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Sentinel asteroid spotters are located in a networked orbit around Earth. These spotters—operated by the United Nations—are the first line defense against any asteroids or space debris that may impact Earth if not spotted and stopped.


Throughout the series[]


In the months leading up to the eventual Free Navy Conflict, the Attack on Sentinel Nineteen was an experiment conducted by Marco Inaros' faction to test the United Nations asteroid spotters' sensory capabilities. The captured colony ship Sojourner was on a trajectory which would possibly impact the satellite, while a transmission from onboard the ship claimed to have technical difficulties and children onboard. Secretary-General Avasarala was rushed on board UN One and quickly briefed on the situation; after a brief exchange, Avasarala ordered the vessel to be eliminated immediately. Torpedoes then intercepted the Sinon and destroyed it completely.[1]

A stealth-coated asteroid destroyed with the aid of the UNN's Watchtower and Sentinel networks.


At the outbreak of the Free Navy Conflict, the Free Navy launched several stealth-coated asteroids at Earth; the stealth coating meant that the asteroid spotters could not detect them, and the orbital railguns around the planet could not stop them. Three asteroids impacted Earth, causing significant ecological and economic damage, as well as killing Secretary-General Nancy Gao and a majority of her cabinet. However, upon Gao's order from Avasarala's suggestion, the Sentinel asteroid spotters were linked to the UNN's Watchtower spy satellites—satellites which could penetrate stealth technology. A fourth asteroid was then spotted and successfully destroyed by planetary defenses, along with two further stealth-coated asteroids.[2]

"Strange Dogs"[]

The remains of three asteroids destroyed with the aid of Sentinel-16.

Sentinel satellites were readily in use during the six-month long continuous bombardment of Earth by the Free Navy, eventually managing to spot numerous asteroids after the Free Navy had stopped using stealth coating. Sentinel-16, in conjunction with the UNN Farragut and MCRN Denali, was able to locate three such incoming rocks, resulting in their interception and destruction by orbital railgun barrages.[3]


  • A "sentinel" is a soldier who stands watch and stand on lookout. Much like its namesake, the UN's Sentinel network stands guard over Earth and watches for space debris that could pose a threat to the planet.


  1. The Expanse TV Series - Season 4, Episode 5: "Oppressor"
  2. The Expanse TV Series - Season 5, Episode 4: "Gaugamela"
  3. The Expanse TV Series - Season 6, Episode 1: "Strange Dogs"