Hello! What diameter would a meteor need to have to pose a threat to all life on earth

Thank you very much!

Apparently, it wasn't just the 1 meteor that killed the Dinosaurs, it was a couple, and then a Massive Super volcanic Eruption to finish them off. It Depends on the Meteors Trajectory and Speed. If it lands in the Ocean the Impact would not be as Cataclysmic then what it would be if it hit Land, how ever It would still Create Tsunamis Bigger than Mount Everest, and As strong as 10,0000 Nuclear bombs.
If your talking about Extinction sized, I would say about the size of our Moon 3,474 km. If you mean Pose a threat as in Almost wipe every species on earth out Half of the Size of our moon roughly 1.7 km. If it were to hit on land, it would Trigger Volcanoes All across the globe, And emit Ash clouds in to the sky so thick that all Vegetation And Life Would Die. The sun would be blocked out.. Blah you have seen deep Impact.. you know how it goes

*(Excuse my Answer editing, Haven't slept in 24 hours lol) Trying to FInd out whether or not Saturn is a Dwarf star or not.

Peace, Light, and Love from Australia x

btw Callum, good luck with your findings!

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<< Earth has collided with several large asteroids in recent geological history.
The Cretaceous-Tertiary asteroid, for example, is theorized to have caused the extinction of the dinosaurs 65 million years ago.
If such an object struck Earth it could have a serious impact on civilization.
It is even possible that humanity would be completely destroyed; for this to occur the asteroid would need to be at least 1 km (0.62 mi) in diameter, but probably between 3 and 10 km (2–6 miles). >>

http://en.wikipedia.org/wiki/Risks_to_ci…

Earth has stronger gravity than Mars.A meteor which would leave such a crater on Mars would leave a larger crater on Earth.