Bad weather has forced Nasa to postpone the launch of Discovery for the second day in a row.
Thunderstorms near the launch site in Florida mean the shuttle will not be able to take off.
Engineers from the American space agency plan to try again on Tuesday.
This is a crucial flight – as it is the second since the 2003 Columbia accident that left seven astronauts dead, and comes after extensive repairs and upgrades were made to the fuselage and fuel tanks.
Nasa is stricing to stop insulating foam breaking off and damaging the shuttle during take-off.
The payload is a mix of much needed supplies and equipment to fix problems on board the International Space Station.
If there are problems with the shuttle launch it’s likely that Nasa will ground the fleet permanently, putting the ISS project on ice too.
The shuttle is only craft capable of carrying the pieces needed to complete the orbiting research centre.