US space agency NASA is ending its shuttle program this summer after two more missions. Shuttle Endeavour is due to launch on Friday on a mission to deliver a high-profile physics experiment and spare parts to the International Space Station.

Here are some milestones of the 30-year-old shuttle program:

* The US space shuttle program debuted on April 12, 1981, with a test flight of Columbia.

* The United States will have spent an estimated $174 billion on the shuttle program by the time the last spaceship is retired this year.                                      

* On January 28, 1986, the shuttle Challenger exploded 73 seconds after liftoff in an accident caused by a faulty rubber seal in the shuttle's rocket booster, killing all seven crew members aboard.

* NASA redesigned the boosters, added safety systems and returned the fleet to flight on September 29, 1988. A replacement ship for Challenger was ordered and crafted out of spare parts. It was named Endeavour.

* The shuttles carried dozens of satellites, planetary probes and other science instruments into orbit, including the Hubble Space Telescope, the Chandra X-ray Observatory, the Compton Gamma Ray Observatory and the Magellan Venus, Galileo Jupiter and Ulysses solar probes.

* The shuttle originally was intended to be a ferry ship to and from a US space station in orbit but it turned out that the first outpost it reached belonged to Russia. The flights to the Mir space station between 1994 and 1998 grew into a multi-national collaboration that built and operates the $100 billion International Space Station today.

* The shuttle also was used as an orbital laboratory, paving the way for many of the science experiments being conducted aboard the space station.

* Disaster struck the shuttle program again on February 1, 2003, when Columbia disintegrated during re-entry because of damage from a piece of insulation that fell off during liftoff. The accident eventually led to a decision to retire the shuttle fleet after construction of the space station.

* NASA is turning over station cargo flights to two commercial operators and hopes private companies will have the capability to fly people into orbit within four to five years. Until then, Russia will operate the only crew flights to the station, a service that currently costs the United States $51 million per person. The price climbs to $63 million a seat in 2014.

* The 135th and final shuttle flight aboard Atlantis is scheduled for launch on June 28. It will deliver a year's worth of supplies to the space station, giving the commercial operators some leeway in case there are problems getting their vehicles into service.

* NASA's next program, which is still in the planning stages, is to send astronauts to asteroids and other destinations in the solar system.