How stratospheric balloons work
What could be simpler than a balloon? It needs no engine or fuel, and the risk of a failure is minimal. Its structure consists of nothing more than some gas inside a plastic bag. It relies exclusively on natural forces: buoyancy for lift, winds for direction and gravity to descend.
The hot-air balloon or Montgolfiere balloon invented in the 18th century by the Montgolfier brothers is the ancestor of today’s lighter-than-air craft. The only difference is that modern balloons no longer need a pilot aboard to steer them and stabilize their altitude.
Today, balloons are a unique tool for scientific research. Only balloons can stay aloft long enough in the stratosphere, a region of the atmosphere too low for orbiting satellites, and traversed too quickly by sounding rockets to obtain meaningful data.
So, how and why do balloons fly? What kinds of balloons are used for science? Here’s a surprising fact: some balloons are bigger than a football field and able to lift payloads of 2 tonnes to altitudes of 40 km.
Different technologies and flight durations
Stratospheric balloons are used mostly by the scientific community to study the atmosphere, its chemistry and dynamics. But they are also valuable tools for astronomers and biologists, or for demonstrating technologies.
Research laboratories conduct a wide range of experiments using balloons. Science payloads may weigh anything from a few hundred grams to several tonnes. And balloons may be required to operate at an altitude as low as a few hundred metres or as high as 40 km.
Another key factor is flight duration. Sometimes a few hours will suffice, but scientists also need balloons capable of staying aloft for several months to achieve the required results. To meet such a broad spectrum of needs, CNES has designed different types of balloons.
Zero-pressure stratospheric balloons have one or more openings on the underside of the envelope. They stay aloft for no more than a few days.
Infrared Montgolfiere balloons are derived from zero-pressure stratospheric balloons. Lift is provided by air heated by the Sun and by infrared radiation from Earth. They are also capable of long-duration flights.
Superpressure balloons are sealed and their envelope is stable enough for long-duration flights.
All these balloons obey the same laws of physics governing a bag filled with gas and left to its own devices in the atmosphere.
The balloon best adapted to short flights—a few hours to a few days—is the zero-pressure stratospheric balloon, which is also the oldest balloon design.
In this type of balloon, the internal and external pressure is always the same.
Zero-pressure stratospheric balloons are filled with helium, giving them the ability to carry heavy payloads at stratospheric altitudes. They have valves to discharge excess gas.
A balloon may rise as far as 40 km. As it ascends, air pressure decreases and the gas inside the balloon expands. The balloon will continue to ascend until the gas completely fills the envelope, at which point any further expansion causes excess gas to be expelled.
To compensate for expansion of the gas, a very large balloon envelope is needed. Sometimes, the envelope may be larger than a football field.
Zero-pressure balloons only stay aloft for a short time. As the surrounding air cools at night, the gas in the balloon contracts and the volume of the envelope decreases. As a result, the balloon inevitably starts to sink after its first night aloft.
In practice, zero-pressure balloons are only used for flights of less than 24 hours. However, longer flights are possible in the polar regions during complete daylight or darkness.
Medium- and long-duration flights
There are 3 types of balloons used for medium-duration flights of up to a few weeks.
Superpressure boundary layer balloons stay aloft for 3 to 4 weeks. The volume of the envelope, which is rigid and sealed, does not change with variations in temperature and pressure. However, because it only spans about 2 m, it can only carry payloads weighing a few kg. Researchers studying the atmosphere favour this kind of balloon
Infrared Montgolfiere balloons are based on the hot-air balloon concept invented more than 200 years ago by the Montgolfier brothers.
The balloon envelope is heated during the day by the Sun, rising to altitudes up to 30 km, and at night by infrared radiation emitted from Earth. Although the lift generated by the greenhouse effect is small, it is enough to keep the balloon flying at a stable altitude of around 20 km.
These lighter-than-air craft can carry payloads weighing up to 50 kg. Their original feature is that they oscillate continuously between 2 altitudes for 2 to 3 weeks. They are very useful for studying the stratosphere.
Infrared Montgolfiere balloons at a glance
45 000 m3
Hot air (helium for launch)
30 km by day, 20 km at night
South America, Equator
Initially conceived to study Mars, the aeroclipper balloon has the ability to simultaneously acquire readings in the air, from the gondola, and in the water, using a probe at the end of a guide rope attached to the balloon. The aeroclipper drifts 50 m above the sea surface for several weeks, making it especially useful for studying ocean-atmosphere interactions.
