Thursday, May 30, 2024

How Lucky Can You Get?

When I first ran across this story a couple of days ago, the emphasis was that some space debris fell near a hiking trail in North Carolina and they thought it was from a SpaceX Dragon capsule. This left me confused for a little while because "everyone knows" that Dragons are reusable. The same handful of Cargo and Crew dragons have been flying for years, with refurbishment between missions. 

Then it occurred to me that both kind of Dragons (Crew and Cargo) also have the equivalent of the old Apollo era service modules, aptly renamed as a trunk. Could it be? 

Today, carried a story about the wreckage with several photos and lots of explanation.  The story was based on going to see the piece because the author lives close to where it came down; near Canton, NC — just outside of the city of Asheville, NC, where he lives.

The piece of debris at the head of the trail on which it was found. Image credit: Future/Brett Tingley

On May 22, groundskeeper Justin Clontz and his father were performing maintenance on a trail at the scenic Glamping Collective, a 160-acre luxury camping property offering private dome-style cabins on a mountaintop with panoramic views of the surrounding Pisgah and Cherokee National Forests. 

Coming around a bend in the trail that day, Clontz and his father stumbled upon an odd piece of junk lying on the ground, not far from the path at all. Roughly 3 feet by 3 feet (1 meter by 1 meter), the debris consisted of shredded carbon fiber composite and scorched metal, with exposed metal bolts and plates poking out of it. It had a faint smell, similar to ozone.

"It landed directly in the middle of the trail," Clontz told "It was just wild. It was crazy-looking. I really didn't know what to think."

They go on to point out that there was no visible damage to trees or anything that they could identify. It was as if someone had carried the piece up to the trail and gently placed it there for everyone to see, but there were no obvious signs people had been there. 

At first they thought it might be from some military aircraft, but the lack of a crash site didn't support that. Astrophysicist Jonathon McDowell looked at the location, looked for some possible sources and didn't take long to conclude that it could well have come from the trunk module of the Crew Dragon that took the Crew 7 mission up and back down.  The actual Tweet allows you to see that map of the reentry path in better detail and it does look to be right over Canton, NC.

The Trunk (AKA Service Module) carries cargo or small satellites and is fitted with solar panels that supply power to Dragon when the vessel is in flight or docked to the ISS. It has fins for aerodynamic control during emergency aborts. While the Dragon itself has a heat shield across its bottom and is slowed to splashdown by parachutes, the trunk is jettisoned before the reentry really gets going, and burns up on reentry. Apparently not completely.

Why do I consider this lucky? Just finding it on the trail is incredibly lucky. Not getting hit by debris from a reentry is really quite the normal result. The surface area of the Earth is 197,000,000 square miles. Of that, the oceans, take up 139,600,000 sq. mi. or about 70% and, of course, stuff that falls into the overwhelming majority of the ocean is lost for good. The area that a person presents is "a couple" of square feet. To turn that 197 million square miles into square feet, multiply it by 27.828 million square feet in a square mile.  The piece of debris needs to hit a target of "a couple" of square feet in 5,492 trillion square feet.  The Aerospace Corporation has estimated the odds of being hit by a piece of space debris is less than a one in one trillion. I think it's quite a bit less than that.

The Glamping Collective plans to build a display case for the debris along the trail where it was found.


  1. For any given US mission where large items are jettisoned prior to re-entry, what fraction of the US land area lies under the re-entry path? What is the average population density for that? What is the descent angle of the debris at impact?Yes, the probability is low but probably not as small as they determined. 1e-9 is the generally accepted civil safety max probability for a catastrophic event in aerospace. Not saying they pencil whipped the numbers but it has been known to have between done to meet a target. And of course the first person hit will be sitting in a kayak just off shore to prove Murphy right.

    1. I tend to think in terms of putting boundary conditions on problems, and that's how I started.

      The only thing that I think can be said with certainty is that the majority of orbits are primarily SW to NE, like that one. The latitude ranges of orbits north to south vary with more satellites in a belt from (rough numbers) +/- 45 degrees than beyond +/- 60 degrees along with a substantial number in polar and sun-synchronous orbits that would reenter on a more north/south trajectory. The descent angle is tangential to the sphere when it starts - parallel to the ground in large scale terms. The angle just before it hits the ground is probably steeper as the thing is losing speed to air drag, but harder to put bounds on.

      To my way of thinking about it, these are distractions that don't change the picture by much. While I'm thinking of the situation of one piece of debris hitting one person on earth, Aerospace's numbers may be from thinking of the total amount of debris reentering in some period of time (1 year? since the start of the space age?) and multiplying probabilities.

      As always, I reserve the right to be wrong but try not to be.

    a hunter-gatherer of the Kalahari Desert whose tribe discovers a glass Coca-Cola bottle dropped from an aeroplane, and believe it to be a gift from their gods