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Why do Microgravity Research?

Most people to assume that any science carried out in space is solely for the benefit of space missions and that any ‘real world ’ applications are purely ‘spin offs’. This is far from the truth, there are many industrial based researchers who find microgravity invaluable in solving ground based problems. This section gives just a few examples of ‘real world’ research together with some of the space based applications that are associated with them. There are no questions offered here as our aim is simply to show a tiny fraction of the range of research areas that are available if you choose to pursue a career in the physical sciences.

Materials Science

This covers a huge range of different research projects and experiments and it includes the IMPRESS project that has supported these educational resources. It should be stressed though that even though the research has an industrial basis there is no intention to carry out industrial manufacturing in space; even if it were possible the costs would be astronomical. For the time being, factories in space are purely a subject for science fiction.

Solidification

Metals are crystalline materials and the properties of the finished metallic casting depend on its ‘microstructure’ (that is to say the size, shape and arrangement of the crystals that make up the solid material). There are two basic types of crystal in a metal; columnar and equiaxed (see the ‘Crystal Growth’ pages for more information). One of the main problems for materials scientists is that it is very difficult to predict when the solidification process switches from one type to the other. All the time in the gradually solidifying casting there are temperature and density differences. These differences drive convection currents that upset the predictions that the materials scientists try to make and so it is extremely hard to check the models to find if they are one the right track. In microgravity, convection does not occur and so the whole solidification process becomes much simpler. It is even possible to introduce controlled amounts of convection like movement by using rotating or alternating magnetic fields. Armed with this information the materials scientists can then turn their attention back to the complicated world of casting in normal strength gravity.

 Hookes Law

Much of this work will be carried out in the European Material Science Laboratory (MSL) that will be installed in the US Destiny module of the ISS. The MSL is pictured above together with ESA Astronaut André Kuipers working in the Destiny module.

Thermophysical Properties

EML ApparatusIn order to produce a high quality casting in a reliable manner it is still necessary to know many of the ‘thermophysical’ properties of the material, such as its melting point and latent heat of fusion together with properties such as coefficient of thermal expansion, viscosity and surface tension at a range of temperatures near its melting point. Viscosity for example determines how easily the molten metal flows in to the mould and surface tension determines whether tiny, trapped gas bubbles can escape before the metal solidifies.

Since many emerging materials are highly reactive in their molten state the accurate measurements can only be made in microgravity with a droplet of the material suspended in a magnetic field, otherwise they would react with the crucible that they were contained in. Without this data, casting of the materials can become little more than inspired guess work. However if the cast object in question is a turbine blade in the aircraft engine that is keeping you flying at an altitude of 11 km you may want a little more certainty than is supplied by guesswork!

As far as space science is concerned it is important to realise that much of the material in a spacecraft is metal and that this tends to be very dense. Every kilogram of the mass budget that has to be allocated to the structure of the spacecraft is a kilogram that cannot be used for scientific instruments. As a result there is a great deal of research in to metallic materials that are strong and at the same time lightweight. Examples are alloys based on magnesium and metallic foams. The space application can then find an application back on Earth as metallic foams have a superb potential in many areas, e.g. for energy absorption in car crumple zones.

Fluid Science

This subject area naturally covers the behaviour of liquids and gases. However it also includes foams and emulsions.

Combustion Science

Flames on Earth are driven by convection currents, this produces the familiar rising flames of candles and bonfires. In microgravity however convection does not occur and so combustion can only occur as quickly as fresh oxygen can diffuse in and the combustion products diffuse out. As a result combustion in microgravity is far simpler than on Earth.

 

Whilst this is critical for engineers and scientists specialising in rocket engines and manned spacecraft safety it is also of great interest to ground based engineers developing for instance clean burn engines and more effective fuel injectors. The reason again is that models developed from ground based measurements have to contend with the disturbances produced by gravity and it can be very hard to separate the two effects. If however the models are developed from the simpler situation of microgravity combustion then the effects of gravity can be added in as a separate factor and so the combined model can be far simpler to use.

Foams and Emulsions

Metallic FoamWhen you next go to your refrigerator try organising the content, on one side place all the things that are either foams or emulsions and on the other side… well actually there isn’t a great deal left. Foams and emulsions are incredibly common in both nature and in man made products including foods, toiletries and cosmetics. They are related to each other as they are both made from thin films of liquid surrounding a ball of another material; in the case of foams a gas and in the case of emulsions another liquid. The manufacturers of these products are eager to work out how to improve the performance and the stability of the foams and emulsions.

