Instead, after contending that the balloon entered the US “due to force majeure” the foreign ministry protested the US downing of the craft as an “indiscriminate use of force against [a] civilian airship” that “seriously violated the spirit of international law and international practice”.

International law experts noted Beijing’s careful wording: the Chinese government has not claimed that the US violated international law – merely the spirit of it, as well as international practice.

Did China violate international law? With the upper limit of sovereign airspace undefined, to what extent can the US even claim sovereignty of its skies?

Sovereign airspace

Steven Schneebaum, an international law expert at Johns Hopkins University, said that the long-standing legal presumption concerning sovereign airspace is that a state maintains sovereignty over the space above the land it controls. It does not have to assert its sovereignty; rather, what must be asserted and justified is any attempt to limit its sovereignty.

“It’s only in the era of aeroplanes, and more recently, space travel … that there have been limitations” set on the ownership of airspace, Schneebaum said.

What’s clear in international law, he added, is that sovereign airspace ends where outer space begins. Objects operating in outer space are not subject to sovereignty claims.

But no one formally agrees on where that boundary lies.

“There is no treaty which clearly defines the upper limit of national airspace. As such, there is no clear dividing line between national airspace and outer space,” said Donald Rothwell, an international law professor at Australian National University.

Beijing says US sent over 10 balloons illegally into China’s airspace

But experts note that international law can still address what is not outer space – and thus, what can be considered sovereign airspace.

Rothwell cited the Concorde, the retired supersonic airliner that had its last flight in 2003, to argue that “customary international law” – widespread state practice followed out of a perceived legal obligation – defines the boundary between airspace and outer space to be at least 60,000 feet (18.29km).

“The supersonic Concorde operated at a maximum operating height of 60,000 feet and was always considered to be within national airspace … So there is an acceptance and state practice that national airspace extends to at least 60,000 feet,” he said.

Customary international law, Rothwell said, was shown by how well countries operating the Concorde abided by airspace regulations as the jets flew across international boundaries at 60,000 feet.

Another example experts cite dates back to 1960 and the Cold War, when the Soviet Union shot down an American U-2 spy plane piloted by Gary Powers. The U-2 reportedly reached heights of 65,000 feet – another indicator of what cannot be considered outer space.

But even lacking a definition of the boundary between sovereign airspace and outer space, legal experts agreed that such a boundary is far above any height that balloons can fly.

Some legal experts contend that the boundary cannot be more than several kilometres lower than the Karman Line at 100km, or about 330,000 feet, above the ground. That’s the approximate altitude where satellites can no longer maintain their orbits around the Earth, which some international science associations define as the lower boundary of outer space.

Balloons can reach no higher than about 100,000 feet (30.48km). The Karman Line also exceeds the reach of communications platforms in the stratosphere, about 164,000 feet.

The Karman Line does not have international legal standing, experts said. But Schneebaum argued that the legal threshold of outer space could not be much lower than the line since the objects that space law is meant to govern – for example, satellites – cannot function below it.

Regardless of what the legal threshold might be, he said, “it is obvious that outer space begins at a lot higher than 100,000 feet”.

Violation of international law

According to legal experts, by invoking the claim of force majeure – a legal term meaning noncompliance with a law or agreement due to unforeseeable forces beyond a party’s control – Beijing essentially recognised US sovereign airspace to be at least the altitude of the balloon, and conceded that it had violated US airspace.

“If the balloon was not within US airspace then China would not have sought to rely on force majeure,” said Rothwell.

Since Beijing did not notify Washington of the balloon’s incursion into US airspace, Rothwell concluded, the US was on “firm legal footing” asserting that the balloon violated US sovereignty and thus in shooting it down about six nautical miles off the US east coast.

The Convention on International Civil Aviation, which both the US and China are party to, allows permission to be sought to enter another country’s airspace, Rothwell says. Article 8 of the convention states that any aircraft flown without a pilot must seek permission.

“As the balloon was within US airspace – which extends to the 12 nautical mile territorial sea offshore the US coast – the US was within its lawful entitlement to respond to a violation of US airspace,” Rothwell said.

Whether the balloon was civilian or military in nature or whether force majeure caused its flight over US territory were irrelevant to the question of whether it violated US sovereignty, Schneebaum said. But he noted, had the craft been staffed, it would have been “unforgivable” – though not illegal – to bring it down.

Beyond the balloon

Countries have a hard time deciding what the boundary between outer space and national airspace is because “there’s a practical question of what a state can defend”, said Henry Hertzfeld, a space law and policy expert at George Washington University.

Science also does not support one particular division line, he continued.

The balloon episode may renew efforts to develop a definition, Hertzfeld said, especially since future technologies – beyond balloons and satellites – may develop.

Rothwell points out that questions are already emerging about a potential “grey zone” at high altitudes: “What is the legal regime that operates above national airspace and is considered to be below traditional outer space?” he asks.

For example, New Zealand’s Outer Space and High-Altitude Activities Act defines 60,000 feet as the boundary between controlled airspace and “high altitude” – without defining the boundary of outer space.

This “high altitude” area could be considered a form of “grey zone”, Rothwell said.