Near-Earth Asteroids Formed Billions of Years Ago. But What Makes Them Tick?

Asteroids are the most plentiful object in the solar system by far, numbering in the millions. They also vary considerably in terms of size and composition, but what exactly are they? Where did they come from? Why is everyone so worried about them in the news?

There's more to these space rocks than most people think, and while they can be incredibly dangerous, they are also fascinating objects for planetary scientists to study to help better understand the evolution of the solar system we live in.

They may also have incredible potential as a source of vital materials for industry. The amount of resources in a single asteroid could be worth trillions of dollars if we were ever able to extract it. 

Needless to say, asteroids are very important for any number of reasons, which is why they are some of the most studied objects in the entire solar system. Fortunately, that means we know a lot more about them than you'd think.

What Exactly Is an Asteroid?

The basic definition of an asteroid is that it's a small rocky object which orbits the Sun and was left over from the formation of the solar system.

When the solar system formed 4.6 billion years ago, all of the material in the massive, densely packed cloud of dust and hydrogen gas known as a molecular cloud, formed into a swirling solar nebula. As gravity pulled more material into the center, the sun was formed, and, according to the disc accretion theory, an accretion disk formed around the Sun. The gas in the disk was pushed out into the outer Solar System, leaving behind dust and other fine particulate matter nearer to the Sun.

Over millions of years, the dust clumped together into larger and larger objects. This process of accumulated growth would eventually form large planetoids that had enough gravity to start drawing in material along its path like a vacuum cleaner.

Eventually, the planets themselves would grow out of this process and entirely clear their orbit of any remaining material. Asteroids, then, are the remnants of this process from billions of years ago. They are the left-over material from this early planetary formation, the metaphorical sawdust on the shop floor of the Solar System.

What is the Largest Asteroid?

Which is the largest known asteroid is something of a debate right now. 

Originally, Ceres was classified as an asteroid when it was discovered in 1801 but it was upgraded to dwarf planet status in 2006. With a mean diameter of 587.82 miles (946 km), it is larger than some of the moons in the solar system, and makes up about a quarter of the asteroid belt's entire mass. 

This, along with other characteristics like its shape and composition — including evidence that it has a large subsurface ocean — as well as evidence of a water vapor atmosphere, led Ceres to be classified a dwarf planet in 2006, and NASA says it is no longer an asteroid.

Still, many still consider Ceres to be an asteroid, arguing that asteroids and dwarf planets aren't mutually exclusive. This includes the International Astronomical Union's Minor Planet Center, which is responsible for cataloging and designating asteroids. It still lists Ceres as an asteroid along with Pallas, Eros, and 2309 Mr. Spock. 

There is no debate about Vesta being an asteroid though, and with a mean diameter of 329 miles (529 km), it is the largest asteroid in the solar system that everyone agrees is an asteroid, so the default consensus is trending towards Vesta.

What Is the Difference Between an Asteroid and a Planet?

Asteroids aren't all that different from planets in terms of composition. After all, planets are made up of the same material from the Sun's formative accretion disk that produced the asteroids in the solar system.

There are some key differences, though. First, they don't have anywhere near the mass of a planet — it's not even close. The dwarf planet Ceres, the only dwarf planet in the asteroid belt between Mars and Jupiter, has only about 1.3% of the mass of the Moon.

The asteroid Vesta has about 28% of the mass of Ceres, which in turn has about 1.3% of the mass of the Moon. That would put the mass of the (likely) largest asteroid in the solar system at just 0.364% the mass of the Moon.

Mercury, meanwhile, is the smallest planet in our solar system and is about 4.6 times more massive than the Moon. This would put the mass of Vesta at just 0.0079% the mass of the Solar System's smallest planet. And Vesta is especially large; if you combined the mass of the asteroid belt, it would still only be about 4% of the mass of the Moon. 

Since asteroids don't have a whole lot of mass, relatively speaking, they aren't big enough to support an atmosphere. An asteroid's mass is also too low for the force of its own gravity to mold it into a sphere.

This leads to asteroids usually having all kinds of wonky shapes, and this is actually one of the official characteristics that a dwarf planet can't have. Dwarf planets must be spherical, having formed under the force of their own gravity.

Why Are Asteroids Called Asteroids?

