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Message: <blockquote data-quote="Nagol" data-source="post: 7486132" data-attributes="member: 23935">The problem with dealing with asteroids with a (very) short timeframe is the any asteroid that can harm the denizens of Earth has to have a lot of mass and since you have so little time to react, tremendous forces need to be brought to bear.  If you want to keep the time pressure, perhaps you could move the asteroid further out and unless its vector is changed in the next 5 minutes, the impact will be unavoidable? Assuming the answer is no, let's do some back of the envelope math!I'll assume the rock is about 100m in radius and has a density of about 3, that means it masses 12 million tonnes.Because the timeframe is short, you have to adjust the vector by a substantial amount.  That mass is moving at quite a clip (~11 km/s).  If you have 300 seconds, you're intercepting the asteroid about 3500 km from Earth.  Earth has a radius just over 6,000 km.  Worst case is a centre strike which would mean you'd need to change its vector by 45 degrees.  But let's assume the impact is projected to be a grazing hit about 1000 km from the edge which means you'd need to shift its angle by about 20 degrees.  Now that shift needs to be instantaneous.  Every second of delay/acceleration means the distance is closing and the angle needs to become more extreme.  Vectors at these speeds are mostly additive so you need to impart a velocity of about 3 km/s which would have the rock just skim the atmosphere -- assuming we ignore gravitational attraction.  Let's bump that up to 3.5 km/s for a basic safety margin.  That change is what you'd need if you could impart it instantly.  Let's assume you spend a whole minute of acceleration and thus need to achieve a higher velocity of about 6 km/s to simplify the math. A = v / t so 6km/60 seconds = 100 m/s-squared or about 10 g of acceleration.  F = ma, so 12 billion kg x 100 m / s-squared is 1.2 trillion Newtons of force which is the same as 1.2 terawatts needs to be applied for the whole 60 seconds.Regardless of how our hero applies the force -- whether directly to the asteroid threat or through a passive intermediary, the amount of energy remains the same.  Since you want the hero to use an intermediary, there needs to be something that acts as a force multiplier.  You don't have sufficient time to use the Earth's gravity well so let's look elsewhere.One simple force multiplier would be a secondary large rock -- say a half the radius of the threat that is moving in a different orbit that is closer to the preferential vector that can be adjusted to impact the threat.  With a mass of about 10% of the target rock, the energy required to change its vector is equally reduced.  If the asteroids are mostly metallic then you could even potentially set up an elastic collision.  In an perfect scenario, the larger mass would have its velocity changed by ~5% of the closing velocity between the rocks.</blockquote>

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