spaceSpace and Physics

What is a comet, and will ISON explode?


Elise Andrew

CEO and Founder

23 What is a comet, and will ISON explode?
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

What is a comet?

Comets are icy dirtballs left over from the formation of the solar system about 4.6 billion years ago. Their icy surface also contains dust, grit and particles from space. Comets usually have elliptical orbits that intersect the orbits of the planets, swinging from close to the Sun to far away, often past Pluto. Some distant comets take more than 30 million years to complete one orbit, while those with tighter orbital paths can take less than 200 years, making their movement more predictable. Halley's Comet is a short-period comet and is visible from Earth every 75–76 years.


Comets are icy and extremely cold when far from the Sun, but begin to warm as they near it. This warming causes volatile materials to vaporize; carrying small dust grain with them to form an atmosphere of gas and dust that appears like a tail when viewed from Earth.

Comets striking the Earth may have played a role in the evolution of Earth billions of years ago. Some scientists suggest comets may have brought some of the water to Earth, and even a variety of organic molecules. The notion that life-building molecules were carried to Earth via comets or asteroids is a hypothesis known as panspermia. 

Asteroids are rocky fragments also left over from the formation of the solar system, but are mainly found orbiting the Sun in a belt between Mars and Jupiter. Asteroids sometimes collide with each other and break up into smaller fragments. These fragments are meteoroids and orbit the Sun. When one of the smaller meteoroids approaches Earth, it burns up in the atmosphere, becoming a meteor. Larger meteoroids that survive Earth’s atmosphere and hit the ground are called meteorites.

Comet ISON


Comet ISON was discovered in September 2012 and was named for the International Scientific Optical Network, or ISON, located near Kislovodsk, Russia, which made the first observation of the comet. When it was first observed, it was considered unusually bright. Astronomers thought it might become visible from Earth even in daylight, however further observations showed that the comet has not brightened as much as predicted as it has moved closer to the Sun.

The comet is likely to be closest to the Sun on November 28 this year when it comes within about 1 million km of the Sun’s surface. ISON will travel 376km per second when it curves around the Sun and will be heated to about 2700 degrees Celsius; rock and metals will be vaporized.

The Sun’s gravity may well pull the comet apart, though recent analysis shows Comet ISON is likely to survive perihelion, which is the point in the orbit of a planetary body when it is closest to the Sun. This is largely due to the size of the comet, as it will lose a lot of material to solar heating. ISON should pass around the Sun with both its nucleus and tail intact. The comet will be visible without binoculars or telescopes on Earth in December in the early morning skies, and throughout the night in January 2014.

ISON’s flight path suggests it is traveling into the inner solar system from the Oort Cloud, which is a reservoir of icy bodies beyond Neptune’s orbit. The comet is likely to be bumped out of the solar system after its dance with the Sun.


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