NASA's Orion spacecraft, atop a United Launch Alliance Delta 4-Heavy rocket, lifts off on its first unmanned orbital test flight from the Cape Canaveral Air Force Station Friday, Dec. 5, 2014, in Cape Canaveral, Fla. (AP Photo/Chris O'Meara)

Photos: Launch of Orion

Orion lifts off on its first unmanned orbital test flight from Complex 37 B at the Cape Canaveral Air Force Station, Friday, Dec. 5, 2014

Police rope off the scene of an accident involving two school buses in Knoxville, Tuesday, Dec. 2, 2014. Two school buses have collided on a Tennessee highway, injuring at least 20 people. Darrell DeBusk, a Knoxville police spokesman, says three people who were most seriously injured were taken to the University of Tennessee Medical Center. At least 17 additional students were being transported to East Tennessee Children's Hospital with minor injuries. There was no immediate word on what caused the crash or the exact condition of those who were injured. (AP Photo/Knoxville News Sentinel, Michael Patrick)

Photos: Tennessee bus crash

Two buses bringing children home from school collided on a Tennessee highway Tuesday afternoon, killing two students.