Many of the more than five thousand satellites orbiting the earth are capable of producing high-resolution images. International agreements aren’t as clear.

A U.S. Air Force U-2 pilot looks down at a suspected Chinese surveillance balloon.

A U.S. Air Force U-2 pilot looks down at a suspected Chinese surveillance balloon as it hovers over the United States, 3 February 2023. (Credit: Department of Defense via AP)

When you look at a map, it is clear where a border begins and ends. But how far up into the air does that border extend?

The downing of a Chinese balloon in February 2023 by the U.S. military off the coast of South Carolina has people looking to the skies with new questions.

The balloon had been tracked travelling across the United States. The Chinese government argued that the balloon was monitoring the weather, but the size of the balloon — 200 feet (61 metres) high — was substantially larger than normal weather balloons. The large bulk of the payload it carried, described as “the size of a regional passenger jet” also raised suspicions that surveillance was the real purpose.

Aircraft have been in existence since 1903. Satellites emerged in 1957. But the upper limit of sovereign or national territory has never been legally defined. 

Passenger jets cruise at altitudes between 35,000 and 40,000 feet or roughly 11,000–2,000 km. Military jets can fly at double that altitude.

Establishing sovereignty of airspace is not so simple.

Both passenger and military aircraft operate according to the Chicago Convention of 1944. Aircraft must seek permission to “engage in international navigation” because “every state has complete and exclusive sovereignty over airspace above its territory.” But the Convention also states that nations must allow civil aircraft a certain level of access.

The Chicago Convention was the basis for the International Civil Aviation Organisation (ICAO), now an agency of the United Nations. But ICAO guidelines do not address the vexed question about the upper limit of sovereign airspace.

Satellites can orbit as low as 99 miles (160 km) above sea level and as high as 20,000 miles (32,000 km). But they are not considered to be violating sovereign airspace. 

That is not the case with aircraft and balloons, which have been targeted and shot down for breaching a country’s territorial integrity.

The issue came to a head in the depths of the Cold War. 

Spies in the skies

In 1960, the Soviet Union shot down an American plane over central Russia. It was a U-2, a state of the art surveillance plane designed to take photographs of strategic targets. The United States claimed that the plane was making weather observations, a cover story that was rapidly disproved by documents found in the wreckage of the plane. 

The Soviet Union captured and imprisoned American pilot Gary Powers. He was returned two years later in a trade for a Soviet spy discovered in the United States. 

But other surveillance missions ended with fatalities. U-2 aircraft were shot down over Cuba in 1962 and four times over China in the 1960s.  

The U-2 may be the most iconic American spy plane, but other U.S. aircraft designed for surveillance include the Blackbird and the AWAC. The Soviets designed the Myasishchev M-55 and various models of the Tupolev aircraft for surveillance missions. Other nations developed their own “spy planes.”

Civilian aircraft have been mistaken for surveillance flights, such as Korean Air Flight 007 targeted by Soviet fighter jets in 1983. The Boeing 747 had apparently strayed into Soviet airspace. All 269 passengers and crew were killed.

Should space be considered a free for all?

After the United States shot down the Chinese balloon in February, U.S. Secretary of State Antony Blinken postponed a planned trip to China and called the balloon a “clear violation of U.S. sovereignty and international law.”

But despite Blinken’s words, there is no definition of sovereign airspace and it remains a gray area in international law.  

Some international lawyers ask whether nations should agree that sovereign airspace ends where space begins. But where is that?

Even within the United States there are two definitions. 

The U.S. Air Force defines the boundary of space at 50 miles (80.4 km) above sea level while NASA uses what’s known as the Kármán line of 62 miles (100 km). The U.S. government has avoided any clarification on the issue. 

Where to draw the line on open skies

Like NASA, the Fédération Aéronautique Internationale (FAI), which has 90 member countries, uses the Kármán line for the boundary of space. Established in 1905, the FAI governs air records and also records human spaceflight missions. 

Other proposals suggest the boundary of space could be regarded as the lowest altitude where satellites can orbit — around 99 miles (160 km) above sea level. 

Spying from the air has been carried out for centuries, initially using observers in balloons during the battles of the Napoleonic wars. Observer balloons were also used during 19th-century conflicts. 

In the 1950s there was a significant investment in air surveillance by the United States and the Soviet Union as well as other nations who wanted to keep an eye on their political adversaries. 

The end of the Cold War changed strategic goals. In 1992, the Open Skies Treaty, which now has 34 members, was signed by the United States and Russia. They agreed that a limited number of surveillance flights, designed to gather intelligence, would be permitted by treaty members.

Not all nations are open to Open Skies.

The Open Skies Treaty, according to the treaty text, was created to promote “greater openness and transparency in their military activities” and to “enhance security by means of confidence.” However, the members are all North American and European nations. 

China has not signed the Open Skies Treaty. But in terms of surveillance, would a definition of sovereign airspace make any difference? 

Many of the more than five thousand satellites orbiting the earth are capable of producing high resolution images up to 3–5 metres in detail. And this data provided by satellites is not limited to governments. 

Determining sovereign airspace and the boundary of space has not been an international priority. The lack of a strict upper limit of national airspace benefits any party who wants to conduct intelligence operations from the air or space.

This area of international law may remain gray for some time.


Three questions to consider:

  1. What did the Chicago Convention of 1944 set out to do?
  2. What is the difference between the altitudes that satellites, passenger jets and military aircraft can fly at?
  3. What would happen if private landowners could control who gets to fly over their land?
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Tira Shubart is a freelance journalist and media trainer based in London. She has produced television news and trained journalists across four continents for international broadcasters, including BBC News, Canadian Broadcasting Corporation and Al Jazeera, over several decades.

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ScienceOn maps, borders pop out. Up above, lines get fuzzy.