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Earth’s Other Moon

January 24, 2009

The Moon inspired the word “lunacy”, and sometimes it inspires actual lunacy. Several astronomers have gone somewhat around the bend trying to make their mark on history by finding a second moon of the Earth.

There’s nothing intrinsically wrong with the idea. Granted, most any sized moon at any likely distance would be bright enough to see with the naked eye; whatever their other flaws, the Babylonians were tremendous naked-eye astronomers. They would have found it first. But if the hypothetical moon were small enough, or far away enough, it would be a very dim spark in even a good-sized telescope. The problem is that in most cases the announced second moons are obviously ridiculous in other ways. For example, one of the most notable “discoveries”, by Frédéric Petit in 1846, was of a moon that approached the Earth as closely as 11.4 kilometers. Unfortunately for M. Petit, the Earth has something called “air”, which is problematic for objects moving at orbital speeds (about 28,000 kilometers per hour at that height, in case you were wondering).

Believe it or not, that was one of the least silly attempts to claim a second earth’s moon. Readers interested in descending the ladder of plausibility even further are directed toward the names George Waltemath and John P. Bagby (often miscalled “John Bargby”), for a start.

The matter of permanent moons of the Earth was essentially settled by Clyde Tombaugh. Tombaugh discovered Pluto after a gruelling systematic search through entirely-too-many photographic plates, so in 1953 he was tapped to carefully survey the space between the Earth and the Moon as a preliminary to the US space program. After six years he announced he’d found nothing closer to the Earth than the moon, though he allowed that something smaller than a football (American or association, take your pick) would be too small for his telescopes.

The important weasel word here is “permanent”. No need to worry about being scooped by a pre-telescopic court astrologer if the moon in question wasn’t there to be seen by him. This avenue of attack turns up more-promising observations.

A prime candidate for a short-lived moon is the Cyrillid meteor shower, a one-time event that took place on February 9, 1913. On that night a series of bright meteors came out of the sky over Toronto in Canada. Toronto is a city whose university was (and still is) active in professional astronomy, so the shower attracted the attention of Professor Clarence Chant, the former president of the Royal Astronomical Society of Canada. He quickly noticed that the fireballs were unusual in ways besides their brightness.

Most meteors are nothing special, and even meteor showers are not much. There’s any number of debris clouds left behind by comets, and the Earth plows into them all the time when orbiting the sun. However, when this happens the geometry of the situation makes the meteors appear to come from a particular spot in the sky—the “radiant”. Radiants are such a given for meteor showers that they’re named after the constellation in which the radiant is located. The Perseids emanate from Perseus, the Leonids from Leo, and so on.

The Cyrillids had no radiant—for lack of one their name is derived from St. Cyril’s Day, the day they fell (that would be St. Cyril of Alexandria, not any of several other St. Cyrils, and you have to be using the Roman Catholic liturgical calendar as it was in the period from 1882-1969). Instead, the meteors’ trails were more typical of a string of objects running along a single highly inclined orbit as the Earth turned underneath them. Professor Chant collated other sightings (over Bermuda, and on a ship in the North Atlantic), as well as noting other places where they weren’t seen. Working backwards from both sets of data, their orbit was calculated in 1956. Whatever they were, they’d been orbiting the Earth before their end.

The phrase “a string of objects along a single orbit” might have rung a bell for readers versed in astronomy. It was one of the intermediate stages traversed by Comet Shoemaker-Levy 9 as it ceased to be an independent comet and became a rain of orbital projectiles bombarding Jupiter. Mapping what happened to Shoemaker-Levy 9 onto the Cyrillids produces an interesting story. A solar system object, after who knows how many loops around the sun, coincidentally passed within our planet’s Roche limit. Not only did gravity warp its orbit into something smaller and focused on the Earth itself, the gravitational tides ripped it into several pieces that slowly separated from one another. These were, temporarily, a small system of moons for the Earth. Unfortunately for them their shared orbit was so elliptical that its bottom end dipped into the atmosphere, just like the hypothetical moon of Frédéric Petit.

This leaves us back where we started, with a series of bright meteors streaking across the sky and someone wondering “What was that?”

Similar things have happened to the Earth since, though never has any object managed to set up permanent home in orbit around us. Astronomers have been paying close attention to Earth-crossing asteroids the last few years and come up with several that looped around us a few times before some extra gravitational kick, usually from the sun, deranged the set up and they flew off into deep space again. The most remarkable of these is 2004 GU9, a 200-meter wide hunk of rock which moved into a semi-stable orbit sometime around the year 1400 and has yet to leave. It’s awfully distant from our planet (at its closest it’s 25 million kilometers away, contrasted with 384,000 for the “real” Moon), so it’s not actually gravitationally bound to the Earth, but an orbital simulation shows how it still manages to stick close to the Earth at all times. (If you do follow that link, set the center of the simulator to “Earth” and the orbits to “No Orbits” otherwise it’s a bit hard to see). This is the same kind of orbit as the more famous quasi-moon Cruithne, but much closer and more tightly associated with Earth. Each of 2004 GU9‘s orbits is somewhat different from the last as it is tugged this way and that by the other planets and its big brother Moon, so sooner or later it too will leave the vicinity and go back to being just another asteroid.

If you want one more object making a grand attempt to be captured by the Earth, you could always try the Tetons Meteor—the 400,000-ton accidental walk-on of one of the most remarkable home movies ever made. Despite a valiant try at aero-braking through the skies from Utah to Alberta, when it skipped away off the atmosphere its orbit had not quite been changed enough. As far as anyone can tell, it’s still in an elliptical orbit around the Sun.

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