The high-profile imbroglio has drawn attention to so-called “spy balloons” and the seemingly old-fashioned technology’s role in modern espionage.

On Saturday, a US fighter jet felled the Chinese balloon off the coast of South Carolina, with officials saying the recovery effort would reveal more details of the Chinese device’s capabilities.

Beijing decried the move as “an obvious overreaction and a serious violation of international practice”, worsening a political maelstrom that had already seen Secretary of State Antony Blinken postpone a planned visit to China.

Why do governments use surveillance balloons?

In the age of satellites, surveillance balloons – which are typically advanced balloons equipped with high-tech, downward-pointing imaging gear – offer close-range monitoring, Iain Boyd, a professor of Aerospace Engineering Sciences at the University of Colorado Boulder, told The Conversation news site. The balloons are sometimes left subject to weather patterns but can be equipped with a “guiding apparatus” to control their path.

While satellites remain the “preferred method of spying from overhead”, the lower-flying balloons, which hover at about the same height as commercial airlines fly, can typically take clearer images than the lowest orbiting satellites, Boyd explained. That’s mostly due to the speed of such satellites, which complete one Earth orbit in 90 minutes.

Another type of satellite is able to rotate in sync with Earth, allowing it to take continuous images of one location, according to Boyd, although such satellites orbit farther away from the planet, and therefore typically produce foggier images.

Surveillance balloons can also be capable of “gathering electronic signals” and intercepting communications, according to David DeRoches, a professor at the Near East South Asia Center for Strategic Studies at the National Defense University in Washington, DC.

He told Al Jazeera the Chinese balloon shot down by the US could also have been used to “gather information on what kind of signals [the US is] using to track it, so it could possibly identify and classify radar hits … which could be of interest if the Chinese wanted to actually launch an attack.”

What has the US said about the balloon?

US officials said the Chinese balloon was about the size of three school buses and entered the US air defence zone north of the Aleutian Islands in Alaska on January 28, moving across Alaska and into Canadian airspace in the Northwest Territories on January 30, before crossing back into US territory over northern Idaho the following day.

Officials have not offered many details about the technology on the balloon, although they have staunchly maintained it was a “surveillance balloon”. Officials have said the balloon was assessed to have motors and propellers, allowing it to be manoeuvred.

“We are confident it was seeking to monitor sensitive military sites,” a senior defence official told reporters on Saturday.

Officials have also maintained that the balloon did not pose any threat to civilian air traffic or people or property on the ground and that US authorities had preliminarily determined it did not significantly increase Beijing’s “intel capabilities” beyond the government’s pre-existing satellite surveillance assets.

The balloon was felled off the coast of South Carolina by a single AIM-9X Sidewinder air-to-air missile fired by an F-22 fighter jet, leaving a debris field that stretched at least 11.2 km (7 miles), according to the Pentagon. The US Navy was leading efforts to recover the wreckage to further analyse the device.

History of surveillance balloons

US officials also revealed that foreign balloons entering into US airspace was relatively common in recent years, with a senior defence official telling reporters on Saturday that Chinese “government surveillance balloons transited the continental United States briefly at least three times” during the administration of former President Donald Trump, who took office in January 2017 and left in January 2021.

US officials have said a second balloon observed over South America last week was also a Chinese surveillance balloon.

Primitive forms of surveillance balloons came into use in the 1800s.

France used crewed balloons for surveillance in the Franco-Austrian war in 1859. Crewed and tethered balloons were again used shortly after during the US Civil War, which stretched from 1861 to 1865.

Surveillance balloons became more common in World War I and II. During the latter war, the Japanese military used balloons to loft incendiary bombs into US territory. No military targets were damaged, but several civilians were killed when one of the balloons crashed in an Oregon forest.

Just after World War II, the US military started exploring the use of high-altitude spy balloons, which led to a large-scale series of missions called Project Genetrix.

The programme says photographic balloons were flown over Soviet bloc territory in the 1950s, according to government documents.