Late in the evening of March 24, residents of the Seattle area got a bit of a light show. And no, it wasn’t aliens.

CosmoQuest community member ThirdRockAstronomy shared an image of the event on our Discord server. You can see the multiple bright streaks crossing the black background of the sky.

Initial reports called it a meteor, but once experienced observers saw the footage, they noticed that the so-called meteor wasn’t behaving as a meteor would. Typically, meteors only appear for a few seconds and don’t usually leave a lasting trail. This particular object, however, was moving across the sky much more slowly  — taking minutes instead of seconds — and left behind multiple trails.

So if it wasn’t aliens and it wasn’t a meteor, what was it?

If you guessed space junk, you are correct. The object in question was identified as the second stage of the Falcon 9 that launched the Starlink 17 mission about three weeks ago.

Space junk reentry differs from meteors in a few important ways. The most significant difference is the slower apparent speed as compared to a meteor. Another difference is that it broke into multiple pieces with trails that persisted longer than a meteor would.

According to astronomer Johnathan McDowell, a piece of space junk weighing more than one ton (about 900 kilograms) reenters the atmosphere about once a week, but most happen during the day or are over the ocean so they aren’t as noticeable to the public. The combination of evening reentry near a major population center meant that this particular event was visible to more people than usual.

People were alarmed, but this reentry was predicted. Space junk is routinely monitored and cataloged by several organizations. Aerospace Corp, an organization that keeps track of CubeSats in space, issued a Tracking and Impact Prediction message at 23:23 UTC on March 25 and gave a reentry window of March 26 at 05:49 UTC plus or minus two hours. This particular piece of space debris had been tracked in orbit since its launch three weeks earlier. The upper stage was supposed to reenter the atmosphere over southern Australia, but the deorbit burn failed, possibly because of underperformance by the first stage on the ascent. This meant the upper stage needed to burn more propellant to get the payloads into orbit, which resulted in not enough propellant to deorbit at the planned location.

Because the second stage didn’t end up where it was supposed to be, calculations had to be done to figure out where it would end up. Experts like McDowell knew it was coming down soon but didn’t know exactly at which point of its orbit re-entry would occur. (That’s what determines the where and when of the light show.) McDowell explained on Twitter that orbiting satellites or rocket bodies travel at least 27,000 kilometers per hour, so a two-hour uncertainty meant that reentry could be basically anywhere in the Western Hemisphere. The uncertainty comes from not knowing its precise location in space and the specific properties of the atmosphere acting on the object. According to McDowell, most of the upper stage would have melted and broken apart in the upper atmosphere, about 60 kilometers above ground.

Denser bits, such as the engine thrust chamber, might reach the ground. And a denser bit did indeed survive reentry: a Composite Overwrap Pressure Vessel, a part on the second stage that holds pressurized helium, was recovered from the ground looking almost untouched in Grant County, Washington State, southwest of Seattle.

More Information

Falcon 9 R/B Reentry Prediction (Aerospace Corp)