Tardigrades, the tiny (less than a millimeter long) organisms also known as water bears or moss piglets, are known to be extremely tough, and able to tolerate the harshest of environmental conditions. But the recent finding by Tarushika Vasanthan and colleagues from McMaster University in Canada is still surprising. One of the tested tardigrades, a species with the name Hypsibius dujardini, can withstand acceleration forces of 16,000 Gs, or 16,000 times normal Earth gravity. By comparison, astronauts in the space shuttle were generally not exposed to more than 3 Gs, and prolonged exposure to 16 Gs can be deadly for humans.
We already knew that tardigrades can withstand temperatures from near absolute zero up to 151o Celsius, pressures ranging from vacuum up to 6,000 times normal sea-level pressure on Earth, and radiation doses of up to 5,000 Gray (1,000 times higher than any other known animal). They also can survive with almost no water. They do this by going into a dormant state called a tun, during which nearly all the water in their bodies (which would ordinarily freeze and form ice crystals at low temperatures) is replaced by sugar.
Given this remarkable toughness, one might think tardigrades would be the ideal candidates for panspermia—the transport of organisms from one planet to another. But as Joseph Seckbach and I have pointed out, it’s not that simple.
Yes, a tardigrade tun would likely survive a meteorite impact on Earth and the resulting high-velocity ejection into space. It could stay alive within the meteoritic rock during its trip from one planet to another, despite the radiation, cold, and vacuum. It could even handle the shock and heat of impact on another world. But that’s where the journey ends, most likely. Tardigrades would not find habitable conditions and food on their new planet or moon, and would be doomed to eventual death in the tun state. Normally, these hardy creatures live in mosses and lichen and feed on plants, algae, and small invertebrate animals. But there is only one place in our Solar System with this type of environment and nutrition, and that is, of course, Earth.