All About Asteroids

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Podcast Transcript

Our solar system is made up of a lot of things.

The biggest thing is the sun, of course which makes up the vast majority of the solar system’s mass. 

Then, of course, there are planets, which come in various sizes, and many of them have moons of various sizes. 

However, that isn’t everything. There are other things in the solar system, things that amount to debris between the much bigger objects. 

Learn more about asteroids, how they were discovered, and how they might serve humanity’s future on this episode of Everything Everywhere Daily.


I’ve done many episodes on all of the major players in the solar system. I’ve covered every planet, the sun, and even the hypothetical Planet X.

However, this doesn’t make up the entire solar system. Floating around between the planets are smaller objects that are known as asteroids. 

The first question we have to address is, “What is an asteroid”? 

If you remember back to my previous episode on whether Pluto was a planet, astronomers have developed a definition to determine what is and what isn’t a planet. 

There were three criteria which were set that had to be met for something to be called a planet:

  1. It is in orbit around the Sun, 
  2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium. In other words, it is nearly round.  And finally
  3. has cleared the neighborhood around its orbit.

Asteroids do not meet all three of these definitions. 

All of them meet the first requirement in that they orbit the sun. Some of them are large enough to meet the second requirement of having enough mass to be round-shaped. 

No asteroids meet the third requirement, which is that they are big enough to clear out their orbit. In fact, as we’ll see, asteroids are usually found with a whole bunch of other asteroids. 

One thing some of you might be asking is what the difference is between an asteroid and a meteoroid. According to the International Astronomical Union, the same group that created the definition of a planet, a meteoroid is “a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom.”

So, that’s a pretty vague definition, but we can probably safely say that if we can observe it from the surface of the Earth, it is big enough to be called an asteroid. 

The discovery of asteroids took some time. It was what I would call a rather deliberate accident, and it involved the asteroid belt.

The ancients knew the planets Mercury, Venus, Earth, Mars, Jupiter, and Saturn. 

By the late 16th century, astronomers had developed an idea of the distances of the planets from the Sun. Once they had an idea of the distances, something seemed a bit….off.

The gap between Mars and Jupiter seemed abnormally large. The German astronomer Johannes Kepler analyzed the data collected by the Danish astronomer Tycho Brahe and concluded, “Between Mars and Jupiter, I place a planet…”

His guess was really just that—a guess based on his sense of proportion of the solar system. 

In the 18th century, Kepler’s hunch developed a more formal mathematical framework.  A series of numbers that seemed to fit the distances of the planets was discovered. 

It was first proposed by the astronomer Johann Titius and later refined by Johann Bode, both Germans. A law known as the Titius-Bode Law was named after them. 

The Titius-Bode Law states that if you take a sequence of numbers starting with zero, then going 3, 6, 12, 24, 48, etc, doubling each time. Then, add four to each number and divide it by 10, and what you get is a close approximation to the distance of the planets from the sun in terms of astronomical units.

The actual distance of all the planets from the sun, up to Uranus, is within 5% of the estimates provided by the Titius-Bode Law.

However, there was one problem. There was a missing planet between Mars and Jupiter. The Titius-Bode Law seemed to confirm the suspicion of Johannes Kepler so many years before. 

It implied that a planet hadn’t been found between Mars and Jupiter. 

The missing planet was apparently discovered by Giuseppe Piazzi, an Italian Catholic priest, on January 1, 1801.

The new planet was named Ceres, and at a distance of 2.8 astronomical units, it seemed to fit the Titius-Bode Law. 

Just before I said, the discovery of the asteroid belt was a deliberate accident. It was deliberate insofar as people were looking for it in the rough orbit where it was found. 

It was an accident because it was later determined that the Titius-Bode Law was pseudoscience. The location of Neptune didn’t conform to the Titius-Bode Law, and there was no physical reason for it. It was just a series of numbers that happened to conform to the orbit of the planets by coincidence. 

The discovery of exoplanets around other stars has subsequently confirmed that the Titius-Bode Law has no scientific basis. 

Nonetheless, Ceres was discovered, but again, there was something wrong. 

Astronomers started finding other things in the same orbit as Ceres. 

In 1802, Pallas was discovered in the same approximate orbit. This indicated that Ceres wasn’t the missing planet. Rather, this was something different. 

The astronomer William Herschel dubbed them “asteroids.”

In 1804, Juno was discovered, and in 1807, Vesta. Throughout the 19th and early 20th centuries, a host of asteroids were found, each one seemingly smaller than the one before it. By 1921, over 1000 asteroids had been discovered.

