Back-up can be an item kept available to replace an item which fails to perform satisfactorily. Something that provides support or help when certain equipment, etc. goes wrong.

A backup crew is a ready team, fully trained, fit and outfitted, able to step in, in a moment’s notice to take one of the primary crews positions, in the Apollo space project one astronaut was exposed to measles ahead of the launch – if that person had contracted measles then one of the back up team would have gone on board Apollo instead.

With space programmes there are many back up bits of equipment on board in case of failure during the mission, including batteries.

Watch this video. The Backup Space Shuttle