Space agencies around the world are collaborating in the largest coordinated planetary defence exercise in history, using the rare interstellar visitor 3I/ATLAS to test and refine Earth’s readiness against potential cosmic threats. This unprecedented drill — involving scientific institutions, telescopes, and monitoring networks from multiple nations — is drawing global attention and sending a clear message: planetary defence is no longer theoretical. What Is 3I/ATLAS? The interstellar comet 3I/ATLAS (C/2025 N1) was discove...