‘Dry’ foams can be as little as 1% liquid by volume (the rest being gas). ‘Wet’ foams on the other hand can be around 35% liquid. The two types behave very differently as dry foams tend to be stable but wet foams drain and then break up under gravity. The use of microgravity to support research is particularly important for ‘wet foams’ and these include the metallic foams mentioned in the previous section. Microgravity research is also showing how wet foams can be stabilised by including tiny solid particles in with the liquid, surface tension between the liquid and the solid particles prevents the films from draining too thinly and then rupturing.

From lipsticks, through car safety devises and on to planetary exploration – quite a range for a seemingly simple subject.

Thermal Physics and Heat transfer

If you were an astronaut on the International Space Station enjoying a well earned cup (actually a pouch) of tea or coffee you would have to wait a long time for it to cool down to a reasonable temperature. On Earth we are used to heat transfer occurring by convection, conduction and radiation (see Chilling Tales for more details). In microgravity though thermal convection doesn’t work; temperature differences still lead to changes in density  but that doesn’t cause the surrounding air to flow because less dense material is no longer buoyant.

In space, heat transfer is not just a matter of cooling the astronauts’ drinks. Heat must be moved around to prevent some sections of a satellite from overheating and others from freezing solid. Because the cost of transporting mass to the ISS is so high, as much water as possible is recycled but this in turn requires the application of an evaporation - condensation cycle and so in turn relies on heat transfer. As a result a great deal of research has gone in to how boiling and evaporation occurs in microgravity and how cooling especially ‘spray cooling’ can be optimised. All of this research, carried out principally for a space environment, can be applied directly and immediately back on Earth when devising any new equipment that requires efficient heat transfer.

Cold Atom Technology

Atomic FountainYou will almost certainly have heard of atomic clocks, the devices that the world now sets and measures its time by. These devices make use of the tightly defined frequencies of certain atoms such as caesium to constantly calibrate a microwave cavity resonator (essentially a sophisticated simple harmonic oscillator). The accuracy of the measured frequency though is limited by the temperature of the gas and so the kinetic energy of the atoms. High temperatures mean high velocities and so the frequencies are Doppler shifted, some to higher and others to lower frequencies. This leads to a smearing out of the central frequency. The answer is to cool the atoms down to a fraction of a Kelvin. However even atoms drop in a gravitational field and so they can only be held for a short period of time. The longer they can be held the more accurate the clock can be. The best we can do on Earth is to throw a packet of caesium atoms in to the air (well a vacuum actually) and ‘excite’ them on the way up and look again at them on the way down. This is the principal of the so called ‘atomic fountain’.

An atomic clock called PHARAO (Projet d’Horloge Atomique à Refroidissement d’Atomes en Orbite) will improve on this though. Sitting outside the European Columbus laboratory of the ISS it will produce packets of extremely cold caesium atoms (less than 1 mK). A stream of these packets will pass first in to a microwave cavity that is tuned close to the resonant frequency of the atoms. In this cavity they receive a shot of microwaves that excites them to the higher energy level. They then pass, very slowly, down the tube to a second detector cavity. There they give up some of the energy that went into their excitation. The more energy they give up the closer the original exciting microwaves must have been to the central frequency. The clock electronics feeds this information back to the first cavity and so the frequency can be automatically adjusted to home in on the central frequency. The result will be an atomic clock with an accuracy of around 1 part in 1016 (or around one second in 300 million years).

 

All this might seem a little academic (and certainly fundamental physics researchers will want to use PHARAO to test Einstein’s General Theory of Relativity to new levels) but because the measurement of time is so central to almost all other measurements the new technology will also help terrestrial applications such as global navigation systems (GPS and Galileo), communications, Earth observation measurements and maybe even ‘quantum cryptography’.

These are just a tiny fraction of the areas of current research in the physical sciences in microgravity. Beyond this there are other research areas in biology, exobiology and human physiology that will have equally important applications on Earth. Much of the research will also be put to use in the next big exploration step for humanity, first a return to the Moon, then permanently manned bases ultimately leading to a manned mission to Mars. Whether you chose to work on purely ground based projects or have your eyes on the skies there are a huge number and range of opportunities for young, talented scientists and engineers. Join us!

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