June 30 - Asteroid Day

Posted on June 30, 2020

Today is Asteroid Day, and tonight is also National Meteor Watch Day (in the U.S.). 

The two are somewhat related. Here's the deal:

The solar system formed from a dense cloud of gas and dust. It started collapsing, much of the gas and dust being pulled by gravity into the center. It started rotating as well. 

About 99.8% of the matter (the gas and dust) in the cloud ended up being pulled into the center and thus created the Sun. A fair amount of the leftover matter formed the eight planets and the various dwarf planets (aka Plutoids). A lot of the smaller bits became moons and rings - and a bunch of smaller-sized rubble that circles the Sun between Mars and Jupiter makes up the asteroid belt.

Asteroids, therefore, are leftovers in the formation of the solar system. They are defined as small rocky bodies that circle the Sun. (Moons and rings primarily circle planets or dwarf planets, and even though they also circle the Sun as their planet orbits our star, since their own orbit is around something other than the Sun, moons and particles that make up planetary rings are not considered asteroids.)

Asteroids range in size from dwarf planets like Ceres, which is about half the size of Pluto, down to grains of dust. Scientists' best guess of the number of asteroids is one to two million.

Meteors are any small rocky or metallic body of matter from outer space that enters the Earth's atmosphere. Most meteors burn up in our atmosphere because of the friction from running into so many air molecules . These are the beautiful streaks of light - often called shooting stars - that we are urged to look for tonight on National Meteor Watch Day.

Any meteor that is large enough to get all the way through the atmosphere and hit ground (or, more likely, ocean) is called a meteorite. 
When we talk about the theory that an asteroid hit the Earth and killed off the dinosaurs, we are talking about a really large meteor that formed a huge crater and probably mostly vaporized - causing an awful climate change event that resulted in lots and lots of species dying off.

And that is why Asteroid Day and National Meteor Watch Day are related. Asteroids can become meteors, and bits of asteroids that get hit by something can chip off and become meteors. The likelihood of a person or even a building being hit by a meteorite is surprisingly small; some sources say that there is only one confirmed case of a human being hit and (mildly) injured by a meteorite in all of history, but there is some evidence that it has happened at least a handful of times. 

But scientists say that a really large impact could have disastrous results, so we should watch for asteroids and other space debris that could become meteors!

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