Flying at 235km will allow little margin for error, especially if the Sun becomes more active.

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When the Sun becomes more active, this isotope becomes more abundant along with beryllium, another radioactive element.

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As mentioned, carbon-14 is radioactive, and as the Sun becomes more active (interglacial times) so does the amount of the radioactive carbon-14.

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"Once someone has a wireless device, she becomes much more active in how she uses the internet -- not just with wireless connectivity, but also with wired devices, " said Pew.

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This means they become much more engaged with the ad, so passive advertising becomes active engagement.

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