For flights longer than a few weeks, only stratospheric superpressure balloons offer a satisfactory compromise between flight duration (several months), payload capacity (up to 30 kg) and altitude (up to 20 km). These balloons use an envelope made of tougher material, making it possible to pressurize balloons with a diameter up to 10 m, to achieve the required flight profile. Stratospheric superpressure balloons are used in particular to measure the ozone hole over the South Pole.
Behind the scenes of a zero-pressure stratospheric balloon launch
Aire-sur-L'Adour is one of 2 launch bases that CNES operates in France. A few days before launch, the science team prepares the gondola containing all the instruments at the launch site.
Launch day begins with a weather update. If the conditions are right, the launch team lays out the flight train on the ground.
This generally consists of the gondola and science instruments, as well as a radar reflector, parachutes for the balloon landing, and a small auxiliary balloon to lift the gondola off the ground until the main balloon is airborne.
One hour later, the main balloon—about 50 times larger than the auxiliary balloon—is unfolded and inflated. Once launched, it can reach an altitude of 30 km in less than 2 hours.
Once the balloon has completed its programmed experiments, which only take a few hours, its descent is remotely controlled by teams on the ground.
They must find a landing site where the balloon and its precious scientific payload can be brought safely and securely back to Earth.
During these tricky final 30 minutes of the descent, the balloon is braked by parachutes also opened by remote control from the ground.
Tight range safety regulations
Any risk of injury must be averted at every stage of a balloon's flight, both on the ground and in the air.
Once the balloon's launch trajectory has been defined, rigorous calculations are performed to ensure the lowest possible probability of causing a fatal accident. In comparison, this probability is much lower than that of a road accident.
The balloon trajectory is chosen so that its flight path takes it over sparsely populated areas. After launch, it may cross over aircraft flight corridors during its ascent and descent. A flight plan must therefore be submitted beforehand to air navigation authorities for approval.
Like airliners, balloons are equipped with transponders so that air traffic control radars can keep track of them at all times.
Near the end of a flight, the ground control centre sends commands to bring down a balloon before it goes out of control.
All the gas escapes from the balloon envelope as it ruptures, and parachutes are opened to brake the gondola’s descent before landing or splashdown—thereby avoiding injuries (not a single injury has been recorded at CNES in 40 years of balloon operations) and preserving the equipment in the gondola.
How do balloons fly?
A simple experiment will help us to understand how balloons fly.
Hold a table tennis ball in your hand and plunge it under the water in an aquarium. As soon as you let go, the ball quickly rises to the surface. In the aquarium, the ball takes the place of roughly 20 cm3 of water and is subjected to water pressure all around it.
- Volume of ball: 20 cm3
- Weight of ball: 2 g
- Weight of ball filled with water: 20 g (1 cm3 of water weighs 1 g)
- Weight of ball filled with air: 0.03 g
A simple calculation shows that this results in an upward force, called buoyancy, equivalent to the weight of a ball filled with water, which would be about 20 g. The weight of the table tennis ball (2 g) takes it downwards, while buoyancy exerts a force 10 times greater in the opposite direction.
Why doesn’t the ball continue rising after it reaches the surface? Quite simply because the volume of air displaced by the ball only weighs 0.03 g. The upward force generated is therefore no longer enough to counter the downward force of the ball’s weight, which is 70 times greater. In other words, it’s all to do with density.
Did you know?
“Any body submerged in a fluid is acted upon by an upward, or buoyant, force equal to the weight of the fluid it displaces”. This physical law of buoyancy was discovered by the ancient Greek mathematician and inventor Archimedes (287 – 212 B.C.). According to legend, he made the discovery in his bath, whereupon he leapt out and ran into the street shouting “Eureka!” (“I have found it!” ). Archimedes was a supreme scholar who formulated many theories and is also credited with inventing the screw.
If we take this reasoning a step further, to lift a gondola carrying one person, all we need to do is hitch it to a bag filled with gas less dense than air. Helium, for example, is 7 times less dense than air. One cubic metre of helium only weighs 180 g and can lift a mass of about 1 kg. So, a balloon containing about 100 m3 of helium can lift an adult.
On the other hand, buoyancy does not exist in a vacuum. So, we couldn’t fly a balloon over the surface of the Moon.