Surviving Thanksgiving: A comet’s moment of truth |

Surviving Thanksgiving: A comet’s moment of truth

The spacecraft STEREO-A captured this image of Comet ISON - and Earth - as the comet speeds toward the sun (on the right). The comet's tail is blown off by sunlight and magnetism.

Comet ISON (first spotted in telescopes of the International Scientific Optical Network) is rounding the sun.

Early next month, it may dominate the sky … or it may no longer exist. It all depends on whether the comet survives Thanksgiving.

Tables and chairs, rocks and people — all are made of atoms — atoms held together by electrical attraction.

Electricity is the force that makes laundry stick together when it’s fresh out of the drier. Just as you can pull your sox away from your undies, you can pull atoms away from each other.

Pull hard enough, and you can disassemble a table or a rock, atom by atom. More easily, you tear atoms apart when you rip a piece of paper in half.

Some five dozen moons circle the planet Saturn. Saturn once had that many plus one, but a decaying orbit brought one moon slowly inward, ever closer to the planet.

A moon’s orbit is a compromise: Inertia — what sends groceries flying into the dashboard when you slam on the brakes — inertia “wants” to send the moon flying off into space.

Gravity, on the other hand, “wants” to pull the moon downward, right into the planet. Compromising between the two, the moon circles the planet.

The closer a moon orbits to a planet, the faster it must travel, lest gravity pull it in.

Sometime in the past, one of Saturn’s moons flew too close to the giant planet. The side closer to Saturn “wanted” to go faster; the side farther away “wanted” to go slower.

When the moon got really close to Saturn — inside the Roche (“roash”) limit — the difference in desired speeds overwhelmed the electrical forces holding the moon together, and the moon was ripped apart.

The debris became Saturn’s rings.

At roughly 11 a.m. today, Comet ISON will fly less than a million miles over the visible surface of the sun. The side closer to the sun will “want” to go faster; the side farther from the sun will “want” to go slower.

The comet as a whole will fly at an average of the two velocities.

Comets are made, not of rock, but of ices: water ice, carbon dioxide (“dry”) ice, ammonia ice (frozen ammonia gas, the gas that’s dissolved in water to make “household ammonia”) and carbon monoxide ice.

Depending on the strength of the electrical forces holding the ices together, the comet may or may not find itself within the Roche limit, may or may not survive the gravitational tug-of-war between one side and the other.

Drawing some 120 times closer to the sun than Earth, Comet ISON will see sunlight 120-squared — more than 14,000 — times brighter than we.

The surface temperature of the crust coating the comet’s ices will climb thousands of degrees Fahrenheit ­— another factor tearing the comet apart.

Dispersed by gravity and sunlight, loose cometary material can be an effective mirror for sunlight. It can also glow — fluoresce — brightly. ISON’s sun graze may thus make the comet a magnificent spectacle in the sky.

Or the encounter may destroy the comet utterly. We’ll know after Thanksgiving.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He teaches classes to students of all ages and may be reached at

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