The kilogram is currently defined as the mass of a chunk of platinum-iridium alloy created in 1889, that is housed at the International Bureau of Weights and Measures in Sèvres, France. (Le Grand kilogram, or Big K, as it is affectionately known).
But what if Big K gets stolen, or damaged? And it has already (mysteriously) lost some 50 micrograms since 1889. So this state of affairs will not do for the 21st century.
This Friday in Versailles, a gathering of the International Bureau of Weights and Measures, is expected to vote to redefine the kilogram by its existence as a unit in the Planck constant. This new definition is very complicated to explain (see here). For it to work, the Planck constant also had to be precisely defined and fixed to 9 significant digits, with the aid of a Kibble balance.