In an announcement on Wednesday, SpaceX said it planned to send an unmanned Dragon capsule to Mars in 2018 and land it on the surface about six months later. (Mars and Earth line up only once every 26 months.)

NASA also plans to send people to Mars, although not as quickly, aiming for the mid-2030s. On Wednesday afternoon, a couple of hours after the SpaceX Twitter message, Dava J. Newman, NASA’s deputy administrator, wrote: “We are closer than ever before to sending American astronauts to Mars than anyone, anywhere, at any time has ever been. A new consensus is emerging around NASA’s plan and timetable for sending astronauts to the Red Planet in the 2030s.”

NASA has emphasized that — unlike the Apollo missions to the moon — it is not working alone, but is enlisting the help of other countries and endeavors. Almost in passing, Dr. Newman mentioned the SpaceX Mars effort.