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If for instance in 2117 when we have the next Venus transit (Venus between the Sun and Earth). There was a huge solar flare or storm right on the line from Venus to Earth. Could this give enough energy to strip some of Venus atmosphere away while partially protecting the Earth.

So on this NASA picture of the 2012 transit, the flare would be directly under the black dot representing Venus if this picture represented the view from Earth.

enter image description here

Here is a NASA picture of a massive Xclass flare as you can see it doesn't take up a lot of the suns surface area although it is huge

enter image description here

Assume we predict this storm and take what measures we can on Earth with near future tech, unlimited budget, fast cheap space travel and whatever resources we need. Basically I'm trying to work out if this would be survivable for Earth while at the same time blasting away a lot of Venus atmosphere.

Alernatively the whole flare/Venus stuff happens while Venus is on the far side of the sun from Earth.

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    $\begingroup$ Venus subtends a maximum of 66.0 arcseconds (just over one arcminute), compared to the Sun's around 32 arcminutes, both figures as seen from Earth as per their respective Wikipedia articles. The protection offered Earth by the Venus transit is thus going to be negligible. Also, if a solar flare or solar storm was enough to strip a planet's atmosphere to any significant degree, you'd expect it to have happened already. $\endgroup$ – a CVn May 8 '17 at 9:11
  • $\begingroup$ @MichaelKjörling true, clutching at straws a bit with this one, but just because something hasn't happened yet doesn't mean it can't... I'm a bit amused at the instant downvotes, I think I have some fans :-) $\endgroup$ – Kilisi May 8 '17 at 9:16
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    $\begingroup$ "just because something hasn't happened yet doesn't mean it can't" True, but on the other hand, if something hasn't happened in a few hundred million solar cycles, I kind of doubt myself that it will happen the one time it would be convenient. Also remember that the solar cycle peaks in 11-year intervals; cycle 24 peaked in 2011 and 2014 (double peak), and cycle 23 peaked in 2000; based on the cycle 23 peak, 2117 would be during a period of solar activity minima, quite the opposite to what your scenario would need. Not impossible, but unlikely. $\endgroup$ – a CVn May 8 '17 at 9:23
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    $\begingroup$ Neither Venus nor Earth's atmosphere can be stripped away by a solar flare. Added to the nonsense of a flare being ejected such that it hits both Earth and Venus, it would additionally have be be magnitudes stronger than any flare ever observed. $\endgroup$ – Karl May 8 '17 at 10:39
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    $\begingroup$ @MichaelKjörling You should write that first comment up as an answer since that is the correct one. $\endgroup$ – kingledion May 8 '17 at 11:05
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All right, let's look at this from a geometric point of view.

As seen from Earth, the Sun subtends anywhere between 31.6 and 32.7 minutes of arc in diameter. The area enclosed by a circle (ellipse with the semi-major axis defined as equal to the semi-minor axis) is given as $A = \pi r^2$ where $r = \frac{d}{2}$. So the Sun is $\pi ( \frac{32.15 \pm 0.55}{2} )^2$ square arcminutes in size. Making the Sun its smallest size, that's $\pi 31.6^2 = \pi (998.56) \approx 3\,140$ square arcminutes.

Also as seen from Earth, Venus subtends a maximum of 66.0 seconds of arc, or a shade over one arcminute. The area covered by it is thus at most $\pi 66.0^2 = \pi(4,356) \approx 13\,685$ square arcseconds or $\frac{13\,685}{60^2} \approx 3.801$ square arcminutes.

That's a difference of about three orders of magnitude.

Venus will cover at most $\frac{3.801}{3\,140} \approx 0.001212$, or about 0.12%, of the Sun's visible surface, as seen from Earth.

  • We can reasonably infer that a large solar flare or solar storm will, from the perspective of Earth, pass by Venus, simply because of Venus' tiny size compared to the Sun. For this to work at all, Venus would have to be perfectly centered over the solar flare, and the solar flare must not be larger than Venus. It wasn't that long ago that a solar flare seven times the size of Earth was recorded.

  • Also, the Sun is a violent beast. It stands to reason that if even a large solar flare or solar storm was sufficient to deprive a planet of significant portions of its atmosphere, that would have happened already. Since Venus has the atmosphere it does, we can reasonably infer that it hasn't happened; given the number of solar cycles the Sun has gone through since the planets formed (which, with an approximately 11-year cycle, is in the hundreds of millions), it seems unlikely that something like that would suddenly happen precisely on schedule.

  • Also, while not a showstopper, keep in mind that 11-year cycle. Cycle 23, which was the most recent one to have a single distinct peak, peaked in 2000; cycle 24 peaked in 2011 and 2014. Based solely on the year 2000 peak, year 2117 will be near a solar activity minima.

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  • $\begingroup$ So no reason that it is impossible then? Can you calculate what size flare would be needed even roughly? Unlikely is fine, our current solar system or even the Earth Moon dynamic is unlikely on top of unlikely on top of unlikely etc, We've known sunspots since the 1850's I think, there might be million and 1 year cycles we don't know about. $\endgroup$ – Kilisi May 8 '17 at 11:41
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    $\begingroup$ @Kilisi The Solar System is a solid thousand times as old as such a million year cycle. As Michael said, if this could happen, it already would have. Plus, such long-term phenomena would likely be byproducts of the Sun's life cycle, and to my knowledge, no such mechanism exists in Sun-like stars. Also, Michael, you might want to mention that Earth's magnetic field gives an additional layer of protection. $\endgroup$ – HDE 226868 May 8 '17 at 17:14

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