The Boom

The city of San Diego issued a warning.

Residents were advised they might hear a loud sound on the morning of April 11. The sound was described as a sonic boom. The source of the sonic boom was identified as the Artemis II capsule, which is returning from the Moon.

This is how you know it worked.

A sonic boom occurs when an object moves faster than the speed of sound. The Artemis II capsule will be moving faster than the speed of sound. This is intentional. The capsule was designed to enter Earth's atmosphere at high velocity so that the atmosphere would slow it down, because the alternative involves landing at high velocity, which has a different outcome.

The residents of San Diego have been asked to be slightly ready for the moment the capsule starts slowing down correctly.

There is something unusual about issuing a warning for evidence of success. Sonic booms from military aircraft are also startling, but those are warnings about practice. This one is a warning about completion. The boom is the mission reporting in. Four astronauts traveled approximately 500,000 miles, flew around the Moon, took photographs, and are now making their way back to a designated patch of Pacific Ocean that has been selected for exactly this purpose.

When they arrive, San Diego will hear it.

The warning recommends remaining calm. This is reasonable. The sound is not a problem. The sound is the problem having been solved. San Diego has been asked to be slightly inconvenienced by the Moon mission succeeding.

The noise has been scheduled.