The 55th element is one of the key ingredients in the smallest-ever spacecraft engine. The European Space Agency's Davide Nicolini says that the LISA Pathfinder, set to launch at the end of 2009, will travel to the Lagrange Point 1, the spot where Earth and the sun's gravitational fields balance each other out.

To stay in that exact spot, the craft will use a field emission electric propulsion engine, which ejects ions of liquid metal cesium to exert a miniscule thrust to counter tiny forces on the craft, such as the sunlight itself.