Why All the Fuzz About Parabolic Flights?

Most of us have enjoyed watching astronauts 'floating' around in the space shuttle, on Mir, or now-a-days on the International Space Station (ISS), but why is this possible, how does humans on the ground benefit from this, and do we have alternative, and cheaper ways to obtain weightlessness than going into space?

What is Weightlessness?

There are a few misconceptions about weightlessness, a common one being that we can create weightlessness in a tank on the Earth, for instance in a vacuum chamber. This is not true, in fact the astronauts working on the ISS are not working in vacuum, but under normal pressure conditions. In fact, it is not possible to shield against gravity.

Another common misconception is that is is possible to create weightlessness experiments in a diving pool. This is not true, since everything in the pool is affected by gravity, but the updrift of the water can be used to allow the astronaut to move in all three directions, which is useful for training astronauts in extra vechicular activities (EVAs). It is, however, important to note that the organs of the astronaut are subject to gravity while working in a diving pool. Also the astronaut can move around in the water by swimming, something which is not possible in space.

So, why is gravity not present in a spacecraft? Erh, actually it is, but this is a third common misconception. Astronauts, the spacecraft, and all equipment is doing nothing but constantly falling towards the Earth - they are constantly free falling. The reason that the spacecraft is not crashing into the Earth is that it is moving at sufficient speed (approximately 7.5 km/s in low earth orbit (LEO, 300-600 km above the ground), and about 1 km/s for the moon's orbit), so that the spacecraft is not falling vertically, but it follows a curved course (known as an orbit) around the Earth. This velocity is obtained by the launch vehicle.

Since everything is falling at the same speed the crew onboard the spacecraft experiences weightlessness. If the astronauts stands on a weight it would read 0, but an observer far away would realize, that the reason for this is that the astronaut and the weight are both following each other in a free fall, so yes, the astronaut is weightless, but he is still affected by gravity!

Astronauts during a break on ISS.

Ways to Obtain Zero-G

So, one way to create weightlessness is to go into space, but as this is quite expensive are there cheaper alternatives? Furtunately there are, and they each have advantanges and disadvantages compared to going into space.

We will here briefly describe a number of options for obtaining zero-g. These are: Drop towers, Parabolic flights, and Sounding rockets.

   > Drop Towers.

It is very few people who know that every day experiments are performed on the Earth in micro gravity, but the concept is fairly old. 175 years ago it was a common practice to cast bullets by releasing drops of lead from the top of a tower. The drops were hardnened when they landed in a water bath, and were almost perfectly round.

Also today drop towers offer unique possibilities. The obtainable quality of the microgravity is better than 10^-5 g, and as low as 10^-8 g when special measures are taken. In comparison onboard the ISS the typical quality level is about 10^-3 g to 10^-5 g, depending on the activities onboard the space station. One problem with drop towers is the short duration, typically 4-5 seconds, and the relatively hard landing into foam pellets also limits the type of experiments. Also note that the inside of the tower must be in vacuum in order to avoid air drag during the free fall. The drop tower located in Bremen is building a unique catapult system, which will shoot the experiment up from the ground, so that the total micro gravity time is increased to about 9 seconds.

Examples of drop towers:

   >Parabolic Flights.

While drop towers are unsuitable for experiments involving human interaction, parabolic flights allows humans to work in slightly longer (20-30 seconds) periods of weightlessness. Parabolic flights happens in a specially adapted aircraft, which flies along a parabola (thus the name), as seen on the picture below, which looks a bit like a giant roller coaster. On the top of the parabola (the gray area on the figure), everything onboard the plane is weightless for 20-30 seconds.

The European Space Agency (ESA) is currently running 2 to 3 professional flight campaigns every year, in addition to an annual student flight campaign. These are performed with a modified Airbus A300 (actually the original prototype), operating from Bordeaux Airport. Each flight consist of 30 parabolaes of approximately 22 seconds of weightlessness. NASA is performing similar flights with a KC-135 from Houston, and Russia is performing similar flights from an airport near Moscow.

The primary factor that limits the duration of the microgravity period is the maximum speed dictated by turbulence at the wings together with the maximum tolerable forces exerted on the wings of the aircraft during the 1.8g injection and pull out phases. Note that the experimenters inside the plane are also free falling during the rising part of the parabola. During the parabolic flight the engines of the aircraft are only used to compensate for airdrag.

Note that you can also experience weightlessness in other planes, for instance a F16 can reach almost one minute of zero-G.

   >Sounding Rockets and the ISS.

Smaller sounding rockets have also been used to create weightlessness periods of 5 to 15 minutes. These missions gives an alternative for going all the way into space, but they are still very expensive. Some of these were originally developed as ballistic missiles for delivering a nuclear payload.

One interesting thing about these different options is the difference in quality of the zero-g environment. For instance the quality of a parabolic flight heavily depends on the skills of the pilots, while the quality onboard the ISS depends on astronaut activities, and mechanical noise from engines and fans. One experiment that has been flown on previous parabolic flight missions is the MIEMA experiment, which is a method to increase the quality of microgravity onboard the International Space Station, by operating the experiment on a controlled floating plate.

Notes to the above table: Unmanned platforms in orbit are launched by e.g. the space shuttle, and later recovered and returned to Earth by a second shuttle mission.

The projected International Space Station

Benefits From Zero-G Experiments

An important part of the research in space is to understand the influence that gravity found on the surface of the Earth has on the human body and the blood regulation systems in the body. The only way to learn about these long-term influences is to remove gravity in a longer period. This can only be done in space, for instance on the International Space Station. Results from space research must then be compared to results found on healthy and diseased people on the ground, in order to gain new knowledge.

Probably the most important result of space research so far is that gravity has a much more powerful influence than expected on life on the planet. Thus you can say that gravity has a stronger daily effect on our bodies than previously expected. Just by lying on a bed affects your internal systems by gravity in a way never previously imagined.

If you for instance are lying on your back, the heart is pushed both on the front and rear side, this is something no-one thought of before, but it has implications for people with heart-diseases. Many of these patients do not like to lie down. It has been thought that the reason for this is because too much blood is pumped to the lungs, but it may also be due to the preassure on the heart, which makes the movement of the blood more difficult. This could explain why many heart patients prefer to lie on the side.

Currently space research is ongoing in many areas, some of which are biology and biochemestry (cellular and molecular biology and plant biology), human physiology (cardio-vascular system, electrolytic regulation, muscles and bone, and neuro-sensory system (i.e. balance)), fluid physics and material sciences (protein and inorganic crystallisation, combustion, micro-structures and melting processes), and fundamental physics.

During the 60'es and the 70'es the questions were: "Can humans work in weightlessness?" and: "Can we create experiments in weightlessness?" In the 80'es and 90'es the question was: "Can we create experiments that will give results?" Currently we have moved on to the question: "Can we use research in space to improve everyday life on the ground?" We are thus reaching into more application oriented research.

Gravity is an omnipresent force, which keeps us up and stresses our body, which is fine when we are healthy, but damaging when we are diseased. One thing the space station can be used for is to discover how we can counteract the weakening, and how gravity is damaging us when we are diseased. An interesting point is that some medical researchers sees space as a tool to weaken the systems of the human body (bones, muscles, and cardio-vascular system), to gain an understanding of what happens. By comparing these mechanisms with what happens with different diseases scientists can use the knowledge gained in space to counteract diseases, eventually developing effective medicine, and better preventive treatments.

A bubble in a bubble, something only possible during microgravity.