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Asteroid Itokawa. Credit: JAXA


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If you look at a model of the Solar System there is a large gap between the orbits of Mars and Jupiter. That gap puzzled astronomers centuries ago. In 1596 Johannes Kepler predicted “Between Mars and Jupiter, I place a planet”. And in 1801 Giuseppe Piazzi found a point of light moving in just the right speed to be the desired planet. But it was just a dot and too faint to be a terribly big object. He gave it the name Ceres, but was it the desired planet? A year later another one was found. And in 1804 a third one followed be a fourth in 1807. It was clear that a new class of solar system objects had been discovered.

Because they were all just dots at the telescopes of the time, points of lights like stars, they were given the name Asteroids meaning star-like.

An asteroid, a point of light, moving against the background stars. Credit Wikipedia

Asteroid Belt. Credit: NASA/McREL

How Many Are There?

Today we know of over a million asteroids. There are billions of them larger than 100 meters across and millions larger than 1 kilometer in size.

What Is An Asteroid?

Generally speaking asteroids are smaller rocky or metallic objects that orbit the Sun in the inner solar system up to the orbit of Jupiter.

They are leftovers from the formation of our solar system 4.6 billion years ago.


Most of them, about 75%, are carbonaceous meaning they are carbon-rich. About 17% are made up mostly from silicate (stony) materials and 8% are rich in metals like iron and nickel.

Silica is one of the most abundant families of materials, existing as a compound of several minerals. In many parts of the world silica is the major constituent of sand. If you wonder about the term Silicon Valley, it’s because the first ingredient in the manufacturing process of a computer chip is sand.

So back to asteroids, they are classified into three types accordingly:

  • C-type (carbon)
  • S-type (stony – silicate)
  • M-type (metallic)

Main Asteroid Belt

Most of them orbit the Sun between the orbits of Mars and Jupiter in the Main Asteroid Belt.

Kirkwood Gaps in the Asteroid Belt. Credit: LPI

In the Asteroid Belt there are several gaps, called Kirkwood gaps from astronomer Daniel Kirkwood. These gaps occur where an asteroid’s orbital period forms a simple ratio (3:1, 5:2, 7:3, 2:1) with Jupiter’s orbital period, which produces a gravitational resonance that perturbs objects from those regions. The inner edge of the asteroid belt is defined by the ν6 resonance with Saturn. In that area Saturn alters the eccentricity of an orbiting asteroid and carries it into the inner solar system. That resonance is a very effective source of asteroids and meteoritic debris that hits the Earth-Moon system.

How Dense Is The Asteroid Belt?

In shi-fi movies they always show spaceships trying to avoid asteroids. But in reality the asteroid belt is mostly empty space. Decent size asteroids are on average millions of kilometers apart. If you stood on an asteroid, chances are that you could not even see another one with your naked eye!

Giant Rocks Or Enormous Rubble Piles?

You might think that asteroids are like giant rocks but that is not the case. Pictured here is Itokawa, a 535 m asteroid as seen from the Japanese spacecraft Hayabusa during its close approach in 2005. It was the first asteroid to be the target of a sample return mission. Credit: JAXA

Close encounters with asteroids reveal that most of them are rubble piles, individual rocks held together by their own gravity.

Rubble piles could have formed in two ways as asteroids have spent billions of years colliding into one another:

  • High speed collisions tear apart the parent bodies leaving behind countless smaller debris. The accumulation of those debris form rubble piles – the asteroids we see today.
  • Slower hits can disrupt an asteroid, crack it, but not necessarily break it apart. Over time numerous such collisions can gradually completely shatter a once solid object, like a shattered car window that still holds its original shape though completely cracked.

When you think of an asteroid you better think it like a bag of gravel or like a shattered car window!

Mass of Main Belt Asteroids

Despite being billions of them, the mass of all the objects in the Asteroid Belt adds up to just 4% of the mass of the Moon (about 0,05% of the mass of Earth).

Why To Study Asteroids?

Asteroids represent the original building blocks that formed the terrestrial planets about 4.6 billion years ago. And while the planets and moons have changed over the millennia, many of these small chunks of ice, rock and metal have not. They are like fossils from the planet formation era.

