GEOFF BENNETT: Today marks 20 years since the space shuttle Columbia disintegrated on its way home.

The tragedy killed all seven astronauts on board.

And it was the beginning of the end for the space shuttle program, ultimately changing how we explore space now.

Miles O'Brien was there that day and has our look.

MILES O'BRIEN: So, where were you when it happened?

There are moments in time that trigger that question.

MAN: Columbia, Houston, comm check.

MILES O'BRIEN: The loss of the space shuttle Columbia is one of those occasions.

It happened the first day of February 2003.

We remember as if it were yesterday.

The junior senator from Arizona, Mark Kelly, certainly does.

SEN. MARK KELLY (D-AZ): I was asleep.

And I had planned to get up and watch the landing, something on TV.

MILES O'BRIEN: The 113th mission of the space shuttle fleet, and the second now to end in catastrophe.

That's where I was, covering the disaster on CNN.

Mark Kelly was then a NASA astronaut with one mission under his belt.

His twin brother, Scott, was also in the corps.

When did you know that there was trouble?

SEN. MARK KELLY: I got a phone call from my brother.

And he just says that they lost -- lost communications and they lost tracking at the same time.

And, at that point, I knew that this was not likely to turn out as any of us had hoped.

MILES O'BRIEN: The orbiter had disintegrated as it streaked across Texas.

The crew of seven were gone.

And Kelly knew it.

He raced to the Johnson Space Center near his home in Houston.

SEN. MARK KELLY: You know, nowhere in our contingency plan did we ever expect the space shuttle to crash within a two-hour drive of Houston.

You would never anticipate that.

I said, we need to send somebody there, like right now.

MILES O'BRIEN: He made some quick calls and hopped on a Coast Guard helicopter.

SEN. MARK KELLY: We go to Nacogdoches Airport.

And there's debris at the airport.

There's pieces of shuttle.

I -- that might be a piece of space shuttle, and then I put it back down, decided, what, are you going to pick up every piece?

MILES O'BRIEN: In East Texas, debris was scattered over 2,000 square miles.

Soon after Kelly landed, he heard a body was found in Hemphill.

SEN. MARK KELLY: We recovered one of my classmate's remains.

MILES O'BRIEN: Three of the seven on board were in his class, Laurel Clark, Willie McCool, and Dave Brown.

He helped recover them all.

Obviously, you go into this business, you know the risk.

But, in a way, it's an abstract.

And then, when you see that, what goes through your mind at that point?

SEN. MARK KELLY: It was -- it wasn't really so much at that point.

For me, it was days later, when you start to process what happened.

MILES O'BRIEN: The last crew of Columbia recorded these scenes on the flight deck moments before they perished; 3.5 years later, when Mark Kelly was in the same place, he and Commander Steve Lindsey took a moment to remember the lost crew.

SEN. MARK KELLY: Occasionally get a little spare time and a few seconds here and there, and we actually mentioned it, mentioned the Columbia accident during our reentry on STS-121.

MILES O'BRIEN: But it was all avoidable.

The orbiter disintegrated in the searing heat of reentry because there was a gaping hole in its heat shield.

MAN: We have booster ignition and liftoff of space shuttle Columbia.

MILES O'BRIEN: It happened 16 days earlier on launch day; 81 seconds after Columbia left the pad, a suitcase sized piece of foam, part of the insulation covering the external fuel tank, hit the leading edge of the left-wing.

Everyone saw it, including me at the launch site.

There was a piece of debris which struck the shuttle as it came off.

And this is made of what's called carbon-carbon.

If something fell on that and caused some damage, who knows what the implications of that might be.

As soon as I saw this video after launch, I called my NASA sources.

Engineers were looking at it.

They determined looking, very closely at these high-speed, very close, close cameras that they have, that this was not a significant issue.

They said that foam had been flying off shuttle fuel tanks since day one.

How much had this been kind of ingrained into being something that just happens?

SEN. MARK KELLY: And we spend a lot of time and effort on engineering and engineering analysis and trying to chase down anything that could be an issue.

And this one, for some reason, it just got dismissed.

MILES O'BRIEN: In July of 2003, the accident investigation board did what NASA should have done years before, run a test.

And there it was, the smoking gun.

Just shy of a year later, President Bush announced the shuttle was on its way to retirement.

Where do you think we would be if the Columbia accident hadn't happened?

SEN. MARK KELLY: I think we would have still retired the space shuttle.

Maybe it would have been a couple years later.

The space shuttle was designed to fly a lot of flights.

It wasn't designed -- designed to last for 30 years.

So we started seeing more problems come up with it.

And we started to realize that this -- we can't fly this forever.

MAN: Liftoff!

MILES O'BRIEN: The seed was planted for an entrepreneurial revolution in space.

Twenty years later, SpaceX leads a vibrant group of space enterprises.

The Astronaut Office was very skeptical, almost to a person, on this whole endeavor, wasn't it?

SEN. MARK KELLY: We were -- there was a lot of skepticism, yes, including with me.

SpaceX, you got to give them a lot of credit.

This has put us on the path to get back to the moon maybe in our lifetimes, see somebody walking on Mars.

MILES O'BRIEN: And what about Mars?

Is that worth the risk?

I mean, we can send robots to Mars and learn a lot about Mars without people.

SEN. MARK KELLY: Well, first of all, I mean, forever, people want to see what's over the next hill, what's on the other side of that ocean.

The risk is big to any individual that decides to go into space.

But the benefit to society is really big as well.

MILES O'BRIEN: It's likely the last crew of Columbia would say the same.

For the "PBS NewsHour," I'm Miles O'Brien in Washington.