Can Comets Destroy Earth?


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Is Earth in danger from comets? Image courtesy of NASA

Comets are among nature’s most spectacular celestial shows – these events should not cause fear, but historically they often have. Comets have been seen as evil omens, yet no scientific evidence for ill effects of passing comets exists.

As of this writing, the ignorance industry has raised considerable fuss and blatantly wrong speculation about Comet Elenin causing severe damage to Earth during the fall of 2012. In 1910 Halley’s comet caused similar rampant fear, yet no damage whatsoever. Elenin, Halley, and other comets pose no significant threat to Earth or its inhabitants. To understand why comets do not threaten Earth, it is necessary to first understand what a comet is.

What Are Comets?

The 1910 apparition of Halley’s Comet caused considerable hysteria but no ill effects. Image Credit: Yerkes Observatory

The nucleus of a comet is a chunk of rock and ice, basically a dirty snowball, about 10 kilometers in size orbiting the Sun in highly elliptical orbits. Astronomers think that comets originate in the Oort comet cloud, which is a cloud of icy debris beyond Pluto’s orbit left over from the solar system’s formation. When something disrupts the orbit of a comet, it falls in towards the Sun in a highly elongated orbit.

When the comet is far from the Sun, it is only a dirty snowball nucleus. The icy material consists of both water ice and many other frozen materials that would be gas or liquid on Earth. As comets approach the Sun, radiant solar energy evaporates some of the icy material. The evaporated icy materials form a gaseous coma surrounding the nucleus.

The Bayeux Tapestry depicts Halley’s Comet and people’s fear of the comet in the 11th century.

The solar wind then blows the evaporated ice to form the comet’s spectacularly long tail. Many comets have two tails, a curved one consisting of dust and a straight one consisting of ionized gas. The largest, most spectacular, comets can have tails extending more than 100 million kilometers. The comet’s tail provides the often spectacular show for earthbound stargazers. Despite its size, a comet’s tail is so tenuous that it would qualify as a good laboratory vacuum on Earth. Hence, if Earth passed through a comet’s tail, it would have no significant effect.

As a comet moves further from the Sun in its orbit, the tail disappears, leaving just the nucleus until the next orbit. The passing comet, however, leaves a stream of dusty debris in its orbit. Meteor showers occur when Earth passes through a comet’s orbit and the dusty comet debris collides with Earth’s upper atmosphere to produce another spectacular show for stargazers.

Can Comets Cause Earthquakes?

There are claims that Comet Elenin caused the major earthquakes of 2010 and 2011. When there was an Earth-Sun-Elenin alignment, Comet Elenin supposedly caused these earthquakes by the increased tidal forces shifting Earth’s tectonic plates.

Neither Elenin nor any other comet generates sufficient tidal forces to cause earthquakes!

The nucleus of Comet Hyakutake from the Hubble Space Telescope Image Credit: NASA HST

Astronomical objects cause tidal forces on Earth. Tidal forces are the difference in gravitational force between the closest and furthest points on Earth to the astronomical object. The amount of tidal force a celestial object causes is directly proportional to the mass of the object and inversely proportional to the cube of the distance to the object. Earth receives the greatest tidal effects from the Moon. Tidal effects from the Sun are about one third as strong as those from the Moon. Tidal effects on Earth from any other astronomical object are much weaker.

By my calculations, a comet having approximately the density of water (appropriate for a mixture of rock and ice) and a radius of about 10 km would have between one millionth (10^-6) and one ten millionth (10^-7) the mass of

Comet Hyakutake in March 1996 had an exceptionally long tail when observed under very dark skies. Image Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria Creative Commons

the Moon. Therefore even if a comet came as close to Earth as the Moon, its tidal effect on Earth would be less than one millionth of the normal tidal effect from the Moon. Comet Elenin will not come closer to Earth than about 35 million kilometers (22 million miles), which is almost 100 times as far away as the Moon. At this distance the tidal effects would be another million times weaker. (100 cubed equals one million.)

Tidal effects from comets are simply far too weak to trigger earthquakes or any other seismic activity on Earth.

Is Comet Elenin a Brown Dwarf Star?

Some have claimed that Comet Elenin is really a brown dwarf star. Such claims are nonsense. Brown dwarf stars are sufficiently different from comets that it would be virtually impossible for astronomers to confuse a comet and a brown dwarf. That would be similar to biologists observing a horse and thinking that it was a horse fly.

A brown dwarf is a stage between the most massive planets and the least massive stars. Brown dwarf stars are more massive than Jupiter, but not quite massive enough to ignite the thermonuclear reactions that power stars. Hence, brown dwarfs are at the right temperature so that they emit most of their light in the infrared. Comets, on the other hand, glow from reflected sunlight and are therefore brightest in visible light. In addition, brown dwarfs are large enough in size that pictures of a brown dwarf in our solar system would show a disk larger than Jupiter’s disk. Unlike a comet, a brown dwarf would have no tail. Also, unlike a comet, a brown dwarf star passing through the solar system would gravitationally disrupt planetary orbits. It would be virtually impossible for astronomers to confuse the two types of objects. Neither Comet Elenin nor any other comet is even remotely similar to a brown dwarf.

Can a Comet Cause Three Days of Darkness?

Comet Hale-Bopp in April 1997 had an exceptionally bright nucleus and coma. Image Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria

Some have claimed that Comet Elenin will cause three days of darkness on Earth. If Elenin, or any other comet, were to pass in front of the Sun, it would, in principle, dim the Sun’s light. In practice, this dimming would be virtually impossible to measure. Remember that a comet’s nucleus is only about 10 kilometers across. The Sun has a diameter of about 1.4 million kilometers. A 10 kilometer comet is too small to appreciably block the light from the 1.4 million kilometer Sun.

Comet Elenin, and comets in general, can not cause the doomsday scenarios that many are suggesting. If a comet were to directly collide with Earth, especially a large populated city, the collision would be among the most extreme natural disasters. Such collisions are fortunately very rare. Otherwise, comets can have no ill effects on Earth.

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