Water bears, also known as tardigrades, are very small, segmented animals. The largest species is just over one millimetre in length. Water bears live in temporary ponds and droplets of water in soil and on moist plants. They are known to survive under conditions that would kill most organisms – they can withstand temperatures ranging from -272 deg C to +150 deg C, they can be without water for a period of 10 years, and they are extremely resistant to radiation.
The tardigrades join a fairly select group of organisms which are able to cope with the extreme conditions in space. Over the past 10 years, other ESA experiments have shown that lettuce seeds and lichen were also able to survive exposure to space. If shaded from direct sunlight, bacterial spores are also known to survive for many years under space conditions.