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Space-junk moonlets, Origin of the Moon, Fall leaves in a drought

Q: About all the space junk (old satellites and so on) orbiting the Earth: Will this material ever coalesce into a moon? How long might it take for this to happen?  Eric, Albuquerque, New Mexico

Astronauts Winston Scott, left, and Takao Doi (both on the shuttle Columbia's cargo bay) get set to grab an errant satellite adrift in space, 1997.  Photo courtesy of NASA.

A: Your question — can space junk now orbiting the Earth ever coalesce into a moon — is an interesting idea. But, no, it can't because the space-junk mass is too small to generate enough gravity to draw the mass together. Also, there is such a scattering of junk in orbit that few pieces manage to snuggle up next to each other in close orbits. Moreover, they've got to get that close so gravity can pull the fragments into a larger mass. It just won't work.

To get a rough idea of the amount of junk in orbit around Earth, I checked Space Command's space surveillance site. The fact sheet lists 8,300 man-made space objects (baseball size and larger) in orbit. About 600 of these objects are operational satellites, the rest are inactive satellites, payloads, platforms, rocket bodies, and plain debris: such as, nose-cone shrouds, lenses, or hatch covers.

About 90 percent of the space junk is too close to the Earth (closest point of its orbit within 16,000 km or 10,000 mi) to withstand the effects of tidal gravitational forces. These unfortunate pieces can never collect into a cohesive body because the Earth's tidal forces constantly work at pulling any such body apart. That leaves only 830 hunks of space junk for moonlet candidates. Eight hundred or so pieces are too small in both mass and numbers — by orders of magnitudes — to pull together into another moon.

Further Surfing:

Orbital space debris, SpaceRef.com

Q: I read your answer "Orbiting orbs and how they started," and it got me thinking about our solar system and how the moon formed. [Current theory says a Mars-sized body hit Earth about four billion years ago — AH.] After the collision threw off a plume of rubble, how did this rubble coalesce into the Moon? Eric, Albuquerque, New Mexico

The rings of Saturn.  Photo courtesy of NASA.

A: The rubble pulled together and formed the Moon in much the same way as the Sun and the planets coalesced from an ancient cloud of gas and dust. The bits of plume eventually took up orbits ringing the Earth, similar to Saturn's rings. It happened like this: Each individual chunk of material, if it were alone, would orbit the Earth much as our present Moon does — a nearly circular ellipse. But the chunks were not alone and so collided with one another until all were moving uniformly — in nearly circular orbits in the same plane.

The size of the rubble varied from dust grains to boulders. The important thing, though, is that all this mass — a moon's worth — traveled about Earth in nearby orbits. Two forces acted upon these co-moving bits to make them coalesce:

• electrostatic (what makes clothes cling together as we pull them out of the dryer) and
• gravitational.

Electrostatic forces acted on the surface areas of nearby dust grains. The surfaces became negatively or positively charged by friction, contact, or induction. Unlike charges attracted each other. The greater the charge, the greater the attraction. The grains grew, resulting finally in pebble-sized dirt clods.

Gravitational forces brought larger pieces together to form comet-sized bodies in a surprisingly short time, perhaps a thousand years. If the bodies collided too fast, they shattered and the process restarted. Those bodies that grew in mass, however, pulled in rubble like gigantic vacuum cleaners.

Within a hundred thousand years or so, the dirt balls enlarged until many were hundreds of kilometers across and one formed a small Moon, . The proto Moon was then large enough to absorb almost all impacting bodies. So it grew until it swept up the last of the rubble — a colossal bombardment — and our Moon was complete.

Further Surfing:

Planetary Science Institute: Origin of the Moon

Q: Much of the country is dealing with a pretty bad drought situation. Will that affect how bright the trees become this fall?  Hilary, Pittsburgh, Pennsylvania

Fallen autumn leaf.  Photo courtesy of Wikipedia.

A: Yes — the leaves probably won't be as bright as usual. "Drought can cause premature coloration of the leaves, but a dry autumn diminishes the brilliance of the foliage, causing leaves to die and fall prematurely," writes biologist Scott Camazine in his Naturalist's Year. Leaf displays tend to drab browns, which suggests that the red pigments (anthocyanins) don't develop well during a dry spell.

Further Surfing:

The Naturalist's Year by Scott Camazine

(Answered Oct. 4, 2002, updated Sep. 20, 2007)