The “comet of the century” died before its time.
Most scientists believe that on comet ISON fragmented on Nov. 28, breaking up into small enough chunks that over the next few days many of these chunks evaporated, while the others became a headless dust-and-rubble stream.
This dust cloud is following the outward path of ISON, but the remaining rubble is most likely too small and dim to be seen with anything less than a very large telescope.
The morning skies of December will not have a visual display of comet ISON, and it will be mid-December before we can view it with telescopes. The Hubble telescope will take a look at that time.
So what caused the comet to break up?
It was most likely a combination of three factors: the fragile makeup of comets, the intense heat of the sun, and the powerful gravitational tidal forces of the sun. Let’s take a closer look at each of these factors.
Comets are bodies of ice, rock and organic compounds that can be up to several miles in diameter.
Most of the ice is water ice, but comets can also contain ices from other materials. These ices act as glue to hold the rocky materials together.
The proportions of rock and ice are different for each comet. The strength of the bonding is also different for each comet.
Some have a lot of ice that forms a dense, strong body, while others have little ice and are only weakly held together.
The larger the comet and the stronger the bonding, the greater the chance it will survive its encounter with the sun.
The second factor is the intense heat of the sun.
All comets melt as they go around the sun. As ISON was traveling through the inner solar system, it was losing about three tons of ice per second — enough to fill an Olympic pool in about 10 minutes.
But ISON had a mass of 2 billion to 3 billion tons, and at this rate it would take 25 years for the comet to disappear.
At closest approach, just 730,000 miles from the surface of the sun, the comet would have been traveling at 225 miles per second. That is hundreds of times faster than a rifle bullet and more than 1,500 times faster than a commercial jet.
At that speed the comet would cross the continental U.S. in about 15 seconds. This means that ISON spent very little time this close to the sun.
But at closest approach the comet’s equilibrium temperature would still have reached upwards of 5,000 degrees Fahrenheit — hot enough to cause much of the rock and dust on the comet’s surface to vaporize.
Apparently at this point much of the ice that held the comet together was lost. It appears that the breakup occurred just minutes before this closest approach, and the intense heat was one of the major factors.
The third and most important factor is the sun’s strong gravitational tidal forces. The gravitational force between two objects increases exponentially as the objects get closer together.
Here on Earth we see the tidal forces of the sun and moon in the ocean tides. The side of the Earth closest to the sun and moon feels a stronger pull than the side opposite, and the Earth gets stretched along that line, creating high tides.
A comet approaching the sun would feel these same forces and be stretched in the same way. As the comet gets closer to the sun, these forces get stronger and the stretch is greater. At closest approach, the sun’s gravitational force is very strong and the tidal forces can stretch the fragile comet so much that it breaks apart.
At the same time the comet is feeling these strong tidal forces, it is also being weakened by the intense heat — and this is why many comets fragment or disintegrate as they pass the sun. It appears that this was the fate of ISON.
Even though we will not get a great show of ISON in December, astronomers collected a large amount of data about the comet and made many observations.
In the coming months this data will provide us a better understanding of the early solar system and its formation.
There are trillions more comets out there. Maybe the next one will be the “comet of the century.”
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.