HOUSTON — Six astronauts are almost ready to fly NASA’s Space Shuttle Endeavour on one last mission — a space voyage slated to blast off April 19.

Endeavour is slated to fly to the international space station (ISS), where it will deliver a $2 billion astrophysics experiment to detect cosmic rays from space. The mission will be the second-to-last orbiter flight before NASA retires its 30-year space shuttle program for good.

Astronaut Mark Kelly will command Endeavour’s 14-day mission, which will feature four spacewalks jam-packed with tasks to upgrade the space station.

“This is my fourth flight to the international space station on the space shuttle,” Kelly said March 24 at a press conference here at NASA’s Johnson Space Center. The shuttle has “been an incredible success over a long period of time. It’s sad to see it go.”

Kelly has been juggling space shuttle training while caring for his wife,  Rep. Gabrielle Giffords (D-Ariz.), who was shot in the head by a constituent in January during an attack outside a Tucson grocery store that killed six people and injured 13 others.

Kelly made the difficult decision in February to resume training with his crew and push on with the mission despite his wife’s injury. He said March 24 that she has been improving every day and credited his experience as a veteran of three space missions for helping him get through the grueling training schedule.

“I think it would have been really challenging if this was my first shuttle flight,” Kelly said. “Having that experience certainly makes it very manageable to be able to handle what’s going on in my personal life and handle this mission. I’ve given this mission everything I would have if the events of January had not happened.”

Riding along with Kelly on Endeavour’s STS-134 mission are pilot Gregory H. Johnson, who piloted Endeavour on the orbiter’s STS-123 mission in March 2008; mission specialists Michael Fincke, who has served two long-duration stints living and working on board the international space station; Greg Chamitoff, who has logged a total of 183 days in space; Andrew Feustel, who was a member of the shuttle Atlantis’ STS-125 mission in May 2009; and European Space Agency astronaut Roberto Vittori, who has flown twice before to the space station.

Endeavour’s primary payload is a science experiment called the Alpha Magnetic Spectrometer (AMS), a particle detector 16 years in the making that will analyze particles called cosmic rays for what they can tell us about the origins and makeup of the universe.

In addition to the AMS, Endeavour will also carry a container of spare hardware and science experiments to help the space station continue running smoothly after the space shuttles retire.