By William J. Cromie

Gazette Staff

The moon was created when an object, about two times as massive as Mars, slammed into a newly forming Earth, according to Harvard astronomer A.G.W. Cameron. The Big Whack, as it is popularly known, came from a sister planet forming nearby, and it may have hit the fetal Earth twice.

Despite billions of dollars, six manned landings, and hundreds of pounds of rocks returned from the lunar surface, scientists still don't agree completely on the moon's origin. Cameron, a professor of astronomy, put forth his theory in 1984, 15 years after the first lunar landing. He has been refining it ever since.

At a recent meeting of the American Astronomical Society in Cambridge, Mass., planetary scientists discussed many details of the "giant impact" scenario that remain to be worked out.

One of the newest twists has the collision taking place before, not after, Earth became fully formed. Another brings in the possibility of a smaller bump preceding the main hit. Cameron thinks such adjustments are needed to account for the present masses and movements of Earth and its moon.

The collision, or collisions, took place about 4.6 billion years ago. A huge, flattened disk of gas and rock orbited the sun, extending out billions of miles. Planets began forming when small particles collided to form larger particles, then chunks of rock. During this process, sister planets began to build in the present neighborhood of Earth. The developing Earth grew bigger faster, and it was only a matter of time before its companion collided with it.

Cameron has done dozens of computer simulations of what might have happened. In the single collision scenario, the hapless sister slams into Earth and breaks up. The heavy iron in its core falls into the still molten Earth and sinks to the planet's center. Most of the remainder of the shattered planet goes into making the moon.

In a dual collision, the sister grazes Earth and "bounces" away, but does not get beyond Earth's gravitational grip. That grip causes it to slowly move back toward Earth, then hit it harder. Fragments of the demolished sister planet end up as a moon, which is almost a quarter of Earth's size but only about 1 percent of its mass.

'Luny' Ideas

That story differs from three theories about the moon's origin popular before, and for years after, the Apollo lunar landings.

One held that a newly born, still-molten Earth started spinning faster and faster until it threw off a piece of itself as big as the moon.

"If something like that happened, then where did the excess angular [spinning] momentum go?" Cameron asks. "Both the Earth and the moon would have to be spinning four times faster than they do today if that had happened."

A second theory pictured a moon assembling itself independently from Earth, much as the impactor planet did. "If that happened, both would have about the same percentage of metallic iron in their bodies," Cameron points out. "Rocks brought back by the astronauts show very little metallic iron on the moon. You have to conclude it is made up of lighter material from the outer parts of Earth and its now extinguished sister."

Finally, the moon might have formed elsewhere in space, then been captured by Earth's gravity. "That possibility still leaves you with the iron problem; a body as big as the moon formed from the same material as other planets would have an iron core like Earth, Venus, and Mars," Cameron notes.

"Also," he adds, "there's no way to capture a moon-sized body that leaves you with the Earth and moon spinning the way they do today at their present distance from each other -- 240,000 miles."

The National Aeronautics and Space Administration (NASA) gave a psychologist a grant to interview various experts and determine which theory they favored before and after they analyzed lunar rocks. NASA thought these scientists would converge on whichever idea made the most sense.

"It didn't work out that way," Cameron recalls. "After examining the lunar samples, everyone clung to the same scenario he or she favored before the moon landings. The new data shed no new light on the moon's origin, the psychologist concluded, but it proved that lunar scientists are opinionated and obstreperous."

Starting Over

Cameron decided so much was wrong with the extant theories, he would start from scratch and see where it led him. "I thought of it as a high school physics problem," he says. "I wanted to hit Earth tangentially with a body and come out with the same masses, spins, and distances now existing in the Earth-moon system. How big a body did I need?"

Cameron and William Ward, a former Harvard colleague now working for NASA, came up with a body at least as big as Mars -- 4,200 miles in diameter and weighing about 660 million trillion tons. Subsequently, Cameron has roughly doubled that mass.

When the collision occurred, Earth and the newly formed moon were significantly smaller than they are today, Cameron believes. Following the impact, they both continued to sweep up rocks orbiting the sun until they reached their present masses, about 6,600 million trillion tons for Earth and about 80 million trillion tons for the moon.

Cameron works with Robin Canup and her colleagues at the University of Colorado in Boulder to tie up the loose ends of the Big Whack theory. "I do the impact calculations, which end with a lot of material orbiting close to Earth," Cameron explains. "She plugs these results into another computer model to determine how that material coalesced into the moon."

Neither Cameron nor Canup claims that the origin of the moon is finally settled. However, three longstanding ideas about its creation have been disregarded, along with the idea that life might exist on the moon. As late as 1959, some scientists talked about the possibility of life below the lunar surface and even of "highly organized" plants and "small, sluggish" animals on the surface.