[Editor’s Note: This article is the second 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. In the tale, Ceres is the home of a major station. Read Part I by pointing your browser here.]
In our previous issue, we explored the asteroid belt, a circumstellar disc of millions of rocks that exists between Mars and Jupiter.
Unsurprisingly, the first body discovered in the band is also the largest. Located in 1801 by a Catholic Priest from Sicily, Giuseppe Piazzi, Ceres takes its name from the Roman goddess of agriculture. The deity’s home on Earth and the first temple dedicated to her lay in Sicily. Piazzi originally thought he had spied a comet, but noted, “since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet.” Indeed, he had located something new.
A quick aside for the mathematical historians out there: when Piazzi announced his findings, the orbit of Ceres had taken it too close to the sun for other astronomers to verify. Mathematical giant Carl Friedrich Gaus – 24 years old at the time – developed a method for efficient determination of an orbit, allowing scientists to spot Ceres in the right place when it reappeared at the end of 1801. Gauss’ method was born.
What was Ceres, though? Was it a comet? Or, as we discovered in the previous examination of the belt, was Ceres the missing planet that would fit the mathematical models, which predicted one to exist between Mars and Jupiter?
The early consensus landed on a planet. Ceres did not match the characteristics of comets and, at the time, other classifications did not really exist. Just a year later, though, another mass in the asteroid belt – Pallas – appeared to astronomers. Clearly, a single planet did not orbit the sun at this distance. Other discoveries followed, which prompted William Herschel to coin the term “asteroid” to describe these new types of rocks.
But that wasn’t the last reclassification of Ceres. Today, we consider it a dwarf planet. Like planets, Ceres is massive enough for its gravity to give it a nearly round composition. However, it is not large enough to have cleared the space around its orbit of other bodies. Ceres joined the stable of dwarf planets, those objects that meet the first designation but not the second. This declaration by the International Astronomical Union is the same that demoted Pluto from planet to dwarf planet. Ceres is the only dwarf planet that resides inside the orbit of Neptune.
With this new designation, is Ceres still also considered an asteroid? Confusion seems to reign. The IAU states dwarf planets can have dual classifications and remains an asteroid. NASA, on the other hand, declares Vesta is the largest asteroid in the belt, inferring they have stripped Ceres of the title.
On average, Ceres sits 2.8 Astronomical Units from the sun, meaning it’s approximately 2.8 times as far from the sun as the Earth is. The dwarf planet features a radius of 296 miles, which is about 1/13th that of our planet. A diameter of just under 600 miles means the width of Ceres’ sphere would stretch from New York City to just past Columbus. The mass of the body is, by cosmic standards, minute, just 1.3% the mass of our moon. The gravity of Ceres clocks in at 0.029 g, or approximately 3% of that of Earth.
The stats attached to Ceres demonstrate the scale of the universe in two seemingly disparate ways. The dwarf planet is so small that we cannot see it with the naked eye, despite its relative closeness to us; even the world’s best telescopes can barely see the surface features of Ceres. Yet, of the millions of objects that comprise the asteroid belt, Ceres contains about 25% of the entire region’s mass. Further, it’s 3.5 times more massive than the second-largest object, Vesta. Our solar system is a gigantic place, filled with extremely tiny entities. That system is just a speck within the Milky Way, which is even a tinier member of the universe.
As the largest member of the belt, we prioritized the exploration of Ceres. Our telescopes couldn’t return wonderful data, so NASA sent a probe toward the belt.
Dawn left Earth on 27 September 2007 and reached Ceres in March 2015, after stopping by Vesta on the way. The spacecraft was the first to use ion propulsion, which produces thrust by accelerating ions using electricity. Dawn zoomed to within 22 miles of Ceres, producing some incredible and gorgeous imagery.
Ceres has a mountain!
The photo above is Ahuna Mons, which scientists believe to be a cryovolcano. Instead of magma, cryovolcanoes spew water, methane, ammonia, or other volatile materials. Ahuna Mons is three miles high on its steepest side! Ceres is the closest body with a cryovolcano to the sun.
Further, the probe discovered the surface of the dwarf planet is a mix of ice and minerals. Even more incredible, Ceres features a transient atmosphere of water vapor. After our home planet, Ceres has the most water of any mass in the inner solar system.
These geological attributes prompt the question: could microbial life exist or develop on Ceres?
Ceres is a fascinating place. It exists in a unique location in the solar system and it straddles the margins of astronomical definitions. The dwarf planet is active geologically and contains water. Ceres begs to be explored more.
The European Space Agency and China have both proposed missions during the 2020s to land on Ceres and bring samples back to Earth. Of particular interest are bright spots that appear in some of the large craters there. These spots are known as faculae. If future missions will produce imagery as glorious as those from Dawn, they can’t come fast enough.
Perhaps, centuries from now, the characteristics of Ceres might make it the perfect spot for a human base, a launching pad to the outer reaches of the solar system.
Further Reading and Exploration
Ceres – NASA
Ceres: The Smallest and Closest Dwarf Planet – Space
NASA’s Dawn mission – Official Website
How Johann Carl Friedrich Gauss rediscovered a lost dwarf planet – Vox