Houston ? Family members of the astronauts who died aboard the space shuttle Columbia watched as officials dedicated a granite memorial to their loved ones Tuesday, the second anniversary of the accident that killed the shuttle’s seven-member crew.

The monument in a downtown park consists of a concrete pedestal topped with a black plaque honoring the seven men and women “who made the supreme sacrifice to advance humankind.”

Surrounded by seven magnolia trees, the memorial is also near a similar tribute to the astronauts who died in the 1986 Challenger disaster.

Johnson Space Center Director Gen. Jefferson D. Howell Jr. said it was fitting that a Columbia memorial be placed in Houston.

“As most of us in Houston know, the first word spoken from the surface of the moon was Houston,” he told visitors at the ceremony.

Columbia disintegrated over Texas as it returned from a 16-day mission on Feb. 1, 2003. Investigators blamed the accident on a hole in the shuttle’s wing caused by a piece of insulating foam that tumbled from the shuttle’s external fuel tank, striking the wing during liftoff.

The searing gases of re-entry entered the gash and melted the wing from the inside, causing the orbiter to disintegrate.

Hemphill Garden Club librarian Loretta Sterle lays one of seven roses to honor the seven Space Shuttle Columbia crew members who were killed two years ago near Hemphill, Texas. Residents and officials gathered for a memorial service in Hemphill to honor and unveil two granite markers for the fallen astronauts and two volunteer workers killed during the search.

The loss is still painful, said Jon Clark, a NASA neurologist who was married to astronaut Laurel Clark, a member of Columbia’s final crew. “It’s not the searing heart ripped out of your chest, it’s more of just a chronic ache,” he said.

Evelyn Husband, who lost her spouse, shuttle commander Rick Husband, planned to go to her 9-year-old son’s school Tuesday because he wanted to release a balloon with a private note attached for his father.

The memorial was paid for by the city and private donors.

A memorial also was held in Hemphill, which was the epicenter of the crash.

The shuttle fleet has been grounded since 2003 as NASA works to meet return-to-flight recommendations put forth by Columbia accident investigators.