Wikipedia говорит, что американцы собираются лететь к Марсу на тепловых ядерных двигателях. Тепловой ядерный двигатель это ионный водородный двигатель, разогреваемый ядерным реактором.
Будет 7 запусков SLS на низкую околоземную орбиту, сборка трех раздельных модулей будет сделана на околоземной орбите: два грузовых модуля, третий пилотируемый с 6 астронавтами. Сначала к Марсу отправятся грузовые модули, через несколько месяцев пилотируемый.
https://en.wikipedia.org/wiki/Proposed_SLS_and_Orion_Missions

Цитата

Beyond Earth Orbit (BEO) Orion missions 2033 Forward Work Martian Moon Phobos/Deimos, a crewed flexible path mission to one of the Martian moons. It would include 40 days in the vicinity of Mars and a return Venus flyby. 2037-2039 Forward Work Mars Landing, a crewed mission, with four to six astronauts,[12] to a semi-permanent habitat for at least 540 days on the surface of the red planet in 2033 or 2045. The mission would include in-orbit assembly, with the launch of seven SLS Block 2 heavy-lift vehicles (HLVs). The seven HLV payloads, three of which would contain nuclear propulsion modules, would be assembled in LEO into three separate vehicles for the journey to Mars; one cargo In-Situ Resource Utilization Mars Lander Vehicle (MLV) created from two HLV payloads, one Habitat MLV created from two HLV payloads and a crewed Mars Transfer Vehicle (MTV), known as "Copernicus", assembled from three HLV payloads launched a number of months later. Nuclear Thermal Rocket engines such as the Pewee of Project Rover were selected in the Mars Design Reference Architecture (DRA) study as they met mission requirements being the preferred propulsion option because it uses proven technology, has higher performance, lower launch mass, creates a versatile vehicle design, offers simple assembly, and has growth potential. A nuclear reactor serving as the power source for Ion propulsion was also an alternative option, in the case NTRs were not