NASA is modifying plans to send astronauts back to the moon in 2024 by taking the Gateway — a mini-space station to be assembled in lunar orbit — out of the critical path in favor of a simplified architecture, according to the agency’s chief of human spaceflight.

SpaceX founder and chief executive Elon Musk has shared images of the Falcon 9 booster’s crash landing on a ship in the Atlantic Ocean after the rocket’s successful Jan. 10 liftoff with supplies for the International Space Station.

After four years of brainstorming, custom tool development and training, two astronauts plan to venture outside the International Space Station Friday for the first of four spacewalks to repair a $2 billion cosmic ray detector. The excursions are considered the most challenging since work to service the Hubble Space Telescope.