The United States recently shot down an object which it described as a Chinese spy balloon. While China later agreed that it was one of its balloons, it said it was meant for scientific purposes and had mistakenly moved away from its course.


The incident throws the spotlight on espionage using balloons. While drones and satellites are more commonly associated with surveillance, the history of surveillance balloons goes back much longer.


History of balloon surveillance


One of the earliest recorded instances of a balloon being used for surveillance comes from a battle between Napoleon’s French army and the Austrian and Dutch combined forces in May 1794. A young French captain, Jean Marie Joseph Coutelle, a scientist and engineer who is recognised as a pioneer of ballooning, spied on Austrian and Dutch troops from a balloon.


According to an article in The Atlantic in March 1946, Coutelle’s balloon floated above the French town of Maubeuge, which had been besieged by Austrian troops. Although the Austrian and Dutch troops fired at the balloon, Coutelle stayed out of range. But he conveyed the positions of the enemy troops to the French army, which fired accurately at them.


Later, Coutelle accompanied Napoleon to Egypt, where his balloon was burnt by Admiral Nelson’s troops. Napoleon eventually discontinued his balloon programme.


In the 1950s, during the Cold War, balloon espionage was also used by the United States government for surveillance of Soviet areas.


The science of flying a balloon


While drones and satellites are more sophisticated, balloons too come with certain advantages. They are easier to send afloat, and are cheaper.


While a drone would require something to give it lift, a balloon can be sent afloat without this.


“Aeroplanes, helicopters and drones generate lift by motion. Helicopters can hover but the rotor will have to keep rotating. Aeroplanes will always have to keep moving forward,” Professor M Ramakrishna, Department of Aerospace Engineering, IIT Madras, told ABP Live. 


“The biggest advantage of a surveillance balloon is that there is no issue regarding its flight. No propulsion is necessary to raise a balloon off the ground,” he said.


Helium or hydrogen are the gases used inside a surveillance balloon. “The gas has to be lighter than air. The balloon can float up to enormous heights when filled with helium or hydrogen. A risk in case of hydrogen-filled balloons is that they can burn because the gas is highly combustible. Hence, hydrogen is more difficult to deal with than helium,” Ramakrishna said.


“But the disadvantage is that it has the tendency to move along with the breeze. Therefore, if one wishes to keep it at some place, they will need some propulsion. 


Moreover, even if one wants the balloon to move along a fixed direction, very low-powered propellers will be required,” he added. 


“Also, balloons cannot escape a particular region quickly. This is because balloons are not fast-moving objects. So, once a surveillance balloon has been discovered, it is like a ‘sitting duck’. This means that the balloon can be taken down easily.”


“In the case of helicopters, drones and aeroplanes, a continuous burning of fuels is required to run the engines. So, this limits the flight time,” said Professor H S N Murthy, Head, Department of Aerospace Engineering, IIT Madras. 


“In the case of a balloon, only a little amount of fuel needs to be burnt in order to manoeuvre it,” Ramakrishna added. 


Long-distance monitoring


The material of a surveillance balloon determines if it can be monitored, according to Ramakrishna. 


“If a surveillance balloon is made of rubber, it might not be easy to capture the object via radar. Things made out of metal deflect electromagnetic radiation more easily, and hence, they can be detected easily using radars. Since the balloon is made out of rubber, it cannot deflect electromagnetic radiation,” Dr Ramakrishna explained.


“Tracking surveillance balloons is tough, but one could use satellites to monitor them. A surveillance balloon has to cross the border of a particular country in order to make it possible to track it,” he said, adding: “Vigilance at the border will need satellite imagery to monitor a surveillance balloon.”


Dr Ramakrishna compared surveillance balloons to self-flying aeroplanes, which consist of plastic components. Also, the metal is shaped in such a manner that radars cannot detect the aeroplane. 


Since the amount of metal used in balloons is very small, radars will not be able to detect them. “Radars can detect materials which deflect electromagnetic radiation. A radar station sends a signal which bounces off an aeroplane and comes back. Surveillance balloons are made out of rubber, plastic or fabric-like materials, and hence, they will not deflect the signals sent from radar stations. The signal is either transmitted through or absorbed by the balloon,” Ramakrishna said. 


“In case of satellite imagery, one can take a picture of a suspicious object, say, a surveillance balloon, wait for some time, and then take a second picture. After this, they should look for differences in the two pictures. If there is a moving object, one can spot it using the two pictures,” Ramakrishna explained.


Once a surveillance balloon has been identified, tracking it is easy because it is slow-moving. 


What equipment can be fit inside?


If one makes a very large balloon, they can proportionately fit an extremely large weight inside.


“Surveillance balloons can carry different types of equipment. The balloons could be used for visual surveillance or electronic surveillance. Any kind of payload can be carried. Most surveillance balloons weigh about 80 to 100 kilograms,” Murthy said.


Surveillance balloons can carry equipment such as cameras in the visual or infrared range, or electronic equipment which are used to pick up signals and conduct electronic surveillance. Synthetic aperture radar could also be carried, according to Murthy.


Dr Ramakrishna said surveillance balloons could also be used to interfere with someone else’s communication systems. 


What is the future of surveillance balloons?


Surveillance balloons are here to stay, but people will also come up with counter measures. According to Dr Ramakrishna, a surveillance balloon could be tethered to the ground in a way such that it not only remains in the territory of the country which owns it, but can also see things beyond the country’s borders. In this way, the other country will not be able to shoot down the balloon, as that would be an act of war. The balloon should be sufficiently high so that it can see what the other country is doing. 


“People will come up with more creative solutions to use surveillance balloons. In the past, they have been used for traffic monitoring and advertising. There are countries which use such balloons during sports events. A camera is placed inside the balloon to take pictures from above,” Ramakrishna said.