Moon

Earth's Moon (often referred to as simply The Moon or Luna) is Earth's only natural satellite, and the the setting of the final sequence of Portal 2 It is also the latest known inhabitance of Wheatley.

History
Current scientific hypotheses posit that the moon was formed roughly 4.5 billion years ago, when a large celestial body impacted with the Earth at high speed. Humans first landed on the moon in 1969, when Neil Armstrong and Buzz Aldrin landed in the Sea of Tranquility.

At some point in the late 1970s or early 1980s, Cave Johnson somehow managed to acquire roughly $70 million worth of moon rock. Upon liquefying it, he discovered that it made "A damn good portal conductor", and put it to use in testing, under the name Conversion Gel.

Several centuries later, when Wheatley had taken over the Aperture Science Enrichment Center and allowed it to fall into serious disrepair, the ceiling fell off his chamber and exposed it to moonlight. Chell took advantage of this, and the moon's portal conducive properties, and shot a portal at the moon, causing both herself and Wheatley to be sucked into the near-total vacuum of the moon's atmosphere. Chell was returned to Earth when GLaDOS, newly restored to power, pulled her back in through the portal. Wheatley, however, was left to orbit the moon indefinitely, along with a corrupt Personality Core.

Trivia

 * In real life, the moon has gravity approximately 1/6 of Earth's gravity, meaning that an object on its surface would remain on the surface unless acted upon. However, in the Half-Life universe, the moon appears to have almost no gravity whatsoever, meaning that almost any body on its surface will be sucked out into the vacuum of space.

Appearances

 * Half-Life 2 (seen from Earth)
 * Portal 2