An open-skies agreement signed in 1992 by 24 nations, including the United States and Russia, made the effort possible.
Aerial Images Inc. posted five images of the site taken by a Russian satellite launched in 1998 to map Earth's surface.
An open-skies agreement signed in 1992 by 24 nations, including the United States and Russia, made the effort possible.
The US Air Force only recently acknowledged that Groom Dry Lake Air Force Base - the official name of Area 51 - even exists.
The base is 120 km northwest of Las Vegas.
The base has been the testing ground for a host of top-secret aircraft, including the B-2 stealth bomber.
The base's airspace is restricted and civilian aircraft are not allowed to fly over it.
But satellite overflights are allowed as part of an agreement to verify arms-control compliance.
'No little green men'
The images are better than earlier telephoto shots from the nearby mountains.
A satellite purportedly shot the only other known image in the 1960s.
"There are runways, there are buildings, there are buses, there are test sites, but there aren't any little green men or super-secret aircraft to be seen," John Hoffman, president of Aerial Images Inc, said of the new photos.
An Air Force spokeswoman did not comment on any security concerns about the images.
"We acknowledge having an operating site there, and the work is classified," spokeswoman Gloria Cales said.
The work involves "operations critical to the US military and the country's security".
The images show craters, some seemingly formed by something dropped from the sky, others possibly by something coming out of the ground.
Alien interview
Rumors have always cited Area 51 as a place where UFOs are guarded by the American military.
A videotape allegedly smuggled out of Area 51 some years ago showed what its distributors said was an alien being interrogated by the US military.
The film's distributors, ILC, said it was shot after the Roswell Incident in 1947, when a UFO is said to have landed in New Mexico.
In the film, a bug-eyed alien was depicted in increasing distress. A heart monitor sped up rapidly, then the alien was sick.
ILC said that the alien died shortly afterwards and that it was this alien whose autopsy was released on video.
The US Government and military deny a UFO landing took place at Roswell.