The space shuttle Columbia flared and broke up in the skies over Texas on Saturday, February 1,2003, killing the seven astronauts on board in what NASA and President Bush called a tragedy for the entire nation. NASA launched an investigation into the disaster and began searching for the astronauts' remains. It said that although there had been some data failures it was too early to nail down a precise cause. The break-up, 16 minutes before the shuttle was due to land at Kennedy Space Center in Florida, spread possibly toxic debris  over a wide swath of Texas and neighboring states. Dramatic television images of the shuttle's  _ clearly showed several white trails  streaking through blue skies after the shuttle suddenly fell apart. It was almost 17 years to the day that the Challenger shuttle exploded on Jan. 28, 1986, killing all seven astronauts on board. Take-off and re-entry into Earth's atmosphere are the most dangerous parts of a space mission. In 42 years of US' human space flight, there had never been an accident in the descent to Earth or landing. Challenger exploded just after take-off. Rescue teams scrambled to search for the remains of the crew, which included the first Israeli to fly on the shuttle, former combat pilot Col. Ilan Ramon. There were warnings that parts of a vast 120-mile-long corridor of debris could be toxic because of poisonous rocket propellant . "We are not ready to confirm that we have found any human remains," Nacogdoches County Sheriff Thomas Kerrs said. He added that among the roughly 1,000 calls reporting debris, some people said they found remains of crew members. "The Columbia is lost. There are no survivors, Their mission was almost complete and we lost them so close to home. ... America's space program will go on," said a grim-faced Bush in a message broadcast on television, which included condolences to the families of the dead astronauts. The word "descent" in the passage means   _  .
taking off landing orbiting walking down