HOUSTON (AP) -- On the second anniversary of the Columbia tragedy, Houston Mayor Bill White and other city officials dedicated a granite memorial in a downtown park to honor the seven astronauts killed as the space shuttle returned from a 16-day science and research mission.

Evelyn Husband, wearing a necklace with a shuttle emblem she got from her husband, shuttle commander Rick Husband, said she still deals with the tragedy every day.

"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.

Evelyn Husband couldn't get her son, Matthew, to come to breakfast. She found him staring at a clock instead.

"He was watching the clock and remembering when exactly it happened," the widow said Tuesday, referring to space shuttle Columbia's disintegration over Texas shortly before 8 a.m. CDT two years ago. "This is the first time he's done that, but he's 9 now. He was 7 when it happened."

Johnson Space Center Director Gen. Jefferson D. Howell Jr. said it was fitting that a Columbia memorial was placed in Houston. "As most of us in Houston know, the first word spoken from the surface of the moon was Houston," he said.

He said Columbia's final mission was a wonderful success because "what they accomplished was incredible. They just didn't make it all the way home."

Investigators determined the shuttle was brought down by a hole in the leading edge of its left wing caused when a piece of insulating foam broke off and struck the wing during liftoff. The searing gases of re-entry entered the gash and melted the wing from the inside out, leading to the breakup of the orbiter.

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.

A similar tribute to the seven astronauts who died in the 1986 Challenger disaster is nearby.

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 in Cape Canaveral, Fla.

Evelyn Husband said she tries to lessen her son's pain.

"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.'"