  The NASA Double Asteroid Redirection Test (DART) mission performed a kinetic
impact on asteroid Dimorphos, the satellite of the binary asteroid (65803)
Didymos, at 23:14 UTC on September 26, 2022 as a planetary defense test. DART
was the first hypervelocity impact experiment on an asteroid at size and
velocity scales relevant to planetary defense, intended to validate kinetic
impact as a means of asteroid deflection. Here we report the first
determination of the momentum transferred to an asteroid by kinetic impact.
Based on the change in the binary orbit period, we find an instantaneous
reduction in Dimorphos's along-track orbital velocity component of 2.70 +/-
0.10 mm/s, indicating enhanced momentum transfer due to recoil from ejecta
streams produced by the impact. For a Dimorphos bulk density range of 1,500 to
3,300 kg/m$^3$, we find that the expected value of the momentum enhancement
factor, $\beta$, ranges between 2.2 and 4.9, depending on the mass of
Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of
2,400 kg/m$^3$, $\beta$= 3.61 +0.19/-0.25 (1 $\sigma$). These $\beta$ values
indicate that significantly more momentum was transferred to Dimorphos from the
escaping impact ejecta than was incident with DART. Therefore, the DART kinetic
impact was highly effective in deflecting the asteroid Dimorphos.
