  We have conducted numerical simulations to reproduce the observed optical
energy profile of the 15 October 2021 (UT) impact flash on Jupiter, which was
the largest and the most well-observed flash event detected by ground-based
movie observations. The observed long-duration ($\sim 5.5~{\rm s}$) optical
emission can be reproduced by an impact of an object with an exceptionally
small angle of entry relative to the horizontal. The apparent lack of the
impact debris feature despite the large impact object was possibly due to the
shallower angle of entry ($\le 12^\circ$), which resulted in the lower ablation
per unit volume at altitudes higher than $50 \, {\rm km}$, and the volume
densities of the ablated materials were too low to allow the debris
particulates to coagulate. The absence of temporal methane absorption change in
the observed flash spectrum is consistent with the best-fit results. The model
better fits the observed optical energy profile for weaker material (cometary
and stony) cases than for metallic ones. Based on the simulation results,
prospects for future observations of impact flashes are discussed.
