Didymos system is estimated at 528 billion kg, with Dimorphos comprising 4.8 billion kg of that total. Choosing a binary asteroid system is advantageous because changes to Dimorphos's velocity can be measured by observing when Dimorphos subsequently passes in front of its companion, causing a dip in light that can be seen by Earth telescopes. Dimorphos was also chosen due to its appropriate size; it is in the size range of asteroids that one would want to deflect, were it on a collision course with Earth. In addition, the binary system was relatively close to the Earth in 2022, at about . The Didymos system is not an Earth-crossing asteroid, and there is no possibility that the deflection experiment could create an impact hazard. On 4 October 2022, Didymos made an Earth approach of . Preflight preparations Launch preparations for DART began on 20 October 2021, as the spacecraft began fueling at Vandenberg Space Force Base (VSFB) in California. The spacecraft arrived at Vandenberg in early October 2021 after a cross-country drive. DART team members prepared the spacecraft for flight, testing the spacecraft's mechanisms and electrical system, wrapping the final parts in multilayer insulation blankets and practicing the launch sequence from both the launch site and the mission operations center at APL. DART headed to the SpaceX Payload Processing Facility on VSFB on 26 October 2021. Two days later, the team received the green light to fill DART's fuel tank with roughly of hydrazine propellant for spacecraft maneuvers and attitude control. DART also carried about of xenon for the NEXT-C ion engine. Engineers loaded the xenon before the spacecraft left APL in early October 2021. Starting on 10 November 2021, engineers mated the spacecraft to the adapter that stacks on top of the SpaceX Falcon 9 launch vehicle. The Falcon 9 rocket without the payload fairing rolled for a static fire and later came back to the processing facility again where technicians with SpaceX installed the two halves of the fairing around the spacecraft over the course of two days, 16 and 17 November, inside the SpaceX Payload Processing Facility at Vandenberg Space Force Base and the ground teams completed a successful Flight Readiness Review later that week with the fairing then attached to the rocket. A day before launch, the launch vehicle rolled out of the hangar and onto the launch pad at Vandenberg Space Launch Complex 4 (SLC-4E);