Can an Asteroid or Comet Impact Earth?

Asteroids occupy, in general, a belt in our solar system located between Mars and Jupiter.  They are space rocks ranging in size from very minute to hundreds of miles across.   As long as they stay in orbit beyond Mars the Earth is safe.  But, they act gravitationally on each other, and are pulled by the planets.  So, whether pulled out of a stable orbit by a planet, or deflected into a path that might impact the Earth by collisions with each other, asteroids can stray into a collision course with the Earth.

A comet is a chunk of ice and rock that is believed to be from the Kuiper Belt. The Kuiper Belt is a region occupied by such objects past the planet Neptune, the planet farthest from the sun.  Some comets, after gravitational interaction and collisions, head in through the solar system in long, elliptical orbits about the sun, or even crash into the sun.  When they get close enough the sun causes gas to be released, and the light and particles from the sun push that gas into a tail.  The tail always points away from the sun, and may have water vapor as a major component.  The path of a comet traveling on such a long orbit can be nudged gravitationally into a collision path with the Earth depending on which planets or other objects it interacts with.

First, not every impact is serious.  It depends on the energy released as to how much damage is done.  Size is a consideration, small objects impact the Earth every day, and in great numbers.  Some are so small that they vaporize high in the atmosphere from heat generated by friction.  An object the size of a grain of sand might make a streak of light across the sky, but never reach the ground.

Larger objects can and do reach the ground.  In the air they are called meteors.  On the ground they are called meteorites.  Some meteorites are rocky, others have a high metallic content.

Very large objects, a mile or so across, or even more, bring in lots of energy.  Remember that they came from way out in space, and have been accelerating not only by the gravity of the Earth, but also by the gravity of the sun and possibly some of the other planets.   Think about dropping a ball.  As it falls it accelerates, so the farther it falls the faster it is going when it strikes the ground.  Having come so far, these objects are travelling at high speeds when they reach the ground.  Their energy is calculated as half the product of their mass and the square of their speed.  Energy is conserved, so all of that energy goes somewhere.  Much of it goes into heat.

If the object is solid rock it may reach the ground intact.  If the object has gas, or something that can vaporize into gas, like water, an explosion above the ground can occur.  In either case a crater can form.

A massive explosion, whether it is airborne or on the ground, carves out a crater, throwing debris into the air.  The energy raises the temperature of the atmosphere, and if the object is large enough this rise in temperature can be worldwide, causing forests to incinerate.  Plant and animal life is lost if an impacting object has enough energy, including humans.  Then, the debris that is thrown high in the atmosphere screens out the sun for years, dropping the Earth into cold.  This impacts the initial survivors.  There will be few plants and animals that will survive such an event.  The food chain would be interrupted.

A large enough impact can bore down to the mantle, especially if it occurs where the Earth’s crust is thin.  This would expose molten magma, and release toxic gases.

The impact, whether or not it causes the mantle to be exposed, would cause seismic waves as from an unbelievable earthquake.  These waves could also cause tsunamis depending on where the impact happens to be, and how great it happens to be. 

As seismic waves move out they drop in intensity, much like ripples in water from a rock having been thrown into a pond does.  The energy spreads out.  But, unlike the ripples in a pond, when the waves get half way around the Earth they start to come back together.  Think about a sphere.  If you draw circles starting at the top, they are larger until you get half way around, then smaller as you near the bottom.  When the seismic waves converge opposite the point of impact they can rip open the Earth if intense enough, and allow new mountains to suddenly be pushed upwards.  If this happens at sea a tsunami can form.  

Normally we do not see this happen, but a large enough object could cause any or all of the above.  Some large objects have impacted the Earth in the past.  So, the doomsday books and movies are literary license being taken with a real basis.  An extinction event is unlikely in the near future, but possible.

Well, collectively objects large enough to do major damage are tracked after being found.  They are collectively called Near Earth Objects, or NEOs.  No one is in a panic regarding any impending impact at the present.

The problem is there is an uncertainty.  Those looking for NEOs look in the most likely places, along and just above and below the plane that includes the asteroid belt and the Kuiper belt.  The problem is farther out there is the Ort Cloud, and it also has large objects, but they are not confined to the same plane as the others.  So, a rare impact from an Ort Cloud object can occur with little warning.  There are too few people and resources to watch the entire sky for NEOs.

Feel safe, but with caution.  The most likely part of the sky shows no danger in the near future, but there is always a chance something will slip through unnoticed.

There are things that are being proposed, including letting sunlight push large sails that would be attached to an errant NEO so the object would deflect enough to miss the Earth.  Other ideas like pushing NEOs with motors, or directional explosions are also up for consideration.  The earlier we detect an NEO, the more options we have of saving the planet, and our options would have a better chance of working.  So, even if one is found, doomsday is not a necessity.  

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