Columbia: 16 minutes from home. (Space/Atmosphere).

On February 1, the morning sky over Cape Canaveral, Florida, was flawless. After a successful 16-day mission, the homeward-bound space shuttle Columbia was scheduled to touch down at 9:16 a.m. EST. At 9 a.m., NASA Mission Control in Houston, Texas, lost contact with the spacecraft.

At 200,767 feet above north-central Texas, flying at Math 18--12,500 miles per hour, or 18 times the speed of sound--Columbia burst into flames. Debris rained mostly over Texas and Louisiana, and all seven astronauts onboard perished.

Columbia's mission had been the first in three years solely dedicated to conducting science experiments. "We'll be looking at flames in space, understanding the ways that fire works, and trying to make cleaner-burning engines," astronaut Laurel Clark had told Scholastic News last summer. "We'll be studying cancer cells. We'll be studying viruses." Some of the 80 experiments were conducted on behalf of elementary school students from Australia, China, Israel, and the U.S.

What went wrong? At press time, members of the accident investigation board still scour for answers.

But shuttle landings are tricky. When a shuttle first lifts off from Earth, it gathers enormous kinetic energy (moving energy) to orbit the planet once every 90 minutes, at a speed of about 28,000 kilometers (17,500 miles) per hour. To deorbit, a shuttle must shed that energy to slow down. One hour before landing, it flies upside down and backwards, then fires its engines for 2.5 minutes--easing the shuttle's speed.

After this deorbit burn, the shuttle flips upright and tilts nose-up 40 degrees; its belly bears the first encounter with the atmosphere (protective layer of gas surrounding Earth). The high-speed meeting between the shuttle and thickening atmospheric gas molecules causes friction (rubbing forces), which generates heat up to 3,000[degrees]F. But almost 24,000 hand-laid, heat-resistant ceramic tiles protect the shuttle's aluminum hull from enveloping plasma (super-heated gas).

At about 38 minutes into descent, Columbia's sensors red-flagged trouble. Mission Control instantly realized the temperatures of the left wheel well (conceals a wheel that descends for landing) and then the shuttle's left side had spiked significantly. Shortly after, a strong drag (pull) began to tug on the craft's left wing. This triggered Columbia's emergency autopilot mode into action, which used extreme steering to counter the drag and correct the flight path. The explosion occurred soon after.

Why? Theories abound:

* A breach in the left wing or wheel well let plasma penetrate. The well's inner wall is made of unprotected aluminum, which melts at 1,220[degrees]F.

* A NASA video shows that 81 seconds after liftoff, a 20-inch, 2.5-pound piece of foam insulation broke off from Columbia's external fuel tank, striking the bottom side of the wing. Could this have damaged protective tiles?

* Could a high-speed encounter with tiny space debris have caused a breach?

As each day passes, fewer pieces of wreckage surface. Missing are some that may hold vital clues to the shuttle's end. On February 18, investigators reported that Columbia actually started dropping wreckage over California, six minutes before it disintegrated over Texas. But fragments were so small they would have burned up in the atmosphere. "We've got more detective work," says shuttle program-manager Ron Dittemore.

1957 Dawn of the Space Age: October 4, when the former Soviet Union launched the first human-made object into space. Sputnik was a basketball-size 183-pound satellite that orbited Earth in 98 minutes.

1961 First man in space: Yuri Gagarin on April 12. The 27-year-old Russian flew one 108-minute orbit around Earth. During reentry, he parachuted from Vostok 1 at 7 km, landing in a field.

1961 First American in space: Alan B. Shepard on May 5. "What a beautiful view!" he said of his 15-minute solo journey in a Mercury capsule. Nearly one year later, John Glenn became the first American to orbit Earth.

1965 In March, Russian Aleksei Leonov became the first spacewalker--attached to a tether for 12 minutes. American Ed White followed in June.

1969 Historic first: On July 20, Nell Armstrong and Buzz Aldrin landed and walked on the moon. Armstrong's live-televised quote: "That's one small step for a man, one giant leap for mankind."

1981 On April 12, space shuttle Columbia debuted as the world's first reusable spacecraft. Before shuttles, space missions ended by nosediving into oceans--astronauts had to be fished out.

2000 The first crew traveled to the International Space Station on October 31. The orbiting space lab is the largest scientific cooperative program in history, with resources and expertise from 16 nations.

DISCOVERY TIMELINE

"To leave behind Earth and air and gravity is an ancient dream of humanity. For these seven it was a dream fulfilled.... Each of them knew that great endeavors are inseparable from great risk. And each of them accepted those risks willingly, even joyfully, in the cause of discovery."

--President George W. Bush

COLUMBIA'S LAST MISSION

Space shuttle Columbia broke apart above north-central Texas on February 1, claiming the lives of all seven astronauts on board. Here, its final minutes:

8:52 a.m.

Sensors indicate first sign of trouble

8:55 a.m.

Temperature rises at an unusually high rate

8:58 a.m.

Sensors near the left wheel well (part of the landing gear) fail

8:59 a.m.

Contact with shuttle is lost

[ILLUSTRATION OMITTED]

WHAT WENT WRONG?

Three leading theories on what caused the accident:

[1] FOAM AND TILE

A chunk of insulating foam broke off the fuel tank and hit the orbiter's left wing during liftoff. This damaged heat-resistant tiles bonded to Columbia's aluminum skin and caused the shuttle to overheat.

[2] SPACE DEBRIS

A small piece of space debris struck and punctured the leading edge of the shuttle's left wing while in orbit.

[3] ALUMINUM SKIN

A hole in the shuttle's aluminum skin allowed plasma (superheated gas) to flow into the left wing.

[ILLUSTRATION OMITTED]