A NASA statement said all personnel were accounted for. A damage assessment was underway late Tuesday afternoon.
Stephen C. Doering, manager of the SLS Stages Element Office for NASA, told The Washington Post that he was watching the twister from the parking lot when it moved toward the assembly facility. He and his colleagues ran inside to shelter in the restrooms, he said.
“You could see it come in the parking lot. It took about a half a dozen cars and picked them up and knocked them over like rag dolls,” Doering said.
After the tornado passed, workers smelled gas and were ordered out of the building. But with other tornadoes reported in the area, everyone had to shelter in place again, he said.
Doering said he had heard of one injury but didn't know the severity. While the hardware for the new rocket was undamaged, he said, workers were busy patching holes in the building's exterior, hoping to prevent water damage to the elaborate and expensive equipment used to weld the big fuel tanks.
“We didn’t get to do a full damage assessment yet, because after the first [tornado] we had to shelter in place as two more of them came by,” he said.
Workers at Michoud built military planes in World War II and later key stages of the Saturn V rocket that launched Apollo astronauts to the moon. More recently, NASA built the space shuttle's external fuel tanks there.
With the SLS, NASA is planning the first uncrewed flight for late 2018. The first mission with astronauts is not expected until early in the next decade; that flight would put astronauts aboard the Orion capsule in orbit around the moon. NASA is not currently building a lunar lander.
A version of the SLS could potentially be used for missions to Mars.