On July 7th, 1988, the Soviet Union launched its most ambitious deep-space exploration project to date. It consisted of two twin orbiters, based on the successful Venera design, but additionally carrying two different types of landers. Their mission was to explore in depth the martian moon Phobos, by delivering their landers and acquiring high-resolution images. In addition to the instruments they carried to image the moon, they also carried several instruments to study the Sun, the planet Mars, the interplanetary medium, and gamma-ray burst sources.
The probes, dubbed Phobos 1 and 2, are usually thought by Mars researchers to be failed missions. Surely, since Phobos 1 never even reached Mars, it could be considered as a failure. Yet it would be unfair to apply that designation to Phobos 2, since the probe survived Mars orbit insertion and was able to send back a total of 37 images before it disappeared, apparently due to mechanical failure.
Phobos 2 operated normally during its cruise phase, travelling the millions of miles from the Earth to Mars with no mechnical problems. It successfully gathered data about the Sun, Earth, Mars, and the interplanetary medium. On March 27, 1989, as it approached within 50 meters of the moon Phobos, it was set to drop a mobile "hopper" lander and a stationary platform. But just before doing so, contact was mysteriously lost. The Russians were never able to conclusively prove the exact reason for the failure, but the consensus was that the mission failed due to a malfunction of the onboard computer.
Much controversy surrounds the last photo taken by Phobos 2. Seen to the right, the photo seems to show a long, slender object just below the moon Phobos. Estimates of the size of the object are up to several miles long. However, critics say the object could be nothing but a camera artifact, since many images from the same camera showed long, white streaks similar to this.
Yet, many claim that the image shows a UFO. This photograph was revealed in December 1991 by Marina Popovich, a famous Soviet test pilot sometimes called the "Russian Chuck Yeager" due to the 17 aviation world records she holds. She claimed that the photograph was given to her by cosmonaut Alexei Leonov, who was the first man to walk in space and also a high official in the Soviet space program, and that she had "smuggled it out" of the former USSR. According to her, the photo clearly shows a UFO hovering near the moon Phobos, and is "the first ever leaked accounts of an alien mothership in the solar system". She says this was apparently the cause of the disappearance of the Phobos 2 probe.
Whether this "UFO theory" is correct or not, we will probably never know the true answer to what happened to Phobos 2.