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the_fungible_man t1_jbw6or7 wrote

>wasn't the Moon formed by a huge asteroid slamming into Earth

The moon was formed by a planet slamming into the Earth. There are no longer any rogue planets waiting to smack into the Earth.

Whether a smaller interstellar object would be detected before impact depends on its size and direction of approach. Such objects travel faster relative to the Earth than local asteroids, shortening to lead time between detection and impact. In the unlikely event the object approached from the sunward direction we might never see it coming.

BTW, the Earth has been struck by meteoroids of interstellar origin as recently as 2014. Our atmosphere affords us a great deal of protection from most of what's out there, interstellar or not.

>How could we stop it though?

For the foreseeable future, we couldn't.

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stanerd OP t1_jbw7k4j wrote

What happened to the planet that slammed into Earth? I assume the Moon formed from the debris from that collision.

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the_fungible_man t1_jbw8pt7 wrote

Most of the impactor (estimated to have been approximately the size of Mars) was incorporated into the Earth, while debris from the collision, partly from the Earth and partly from the impactor, coalesced to form the Moon.

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