A Falcon 9 rocket making an uncontrolled re-entry looked like an alien armada

Enlarge / A Falcon 9 rocket launches from Kennedy Space Center on March 4, 2021. Its second stage returned to Earth three weeks later. (credit: SpaceX)

A little more than three weeks ago, a Falcon 9 rocket launched from Florida's Kennedy Space Center carrying a payload of 60 Starlink satellites into low Earth orbit. It was the first of four such missions flown this month by SpaceX.

The entire mission was nominal, except for a problem with the rocket's second stage. Typically, within an orbit or two of launching, the Falcon 9 rocket's Merlin vacuum engine will relight and nudge the second stage downward so that it harmlessly re-enters Earth's atmosphere into the Pacific Ocean.

Ummm... just caught this flying over my home in SW Portland. pic.twitter.com/CvQJwvWsyj

- Vince LaVecchia (@vincelavecchia) March 26, 2021

However, there was not enough propellant after this launch to ignite the Merlin engine and complete the burn. So the propellant was vented into space, and the second stage was set to make a more uncontrolled re-entry into the atmosphere. In this case, the Falcon 9 upper stage still comes back because when it released the Starlink satellites, the vehicle was orbiting Earth at an average altitude of about 250 km. This high above the Earth, it was still susceptible to drag from the thin wisps of our planet's upper atmosphere.

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