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We have studied the stability of C _59 anions as a function of time, from their formation on femtosecond timescales to their stabilization on second timescales and beyond, using a combination of theory and experiments. The ${{\rm{C}}}_{59}^{-}$ fragments were produced in collisions between C _60 fullerene anions and neutral helium gas at a velocity of 90 km s ^−1 (corresponding to a collision energy of 166 eV in the center-of-mass frame). The fragments were then stored in a cryogenic ion beam storage ring at the DESIREE facility, where they were followed for up to 1 minute. Classical molecular dynamics simulations were used to determine the reaction cross section and the excitation energy distributions of the products formed in these collisions. We find that about 15% of the ${{\rm{C}}}_{59}^{-}$ ions initially stored in the ring are intact after about 100 ms and that this population then remains intact indefinitely. This means that C _60 fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C _59 , that are fed into interstellar chemical reaction networks.