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> Icy Visitor Makes First Appearance to Inner Solar System

OCT. 2013

Icy Visitor Makes First Appearance to Inner Solar System

Comet ISON viewed by Hubble in May

Comet ISON, May 2013:
One of several Hubble observations

For thousands of years, humans have recorded sightings of icy visitors sweeping across Earth's skies. These celestial wanderers are comets, dusty balls of ice that have traveled billions of miles from their frigid home in the outer solar system. They periodically visit the inner solar system during their long, looping journeys around the Sun. These "dirty snowballs," as they are sometimes called, hail from the Oort Cloud, a swarm of billions to trillions of comets that surrounds our solar system.

The most famous comet to appear in our skies is Halley's Comet, which visits the inner solar system every 76 years. Now, another comet is making an appearance, and you just might have a chance to see it this fall.

Comet ISON's grand entrance

Comet ISON viewed by Hubble in April

Comet ISON in April:
Another Hubble view

Comet ISON is making its first voyage into the inner solar system, and has traveled for about 5 million years from its home in the Oort Cloud. Officially named Comet C/2012 S1, it has been nicknamed for the organization of its discoverers. ISON stands for the International Scientific Optical Network, a group of observatories in ten countries who have organized to detect, monitor, and track objects in space.

Astronomers have been tracking the comet with many telescopes, including the Earth-orbiting Hubble Space Telescope, since it was first detected in September 2012. Hubble has made a number of observations of Comet ISON over the past several months, examining its size and the structure of the surrounding cloud of gas, called the "coma." The coma consists of ices evaporated from the surface of the comet, which are then pushed back by the solar wind into a tail.

Calling all comet watchers

Partial diagram of comet anatomy

A comet's anatomy
changes as it approaches the Sun

Beginning in late October, sky watchers might not need a professional telescope to view the comet. Comet ISON may become bright enough to be seen with binoculars or a backyard telescope. Through November, the time to view the comet is in the morning before sunrise.

The first weeks of December should be the best show, if the comet survives its very close approach to the Sun on November 28. Comets are unpredictable. The Sun’s heat could break up Comet ISON, making it dimmer than expected.

If the comet survives its brush with the Sun, it could develop a long tail and brighten to the point where it can be seen by the unaided eye. In December, the comet will appear in both the early morning and early evening in the northern hemisphere, but it will rise with the Sun in the southern hemisphere. After that, it will start fading fast as it travels farther away from Earth.

Comets visible to the human eye are rare. The most recent naked-eye comet was Comet McNaught in 2007, largely visible in the southern hemisphere.

All eyes on ISON

part of comet viewing map

When and where can I see Comet ISON?

Observatories, such as Hubble, will continue to take images of the comet. Hubble will observe Comet ISON again during October. Astronomers are using Hubble to study the comet’s icy nucleus, shrouded deep within the gaseous coma. Based on Hubble images, astronomers have estimated that the nucleus is only three or four miles across. The size is important, as a larger comet is more likely to survive its close passage by the Sun.

Of course, many other telescopes around the world will be watching as well. In fact, during early October, the viewing will literally be “out of this world.” NASA missions at Mars will be looking as Comet ISON sweeps past the red planet. More than a dozen NASA missions, both at Earth and Mars, will join the observing campaign, adding to the data from thousands of ground-based telescopes. What would really be nice is if billions of human eyes can join the viewing as well.

To find out more about Comet ISON, check out the ISONblog at


The Star Witness

brings you "tele-scoops" from the Hubble Space Telescope