Sentinel asteroid spotters are located in various spots of space surrounding Earth. These are the first line defense against any asteroids that may impact Earth if not spotted and stopped.
The attack on Sentinel Nineteen was an experiment conducted by Marco Inaros' faction to test the asteroid spotters' sensory capabilities and to partially knock out its asteroid spotting capability alltogether. The captured colony ship Sojourner/Sinon was on a trajectory possibly hitting the satellite, claiming 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.
When the Free Navy launched several stealth-coated asteroids at Earth the coating meant that the asteroid spotters could not detect them. When three asteroids then impacted Earth the asteroid spotters were linked to the UNN's Watchtowers spy satellites which could penetrate stealth technology. A fourth asteroid was then spotted and destroyed by reacting planetary defences.