Will a large asteroid strike Earth?

This is a good question, but the answer is yes, a large asteroid will eventually strike the Earth. They have in the past, and the chances are good that it'll happen again. That is, unless we track it before it does and then alter its trajectory.

First task is tracking all possible Earth orbit-crossing objects large enough to cause damage. How large is considered dangerous? Most scientists believe an object 100 to 500 meters in size would do considerable damage by either striking the ground or exploding in the atmosphere. There is also the possibility of it hitting the ocean and causing a massive Tsunami. The probability of this sized object hitting Earth is once every 1000 to 10,000 years.

The 1/2 to 1 kilometer sized objects would represent an extinction catastrophe. In other words, an object this big would wipe out life. Here we're talking once every tens of millions of years.

So, how many of these big bad boys are out there waiting to knock us into the Stone Age? By definition, a potentially hazardous near-Earth object (NEO) is an object with an Earth minimum orbit-insertion distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or brighter. An AU is the distance from Earth to the Sun or 93 million miles; so 0.05 AU would be 4,560,000 miles. The absolute magnitude has to do with how bright the object is. The brighter, the larger it is. An H of 22 would be an object around 150 meters.

There are around 13,000 near-Earth asteroids (NEA) and 100 near-Earth comets (NEC). As of June 2015, 872 NEA's larger than 1 kilometer have been discovered. Of these, 153 are considered hazardous. The bad news is that 20% of these have yet to be found.

How do they track possible Earth collision objects? The main process has been used for many years and involves astrometry, the art of measuring the orbits of sky objects. You need a telescope for this, one that can accommodate a camera. The telescope has to be mounted so that it can compensate for the rotation of the Earth. Distant stars stay fixed in the sky (over the short time), but asteroids and comets don't. One must take photographs of the same area of sky hours apart. The photographs then must be examined in a measuring microscope that can look at two consecutive photos in order to detect motion. If an object is seen as moving, then it's a matter of measuring its motion and brightness with time. The other thing is to identify the position of known stars in the image. These allow one to determine a position of the object relative to the Earth. Basically, one is determining the orbit of the object.

Most objects travel in the plane of the ecliptic (the plane of the Earth's orbit around the Sun). If the object in question crosses this ecliptic then it could be a possible risk if it and the Earth arrive at the same location at the same time.

Most of these measurements and calculations are done with computer programs. NASA relies on amateur astronomers for a lot of the grunt work in finding potentially dangerous objects. NASA also uses radar to measure these bad boys. A recent near event was Asteroid 2105 TB145 that passed within 300,000 miles of Earth. The most recent event was 2015 WP, 1-5 miles in size, passed within 142,000 miles.

It appears that these objects are passing us quite a bit. However, we have an advantage over the Dinosaurs that were wiped out by a large asteroid collision 65 million years ago. We can track them and have the technology to change their courses if needed.

Thanks for reading.