Saturday, February 20, 2010

Clearing Misconceptions About Near Space Missions

I was thinking about these topics earlier this week and thought they ought to be cleared up. So if you'll permit me.

Most people are familiar with the concept that motion is relative. This means that motion to one person looks just the opposite to another person who is not sharing that motion. It's all a matter of your frame of reference.

When we discuss things like the ascent rates and maximum altitudes of a balloon, we really should be discussing these issues in their more accurate frame of reference, that of the balloon. In reality, the balloon is holding still and the earth (along with the atmosphere which is firmly attached to the earth via gravity) is falling. Apparently this occurs because when we put helium into a balloon, we're removing it from the earth and its atmosphere (I'll refer to these as the earth-atmosphere system). When you remove low density material from the earth-atmosphere, you're increasing its average density. Recall that dense objects sink and less dense objects can float. The denser earth-atmosphere now wants to sink. And as long as the filled balloon is firmly attached to the earth's surface (via gravity), like by a person holding the balloon's load line or by tying the load line to a helium bottle, the balloon will hold up the earth. So those of you who are holding the filled balloon before launch, you're really holding the earth-atmosphere up. Think about that next time.

Once the balloon is no longer tied to the earth, the earth-atmosphere falls away. As the earth and its atmosphere fall away, the balloon is surrounded by less and less dense air. The balloon expands as a result. Since the helium is trapped inside the balloon, there are no further changes in the earth-atmosphere's density and it falls away at a constant rate that is dictated by the friction of the air around the balloon. The atmosphere, which remember is firmly attached to the earth, can only slide pass the balloon at a limited rate. Friction is why the earth does not fall away from the balloon infinitely fast. Many of you have no doubt noticed that at some where around 30-40,000 feet, the balloon appears (and let me stress appears) to rise faster. This is the result of the earth-atmosphere slipping around the balloon faster because of changes in air density and balloon size. This is pretty obvious if you recall that the force of friction is based on factors like surface area and density.

At the point where the atmospheric pressure around the balloon is low enough, the balloon bursts and releases its helium back into the atmosphere. This mixing of helium back into the earth-atmosphere system decreases its average density and let's the earth and atmosphere float back up to the balloon. The air rising around the balloon payload makes it tumble (due to turbulence) and inflates the parachute. The mixing of the balloon's helium with the atmosphere occurs very rapidly and therefore, the change in the density of the earth-atmosphere is very fast. This makes the earth-atmosphere begin to rise very quickly. At the earth-atmosphere rises back up to the balloon, the air becomes denser and the parachute creates more drag, slowing the ascent of the earth-atmosphere. Therefore, we see the initial ascent of the earth-atmosphere is very fast at the start, but over time, the ascent rate slows down until the balloon and earth make contact. At that point, the earth-atmosphere system and balloon are back in equilibrium and the motion comes to an end.

Now, since the days of the Greeks, we've known the world is round or spherical. There is no friction between the earth-atmosphere system and outer space. So when you go on a balloon chase, your car tires are pushing the earth and making it rotate the opposite direction. Let me stress, your car is NOT MOVING!! Therefore, it would help if everyone in their cars would travel together and go the same direction. If your chase teams will push the earth in the same direction, you'll rotate the earth in the same direction more quickly and get the earth rotated into the proper alignment with the balloon more efficiently. Therefore, it is imperative that we prevent chase crews from leaving their homes from the opposite direction, as this pushes the earth in another direction at the same time. When one big and heavy chase truck tries to push the earth to the west, the rest of our lighter cars trying to push the earth to the east suffer. I for one do not want to see my gas mileage decrease because of this. So please be polite to everyone else and follow along with the rest of the pack.

This also highlights the importance of using the balloon launch announcements system. There are some weekends with multiple balloon launches. If they are occurring at the same time, our cars are fighting each other to rotate the earth to our proper positions. So be considerate and coordinate your launches with other teams across the country.

Just doing my part to clear things up,
Paul

(Next time I'll explain the relativistic effects of a balloon launch and why the Twin Paradox makes use younger after each balloon flight)

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