Seventeen years ago, students in classrooms across the nation tuned in to live coverage of the launch of a space shuttle -- the first space flight to include a teacher. To everyone's horror, the space shuttle Challenger's fuel tank blew up shortly after lift-off, breaking the craft into pieces.

All seven people on board, including teacher Christa McAuliffe, died. After the disaster, many thought that the space shuttle program would cease to exist. But on Sept. 29, 1988 the shuttle Discovery went into space.

On Saturday, the space shuttle Columbia broke apart in space 15 minutes before it was to land at the Kennedy Space Center in Florida. Once again, all seven astronauts aboard died.

The tragedy raises questions about why humans want to go into space, the goals of the American space program and what happens next.

Why send people into space?

To many Americans, the desire to explore is part of being human. "I believe that people go into space not just because it's in the culture, because it's actually wired into our DNA to explore," said Donna Shirley, former manager for Mars Exploration at NASA.

The families of Columbia's crew urged the U.S. to continue exploring space. Evelyn Husband, wife of Commander Rick Husband, read a statement from the families on Monday. It said the astronauts' hearts were full of "a willingness to accept risk in the pursuit of knowledge -- knowledge that might improve the quality of life for all mankind…Once the root cause of this tragedy is found and corrected, the legacy of Columbia must carry on for the benefit of our children and yours."

On Monday, President Bush proposed increasing the budget for the space shuttle program to $3.9 billion from the $3.2 billion it received in 2003. According to White House spokesman Ari Fleischer, the budget request had been decided before the loss of the Columbia. The budget was not changed over the weekend.

Some individuals argue that unmanned spacecraft are cheaper and less dangerous. "What we need to do now is end the program that explodes in front of our eyes and replace it, use the money ... to research a new more reliable and lower-cost system. Once we have that, then grand human ambitions in space might become possible," Gregg Easterbrook, who writes on science as a senior editor for the New Republic Magazine, said.

Future of the shuttle program

The U.S. has three space shuttles remaining: Atlantis, Discovery and Endeavor. It is uncertain if the U.S. will build a replacement for the Columbia due to logistical and budget issues.

The space shuttle, first sent into orbit in 1981, is a reusable manned spacecraft. Originally, the project was going to pay for itself by delivering and retrieving satellites for the military and private industry.

But after the Challenger accident, the military returned to the use of unmanned rockets for many of its satellite needs.

The goals shifted towards providing transportation to and from the International Space Station. The shuttle also became a laboratory in space.

The STS-107 mission

The most recent Columbia mission, labeled STS-107, was mainly scientific. The astronauts were involved in round-the-clock experiments, many of which have practical implications back on earth.

The astronauts were researching the absence of gravity on granular materials in order to help engineers provide stronger foundations for structures in areas where earthquakes, floods and landslides are common. They also worked on projects that hoped to combat prostate cancer and improve crop yield.

They hoped to help to create drugs with a decreased risk of side effects for several diseases. And they aided students from around the world, bringing some schools' experiments aboard the space shuttle.

Now, the future of the shuttle program is uncertain until the cause of the Columbia accident is determined and corrected. NASA has said the remaining three shuttles are grounded until then.

NASA's investigation was focusing on whether foam insulation that flew off during liftoff damaged the shuttle's system of ceramic tiles that protect it from extremely hot temperatures reached on re-entry from space.

However four days after the disaster, shuttle program manager Ron Dittemore said the foam simply was not heavy enough or traveling fast enough to damage the shuttle's heat resistant tiles.

"We're focusing our attention on what we didn't see. We believe there was something else ... there's got to be another reason," he said.

-- By Annie Schleicher, NewsHour Extra