The name asteroid was officially coined by renowned British astronomer William Herschel in 1802, though he wasn't the one who came up with it. The word itself is derived from the Greek word for "starlike" and originated with a Greek scholar named Charles Burney Jr.

When Herschel was writing up his observations of Ceres and the large asteroid Pallas, he searched for a word to describe them, since they were not planets, moons, comets, or stars, but rather something entirely new. He asked his close friend Charles Burney Sr. for help.

The senior Burney in turn asked for his son's assistance, since Burnley Jr. was a specialist in Greek, the language of scientific nomenclature.

Though the correspondence between the two is incomplete and we have not uncovered Burney Jr.'s reply to his father, Herschel used the term asteroids when he presented his findings to London's Royal Society in May 1802.

Burney Sr. later wrote in a letter discussing Herschel's just-published paper: "They are not allowed by Herschel to be either Planets or Comets, but asteroids...a kind of star — a name my son, the Grecian, furnished." 

What Are the Three Types of Asteroids?

There are three major types of asteroids in our solar system: C-type, S-type, and M-type.

C-type asteroids are also called chondrite asteroids, or carbonaceous chondrites, and are the most common type, making up about 75% of all the asteroids in the solar system. They have a very low albedo of about 0.03 to 0.09, where 1.0 is totally reflective, and 0.0 is totally absorptive.

This makes them very dark in appearance, leading many to believe that they were carbonaceous in nature (essentially giant lumps of space coal), but this isn't the case. They are composed of clay and silicate rocks, and devoid of volatiles.

They have a mean density of about 1.4g/cm³, so they aren't as tightly packed as other types of asteroids. For reference, the density of Earth is about 5.5g/cm³ and the Moon's is about 3.3g/cm³.

These are also some of the most ancient objects in the solar system, having formed 4.6 billion years ago, and reside mostly on the outer edge of the asteroid belt. This makes them of particular interest, as their composition and structure can tell us a lot about what conditions were like when the solar system first formed.

S-type asteroids account for about 17% of known asteroids. They are about twice as dense as C-type asteroids, with a mean density of 2.69g/cm³, and they are believed to be composed of iron-nickel and silicates, making them more reflective and easier to spot, with an albedo of 0.10 to 0.22.

It also helps that S-type asteroids dominate the inner edge of the asteroid belt, so there is relatively little obstructing our view of them.

M-type asteroids make up nearly all the remaining asteroids and are believed to be predominantly made of iron and nickel, giving them a rusty, reddish hue.

They also have a relatively high albedo of 0.10 to 0.18, and occupy the middle region of the asteroid belt. They also have about twice the density of S-type asteroids, with a mean density of 4.7g/cm³. This is comparable to the density of the Earth's mantle, which is 4.5g/cm³, though much lower than the 9.9 to 12.2g/cm³ of the Earth's inner core and 12.6 to 13g/cm³ of its liquid outer core, both of which are almost entirely made of an iron-nickel alloy.

There are also some other minor types of asteroids based on their composition, like Pallas, which is the best-known example of the heavily basaltic V-type asteroid. 

What Are the Three Classifications of Asteroids?

In addition to the three primary types of asteroids, there are three ways to classify them based on their locations: main belt asteroids, trojans, and near-Earth objects.

Main belt asteroids are the most common asteroid and are found in a belt in orbit around the Sun between Mars and Jupiter. By and large, these asteroids stay in their lane, as it were.

Trojans are special types of asteroids that share an orbit with a planet, usually orbiting at the fourth and fifth Lagrange points where the gravity of the Sun and the gravity of the planet balance each other out enough that an object can safely orbit there without being disrupted.

Any planet can have a trojan (named for the warriors of Troy in Homer's Iliad) but Jupiter has the most by far, possibly as many as are found in the entire asteroid belt. Mars and Neptune are known to have trojans, and it was discovered in 2011 that Earth has one as well.

Trojans are not to be confused with the final classification of asteroid though.

Near-Earth objects are asteroids whose orbit closely approaches, follows, or intersects Earth's orbit in some way. Unlike a trojan, which can safely orbit along with Earth at a distance like a car in an adjacent lane on a highway, near-Earth objects are more like cars that merge through the lane Earth is driving in.

