became clear that if CloudSat restricted its primary operations to the sunlit portion of its orbit, it had sufficient electrical power to maneuver and possibly continue science operations. More time was needed however to develop the procedures that would allow safe operations.


A-Train Configuration Prior to CloudSat's Anomaly (April 2011)

On June 18, CloudSat successfully fired its thrusters and lowered its orbit by 2.6 km, safely passing under Aqua the following day. In the months that followed, the CloudSat team devised a new operations concept where radar science measurements and maneuvers would be restricted to the sunlit portion of the orbit. CloudSat would reduce its power consumption and essentially hibernated while not in sunlight. This approach, referred to the Daylight Only Operations (DO-Op) mode, allowed CloudSat to collect cloud data for approximately 55% of each orbit.

The next concerns were where and when it could return to the A-Train. CloudSat requested a new location that provided additional safety buffer in the event of another failure. This location and the specific maneuvers used to return to the constellation were approved by the MOWG in a meeting with all stakeholders in January 2012. CloudSat raised its orbit and returned to the A-Train in June 2012. It has operated without major incident since then.


A-Train Configuration after CloudSat's Return (June 2012)

Fermi - Cosmos 2251 close approach
April 4, 2012

Fermi was just one week away from an unusually close encounter with Cosmos 1805, a defunct spy satellite dating back to the Cold War. The update from NASA's Robotic Conjunction Assessment Risk Analysis (CARA) team indicated that the satellites would occupy the same point in space within 30 milliseconds of each other. "The maneuver, which was performed by the spacecraft itself based on procedures we developed a long time ago, was very simple, just firing all thrusters for one second," Goddard's Eric Stoneking explained. "There was a lot of suspense and tension leading up to it, but once it was over, we just sighed with relief that it all went well." [More Details]

Aura and Shijian (SJ)-11-02 satellite
September 3, 2013

On September 3, 2013, there was a close approach predicted between the Aura satellite and the Shijian (SJ)-11-02 satellite. SJ-11-02 is the second in a series of SJ-11 satellites launched by the People's Republic of China into an orbit very similar to that of the Morning and Afternoon Constellations.

Aura flight controllers prepared a risk mitigation maneuver to avoid the close approach, but they did not know whether SJ-11-02 was capable of maneuvering and might do so at the same time (potentially making the situation worse). A request was sent through the US State Department to its counterpart in China to let their space agency know of NASA's planned maneuver, but there was no direct two-way coordination. In the end, both satellites maneuvered, but (thankfully) they passed safely without incident. From this episode, it became clear that the coordination between nations should be improved for future close approaches.


Afternoon Constellation (A-Train) Orbital Configuration

EO-1 and HELIOS-2B close approach
February 4, 2014

The NASA Robotic CARA Team identified a conjunction between the US satellite Earth Observing-1 (EO-1) and the French satellite HELIOS-2B in a screening on January 28, 2014. The time of closest approach (TCA) was 7 days away (on February 4). The CAESAR Team of CNES contacted the CARA Team via e-mail on February 1, when the latest predictions showed a low predicted miss distance of less than 100 meters. The CARA Team responded within 90 minutes to indicate they were also aware of the conjunction, and to provide additional information about EO-1.

Close cooperation continued between CARA and CAESAR. CARA was able to provide an EO-1 predictive ephemeris to CAESAR. The teams agreed that if an avoidance maneuver was necessary, it should be performed only by HELIOS-2B (due to the fact that EO-1's propulsive capabilities are limited at best and due to limitations on distribution of HELIOS-2B orbital data).

CARA and CAESAR exchanged e-mail messages over the following days with the information needed to ensure that the two spacecraft passed safely. The CARA Team simultaneously kept the EO-1 mission operators and management informed regarding both the conjunction data and the cooperation between the CARA and CAESAR teams. As the TCA approached, the risk dissipated and HELIOS-2B did not have to perform an risk mitigation maneuver (RMM).

Sentinel 1A - ACRIMSAT close approach immediately after launch
April 5, 2014

At the end of the first day after the launch (4 April): all deployments have been executed during the night and completed early in the morning at the beginning of the first 'day shift'. As the first day shift nears its end, a serious alert is received: there is a danger of a collision with a NASA satellite that can no longer be manoeuvred. It was decided to manoeuvre Sentinel-1A. The orbit altitude needed to change to escape the chaser. The sequence of commands was uplinked during pass 37 in Alaska/Svalbard/Kiruna/ at 04:33 UTC for execution at 05:14 UTC, outside visibility.

Eyes in the main control room were looking up at the big screens on the wall, waiting for a sign. As the satellite approached Troll ground station on the next pass and the telemetry started to scroll down in the twilight of the control room, the satellite was in Orbit Control Mode and the GPS on-board showed a change in the orbit status. The manoeuvre was successful!