The United States has marked the 30th anniversary of the explosion of the Challenger space shuttle, which killed seven astronauts 73 seconds after lift-off.

At the Kennedy Space Centre in Florida, where the shuttle blasted off on January 28, 1986, singers in red and blue belted out the Star Spangled Banner for a crowd including relatives and friends of those killed.

Six NASA astronauts and Christa McAuliffe -- who would have been the first teacher in space -- died when a failed booster engine caused the shuttle to explode.

"These brave women and men are forever a part of a story that is ongoing," US President Barack Obama said.

"It is a story that will bring human beings to Mars and out into our solar system -- and beyond. It is a story made possible by their sacrifice and heroism."