  Quantum technology based on cold-atom interferometers is showing great
promise for fields such as inertial sensing and fundamental physics. However,
the best precision achievable on Earth is limited by the free-fall time of the
atoms, and their full potential can only be realized in Space where
interrogation times of many seconds will lead to unprecedented sensitivity.
Various mission scenarios are presently being pursued which plan to implement
matter-wave inertial sensors. Toward this goal, we realize the first onboard
operation of simultaneous $^{87}$Rb $-$ $^{39}$K interferometers in the
weightless environment produced during parabolic flight. The large vibration
levels ($10^{-2}~g/\sqrt{\rm Hz}$), acceleration range ($0-1.8~g$) and rotation
rates ($5$ deg/s) during flight present significant challenges. We demonstrate
the capability of our dual-quantum sensor by measuring the E\"{o}tv\"{o}s
parameter with systematic-limited uncertainties of $1.1 \times 10^{-3}$ and
$3.0 \times 10^{-4}$ during standard- and micro-gravity, respectively. This
constitutes the first test of the equivalence principle in a free-falling
vehicle with quantum sensors. Our results are applicable to inertial
navigation, and can be extended to the trajectory of a satellite for future
Space missions.
