Slide 1 of 13
Welcome to WeightlessnessWatched pots, as they say, never boil. Besides, even if they did, why watch them? You know exactly what boiling water looks like.
But I bet you don't know what it looks like in outer space.
Here are six everyday occurrences — including the boiling of water — that happen very differently in microgravity, plus explanations as to why.
This countdown was provided by Life's Little Mysteries, a sister site to Space.com. Follow Natalie Wolchover on Twitter @nattyover
Water Boils in a Big BubbleSlide 2 of 13
Water Boils in a Big BubbleOn Earth, boiling water creates thousands of tiny vapor bubbles. In space, though, it produces one giant undulating bubble.
Fluid dynamics are so complex that physicists didn't know for sure what would happen to boiling water in microgravity until the experiment was finally performed in 1992 aboard a space shuttle. Afterward, the physicists decided that the simpler face of boiling in space probably results from the absence of convection and buoyancy — two phenomena caused by gravity. On Earth, these effects produce the turmoil we observe in our teapots.
Much can be learned from these boiling experiments. According to NASA Science News, "Learning how liquids boil in space will lead to more efficient cooling systems for spacecraft ... [It] might also be used someday to design power plants for space stations that use sunlight to boil a liquid to create vapor, which would then turn a turbine to produce electricity."
• Infographic: How Big Is the International Space Station?Slide 3 of 13
Flames Are SpheresSlide 4 of 13
Flames Are SpheresOn Earth, flames rise. In space, they move outward from their source in all directions. Here's why:
The closer you are to the Earth's surface, the more air molecules there are, thanks to the planet's gravity pulling them there. Conversely, the atmosphere gets thinner and thinner as you move vertically, causing a gradual decline in pressure. The atmospheric pressure difference over a height of one inch, though slight, is enough to shape a candle flame.
That pressure difference causes an effect called natural convection. As the air around a flame heats up, it expands, becoming less dense than the cold air surrounding it. As the hot air molecules expand outward, cold air molecules push back against them. Because there are more cold air molecules pushing against the hot molecules at the bottom of the flame then there are at its top, the flame experiences less resistance at the top. And so it buoys upward.
When there's no gravity, though, the expanding hot air experiences equal resistance in all directions, and so it moves spherically outward from its source.
• What's an Air Pocket?Slide 5 of 13
Bacteria Grow More ... and Are More DeadlySlide 6 of 13
Bacteria Grow More ... and Are More DeadlyThirty years of experiments have shown that bacterial colonies grow much faster in space. Astro-E. coli colonies, for example, grow almost twice as fast as their Earth-bound counterparts. Furthermore, some bacteria grow deadlier. A controlled experiment in 2007 testing salmonella growth on the space shuttle Atlantis showed that the space environment changed the expression of 167 of the bacteria's genes. Studies performed after the flight found that these genetic tweaks made the salmonella almost three times more likely to cause disease in mice than control bacteria grown on Earth.
There are several hypotheses as to why bacteria thrive in weightlessness. They may simply have more room to grow than they do on Earth, where they tend to clump together at the bottom of petri dishes. As for the changes in gene expression in salmonella, scientists think they may result from a stress response in a protein called Hfq, which plays a role in controlling gene expression. Microgravity imposes mechanical stresses on bacterial cells by changing the way liquids move over their surfaces. Hfq responds by entering a type of "survival mode" in which it makes the cells more virulent.
By learning how salmonella responds to stress in space, scientists hope to learn how it might handle stressful situations on Earth. Hfq may undergo a similar stress response, for example, when salmonella is under attack by a person's immune system.
• How Does Salmonella Contaminate Vegetables?Slide 7 of 13
You Can't Burp BeerSlide 8 of 13