Family members of the astronauts who died aboard the space shuttle Columbia (search) 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 consisted 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 (search).

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.

The loss is still painful, said Jon Clark, a NASA (search) 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 lost her spouse, shuttle commander Rick Husband. "There is such a desire in my heart to return to normal, and yet this is something we are never going to be able to forget," she said.

She 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.

"I told Matthew the other day, 'Feb. 1 was not a bad day for Daddy. He had a bad minute or so, but he didn't have a bad day. He went from flying the shuttle to being in the presence of God."'

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

Elsewhere in Texas, residents laid out roses during a remembrance ceremony in Hemphill, a small town where shuttle debris was found after Columbia broke apart. Mourners also placed flowers at a memorial at the Kennedy Space Center (search) in Cape Canaveral, Fla.

The shuttle fleet has been grounded since 2003 as NASA works to meet return-to-flight recommendations put forth by Columbia accident investigators. The agency plans a May or June launch of space shuttle Discovery.