A classified experimental US spy satellite has failed and will be allowed to drop out of orbit and burn up in the atmosphere.

The L-21, built by Lockheed Martin and launched in December last year, has been out of touch with earth since reaching its low-earth orbit – approximately 22 miles above earth.

Officials from the US National Reconnaissance Orbit Office told Reuters it will take decades for the satellite, worth hundreds of millions of dollars, to return to earth.

They declined to say what technology the satellite was supposed to be testing but said the Pentagon is likely to ‘piggyback’ the technology onto other satellites, like the TacSat-3, due to be launched later this year.