That's the main engine cut-off when the shuttle's engines turn off and you're in orbit.

NPR: Former Astronaut on NASA Risk Assessment

Is it your dream to see the space shuttle lift off -- in person?

CNN: The Internet: the final frontier for space watchers

And they're looking at--their concern is that it could break off during re-entry and maybe damage the shuttle further back.

NPR: Shuttle Spacewalk Repair Mission a Success

The waterproofing burns off during the shuttle's fiery re-entry into Earth's atmosphere, leaving the tile-encrusted spacecraft vulnerable to the elements after landing.

CNN: Technicians finish toasting shuttle tiles

In 1986, seven astronauts died after the Challenger space shuttle exploded 72 seconds after lift-off from Cape Canaveral.

CNN: Wednesday,

The bottom part of the proposed Liberty rocket would be based on the solid-fuelled boosters that help get the shuttle off the ground.

BBC: New rocket could lift astronauts

The shuttle blasted off on Monday with six astronauts on an 11-day voyage to deliver new equipment to the station.

BBC: Shuttle docks with space station

The space shuttle uses ATK's four-segment solid rocket boosters (SRBs) to do the major part of lifting the shuttle stack off the Earth.

BBC: New rocket could lift astronauts