Should I be worried about asteroids hitting Earth?
No, and yes. You shouldn’t lose sleep over it because damaging asteroids do not hit very often.
But you should care about it: they have hit and dangerous asteroids will hit again, unless we prevent that from happening—which we can if we put in the work to find, track, characterize asteroids, develop methods to deflect dangerous asteroids, and internationally coordinate and educate.
What is an asteroid, exactly?
An asteroid is a small, rocky or metallic object orbiting the Sun. They are now usually defined as being larger than 1 meter in diameter with objects smaller than that being called meteoroids.
The largest asteroid is Ceres at 965 km (600 mi) diameter. Most asteroids, including Ceres, are located in the asteroid belt between Mars and Jupiter, but some asteroids come near to or cross Earth’s orbit.
How do asteroids form? Where do asteroids come from?
Asteroids are typically material left over from the period of planetary formation 4.5 billion years ago, the stuff left over that didn’t form into planets in the inner solar system. Often they are fragments of collisions between asteroids in the past.
How many near-Earth asteroids are there?
Using the cut-off for asteroid diameter of 1 meter, there are estimated to be more than half a billion near-Earth asteroids.
For objects that cause major damage if they hit Earth (larger than about 30 meters), there are about a million. So far, we are approaching 20,000 found.
It is easier to find larger objects, so we think we have found more than 90% of the asteroids 1 km and above, but for smaller asteroids still capable of causing major regional damage, we have only found a small percentage.
Are there any asteroids heading for Earth?
There are a few asteroids that currently are known to have a low probability of hitting Earth in tens to hundreds of years.
For example, one of the highest probabilities currently is an approximately 37 meter diameter asteroid called 2000 SG344 that has a 1 in 1100 chance of impact in 2071.
But these always are based on asteroid observations that have uncertainties in them. Usually, as more observations are obtained, the impact probability will drop to 0; in other words we know it won’t hit.
JPL keeps an online list of all asteroids with any probability of hitting Earth.
The big uncertainty is that we haven’t discovered most of near Earth asteroids yet, so we don’t know if they are on a collision course with Earth, which is why finding and tracking them is crucial.
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