Balloon on the Moon

Imagine that you could take a helium-filled balloon to the moon. If you stood on the lunar surface, holding the ribbon tied to the bottom of the balloon, what would you expect to happen when it was released? We’ll assume that the balloon doesn’t burst and I’ll give you four possible outcomes;

1. The balloon rises up, faster than it would on Earth.
2. The balloon rises at the same rate as it would on Earth.
3. The balloon simply floats, staying where it was released.
4. The balloon falls to the ground.

I asked this question of a group of Year 3 (8-year-old) students recently when I was invited in to a local primary school to help celebrate this year’s Science Week. Before the students volunteered their answers, and not having access to the lunar surface, we discussed the reason why helium balloons go upwards when released on Earth and tried to use this information to decide what we would expect to happen on the moon.

You may already know the correct answer but, if not, think about a football here on Earth. If you drop it, it will fall to the ground. But if you hold it underwater then let go, it will rise upwards. Why? The short answer is that the football is less dense than water but more dense than air.

A better answer would talk about buoyancy, which is an upwards force that is created when an object displaces (pushes out of the way) the particles that surround it. The upwards buoyancy force depends on the volume of the object. At the same time, there is a downwards force created by gravity, which depends on the mass of the object. These two properties (volume and mass) are combined to give the object’s density and we can say that when an object is more dense than the surrounding fluid (air or liquid) it will move downwards whereas when it is less dense that the surrounding fluid it will move upwards.

To be a bit more exact, the football’s buoyancy in air is less than its weight whereas its buoyancy in water is greater than its weight, so the balloon falls in the air and rises in the water. It is important to recognise that the football has buoyancy in both cases but the buoyancy force is determined by the weight of the fluid that is displaced and when that weight is greater than the weight of the football there is an overall upwards force but when the weight of the displaced fluid is less than the weight of the football, the overall force is downwards.

So what about the helium balloon? There is no atmosphere on the moon and the balloon is therefore denser than the surrounding space, so it cannot rise. At the same time, gravity works on the moon just as it does on Earth (albeit with a smaller force owing to the lesser mass of the moon) and that means everything will be pulled towards its surface. The correct answer, therefore, is that the helium-filled balloon will fall to the ground.

Many of the Year 7 students thought the balloon would just float but a good number worked out that it would drop and I don’t think anyone thought it would go upwards. Even though some students didn’t get the answer exactly right, there is clearly an encouraging amount of solid scientific reasoning going on in young brains today.