Just how dangerous is this scenario, anyway?

Question

Thanks to movies like "Armageddon" and "Deep Impact", we all are aware of asteriods and comets and how they can crash into the Earth, causing death, destruction and doom for life here. (Actually, I knew this stuff even before such movies.) But the planet has had numerous near-misses over the years, and we think nothing of it--which is okay, of course. But the other day I was asked by my nephew about the moon's effects on the Earth's tides, and it just occurred to me that even a near miss by an object that is massive enough could cause serious problems for our world through its gravitational forces alone. My question is, how massive would such an object have to be before being a threat? For example, say a white dwarf star (or some other such star) goes supernova, and in the process throws out a fragment of white dwarf matter. How massive would that fragment have to be to cause gravitational chaos for the planet? I know it wouldn't have to be big; how massive would it need to be?

Answer 1

extremely big. the moon does impact life on the earth, and without the moon life might not be possible because the moon keeps our axis in place. without the moon the earth would be wobbling all over the place and all life would be destroyed. imagine going from -50 to 120 degrees.

if a comet or asteroid was big enough to impact life on earth without hitting earth, it would have to be about as big as the moon to make any severe changes and it would only be temporary. the comet or asteroid would be going so fast that it would escape our gravitational pull easily, we might alter its course and might hit the sun. i dont know about the white dwarf situation though.

Answer 2

The destructive power of a large asteroid impact is truly horrifying. If we assume a silicaceous asteroid with an average density of (for example) 2700 kg/m³, a 30 km wide asteroid moving at typical Earth-crossing velocity of 25 km/s would hit our world with the kinetic energy equivalent of nearly three billion megatons of TNT. This is a number which is so staggeringly large as to boggle the imagination; just try to imagine detonating a Hiroshima bomb every second for more than six thousand years, and then compressing all of that into a single titanic blast.

A one billion megaton event would be sufficient to cause global annihilation of complex life-forms like ourselves. This calculates down to a six-mile wide asteroid.

There are about 500 identified and classified Near Earth Objects that periodically cross our orbital path, sometimes when Earth is actually in the vicinity and often times not. There are another 500 NEO's that are believed to exist in our solar system that have yet to be categorized. Most of these should be within the next ten years. If they come out of the darkness, our scopes should be able to pick them up decades before anything might need to be done. If they come out of the glare of the Sun, that's a problem. It is the one kind of "sucker punch" we are not presently capable of dealing with.

Answer 3

Something large enough to just cause gravitational damage as it goes by would have to be pretty big. Probably big enough where we could see it years before it got here. Much bigger than asteroids or comets. If it passed close enough, a moon sized object could cause some serious tidal waves. However, like I said, we would see something like that coming. It would be very visible. Also, the odds of something that big coming at us are pretty small. I would not worry about it too much.

Answer 4

I seriously doubt that a piece of a supernova would be able to travel the distance to earth intact. There are no white dwarf stars close enough to our system to ever pose any kind of gravitational threat.

Answer 5

Relax. The frequency of impact drops off dramatically with size. So dust hits us by the million every day, while a 10km asteroid buzzes us only every 10s of millions of years.

It is unlikely that anything large enough to effect our gravity has come by in the last few billion years. So, why should it in your lifetime?

In reality, space is very very empty.