Asteroids Delivered Earth's Water

Studying asteroids not only we learn about the early solar system formation, but also about the source of our oceans water.

How unique is our water- rich planet? Mars is dry, Venus is hot and dry, Earth on the other hand is an ocean planet. Earth formed from the same materials as these other planets. It went through a process of melting, forming it’s iron core and basaltic crust and it was hot enough to boil off its internal water. So the fact there is an ocean today is a little perplexing.

Water was delivered to Earth after it formed. Earth’s water came from water rich objects, like asteroids and comets, impacting and leaving the water behind. Earth was able to keep that water for two reasons. First Earth’s gravity is high enough that can retain the volatiles that were delivered. And second Earth has a protective magnetic field that keeps the solar wind for striping the atmosphere and oceans away.

Studies revealed that Earth’s water was delivered 90% from asteroids and 10% from comets. That conclusion came from studying levels of hydrogen isotopes, the ratio of deuterium to hydrogen, both at asteroids and comets and then compare it with Earth’s. Scientists found that asteroids isotope composition better match our ocean water.

But how do we know isotopes levels on asteroids and comets in the first place. Well, probes have been visiting asteroids and comets returning samples back to Earth!

How Big Are They?

The largest asteroid Ceres with Moon and Earth to scale for comparison. Ceres surface area is approximately the same as the land area of India. Credit Wikimedia Commons

Credit Wikimedia Commons

The largest asteroids are large enough to be called protoplanets.

Ceres qualifies to be called a dwarf planet due to its size and spherical shape.

Enhanced color image of Ceres. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA


Ceres is the biggest asteroid, accounting for 25% of the total mass of all asteroids.

It contains large amounts of water ice beneath its surface. It’s low density indicates it could be composed of as much as 25 percent water. If that is correct, Ceres has more water than Earth does!

Its surface is saturated with impact craters, but many of them has bright features in them. These are briny patches, left over from salty water bubbling up from the interior. When the water ice sublimates away (transforming directly from solid to gas) it leaves behind the salt crust. Salt deposits look like they had built up within the last 2 million years – the blink of an eye in space time. This suggests that the brine may still be ascending from the planet’s interior, hence Ceres is a geologically active world.

The Dawn spacecraft studied Ceres from orbit from 2015 to 2018, so we have a lot of detailed pictures. Although the bright spots look very shiny, Ceres is actually very dark. It only reflects about 9% of the sunlight hitting it, making it roughly as dark as asphalt. The images from Dawn are exposed and processed to make the contrast better. The below image is closer to how dark it really is.

Ceres – shiny spots in Occator crater and Ahuna Mons at far right silhouetted against space. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

3D visualization of Ahuna Mons based on Dawn data. Credit: Dawn Science Team and NASA/JPL-Caltech/GSFC

The mound poking up out of the right side is a huge mountain called Ahuna Mons. It’s 4 km high and 17 km long. The most likely explanation of it is that it’s a cryovolcano: A volcano not of lava, like on Earth, but of water ice! Well not pure ice, data from the Dawn mission reveal the material to be 55 – 70% water by volume, with the remaining 30 – 45% non-soluble, solid particles. In other words, mud.

The findings resemble New Horizons mission that suggests that Pluto hides a global liquid water ocean beneath its icy crust.

Oceans could be common features of dwarf planets, based on what we learned at Pluto and Ceres, raising interesting questions about worlds that could be habitable by alien life.

Here a cool video Highlighting Bright Areas of Ceres. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA


Vesta is the second most massive body in the main asteroid belt, accounting for 9% of the total mass of all asteroids. It has one of brightest surfaces observed on any rocky body in our solar system. It reflects 43% of the light it receives from the Sun when the Moon reflects just 12% and Ceres 9% for comparison.

Vesta is the brightest asteroid as seen from Earth, just barely visible to the naked eye. If you are curious you can spot it through our telescopes in an Astro Tour Greece event.

Vesta 3d model. Credit: NASA/Dawn mission

Vesta is believed to have suffered a huge impact less than a billion years ago, responsible for the enormous Rheasilvia crater, which is 500 kilometers wide — 95% of the diameter of Vesta itself!

