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Seeing the news about A/2017 U1 made me wonder what would happen if a similar object, but with a mass close to Earth's, had a close call with Earth. Could it shift Earth's orbit enough to quickly and significantly change the conditions for life?

This answer sort of gets it it, but it's not clear how much of an effect the change in orbit would have.

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    $\begingroup$ What is your definition of quick? A few months? A few years? A century? It's impossible to say how far without knowing how quick. For a ballpark estimate, take your orbital variation (calculate by the method in the other thread), square it and that's your solar insulation variation. Variations as small as 1% can trigger ice ages, but you have to look closer because ice ages depend more on seasonal variation. The specifics are more Earth Science. Also, as noted in the answer below, you should consider tidal effects such as Earthquakes and Tsunamis happening as the object passes. $\endgroup$
    – userLTK
    Commented Oct 30, 2017 at 9:16
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    $\begingroup$ An answer to this would require some serious work. This is no easy question. $\endgroup$
    – zephyr
    Commented Oct 30, 2017 at 14:23
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    $\begingroup$ There is an obvious, crude answer to this question: the habitable zone. If the orbit changes too much (less than 0.95 AU or more than 1.37 AU; see Kasting et al., 1993), the climate would dramatically change and the Earth would become inhabitable, either turning into something like Venus or Mars. It would need more serious calculations to say anything fancier. $\endgroup$
    – MBR
    Commented Nov 30, 2017 at 18:15
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    $\begingroup$ @MBR I guess that just means some part of the planet would be habitable as long as it's within the zone, but it does provide an outer bound. $\endgroup$
    – David Ehrmann
    Commented Nov 30, 2017 at 22:52
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    $\begingroup$ @DavidEhrmann Yes, that is it. But there would be strong ecological changes before that. From the top of my mind, the first example I can think of are the Milankovitch cycles that are responsible from climate change over geological times, and that can cause mass extinction (see for example Mayhew et al. 2008). $\endgroup$
    – MBR
    Commented Dec 1, 2017 at 9:34

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Depending on how one defines "quickly" and "significantly," I think a gravitational influence strong enough to do that would tear apart the Earth through tidal effects long before the change in orbit would make the planet unsuitable for life.

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    $\begingroup$ That's my thinking as well with an Earth mass planet passing through. For ideal orbital change without killer tidal effects you'd want a more massive object passing through, somewhere in the heavy Jupiter/small brown dwarf range. Passing 5-15 million miles away the tidal forces would be manageable but the orbit could change enough to be a problem. $\endgroup$
    – userLTK
    Commented Oct 31, 2017 at 11:19

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