Twice in a blue moon.

• Author: Braffman-Miller, Judith
• Position: ASTRONOMY

OUR GLEAMING, beguiling moon is the largest object in the night sky--a magical companion-world--and an ancient symbol for love and evanescent beauty. It comes to us at the time of dreaming, disappearing and reappearing every month, growing from a luminous slender crescent, ever larger and larger, until it is flail and spherical, looking like a shining white-gold mirror that reflects a man's face. From time immemorial, it has been a source of inspiration for the lunatics, lovers, and poets among us.

Even though the moon is surrounded by a crowd of fiery stars, it seems lost and alone--a solitary object sailing in a sea of starlight. Although it shines brightly, its surface actually is quite dark. It is the only body beyond Earth that humans have set foot upon, and yet, in many ways, our nearest companion in space remains veiled in mystery.

Where did the moon come from, and has it always been alone? Several theories exist explaining how it was born. One suggests that the moon once was part of Earth and then somehow budded off when our solar system still was very young. Another states that our moon was born elsewhere in the solar system and was captured when it wandered too close to Earth's gravitational grasp. A third theory is that the Earth and its moon condensed individually from the same protoplanetary nebula that gave birth to the solar system, and that the moon formed in orbit around the Earth.

The one that is favored by most planetary scientists is termed the Giant Impact Theory or, much less formally, the Big Whack, in which a Mars-sized protoplanet, named Theia by astronomers, smashed into the primordial Earth about 4,500,000,000 years ago, when our solar system was taking shape. The violent collision blasted part of Earth's crust--mixed well with material from the pulverized Theia--into space. Some of this debris was ensnared by Earth's gravity into orbit, where it at last was pulled together by gravity's relentless embrace to become our moon.

Most of the Giant Impact Theory first was proposed in 1975 by William K. Harlmann and Donald R. Davis of the Planetary Science Institute in Tucson, Ariz. Their theory is based on geological samples brought back to Earth by Apollo astronauts after they had made their historic journey to the moon in July 1969. Oxygen isotopes within moon rocks were found to be nearly identical to rocks on Earth. Furthermore, additional evidence suggests that the moon is composed of, at least in part, the same material that makes up Earth's mantle.

Such giant impacts are believed to have been a common occurrence in our early solar system. Young solar systems are violent places--often likened to "cosmic shooting galleries"--where planetary building blocks (planetesimals) frequently crash into each other, shatter one another, and meld together into ever...