The Power and Propulsion Element (PPE), previously known as the Asteroid Redirect Vehicle propulsion system, is a planned element of the Space Reactor‑1 Freedom (SR-1 Freedom) spacecraft. Developed by Lanteris Space Systems for NASA, PPE will use ion thrusters for nuclear electric propulsion supplemented by separate, higher-thrust bipropellant chemical propulsion.

The PPE development effort started in 2013 at the Jet Propulsion Laboratory as a part of the Asteroid Redirect Mission (ARM), but is now managed by the NASA John H. Glenn Research Center. The PPE was designed to have a launch mass of 5,000 kg (11,000 lb) with propellant accounting for half that mass and the capability to generate 50 kW of solar electric power using Roll Out Solar Arrays for its Hall-effect thrusters, which can be supplemented by chemical propulsion.

When ARM was cancelled, the solar electric propulsion was repurposed as the PPE for the Lunar Gateway. The PPE was designed to be able to transfer the reusable Gateway to lunar orbit and serve as the communications center of the Gateway. NASA originally planned to integrate PPE with the HALO module and launch them together on a Falcon Heavy no earlier than 2027.

In March 2026, NASA announced that PPE would be repurposed as the propulsion element of the nuclear-powered SR-1 Freedom spacecraft.

The Asteroid Redirect Vehicle was a robotic, high performance solar electric spacecraft for the Asteroid Redirect Mission (ARM). The mission was to send the spacecraft to a near-Earth asteroid and capture a multi-ton boulder from the surface with a grappling device. It would then transport the asteroid into orbit around the Moon where crewed missions to study it could be conducted more easily. The mission was cancelled in early 2017 and the spacecraft's propulsion segment became the Power and Propulsion Element (PPE) for the Deep Space Gateway, now known as the Gateway.

During the Asteroid Redirect Mission, space tug missions were proposed to separate Mars logistics that can spend a longer time in space than the crew into a separate mission, which could have reduced the costs by as much as 60% (if using advanced solar electric propulsion (ion engines) ). They would also reduce the overall mission risk by enabling check-out of critical systems at Mars before the crew departs Earth. This way if something goes wrong in those logistics, the crew is not in danger and the hardware can simply be fixed or relaunched.

Not only would the solar electric propulsion (SEP) technologies and designs be applied to future missions, but the ARM spacecraft would be left in a stable orbit for reuse. The project had baselined any of multiple refueling capabilities. The asteroid-specific payload was at one end of the spacecraft bus, either for possible removal and replacement via future servicing, or as a separable, reusable spacecraft, leaving a qualified space tug in cislunar space. This made adaption for Gateway easy, as the propulsion system was already designed to be multi-mission reusable. When the ARM was cancelled however, development on the bus and any reusable tug ideas died, temporarily.

In 2017, a year after the Artemis program came into existence, the ARM space tug/propulsion bus was repurposed as the main propulsion system for the Gateway space station. It officially became known as the Power and Propulsion Element or PPE. The PPE will be a smaller version of the Asteroid Redirect bus. In 2018, the Gateway was split off from Artemis as a separate program to allow a Moon landing by 2024 without having to wait for the Gateway to be completed.