And just like cars, they can merge safely or they can do so dangerously, and so these are the asteroids that are of special interest to us. Just like driving on a highway, seeing the danger coming is your best chance of avoiding disaster.

What Happens When an Asteroid Hits Earth?

Asteroids have been smacking into Earth ever since the planet formed. In fact, impacts from water-rich asteroids similar to Pallas and Ceres might explain where Earth got its water.

There is evidence that Earth already had its oceans during the Late Bombardment Period of the Hadean Eon about 4 billion years ago when it was clearing out its orbital path of material. So asteroid impacts aren't always terrible, awful things.

We wouldn't be here without them, most likely, but the fear that an asteroid impact has in our consciousness is very understandable. So let's sort out the reality from the reckless speculation.

First, we're being hit with asteroids all the time. Asteroids can be as small as a few meters across, and these are very common in our neck of the woods. Fortunately, these asteroids burn up very quickly in our atmosphere as meteors, and even give a little bit of a light show in the form of a "shooting star". Everyone loves a shooting star, so you can thank itty-bitty asteroids for that.

Getting hit with the bigger asteroids though is a different matter, on the other hand. Any time an asteroid hits Earth (any object that, whether comet or asteroid, enters our atmosphere, officially becomes a meteor), it has to get through the atmosphere first, and most never do.

If the asteroid is big enough though, say less than 200 feet in diameter (~61 m), it can make it almost to the ground before the pressure difference between the hot air underneath the meteor and the cold, low-pressure, skyward-facing side manages to squeeze into all of the cracks and imperfections in the meteor's structure to eventually punch a hole through the meteor. 

When that happens, the rush of superheated air from below through to the low-pressure side of the meteor is so violent that the meteor is blown apart from the inside.

The incredible speed of the meteor and the heat energy it has generated up until this point suddenly transforms into kinetic energy, and the amount of energy released from this detonation can be comparable to some of the largest bombs humans have ever made, and even dwarfs them by orders of magnitude.

This energy essentially travels out in all directions, including toward the ground. If the meteor is small, the only thing you might experience is a loud booming sound. If it is larger, say the diameter of a typical train car, then it can cause serious injury to those on the ground, including from broken glass as the shockwave shatters windows miles away from the epicenter.

If you go bigger than that, the air will ignite and create a fireball similar to an atomic weapon. And since the energy released scales exponentially with an object's mass, You don't need to increase the size of the asteroid that much before you get meteors that explode with more force than anything humans have ever produced.  

What happens when the asteroids are large enough that the air pressure can't blow apart the meteor before it hits the ground?

Well, from there, you start getting into the really terrifying stuff. Generally, if an asteroid is less than a couple hundred feet in diameter, we can assume it is going to explode in the atmosphere. This is especially true if it is a C-type asteroid, which is much more likely to fragment under pressure due to its density.

If a C-type asteroid is big enough though, the air pressure won't have enough time to blow it up, so these can still reach the surface, where all of that energy explodes with just as much force as in the air. 

What that energy does will largely depend on where it hits, such as whether it hits the ground or in the ocean. The physics of actual ground impact is a lot more complicated than airbursts, but there is a funky calculator you can play with to simulate the effects of different impacts based on size, speed, density, and angle of impact.

For example, a 250-foot (76 m) wide standard C-type asteroid traveling at 38,000 miles an hour (61,150 km/h) will produce a blast equivalent to about 715 Hiroshima bombs at about 25,000 feet (7,620 m) above the ground.

If the same size of a standard M-type asteroid hit the ground in some manner, around 600 Hiroshima bombs worth of energy would be directed into the surface, carving out a nearly a mile (1.5 kilometer-wide) crater more than 984 ft (300 meters) deep, while directing nearly 2,000 Hiroshima bombs worth of energy into the atmosphere.

If that M-type asteroid hit the ocean 6.21 miles (10 km) off the coast in 3.1 miles (5 km) deep water, the tsunami generated by the impact would arrive at land two minutes later with a wave just over 82 feet (25-meters) high. 

What happens when you go bigger than 0.62 mi (1 km) in width? Regardless of the composition of the asteroid, it's going to hit the ground. A C-type asteroid is going to hit with the force of nearly 5,500 Hiroshima bombs. This is enough to knock down buildings 62 miles (100 km) away and set fire to grass, trees, and structures, while causing third-degree burns to anyone exposed to the thermal radiation generated by the impact.

