The Apollo 11 samples, brought back in pieces for all mankind, rocked astronomy.

For openers, they neatly resolved the conflict between the meteoric and volcanic theories of the lunar landscape: Both were correct. Many iron-rich rocks from the Sea of Tranquility were plainly basalt, very similar to basalts produced by volcanoes on Earth.

But the specimens also contained substances that could be formed only by meteoroid impact. There were, for example, tiny orangish bits of glass typical of material liquefied by impact and then rapidly cooled. And many of the samples were a form of breccia, fragments of several rock types welded by the heat of collision.

Gradually, most scientists came to agree that meteoroids punctured the lunar surface to form craters and basins during the period of furious bombardment from 4.5 billion to 3.5 billion years ago. Then the maria, or seas, were created by lava that oozed up, often to a depth of 300 feet, from below the crust.

What caused the melting -- presumably heat from radioactive elements -- remains unclear. It seems certain that the process seems to have ended about 3 billion years ago, by which time the moon's innards had cooled considerably. Things have been extremely quiet since then.

There are more maria on the near side of the moon because Earth's gravity tugs harder at that side, shifting the lunar center of mass off geometric center and causing the crust to be thinner on our side. On the far side, the crust is much thicker, making it less likely that basalts could well up to fill impact holes.

Some of the Apollo samples, it turned out, were more than 4 billion years old, far more ancient than nearly all existing Earth rocks. Their chemistry was very similar to, yet intriguingly different from, the terrestrial crust.

Lunar surface rocks, predominately made up of low-mass elements such as aluminum, calcium and silicon, revealed the same telltale mix of oxygen isotopes found in Earth's part of the solar system but in no other.

Many Apollo 11 samples and the nearly 800 pounds retrieved by later Apollo missions were similar to common Earth minerals such as olivine or pyroxene. But all were surprisingly deficient, compared to Earth, in the kinds of volatile elements that would boil off easily when heated.

These findings seemed to rule out all three leading theories of lunar origin. Instead, they supported a radical new "giant impact" model: About 4.5 billion years ago, an object perhaps one-fourth the diameter of Earth -- and as much as three times the mass of Mars -- whacked our planet obliquely. The collision blew off a huge amount from the outermost layers of both objects, spraying dust and vapor into space.

The most volatile elements evaporated into the vacuum; much of the rest condensed again into orbit around Earth, where it eventually formed the moon. This impact theory also explains the moon's unusual orbit, which is tilted with respect to Earth's path around the sun, as well as the Earth's rapid spin rate.

Thereafter, incoming rubble pounded our new satellite for a billion years or so, heating the outermost layer and engraving the craters.

The entire surface probably was molten at one time, as suggested by the preponderance of lighter minerals and breccias in the upper crust. And certainly some element of the mantle was once liquid, or lavas could not have filled the mare depressions.

Whether the moon has a small, partially molten iron core analogous to Earth's is a matter of debate. If so, it is not creating a magnetic field these days, although there appears to have been at least a weak field about 3.8 billion years ago, judging from remnant magnetic evidence in lunar rock samples. And an iron core is hard to reconcile with the mascons, or dense lumps below the surface: If the entire moon had been heated to the melting point, all the heaviest materials would have separated out and gravitated to the center.

Finally, if the entire moon had been fairly plastic at one point, it would have assumed a much more uniformly spherical shape. On the other hand, seismometer readings from instruments left by various Apollo missions suggest that a quasi-melted mantle layer may begin at a depth of 500 or 600 miles.

In short, there is still no definitive explanation for the moon's origin.

Much, however, is known. Its crust makes up 12 percent of its volume, versus 0.5 percent for Earth, and it is now clear that, although the moon is not one solid piece of rock, it is only about three-fourths as dense as our planet.

The moon's diameter is about 2,160 miles; Earth's is about 7,930. If they had exactly the same composition, we would expect the moon to have approximately 1/60th the mass of Earth.

Volume increases as the cube, or third power, of radius. So an object whose dimensions are four times larger than another will have 4 x 4 x 4, or 64 times, the volume. In fact, the moon has about 1/81st as much mass.

If the moon has only 1.25 percent of Earth's mass, why do moonwalkers feel attracted by about one-sixth Earth gravity? The answer is that the distance between an astronaut and the moon's center of gravity is only about 1,080 miles, versus about 3,965 miles on Earth. And although gravitational force is a function of both mass and distance, it varies with the square of the distance from the center.

GOING HOME

With the first part of their mission accomplished, the Apollo astronauts still had to travel a quarter of a million miles back to Earth.

1. Liftoff

July 21, 1:54 p.m. EDT

Eagle's ascent stage lifts off from the moon, leaving behind the descent stage.

2. Docking

5:35 p.m.

Eagle and Columbia reunite after a separation of nearly 28 hours. The crew reassembles and transfers the lunar samples from Eagle to Columbia.

3. Eagle separation

7:42 p.m.

Eagle's ascent stage is jettisoned from Columbia 61 miles above the lunar surface. It gradually loses altitude and crashes on

the lunar surface.

4. Transearth injection

July 22, 12:56 a.m.

To increase Columbia's velocity from 3,600 mph to 5,500 mph, the Service Module engine is fired for 2 minutes, 31 seconds. This firing inserts Columbia on a trajectory to "coast" back to Earth. Only one small correction is needed to keep Apollo 11 on course.

5. Reentry

July 24, 12:21 p.m.

After almost 60 hours traveling back to Earth, Columbia prepares for reentry by jettisoning the Service Module. The Command Module, the only remaining piece of Apollo 11, reenters Earth's atmosphere at a velocity of more than 36,000 feet per second; its heat shield withstands temperatures of 5,000 degrees.

6. Splashdown

12:51 p.m.

The Command Module's parachutes deploy, setting the craft down in the Pacific Ocean, where it is recovered by the USS Hornet.

TOTAL MISSION TIME:

8 days, 3 hours, 18 minutes, 18 seconds

DID YOU KNOW . . .

After splashdown, the astronauts were required to don biological isolation garments and scrub down in iodine to protect against any possible lunar "germs" that might have traveled with them. The Apollo crew was transported in an airtight Mobile Quarantine Facility to the Lunar Receiving Laboratory in Houston, where the spacecraft and lunar surface samples were isolated for study. President Richard M. Nixon visited the astronauts during their three-week quarantine. Scientists found no biological material in the moon samples, and later Apollo crews were not quarantined.

At first, Earth's new satellite had an upper layer of magma covered by a thin outer crust of lightweight rock. When meteorites dug new impact holes, the liquid rock flowed up to create the basalt floors of the maria.

By 3 billion years ago, however, the magma had chilled out, freezing the moon into the configuration we see today.

(Illustration is based on current theory)

SOURCES: NASA; "The New Solar System" (Fourth Ed.)

CAPTION: How the moon was formed:

About 4.5 billion years ago, Earth itself had barely been assembled from gravitational agglomeration of stony flotsam and rubble in orbit around the sun. The heat of repeated collisions plus radioactivity made our planet so hot that much of the iron and other heavy elements sank into the center, where molten rock surrounded a massive core. Then, scientists now suspect, an object the size of a small planet struck Earth, blasting some of the crust into space. As that swirling debris coalesced, it formed the moon.

CAPTION: Vesicular basalt flowed to the surface as lava 3.4 million years ago.

CAPTION: Breccia is a conglomeration of materials fused by impact.