Bill Shepherd, a former Navy SEAL-turned astronaut who arrived at the International Space Station 20 years ago Monday to begin the continuous occupancy of the orbiting research outpost, says the experience gained by NASA and its global partners over the last two decades provides a blueprint for missions to the moon, Mars, and beyond.

The second fully commercial astronaut flight to the International Space Station will have just two opportunities to launch Sunday and Monday, or else wait until after an upcoming SpaceX resupply mission next month to deliver a new set of high-priority solar arrays to the complex.

Eight days after a dramatic spacewalk to inspect the site of a leak in the hull of his Soyuz ferry ship, Russian commander Sergey Prokopyev, German flight engineer Alexander Gerst and NASA astronaut Serena Auñón-Chancellor geared up to depart the International Space Station Wednesday for a fiery plunge back to Earth.