If anyone was looking directly at the impact when it hit, in half a second, they'd see a fireball that would appear nearly 20 times larger than the Sun and it would almost certainly be the last thing that they ever saw. Thermal radiation from the impact would reach its maximum in just 0.56 seconds, causing severe burns to the eyes and subsequent blindness.

Needless to say, the power of these kinds of geological forces is beyond most people's capacity to comprehend, once you start dealing with asteroids bigger than .62 mi (a kilometer) in width. But what if we went whole-hog?

What if the Largest Asteroid in the Solar System Hit the Earth?

Ok, so let's game out what would happen if the largest asteroid in the solar system hit Earth. For our purposes, we're going to go with Vesta, since the consequences of a Vesta impact are so apocalyptic that a Ceres impact is pretty much like taking the square of Hell on Earth, making it really just a difference in scale, not a difference of kind.

So what would happen if Vesta hit the Earth? The 329-mile-wide (530 km) asteroid would be a "planet killer" in the sense that it will kill everything on the planet, but Earth itself has been there, done that.

Earth chewed through Vesta-sized asteroids for millions of years in its early days, so it's absolutely got the bulk to absorb the blow and barely skip a beat.

Its mass would remain pretty much the same, it wouldn't shift on its axis, and it wouldn't affect Earth's orbit in a noticeable way. The worst that would happen to the planet itself is that the length of a day on Earth would be affected by several minutes in either direction depending on how Vesta hit, but that's about it.

Its surface, though, is another matter entirely. No matter where it hits, Vesta is hitting rock, since the incredible heat and air pressure building up beneath it would vaporize any ocean water in front of it. Given its size, Vesta would also be hitting Earth at full speed, since it is simply too big for the atmosphere to slow it down any appreciable amount.

After impacting the crust, Vesta would create something both Redditors and NASA scientists like to call a "crust tsunami." The Earth's crust would be peeled away from the point of impact, rolling up in a towering wave of material stretching out far beyond the upper atmosphere and into space itself.

The material ejected from the impact site will eventually rain back down onto the entire surface of the Earth, even points on the other side of the planet from the impact.

The rim of the crater produced by the impact would be about 23,000 feet (7 km) tall, dwarfing most of the mountain ranges on Earth. The crater would be about 2,500 miles (4,000 km) across, and blooming out of it would be an expanding shroud of vaporized rock as hot as the surface of the Sun, that would soon begin to spread out from the point of impact like a ring of fire.

It will take at least a day for this vaporized rock to fully envelop the Earth, but if you were at the farthest possible point from the impact, the rush of superheated air being pushed ahead of the vaporized rock would get to you first, setting everything in its path on fire and boiling the oceans.

If you somehow managed not to be roasted alive by the firestorm ahead of the vaporized rock cloud, that will reach you in no more than a day, and it would vaporize you then. Some estimates calculate that this thousands-of-degree vaporized rock would cover the entire planet for about a year, turning the entire surface of the planet to ash.

The oceans would boil and evaporate at a rate as fast as two inches every second. The salt left behind by the evaporating ocean also vaporizes, exposing the ocean floor to the thousands-of-degree heat, turning the basalt ocean floor into a sea of lava. Within a month, the Earth is completely sterile.

The rock vapor wouldn't last forever though. A year after the impact, it would start to dissipate and the temperature would start to fall. Because the Earth is large enough, all that boiled water wouldn't escape its gravity, but would instead remain in the atmosphere.

When the Earth was sufficiently cooled, in as few as a thousand years all of that water would start to condense and return to the surface as rain, refilling the oceans and performing something of a great reset for the planet.

We're pretty confident that this is how things would play out because we have evidence that just such an apocalyptic scenario has played out on our planet before. In fact, some argue that during these periods the planet is never completely sterilized, as some extremophile microbes might be able to tolerate the otherwise inhospitable environment.

Either way, the largest asteroid in the solar system, whether you think it's Ceres or Vesta, is more than enough to send the planet temporarily back into the Hadean Eon.

What Are Some of the Most Famous Asteroid Impacts?

