Several space mining companies plan to mine asteroids, and they want to bring back to Earth either the entire asteroid or just the part that they mined from it. Either way, obtaining resources in space will be revolutionary for exploration and for the advancement of human civilization into space, so bringing back an asteroid for the first time may well be one of those special moments in human history that we never forget.

The two previous blog posts discussed what size asteroid we may want to bring back to the Earth and what kind of location and trajectory it needs to be in for us to affordably bring it back. The next few posts will discuss what class of asteroid we want to bring back, i.e., its composition. Here, we discuss the composition of asteroids in relation to where they formed near the solar system frost line.

The Solar System’s Frost Line

Near Earth Asteroids (NEAs) are mostly from the main asteroid belt, kicked into the inner solar system by gravitational disturbances from Jupiter. As such they have a geochemistry that represents the main asteroid belt,somewhat different than the geochemistry of bodies that formed closer to the sun such as our own dear planet Earth. Closer to the sun it was hotter and so fewer volatiles were there while the planets were forming. A volatile is a substance that evaporates in the hot sunlight and is driven outward by solar pressure to the outer, cooler parts of the solar system. It’s like heating a pot of water on the stove during a cold winter’s night: the water is driven from the pot and condenses onto the cold glass of the windows of your home. The “frost line” of the solar system is a hypothetical circle drawn at a radius from the sun just beyond the asteroid belt, just inside of Jupiter. Beyond that circle it was cold enough for volatiles to condense and to form icy and gaseous bodies, like the water that condenses on your cold windows. That is why Jupiter formed where it did; it is composed of the volatiles driven beyond the frost line. And just inside the frost line is the asteroid belt. It is composed of rocky material (mostly non-volatile) that never successfully merged into a planet, and again the frost line explains why the asteroid belt is where it is. Because the frost line caused Jupiter to form so closely nearby, the huge gravity of Jupiter kept stirring up all the other material near the frost line. That caused the material to slam around too quickly so it couldn’t merge into a new planet. Some pieces did grow big, big enough to be called protoplanets, and one of them grew big enough to be called a dwarf planet (that is Ceres), but no object in that region grew big enough to dominate the orbit. Today, the asteroid belt is a collection of protoplanets and smaller rocks including some busted-up protoplanets, still stirring around. So the frost line is straddled by the biggest planet on one side and a failed planet on the other, and the physics of the frost line explains both.

Volatiles in the Asteroid Belt

Now it’s true that most volatiles condensed beyond the frost line, but it’s too simplistic a story to leave it at that. Nature is too wonderfully messy for simplistic explanations. Because lots of protoplanets were forming just inside the frost line where it was almost cool enough for volatiles, they provided some shade and some protection beneath their surfaces from the sun so that plenty of volatiles actually did collect in the asteroid belt, too. And that’s why the chemistry of the asteroids is so different than the chemistry of Earth. The Earth is just silicate materials and metals, plus relatively tiny amounts of water and other volatiles that were brought back into the inner solar system at a much later date by the comets and asteroids. The asteroids contain silicates and metals, like the Earth, but they also contain huge stores of water-bearing minerals and carbon compounds and other volatiles. And that’s why we want to mine them! They’ve got lots of the stuff we need.

Now again, just saying the asteroids have all those kinds of materials is too simplistic a story for this wonderfully messy universe. Not all the asteroids have all these different materials in them. The next post will explain why and will discuss the different types of asteroids that formed.