Phobos is the inner and larger of the two moons of Mars, an irregular body about 26 by 23 by 18 kilometers that circles roughly 6,000 kilometers above the planet's surface every 7 hours and 39 minutes. Asaph Hall discovered it in August 1877. Its surface is marked by the 9-kilometer Stickney crater and by long grooves, and scientists still debate whether it is a captured asteroid or debris from an ancient impact on Mars. Tidal forces are lowering its orbit, and it is expected to break apart within about 30 to 50 million years.

In the late 1950s Soviet astrophysicist Iosif Shklovsky suggested that Phobos might be hollow, an idea that fed talk of an artificial moon until Viking orbiter images in the 1970s showed a natural object. The Soviet Union launched two probes to study it in 1988. Phobos 1 was lost on the way, and Phobos 2 reached Mars orbit on January 29, 1989, returning 37 images of the moon before contact was lost on March 27, ahead of a planned lander deployment. The failure was attributed to a malfunction of the onboard computer.

UFO writers soon presented the probe's last frames as showing a cigar-shaped object or its shadow, and as evidence that something had disabled the spacecraft. Two Soviet space engineers, Selivanov and Gektin, wrote in The Planetary Report in 1993 that the "mystery object" was the shadow of Phobos itself, recorded by the probe's thermal scanner and stretched into an ellipse largely by drift in the spacecraft's axis. A second claim centers on a boulder near Stickney, about 85 meters across, in a 1998 Mars Global Surveyor image. Buzz Aldrin, urging a mission to Phobos on C-SPAN in July 2009, called it a monolith and said, "The universe put it there." In July 2026 Ross Coulthart asked physicist Max Derakhshani whether a craft had knocked out the probe, and American Alchemy promoted the claim that NASA had proposed a 2007 mission to sample the monolith.

That 2007 concept, PRIME, was a study funded by the Canadian Space Agency and led by Optech and the Mars Institute, and it was never funded for flight. NASA's caption for the 1998 image says most of the large boulders on Phobos may have been ejected by the Stickney impact, and scientists treat the monolith as a natural rock. Aldrin's remark has kept the moon in UFO media, while Phobos itself remains a target for sample-return missions seeking the origin of Mars's moons.