In 2013, a meteor over the Russian city of Chelyabinsk exploded in just such a fashion. Starting out as an asteroid about 60 feet (18.2 m) in diameter, weighing 22 million pounds (~10 million kg), and traveling through the atmosphere at 40,000 miles an hour (64,000 km/h), the Chelyabinsk meteor detonated about 12 to 14 miles (19.3 - 22.5 km) above the ground and released the energy equivalent of 20 to 40 Hiroshima bombs.

Detonating well above the cruising altitude of commercial airliners, this blast was still powerful enough to shatter windows miles away and injured hundreds of people, mostly from flying glass. Fortunately, no one was killed.

An asteroid about twice the size of the one from the Chelyabinsk event famously exploded on June 30, 1908, over the remote Podkamennaya Tunguska River region of Siberia. Estimated to be about 120 feet (36 m) across, weighing 220 million pounds (100 million kg), and traveling about 33,500 miles an hour (~54,000 km/h), the Tunguska meteor exploded about 28,000 feet (8.5 km) in the air.

This would only be a few thousand feet short of where a modern regional airliner might cruise today, but the devastation from the explosion is literally legendary.

The explosion unleashed the force of about 185 Hiroshima bombs and produced a shockwave of hot air that was able to knock one man out of his chair in front of a trading post 40 miles away (64 km).

According to NASA, Tunguska's "resulting seismic shockwave registered with sensitive barometers as far away as England. Dense clouds formed over the region at high altitudes which reflected sunlight from beyond the horizon. Night skies glowed, and reports came in that people who lived as far away as Asia could read newspapers outdoors as late as midnight.

Of course, we can't talk about asteroids without talking about the most famous of them all.

How Big Was the Asteroid that Killed the Dinosaurs?

About 65 million years ago, a massive asteroid hit the Earth in the Gulf of Mexico off the Yucatan Peninsula and caused a calamity so devastating it eventually killed off 75% of all life on the planet, including every species of non-avian dinosaur.

The Chicxulub impactor, as it is called, is believed to have been anywhere from 6.2 to 50.25 miles wide (10-80 km), depending on its composition and mass. When it struck though, it released something on the order of 100 million megatons of energy, or about six and a half billion Hiroshima bombs.

The impact ejected so much material into the atmosphere that the resulting dust in the atmosphere killed off the plants that most species depended on for survival, and when they died off, the carnivores who ate the plant-eaters died off too. It took years for the skies to clear, and when they finally did, the entire food chain had been upended and large parts of the planet were essentially a lifeless husk.

Fortunately, this opened up niches that had previously been occupied by dinosaurs and other now-extinct species, providing the space necessary for mammals to take over.

Why Do Asteroids Come to Earth?

While most asteroids are neatly contained in the asteroid belt or in orbit with Jupiter as trojans, Jupiter's gravitational influence often sends asteroids flying off in different directions, and more often than not, it sends them flying into the inner solar system.

Now, that in itself isn't that big a deal. Space is huge, after all, so Jupiter isn't guaranteed to land a hit with any particular asteroid it sends flying our way, but on the time scale we're talking about, something on the order of hundreds of millions and even billions of years, that provides a lot of opportunities for an asteroid to make a pass at Earth.

Most asteroids aren't projected to come anywhere near us, and so are more suitable as objects of study than they are causes for concern. Near-Earth asteroids, however, are a different matter, and NASA has a special program known as the Center for Near-Earth Object Studies (CNEOS) tasked with managing that risk.

CNEOS's purpose is to track these potentially deadly objects so that if there is a significant change (greater than 1%) of an impact with Earth, we can have advanced notice and begin to take steps to mitigate the potential threat, and hopefully even prevent a disaster by redirecting an asteroid away from us. 

But even though it's definitely a good idea to keep an eye on potential threats, you also don't want to overthink the threat that asteroids pose. NASA might say an object is "near" to Earth, but this is a relative term when talking about the solar system. Even a million miles is considered "near" to Earth, even though you could fit 126 Earths in a row in that same distance, and a kilometer-wide asteroid is absolutely minuscule in this context.

So whenever you hear about some asteroid NASA has identified as coming close to Earth, it's not something you should be losing sleep over, much less cause you genuine anxiety. There are a great many other things in the world that are a much bigger cause for concern than a potential asteroid impact, and those are usually things you can actually do something about.