By Steve Schifferes BBC News Online Washington correspondent

President George W Bush travelled to Houston to show his respect for the astronauts who perished in the Columbia shuttle disaster, and take part in a memorial service with the families of the lost crew members.

"They go in peace for all mankind, and all mankind is in their debt," he told the nation in words tinged with deep emotion.

The president's job is to comfort the families and the nation and provide them context that helps them in their grief Presidential Advisor Karen Hughes

It is the second time in the week that the President has addressed Americans on the loss of the Columbia.

Mr Bush's prompt response to the disaster, and dignified words of comfort, have boosted his standing on the eve of a possible war.

While the administration has made it clear that it will not be deterred from its timetable on Iraq, the tragedy has provided the president with an opportunity to bind together a nation which is still divided over his approach to foreign and domestic policy.

Space travel will go on: George W Bush

"Part of the president's job is to comfort the families and the nation and provide them context that helps them in their grief," said presidential adviser Karen Hughes, who helped to write the State of the Union speech.

The country is still nervous about the possibility of further terrorist incidents, and worried about possible casualties in the event of military action in the Gulf - all of which made the Columbia disaster cast a longer shadow than the loss of the Challenger 17 years ago.

"I think the country is looking for a little help and a little uplift and in midst of this sombre occasion," said former Reagan speechwriter Ken Khachigian.

Mr Bush hopes to give Americans a sense of purpose, a belief that they are engaging in a higher calling, and he believes that the space programme furthers a noble aim: the discovery and exploration of the universe for the benefit all mankind.

"America's space programme will go on. It is a desire within the human heart," he told the families and workers at the Johnson Space Centre in Houston.

Approval ratings

Mr Bush's State of the Union speech last week was less effective in inspiring the nation.

According to opinion polls, his overall public approval rating was unchanged, and he received lower approval ratings for his handling of the economy.

But this time Mr Bush seems to have learned the lesson of September 11, which became the defining moment of his presidency after a shaky start.

Polls show over 80% backing for the space programme, vindicating Mr Bush's approach.

In this context, it has made it harder for Democrats to carry on with politics as usual, and on Monday Senate Democratic leader Tom Daschle cancelled a press conference called to fight Mr Bush's nomination of a conservative judge.

Backing Nasa

The President also has another purpose in mind by devoting so much time to the Columbia tragedy.

He wants to demonstrate his support for Nasa, an organisation under siege after the disaster.

George Bush demands intense loyalty from his staff and supporters, and in return he is prepared to give them his full backing.

On Monday he summoned the head of the Nasa, Sean O'Keefe, to the White House, and told him that "you make us proud."

In contrast to the Challenger disaster, Mr Bush has made it clear that he has no intention of creating a presidential commission to investigate the disaster, leaving it instead to Nasa to create its own independent panel.

Although the space programme is a civilian one, many of the astronauts are former members of the US military, and the investigating panel will be dominated by military figures.

And Mr Bush's attitude may also be influenced by the possibility that in a few weeks he may have the heavy responsibility of sending American troops in the Gulf into a war.

Those troops also will have to face significant risks and possible losses in a cause Mr Bush believes to be just - defeating evil and promoting freedom throughout the world.

And, if it comes to war, the president will want the nation to give those troops the same unconditional backing - and to set that campaign in terms of a higher moral purpose.