If We Had No Moon, Discovery DVD Classics

After a dramatic opening sequence of planets colliding and the possibility that a Mars-size body could hit Earth, the narrator Stewart says "The odds are that such an impact has already rocked Earth to the core and dealt us the luckiest blow."

From William K. Hartmann, who is credited with coming up with the collision model "The Earth apparently was hit by something nearly the size of Mars. We got a Moon out of the deal and it was a good job too, because the Earth ..."

From Robin Canup, who ran the computer simulations of the impact for her doctoral thesis, comes the comment that Earth is a "completely different planet than it would have been had the moon-forming impact not occurred."

From Lynn Rothschild, evolutionary biologist "If the moon had never formed, we wouldn't be here to worry about it."

Comparing Earth's Moon to other moons in the solar system, the phrase "one big lucky Moon" was used in contrast to Mars' tiny captured asteroid moons.

Fission discussion at 47:15

Outlines the three alternative scenarios for moon formation and dismisses them. "Formed together " dismissed because of no iron core in the Moon, "Capture" no dice, "Fission" - the physics doesn't add up. Hartmann quotes Urey "None of the three theories worked, so the Moon must not be there at all."

Apollo program brought back 840 pounds of rock and moon dirt. Rocks resemble Earth's mantle, but no water and little iron.

Dr. Jay Melosh got collision modeled by Sandia Labs computers. Says collision would take 20 to 30 minutes. Modeled with 11km/s projectile half diameter of Earth.

Narrator "In the chaos of the early solar system, the luckiest happening."

Robin Canup "Its really sort of a random chance, what body collides with what other bodyto yield the final planet. One of these chance collisions happened to occur at just the right angle with just the right speed to form our Moon. And when you view it in that way, the Earth ended up such a habitable distance from our Sun with our large Moon. It is seen to be a very fortuitous event."

35:12 Moon impact. Narrator --- head on, no moon. Would throw up a ring of debris, but within the Roche limit.

Canup "In other cases we find that after an impact we form two moons. They are actually stable for tens of thousands of years."

Narrator: To model one moon, Canup takes the most oblique approach.

Canup "We found that if you make th eimpact occur at a very off-center angle so that the impactor almost missed the proto Earth entirely, that that configuration was very good at placing material into orbit. An off center impact destroys the impacting body, which then passes away from the Earth and barely recoalesces before coming back in for a second hit. " "The entire impact process probably took on the order of a couple of days. But it would have been a very bad weekend to be on the Earth."

Narrator: Moon forms just outside the Roche radius, 14,000 miles out. Coalesces in period from 1 to 100 years.

Canup Newly formed Moon would be "15x as large in the night sky ."

Discusses increase of length of day from 4 to 24 hours -- evidence from tidal rhythmites studied by Marjorie A Chan. The principal tides more frequent, days shorter.

Discusses Moon being same angular size as Sun, comments that it is an "astonishing cosmic coincidence".

From record of eclipse in ancient literature, solar eclipse was dramatically different in path - something like over Bagdad rather than Cairo, from just a 1/20 sec chande in day length.

Apollo program mirrors tell us Moon receding 1 1/2 inches per year.

Discusses stability of Earth's spin axis due to Moon, while Mars' swings from 0 to 90 deg.

Lynn Rothschild discusses evidence that a change in obliquity of 1 degree made a desert out of the Sahara.

Rose, Greenland life at 3.8 billion.

Norman Sleep Theory that Orpheus could have bundled life, but impact would have killed life on it and Earth. Orpheus presumed to have iron core that joined Earth's core, impact produced reducing atmosphere which would possible make Urey Miller hypothesis work to get life going. Narrator says Miller-Urey "lost favor", but that reducing atmosphere might make it work.

Then go to a series of speculative projects

Jack Lissauer (One of authors of Planetary Science) comments on Moon as a symbol of luck, role in planetary exploration

Alan Binder , Shoemaker & Levy, Zupperov, Zolen space spinnaker powered by nuclear explosives, Canup and Hartmann with final comments on likelihood of other planets with moons.