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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1$\begingroup$ I suppose it is a bit of a pun because of Halloween. But with a dead comet I would understand one which has lost its volatiles and no longer forms a coma or tails as it approaches the Sun. $\endgroup$– LocalFluffCommented Nov 1, 2015 at 9:42
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2$\begingroup$ @LocalFluff Quite right, just a Halowe'en pun, it's good for science to engage with the Zeitgeist. Obviously, having a dead comet which looks like a skull on the 31st of October is a great way of promoting astronomy. $\endgroup$– MichaelJRobertsCommented Nov 1, 2015 at 10:43
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2$\begingroup$ Also, comet is from the latin coma "Hair of the head". So it's even more punny. $\endgroup$– Tony EnnisCommented Nov 1, 2015 at 14:11
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$\begingroup$ @TonyEnnis So this was a bald dead comet, a coin with head but no tail. :-) $\endgroup$– LocalFluffCommented Nov 1, 2015 at 15:49
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$\begingroup$ If it has a tail,it might be even more freakish,more like a gastly from pokemon ;) $\endgroup$– r2_d2Commented Nov 1, 2015 at 16:13
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3 Answers
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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.
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2$\begingroup$ It's the first time it's been detected.So why don't we call it asteroid?. $\endgroup$– r2_d2Commented Nov 1, 2015 at 10:15
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1$\begingroup$ Asteroids formed closer to the Sun, and have never really developed a tail or coma - they have never been comets. A dead comet, on the other hand, while similar in morphology now has quite a different history (and formed further out in the solar system). $\endgroup$– user1991Commented Nov 1, 2015 at 13:26
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$\begingroup$ How did we come to know about the history of this comet that once it had a tail or coma? $\endgroup$– r2_d2Commented Nov 2, 2015 at 11:14
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1$\begingroup$ Let me emphasize that in a way this discussion is mere semantics. A comet develops its tail when it is relatively close to the Sun. Therefore, it has to have a rather eccentric orbit. The eccentricity of the orbit can be constrained observationally. In addition (from what I gather from the NASA press release), the albedo (reflectiveness) is reminiscent of values typical of comets. $\endgroup$– user1991Commented Nov 2, 2015 at 12:43
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1$\begingroup$ I am not an expert on comets, but it is not the first time this term is used, no. So it was not invented just for Halloween, if you were wondering. I've done a quick literature search, and the first mention I found was in a paper from 1984. And that is just by a title search, the term was quite probably used before that as well. $\endgroup$– user1991Commented Nov 2, 2015 at 14:57
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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."
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1$\begingroup$ The coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate. $\endgroup$ Commented Nov 1, 2015 at 11:19
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$\begingroup$ Can you give a source for the quote? $\endgroup$– HDE 226868 ♦Commented Nov 1, 2015 at 16:30
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4$\begingroup$ @MichaelJRoberts, first you say, "It is actually an asteroid." Then your blockquote says, "... it is a dead comet." It kind of seems like you're contradicting yourself. $\endgroup$– BillDOeCommented Nov 1, 2015 at 17:15
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$\begingroup$ It's not a contradiction, asteroids were at some point, it is believed, a comet. Hence, they can be seen as dead comets. $\endgroup$ Commented Nov 1, 2015 at 18:53
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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.
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$\begingroup$ So the word Dead Comet is coined even before this hallowe'en asteroid. $\endgroup$– r2_d2Commented Nov 2, 2015 at 4:03