Launch day is finally here: Boeing's Starliner capsule blasts off Monday to the International Space Station on its first crewed mission—several years after SpaceX first achieved the same milestone.

The flight, a final test before Starliner takes up regular service for NASA, is critical for the US aerospace giant, whose reputation has suffered of late due to safety issues with some of its passenger jets.

Starliner, which was first ordered a decade ago by the US space agency, has had a bumpy ride to the finish line, with surprise setbacks and multiple delays—a saga Boeing is eager to complete.

Astronauts Butch Wilmore and Suni Williams are set to leave Cape Canaveral at 10:34 pm Monday (0234 GMT Tuesday) aboard the capsule.

Throughout the annals of American space exploration, a select few spacecraft have had the distinction of carrying human beings beyond Earth.

Next week, Boeing is poised to join this elite group with the long-awaited launch of its Starliner capsule, just the sixth class of vessel built in the United States for NASA astronauts.

Here's a recap of their storied past, marked by groundbreaking triumphs and some devastating setbacks.

Mercury

Known as America's "man-in-space" program, Project Mercury was born just days after NASA itself was formed in 1958, and officials settled on the term "astronauts" for its space explorers.

On May 5, 1961, Alan Shepard became the first American to fly in space during a 15-minute suborbital flight in the one-man, cone-shaped capsule—about a month after the Soviet Union's Yuri Gagarin became the first human to achieve the feat.