On Mar 6, 1:27 pm, Robert Clark <rgregorycl...@yahoo.com> wrote: > ... > This comet to make a close encounter to Mars is *huge*. To put it > perspective it dwarfs the asteroid that destroyed the dinosaurs. Such > close encounters to any of the terrestrial planets must be very rare. > For instance the puny, in comparison, asteroid 2012 DA14 would be > expected to get so close to the Earth once in 40 years. That such a > large comet would get so close to Mars must be much rarer than this. > So the chance is less than 1 in 40 in a year. Say it happens for > either of two planets; that's a chance of less than 1 in 20 in a year. > Say then it happens within a 2 year period; that's 1 chance in 10. > Now the chance of the three encounters occurring within such a close > time span is greater than one in several billion. The unlikelihoods > begin piling up greater and greater. >
Another recent comet came unusually close to the Earth in 2010:
Comet and Earth to Have Rare Close Encounter. by Joe Rao, SPACE.com Skywatching Columnist Date: 01 October 2010 Time: 08:41 AM ET [quote]Rare close encounter This fall, Comet Hartley 2 will again be passing through the inner solar system, reaching its closest point to the sun (called perihelion) on Oct. 28 at a distance of 98.4 million miles (158.4 million km). And while en route to the sun, it will also make a very close approach to the Earth. In fact, at 3 p.m. ET on Oct. 20, the comet will be at its closest point to our planet at a distance of 11.2 million miles (18 million km). It's quite unusual for any comet to approach this close to Earth. Such an event only happens on average perhaps three or four times a century. [url]http://www.space.com/9240-comet-earth-rare-close-encounter.html[/ url][/quote]
So in a year the chance of a comet getting this close is between 1 in 33 to 1 in 25. Then over a 3 year period about 1 chance in 10. Now the chance of all these rare events occurring within the same short time frame is in the range of tens of billions to one.
Additionally we have the unusual comet coming up in November this year, Comet ISON, expected to be one of the brightest in history:
The 9 Most Brilliant Comets Ever Seen. by Joe Rao, SPACE.com Skywatching Columnist Date: 05 October 2012 Time: 12:24 PM ET [quote]Comets that are visible to the naked eye during the daytime are rare, but such cases are not unique. In the last 332 years, it has happened only nine other times. Here is a listing of past comets that have achieved this amazing feat. [url]http://www.space.com/17918-9-most-brilliant-great-comets.html[/ url][/quote]
Now based on this rarity, the combination of unlikely events happening on such a short time scale raises to an unlikelihood of hundreds of billions to one.
This comet also will get unusually close to Mars at 0.07 AU, 10 million km:
Comet C/2012 S1 (ISON) is on its way to skirt around the Sun in November 2013. Will Comet ISON achieve Great Comet status? I summarize what we know about this promising comet and what it might become. [quote]Earth and Mars Close Approaches. Comet ISON makes two interesting close approaches. The first, in October 2013, we will watch the comet pass by Mars at the small distance of only 0.07 AU. This distance is small on the grand scale of the Solar System, but it is still 10 million kilometers. The close encounter may make comet ISON observable to NASA and ESA's spacecraft orbiting Mars. Maybe we can even hope to see a picture of ISON from NASA's Curiosity rover? Although there is no possibility of such a close approach between the Earth and comet, the second close approach I want to bring up will be between the comet's orbit and the Earth. In January 2014, the Earth will swing past a part of space that Comet ISON already traveled through, at a small distance of only 0.03 AU. This encounter brings up the possibility of a meteor shower on Earth. Meteor showers from Oort Cloud comets are rare events indeed. Stay tuned while astronomers consider this possibility. [url]http://www.astro.umd.edu/~msk/blog/articles/comet-ison-jan13[/ url] [/quote]
Assuming comets get so close as a small fraction of an AU to Mars as rarely as they do to Earth, then on that basis we would also conclude, aside from the brightness issue, that this comet adds another order of magnitude to the unlikelihood of all the events occurring on such a short time frame. (This consideration does not tack on another factor of ten since this comet is already counted. It just offers another way to draw the same conclusion.)
Note also the unlikelihood is almost certainly actually trillions to one because the case of a comet the size of Comet C/2013 A1 (Siding Spring) getting within just thousands of kilometers of a terrestrial planet has to be [I]extremely[/I] rare, much worse than the 40 to 1 I estimated previously.
This comet remember dwarfs the size of the asteroid that wiped out the dinosaurs and would cause an even worse global extinction level event if one of such size were to impact Earth.
Now note that trillions to one odds are such that we would not expect it to happen during the entire age of the Solar System.