An artificially produced, highly unstable, transuranic element with no detectable natural occurrence on Earth. Alpha-alpha correlation, in which the alpha decay of the unknown isotope and that of its alpha decay products are correlated in time, can be used to positively identify short lived isotopes and was used by Peter Armbruster at the Heavy Ion research centre at GSI in Germany to identify hassium in 1984. It was only named hassium in 1993.

The isotope with the longest known halflife of hassium is hassium-265 with a very short halflife of just 1.8 milliseconds decaying by alpha decay into seaborgium-261 which decays by either alpha decay or by spontaneous fission with a halflife of just 260 milliseconds.

Isotope hassium-265 was created at the GSI research centre by bombarding a lead-208 target with iron-58. This created a compound nucleus of hassium-266* which upon losing a neutron became the hassium-265. After 75 such atoms were observed to decay by alpha emission its halflife of 1.55 milliseconds was determined.

Altogether, just 4 isotopes of hassium are known, ranging from the alpha decaying hassium-263 to the alpha decaying hassium-265 mentioned above.

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