How is a Dead Comet different from the normal comet?
How are they formed?
And why is the Halloween asteroid 2015 TB145 called a dead comet?.
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A comet is usually characterized by its tail. A dead comet has lost all its ices and gases (responsible for producing this tail), leaving just a rocky core.
The Halloween comet is such a dead comet, in that it has no tail, but furthermore it resembles a skull, making it particularly relevant for Halloween.
It is actually an asteroid. I think it just helps to promote astronomy near Hallowe'en by calling it a dead comet and releasing some pretty nice photos in which it can be likened to a skull. But yes, a dead comet has been shed of all of it's ice and gas, and is left as the rocky core.
This is the quote relating to the comet you are refering to and perhaps a theory on its origins:
"We found that the object reflects about six percent of the light it receives from the sun," said Vishnu Reddy, a research scientist at the Planetary Science Institute, Tucson, Arizona. "That is similar to fresh asphalt, and while here on Earth we think that is pretty dark, it is brighter than a typical comet which reflects only 3 to 5 percent of the light. That suggests it could be cometary in origin - but as there is no coma evident, the conclusion is it is a dead comet."
Comets are bodies that formed in the outer solar system, and are composed largely of ices (water, CO2 and others). The Rosetta mission is discovering lots of new science about the compostion of comets right now.
The asteroids are more varied. Some are rocky, some metallic and some have a lot of ice. The asteroids are have various origins, but most orbit between Mars and Jupiter and in the plane of the solar system.
In contrast, comets tend to have highly elliptical orbits and often their orbits are highly inclined, relative to the rest of the solar system. This is because they are falling towards the sun having been disturbed from their birthplace in the outer solar system.
The chief characteristic of a comet is that, as it comes near the sun, the ices sublimate off the comet and form an a coma: a giant sphere of gas and dust that surrounds the comet's icy nucleus. The solar wind pushes this into a tail that point away from the sun.
After many orbits of the sun a comet will eventually run out of ice, at least on its surface, and will no longer form a coma. How long this takes depends on the orbit of the comet, but about 1/2 million years seems to be an estimate of the average life span of a comet. As it is no longer active, it is a "dead comet". Although having a "dead comet" on Halloween is a bit of spin, the term is real and has been used prior to 2015-TB145, for example in this page about near earth objects
In fact, as noted there, many near earth "asteroids" may actually be dead comets. Evidence that 2015-TB145 is one is that it has a highly inclined elliptical orbit, and it is very dark. Other comets, such as Halley, are blacker than soot.