The Belt


[Editor’s Note: This article is the first in a series exploring real locations featured in a novel and television series, called The Expanse. In this sci-fi epic set in the 2300s, Earth and Mars have become independent “nations,” while millions of humans inhabit the asteroid belt, where they mine lucrative resources for the Inner Planets.]


Of all the sumptuous centuries in human history, we can thank the 16th for the advent of modern astronomy. The Father of Observational Astronomy, Galileo, popularized the telescope, which fundamentally altered how we view the universe. Nicolaus Copernicus finally put the sun at the galaxy’s center, instead of the Earth. By the end of the century, Johannes Kepler had taken incredible information gathered by Tycho Brahe to map the movement of the known planets. Kepler’s laws of planetary motion keyed us into how the orbs of the solar system revolved around our star.

While analyzing Brahe’s data, Kepler predicted a planet would exist between Mars and Jupiter. Based on his calculations, too much space existed between the two planets to fit how his model explained the system.

Almost two centuries later, Johann Daniel Titus noticed something incredible. Take the series that begins 0,3,6, 12 – i.e. double the previous value – and then add 4. Divide by 10. These values oddly matched the orbits of the known planets in terms of Astronomical Units, the distance from the Earth to the Sun. In 1781, William Herschel discovered Uranus. Its orbit matched the next number in the series. Only one problem existed with the theory: it predicted a planet between Mars and Jupiter, too.

How could we find Uranus but not spy a planet much closer? Astronomers went on the hunt. At the beginning of the 19th century, skywatchers discovered two tiny bodies in the region of the missing planet. They were much tinier than all the other planets. Herschel proposed they be called asteroids, from the Greek asteroeides, which means “star-like.” From there, new discoveries kept coming. And coming. Because there were no normally sized planets in the space, a multitude of small rocks filled a band where the models predicted an orbit. The mystery had been solved; we had found the asteroid belt.

Portrait of Johannes Kepler by August Köhler

Sitting exactly in the space predicted by Kepler and Titus, the asteroid belt is a torus-shaped grouping of millions of individual space rocks. The masses in the belt vary incredibly in size. The largest body in the circumstellar disc is Ceres at 950 kilometers in diameter, which classifies it as a dwarf planet. On the other end of the spectrum, the smallest pieces are the size of dust particles.

Despite the huge number of distinct rocks in the belt, most of the space it occupies is empty. The four largest asteroids – Ceres, Vesta, Pallas, and Hygiea – comprise about half of the total mass of the whole. That total mass is just 4% of the mass of the Moon. When we started to explore the outer regions of the solar system, the sheer number of bodies worried scientists. Would probes run into asteroids? Would the number of minuscule particles damage hardware during the flight?

In 1972, Pioneer 10 traversed the belt on its way to Jupiter without incident. To date, 12 spacecraft have braved the asteroid fields and not a single one took a hit. Collisions do happen in the belt, though.

The asteroids in the belt (white) - graphic by Mdf

The bodies of the belt sometimes slam into each other. Occasionally, this interaction forms asteroid families, which are groupings that share orbital and compositional characteristics. Other times, the collisions send pieces of the belt to Earth. We have discovered approximately 50,000 meteorites on our planet. Scientists believe 99.8% of those originated in the asteroid belt!

When a solar system is born, dust and rock typically swim around its sphere of influence. A star’s gravity slowly starts to coalesce these masses into larger pieces. Planetesimals and protoplanets eventually become full-fledged orbs. The rocks of the asteroid belt might have once become a planet, but Jupiter’s gravity wreaked too much havoc. The largest planet in our system gave them too much energy to accrete. In the early years of the solar system, these interactions caused more collisions than currently occur, causing many asteroids to shoot off the orbit of the belt. Astronomers believe 99.9% of the mass originally contained in the belt is now gone. That loss of mass doomed the belt from ever becoming larger bodies.

951 Gaspra, the first asteroid imaged by a spacecraft, in 1991 - NASA
The asteroid Vesta imaged in 2011 and 2012 - NASA

Many space operas foresee the days when we’re able to mine the vast resources of the asteroid belt. While that reality might be closer to The Expanse than the 21st century, it’s not necessarily a far-fetched prediction.

NASA proposed a mission to reach an asteroid named 16 Psyche. They estimate a launch in either 2023 or 2024, with the probe reaching Psyche by 2030. Scientists estimate this single asteroid might contain $10 Quintillion in metallic ore.

With millions of rocks floating around, the potential boon is mind-boggling. The future land grab won’t be on Earth. What could possibly go wrong?

Thankfully, right now, we’re focused on scientific discovery. And the photographs are rather striking, too!

Further Reading and Exploration

Asteroids – NASA

Asteroid Belt: Facts & Formation – Space

This Metal-Rich, Potato-Shaped Asteroid Could Be Worth $10 Quintillion – Smithsonian Magazine

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