By Jon Kelvey
The Chinese balloon is gone, its remnants being examined by the FBI and the Naval Criminal Investigative Service, but the debate lives on over what precisely China’s intent was. The history of balloons, and the likely capabilities of this particular one, can’t resolve the debate, but they do provide some tantalizing evidence.
In a statement posted to its website, the Chinese Foreign Ministry said the balloon was “a civilian airship used for research, mainly meteorological, purposes. Affected by the Westerlies and with limited self-steering capability, the airship deviated far from its planned course.” The United States maintains that the balloon’s purpose was to spy.
Who is right?
To explore that question, first it’s important to make a critical distinction. Despite the Chinese Foreign Ministry’s choice of words, the object that floated through U.S. airspace for eight days was a balloon and not an airship. Think of the familiar cigar shape of a rigid-bodied Zeppelin or the more bulbous football shape of a blimp. Those are aerodynamically shaped. A balloon, meanwhile, “is an envelope filled with a lift gas, and then that is not generally steerable,” explains balloon entrepreneur Taber MacCallum, co-founder of World View Enterprises and more recently co-founder of Space Perspective, a Florida company that aims to take tourists to the edge of space.
But lacking a direct steering mechanism doesn’t necessarily mean a balloon would be entirely uncontrollable. Here’s why.
High altitude balloons come in two flavors, zero-pressure and super-pressure. “You can think of a zero-pressure balloon sort of like a sack of gas,” MacCallum says, usually helium. “It just has lift gas floppily inside of it.”
This kind of balloon rises in altitude as the sun warms the lift gas, causing the envelope to increase in volume, and descend when the sun goes down and it cools.
MacCallum believes the Chinese balloon wasn’t one of those, based on publicly available images and the balloon’s flight path. It was likely a super-pressure balloon, and if so, its envelope was pressurized and its internal volume fixed. “With a pressurized sphere, as the sun hits it, or it gets warmer, it doesn’t change volumes,” MacCallum says. “So the amount of lift you have for the altitude that it’s at stays the same.”
For this reason, a super-pressure balloon will fly at the same altitude all the time, unless a design twist is employed. One could add a ballast system, and MacCallum believes China may have done just that, as he did with the Worldview Stratolite balloon, a zero-pressure design that vents excess gas via ducts. A ballast system would require a compressor to pump air into a smaller balloon, or ballonet, likely nestled inside the larger balloon in the Chinese case. Drawing air in makes the balloon heavier, lowering its altitude, while pumping air out does the opposite. The equipment slung beneath the Chinese balloon looked large enough to house the necessary batteries and compressor. The rectangular objects looked like solar arrays.
“You use solar panels to charge up batteries, those batteries run a compressor, that compressor allows you to add or subtract ballast all you want,” MacCallum says.
For navigation control, the peculiarities of the tropopause come into play. In this region, generally between 55,000 to 65,000 feet, winds crisscross at various altitudes. By finely controlling altitude, balloons can “surf the tropopause,” MacCallum says, choosing winds to carry a balloon over a target and even loitering for weeks at a time. A well-engineered super-pressure balloon of this type — paired with good weather modeling, he adds — would be more than capable of making a journey from Northern Asia, across the Pacific, to loiter over Montana, home to the nuclear intercontinental ballistic missiles kept at the ready by the 341st Missile Wing at Malmstrom Air Force Base.
As to whether China intended to fly the balloon on such a course, MacCallum cautions that it’s impossible to know for certain based on the balloon’s capabilities, but “if they’re actively controlling it and they didn’t want it to go in the U.S., they could have kept it out of the U.S. based on the winds that are there,” he says.
At the same time, it’s possible that whatever the original mission of the balloon was, its operators had, in fact, lost control by the time it reached U.S. airspace.
“A rogue balloon is not out of the question because you make a balloon that is designed to be rugged and fly for long periods of time,” MacCallum says. He notes that in some of the pictures he had seen of the balloon, the solar panels don’t appear to be doing a great job of pointing at the sun. “If it were to have a major electrical failure, I can totally imagine that this is what would result.”
Regarding the decision to wait to shoot down the balloon, the Department of Defense estimates the payload was about the size of a regional jetliner suspended from a 60-meter tall balloon. That, MacCallum says, explains why the balloon wasn’t shot down until it was back over water on Saturday.
“This isn’t a little weather balloon, a party balloon,” he says. “It’s a big heavy object that would go through a roof, and it’s impossible to know where the balloon will go once you start shooting at it.”
In fact, MacCallum is among those perplexed as to how prior balloon incursions during the Trump administration were initially missed by NORAD, the North American Aerospace Defense Command. As the images taken by amateurs attest, large stratospheric balloons are typically quite visible, even to the naked eye.
“Ground-based telescopes and radar would certainly be able to give us all the information that I just described, very easily,” he says.
But MacCallum also offers a final note of caution about drawing conclusions from the balloon’s performance. He has, in his own balloon experiments, more than once had a balloon seemingly fly the exact path he had hoped, only to realize the balloon was not under control.
“The chances of serendipity are so high,” he says.