You can see the flattened south pole and the huge mountain at lower right at the center of that crater. That mountain is 22 km high, one of the tallest known mountains in the Solar System!

During the huge impact one percent of Vesta mass got ejected into space. Those ejecta have created the Vesta family of asteroids with more than 15.000 members.

Another think is that Vesta happens to lie close to a Kirkwood gap in the Asteroid Belt that we talked before. Small pieces of Vesta that were excavated from the surface during impacts would move into that gap, in resonance with Jupiter, and that would sent the material into an Earth crossing orbit. That’s like a highway to send pieces of Vesta to Earth. As a consequence we have a lot of pieces of Vesta as meteorites found on Earth, more than 1.500 of them! They form the HED meteorites – Howardite, Eucrite and Diogenite groups.

And that process still continues, new HED meteorites are found every year. In 2018 we saw a piece of Vesta fallen to Earth and collected the meteorites.

Here are some other interesting asteroids.

Asteroids with moons - Ida

Asteroid Ida and its moon Dactyl. Scientists found the moon – the first discovered orbiting and asteroid – when the Galileo spacecraft flew past Ida in 1994. Image Credit: NASA/JPL

Some asteroids even have moons. Ida is 60 km in length and Dactyl about 1,4 km.

Binary asteroids - Didymos

Asteroid Didymos and its small moonlet make up what’s called a binary asteroid system. Image Credit: ESA

Didymos – which means “twin” in Greek – is a binary asteroid. The primary body is about 780 m in diameter and the secondary 160 m.

Didymos is classified as a potentially hazardous asteroid, a near-Earth object that can make close approaches to the Earth and is large enough to cause significant regional damage in the event of impact.

NASA’s DART mission expected to lunch later this year, is going to test in reality our ability to alter the orbit of an asteroid.

The DART probe will impact the moonlet (secondary body) of Didymos at high speed – about 6.6 kilometer per second. The goal is to determine how much the impact alters the moonlet’s velocity in space, by measuring the change in its orbit around Didymos. Scientists think the collision will change the speed of the moonlet by a fraction of one percent and alter its orbital period around the larger asteroid by several minutes – enough to be observed and measured by telescopes on Earth.

Ryugu - Sample return mission Hayabusa2

Image sequence showing the rotation of Ryugu. Credit: Japan Aerospace Exploration Agency (JAXA)

Ryugu has a mean diameter of 490 m. It has been visited be JAXA’s spacecraft Hayabusa2 which touchdown twice and collected samples from the surface. The capsule have been successfully return to Erath in December 2020.

Hayabusa2 touchdown on Asteroid Ryugu

Bennu - Sample return mission OSIRIS-REx

Asteroid Bennu as seen by the OSIRIS-REx spacecraft. Image credit: NASA/Goddard/University of Arizona

OSIRIS-REx Touches Asteroid Bennu

In October 2020, OSIRIS-REx successfully touched down on the surface of Bennu and is intended to return its samples to Earth in 2023.


An artist’s concept of asteroid Psyche. Credit: Maxar/ASU/P.Rubin/NASA/JPL-Caltech

One of the most intriguing targets in the main asteroid belt, 16 Psyche is a giant metal asteroid, with an average diameter of about 225 kilometers. Unlike most other asteroids that are rocky or icy bodies, scientists think the M-type (metallic) asteroid 16 Psyche is comprised mostly of metallic iron and nickel similar to Earth’s core.

Scientists wonder whether Psyche could be an exposed core of an early planet that lost its rocky outer layers due to a number of violent collisions billions of years ago.

This intriguing asteroid is the primary target of NASA’s Psyche mission targeted to launch in August of 2022. The mission’s goal is, among other things, to determine whether Psyche is indeed the core of a planet-sized object.

The Psyche mission will be the first mission to investigate a world of metal rather than of rock and ice. Deep within rocky, terrestrial planets – including Earth – scientists infer the presence of metallic cores, but these lie unreachable below planets’ rocky mantles and crusts. Because scientists cannot see or measure Earth’s core directly, Psyche offers a unique window into the violent history of collisions and accretion that created terrestrial planets.

Konstantinos Sakkas