<p>Using containerless electrostatic levitation (ESL) facility aboard China Space Station (CSS), we fulfil on-orbit undercooling and density measurements for a binary Ni₆₀Nb₄₀ eutectic melt in a microgravity environment. It is found that at a short isothermal time of overheating, the undercooling gradually increases with overheating but saturates after passing a threshold when combined with ground-based ESL experiments and molecular dynamics (MD) simulations. In contrast, at a long isothermal time, the undercooling remains almost constant within the studied overheating range. It demonstrates that the microstructure of the melt does not reach equilibrium instantaneously after melting. Instead, a certain time duration is required to complete the melting process. Our results propose that the residual medium range ordered structures (MROs) significantly influence the deep undercooling behavior of the melt during the solidification process, which might be applicable across different alloy systems as well.</p>