All of these minor planets were in a belt around the Sun, which was dubbed the asteroid belt. 

People have some very mistaken views of the asteroid belt, mainly due to movies and television. It is often viewed as a region filled with rocks. If you were a spacecraft trying to navigate the asteroid belt, it is often assumed that you have to be dodging rocks so they don’t get hit constantly. 

That is not what the asteroid belt is like. If you were in the middle of it, odds are you’d see absolutely nothing. Yes, there are objects in the asteroid belt, but they are spread so far apart that you couldn’t probably see one if you were on another. 

The current estimate is that there are 1.1 to 1.9 million asteroids larger than 1 kilometer and probably many millions more that are smaller. 

Despite the seemingly large number of asteroids, they don’t add up to much. All of the asteroids in the asteroid belt have less than 4% of the mass of the Moon. 

The next big question with regard to the asteroid belt was how it was formed. 

One theory held that the asteroid belt was at one time the location of a planet, just as the Titius-Bode Law. According to the theory, at some point in the early solar system as Mars-sized planet or smaller either broke up or was destroyed. 

This theory has subsequently fallen out of favor because there isn’t nearly enough mass in the asteroid belt to have once compromised a planet the size of Mars. 

The current popular theory is that the material in the asteroid belt was simply never able to coalesce into a single object because of Jupiter’s gravitational interference. 

Despite not being as sexy as the larger planets and moons in the solar system, asteroids continued to be studied. It is a good thing that they were because what was found out about them has groundbreaking potential. 

There are, roughly speaking, three categories of asteroids: C-type, S-type, and M-type.

C-type asteroids contain a high proportion of carbon along with rocks and minerals. They are the most common, making up about 75% of known asteroids.

S-type asteroids are made up of silicate materials and nickel-iron. These asteroids are less common.

Finally, M-type asteroids are composed mostly of metallic iron and nickel and are relatively rare.

It is the composition of these asteroids which has such enormous potential.

If humans are ever going to build large-scale space stations or spacecraft, they are going to need an enormous amount of raw materials to do so. These materials can’t come from Earth because of the difficulty and energy required to get them out of the Earth’s gravity well.  

Asteroids make for an almost perfect source of raw materials in space. They are rich in metals, and there are millions of relatively small objects that could be towed in full wherever they are needed in the solar system. 

Asteroids are abundant and contain precious metals like platinum, gold, and rhodium, as well as rare earth elements that are used in electronics and other advanced materials. However, they have abundant amounts of iron and nickel as well as the most precious substance in space…….water.

The total economic value of asteroid mining is mind-blowing. A single asteroid that was visited by a spacecraft was estimated to have an economic potential of $100,000 quadrillion dollars. 

To put that into perspective, the economic value of everything on Earth, including all objects and real estate, is only around $400 trillion dollars. A more expansive estimate that puts a value on all the mass of Earth is 5 quadrillion dollars.

We are still a long way away from asteroid mining, but asteroid exploration has begun.

A few missions have done flybys of asteroids, but they were only able to obtain images. To date, there has been only one mission to an asteroid in the asteroid belt as an objective. 

In 2007, NASA launched the Dawn mission. It orbited the largest asteroid, Ceres, as well as Vesta, 

The Dawn mission was great, but what was really needed was a sample of material from an asteroid. This finally took place in 2016 when NASA launched the Osirius mission. 

Osirius traveled to a near-Earth asteroid named 101955 Bennu. It was launched on September 22, 2017, and it arrived at Bennu on December 3, 2018.

It spent over two years analyzing the surface before landing on October 20, 2020. It then took a sample of the asteroid and returned it to Earth. It arrived back on Earth with its sample on September 24, 2023.

I should note that the asteroid belt is not the only place in the Solar System with asteroids. Some have their own orbits around the sun.

Other asteroids can be found in groups. The orbit of Jupiter has two groups of asteroids that have been captured and are now stuck in orbital resonance with Jupiter. Collectively, they are known as the Hildian asteroids. 

There are currently over 5,000 known Hildian asteroids that are gravitational locked in Jupiter’s orbit. 

The generic term for any asteroid that is gravitationally caught in a planet’s Lagrange point is a Trojan asteroid. 

There are probably millions more asteroids floating around the solar system that we are unaware of. 

Asteroids aren’t as enchanting as planets. They don’t have colorful atmospheres and there is almost certainly no possibility of life on them. But in terms of potential future usefulness, they might be the most important things in the solar system.