Dubbed a 'Christmas miracle' by anxious astronomers, the massive, next generation Jame Webb Space Telescope is finally on the launch pad, ready to go to space.

The telescope has been hit by months of launch delays, which followed years of delays to its construction, with its first launch date supposed to be in 2007.

The observatory is now set to take off on December 25, with a launch window between 07:20 ET (12:20 GMT) and 07:52 ET (12:52 GMT). 

Ahead of launch, it was loaded into the fairing of an Ariane 5 rocket, and moved on to the launch pad at the Europeans Space Agency facility in French Guiana.

Live coverage of the launch will be shown on NASA's TV channel and website on Saturday, starting at 06:00 ET (11:00 GMT). 

Officials confirmed that the Ariane 5 was in good shape, and the only problem facing the launch was the start of the rainy season in French Guiana. 

Work on the James Web Telescope, also known as JWST or Webb, first began in 1996 and at the time NASA had just a $500 million budget to complete it.

The agency was set to launch it in 2007, but cost overruns and technical issues forced a major redesign in 2005 that led to its first delay.

Construction of the telescope was completed in 2016, allowing testing to begin, but two years later the massive sunshield ripped during a practice run that led to another postponement.