If an Asteroid was to strike the Earth, would it affect noticeably the Earth's rotation, and if so, how large would this Asteroid have to be?

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    $\begingroup$ It'd be the least of our worries. $\endgroup$ – gerrit Sep 16 '15 at 14:10
  • $\begingroup$ haha XD @gerrit still an interesting thought though because if it's possible, then it is also possible to have a different rotation than that existed during the dinosaur era- before an asteroid wiped them out. $\endgroup$ – JonathanScialpi Sep 16 '15 at 14:19
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    $\begingroup$ There certainly is a different rotation now that 65 million years ago, as the Earth rotation is slowing down. I don't know the answer to your question, though. $\endgroup$ – gerrit Sep 16 '15 at 14:34
  • $\begingroup$ Right, well I meant in terms of an outer space projectile directly affecting its rotation. +1 for the cool fact though $\endgroup$ – JonathanScialpi Sep 16 '15 at 14:38
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    $\begingroup$ Everything has some effect, but it's pretty tiny. Even the Chicxulub impact, about 5 miles across, the largest in over 100 million years had a tiny amount of energy compared to the Earth's rotation. I can try to run the math later, but you'd need a really big impact to have a significant effect. A 100 mile in diameter space rock would obliterate the Earth's surface and boil the oceans but it's mass would be just a bit over a millionth that of the Earth. Even at a perfect impact angle at a few tens of thousand MPH, it would only add or remove a tiny bit of rotational velocity. $\endgroup$ – userLTK Sep 16 '15 at 18:15

To have a noticeable effect the impactor needs to be BIG.

Most questions about "what would happen if ... hits" can be answered by the "Earth impact effects program" (http://impact.ese.ic.ac.uk/)

Here are calculations for a 100km stony asteroid...

A brute like this would have a good chance of wiping out most complex life on the planet. There has been nothing like this in the last 4 billion years (or so)... It could cause the length of the day to change by "up to 2.42 seconds"

As gerrit said, it would be the last of our worries.

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