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Okay, let's analyze this for a few moments.
The size of Comet 2013 A1 (Siding Spring) was estimated by Dr. Tony Phillips of NASA Science News

"The nucleus of the comet is probably 1 to 3 km in diameter, and it is coming in fast, around 56 km/s (125,000 mph)."

Don Yeomans of NASA's Near-Earth Object Program at JPL estimates

"It if does hit Mars, it would deliver as much energy as 35 million megatons of TNT."

But Leonid Elenin of SpaceObs.org stated the size of 2013 A1

" With the current estimate of the absolute magnitude of the nucleus M2 = 10.3, which might indicate the diameter from 10 to 50 km, the energy of impact might reach the equivalent of staggering 2×10¹º megatonnes! "

That's compared to Comet Ison's first estimates, very similar.
But yet the government couldn't tell us about Ison's data, and kept most of the data they did release on a 2 day delay?
And they can tell all of this when Comet Siding Spring is 548,226,055 km (or less now) from Earth?

Some say

"Interestingly, the closest approach has tightened a bit to 68,000 miles."

Dr. Tony Phillips stated

"ven a near miss is a potentially big event. The latest orbit solutions put the comet somewhere within 300,000 km of the red planet at closest approach. That means Mars could find itself inside the comet's gassy, dusty atmosphere or "coma." Visually, the comet would reach 0th magnitude, that is, a few times brighter than a 1st magnitude star, as seen from the Red Planet."

Now, my question is:

WHY CAN THEY TELL THIS ABOUT COMET SIDING SPRING AT THAT GREAT DISTANCE,
YET COULDN'T TELL THE SAME DATA ABOUT COMET ISON WHEN IT WAS MUCH, MUCH CLOSER TO US?

I THINK SOMEONE IS LYING TO US, OR NOT TELLING US THE WHOLE TRUTH !

the thermal pulse due to comet passage lasts approximately 5 hours and deposits on the order of 10^13 Joules in the Martian upper atmosphere.

Expressed that way, it seems like a lot of heat energy. Keep in mind, though, that a joule is a watt-second, so a kilowatt hour is 1000 * 3600 = 3.6 * 10^6 joules. So that energy release is about what you would get if 370,000 people ran their 1500 watt hair dryers for the 5 hour duration.

Effect on the surface temperature of Mars? Too small to measure.

WHY CAN THEY TELL THIS ABOUT COMET SIDING SPRING AT THAT GREAT DISTANCE, YET COULDN'T TELL THE SAME DATA ABOUT COMET ISON WHEN IT WAS MUCH, MUCH CLOSER TO US?

I THINK SOMEONE IS LYING TO US, OR NOT TELLING US THE WHOLE TRUTH !

Shush, there there, nothing is going on. You just had a bad dream. Look over there, some shiny things. What's on TV? Isn't NASA wonderful? Just ask them, they have white lab coats, you can trust them. Don't worry your head with all of those big words and complicated diagrams, how boring. Don't you want to be popular? Isn't it fun when stars twinkle? Aren't they pretty? Do you know how to find the Big Dipper? There now, don't you feel better? Sleep... sleep... sleep...