In the absence of gravity, surface tension dominates the physics of fluids. Here, in an image taken on the International Space Station, it causes water to extend from a metal loop as if it were stirred by an invisible spoon.



This stirring effect was created by using a flashlight to unevenly heat the water. The resulting temperature difference induced an imbalance in the surface tension, causing the fluid to rotate.



Such surface-tension-triggered movement, called Marangoni convection, is less obvious on Earth, but can be seen in environments such as cooling puddles of molten steel.



(Image: NASA)

Microgravity tends to produce rounder, cooler flames, as this comparison of combustion in normal gravity (left) and microgravity (right) illustrates. Unlike on Earth, hot, less-dense air does not rise in microgravity. As a result, other processes, like the diffusion of particles from a high temperature to a low temperature area, dominate.



Studying combustion in space reveals more about the fundamental physics of the phenomenon and could help develop fire-suppression technology for future space missions.



(Image: NASA-JSC)

Crystals tend to grow bigger in microgravity (right panel), as evidenced by these cubes of the mineral zeolite. That's because liquid-grown crystals feed on material dissolved in solution, leaving a less-dense liquid behind. On Earth, this liquid floats upwards, creating a convection current in experimental containers that introduces flaws and limits the size of crystals. The effect is virtually absent in microgravity.



Creating larger, purer crystals can reveal more about their basic structure and properties. Zeolite, for example, is full of microscopic pores that can be used to filter and store materials, such as hydrogen for use in future fuel cells.



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Japanese Medaka fish (an embryo is shown here) were among the first animals used to study embryo development in space when they flew aboard the shuttle Endeavour in 1994.



The importance of gravity at the start of an animal's life cycle is still a big mystery. While Medaka fish born in space grew to resemble their Earth-born brethren, experiments on other animals, from mice to clawed toads, have shown that weightlessness can have a significant impact on early development, making many more prone to physical abnormalities.



(Image: NASA Marshall Space Flight Center)

The absence of gravity isn't the only environmental factor that changes when animals fly in space. They must also withstand heavier doses of solar and cosmic radiation. Lichen and bacteria have been able to survive exposure to the combination of the airless vacuum and intense radiation of space.



But so far only one animal – a microscopic invertebrate known as a "water bear", or tardigrade, has done the same. During a European rocket experiment in 2007, some tardigrades were exposed to both the sun's intense ultraviolet radiation and the vacuum of space, while others were shielded from radiation and only exposed to the vacuum.



Only a handful exposed to radiation could be revived, but many survived the vacuum alone.



(Image: Ralph Schill)

A large part of space research has focused on the physiological effects of microgravity. Free-fall has been shown to affect astronauts' ability to judge size and distance and to cause the loss of red blood cells and muscle mass. But the biggest toll may be on the bones.



Even with a rigorous exercise regimen, most people lose an average of about 1.5 per cent of their bone mass in certain body parts – such as the hips – for every month in space. That is about the same amount of bone mass that a post-menopausal woman loses in a year. Researchers are working to reduce this effect, in part with vertical treadmills that simulate microgravity conditions.



(Image: NASA/Quentin Schwinn/RS Information Systems, Inc.)

Weightlessness may be doubly dangerous for fending off infection. Space travel seems to weaken the immune system, and it may also make a range of microbes deadlier than they are on Earth. A flight of the space shuttle Atlantis in 2006 showed that the bacterium Salmonella typhimurium (shown here in red) was nearly three times as likely to kill mice.



Microgravity also seems to boost the virulence of methicillin-resistant Staphylococcus aureus, an antibiotic-resistant superbug that is a common cause of infection in hospitals.



A firm called Astrogenetix is now studying this increased potency to isolate factors responsible for virulence in the hopes of producing vaccines.



(Image: NASA)

This suitcase-sized experiment may look like the ultimate cosmetics kit, but it is used to test the effect of radiation on a range of materials, from ceramics to spores. The first Materials International Space Station Experiment (MISSE) box was attached to the station in 2001.



Astronauts from the space shuttle Discovery removed the sixth set of boxes from the station on 1 September this year.



A recent NASA report on space station science called MISSE "perhaps the most prolific suite of experiments to date" aboard the outpost.



(Image: NASA-MSFC)

The International Space Station has played host to a clutch of mini-satellites, soccer-ball-sized devices that are part of a project called Synchronized Position Hold Engage Re-Orient Experimental Satellites (SPHERES). This trio has been used to test control programmes that allow satellites to fly in formation with minimal human intervention. Individual space satellites could be coordinated to make powerful telescopes.



Better control procedures could also enable spacecraft to dock autonomously, an ability that could be useful for assembling objects in orbit.



(Image: NASA)