HOUSTON, Texas  -- Hundreds of people gathered Tuesday at Johnson Space Flight Center in Houston, as NASA paid tribute to the seven astronauts who died aboard the space Shuttle Columbia.

The service began with a prayer, in English and Hebrew from Capt. Harold Robinson, a Navy rabbi, and a solemn rendition of "God of our Fathers," by the U.S Navy Band chorus.

"When we view our little planet from out in space, we learn unity of all humanity," Robinson said. "We are one as you are one."

NASA Administrator Sean O'Keefe today told the crowd that the agency must "honor the legacy of these fallen heroes" by determining what caused the disaster.

"Throughout our proud NASA family, the bond between those who venture into space -- the astronaut corps -- and those who make space flight possible, this bond is incredibly strong and today our grief is overwhelming," O'Keefe said.

President Bush and First Lady Laura Bush, who could be seen wiping away tears, sat with the families of the seven astronauts -- commander Rick Husband; pilot William McCool; payload commander Michael Anderson; mission specialists David Brown, Laurel Clark and Kalpana Chawla; and Israel's first astronaut, Ilan Ramon.

John Glenn, the former senator and astronaut, and Neil Armstrong, the first astronaut to walk on the moon, accompanied Bush on the trip.

The president plans to speak at the service and meet privately with the families of Columbia's seven crew members.

The memorial will be one of the largest in the three days since the shuttle broke up on re-entry, leaving a trail of debris scattered across several states.

"We're going to pause and reflect upon the crew of Columbia, their lives, their contributions, their memory, and although we cannot stop our investigation and the recovery effort, we will pause in this location to take the time to reflect upon their lives, their sacrifice," space shuttle program manager Ron Dittemore told reporters Monday.

"It's a day of remembering. It's a day of remembering our friends, and for us it's a day of mourning."

Texas Gov. Rick Perry also is expected to attend the service. A congressional delegation, led by Senate Minority Leader Tom Daschle, D-South Dakota, and Senate Majority Leader Bill Frist, R-Tennessee, also will go to the service in Houston.

The Senate on Monday passed a resolution commemorating "with deep sorrow and regret the fate of the Columbia space shuttle mission."

Each of the chamber's 100 senators co-sponsored the resolution, including Sen. Bill Nelson, D-Florida, who was a shuttle crew member in 1986 while a member of the House.

The Russian Space Agency said that the three crew members on the international space station will be listening to Tuesday's ceremony. They are U.S. astronauts Ken Bowersox and Don Pettit and Russian Soyuz commander Nikolai Budarin.

In other memorials, a vigil was held Monday for Clark at Olympia Brown Unitarian Universalist Church in her hometown of Racine, Wisconsin. A similar service was held Sunday in the same city with some of her family members, and a memorial service is scheduled for Wednesday at Racine's Festival Hall.

In Washington, the Israeli Embassy has put out a condolence book for people to sign for Ramon. The Israeli community in Philadelphia, Pennsylvania, held services Monday for Ramon.

A service in memory of the entire Columbia crew will take place at 10 a.m. EST Thursday at the Washington National Cathedral, and NASA officials will have another memorial service Friday for employees at Kennedy Space Center in Florida.

An honor guard at Barksdale Air Force Base in Louisiana held a military reception Sunday for astronauts' remains that were recovered in Texas, authorities said.

The remains arrived in two separate caskets and were greeted by the Second Bomb Wing Honor Guard for a "very honorable and respectful reception ceremony," said Lt. Col. Larry Hahn.

"Everyone was dressed in their full dress uniforms," Hahn said. The honor guard received the caskets as it would in a military funeral and put them in an ambulance, which brought the remains to a place where they are being housed temporarily.

There were no flags on the caskets, he said, because "there is no way to discern what remains there are at this point." Six of the crew members aboard Columbia were American and one was Israeli.

Hahn said an Israeli representative was present, as was a color guard with U.S. and Israeli flags.

In a statement Monday, the astronauts' families thanked NASA for its support while telling the agency "the bold exploration of space must go on."