Cannibal galaxies and asteroid mining

Our galaxy is a cannibal, and we have quite an appetite for resources in our own little corner of the Milky Way, too. That’s what we learned at the latest Astronomy On Tap event in Seattle, held last week at Bad Jimmy’s Brewing Company in Ballard.

John Lurie talked about the cannibal
Milky Way galaxy at Astronomy on
Tap Seattle. Photo: Greg Scheiderer.

John Lurie, a graduate student in astronomy at the University of Washington who studies the structure of the Milky Way, started his talk with a bit of history. For millennia, up until recently when light pollution made the Milky Way invisible to a great many of us, people saw it and made up stories about what it was. To Lurie’s mind, some of the violent images of Greek mythology seem fitting.

“Our Galaxy is actually a cannibal, and it likes to eat other galaxies,” he said. “Not only that, but the entrails of its victims are strewn across the heavens.”

We didn’t know much at all about the Milky Way until Galileo pointed his telescope at it four centuries ago and wrote down that he saw individual stars.

“Up until the beginning of the 20th century that was basically it,” Lurie said. “The entire universe, as far as we knew, was contained in the Milky Way.”

New learning

Fast forward to Edwin Hubble, who used a much larger telescope, the 100-inch at Mt. Wilson, to look at cepheid variables. Hubble calculated that what was then known as the “Andromeda nebula” was about 2.5 million light years distant—way too far away to be part of our galaxy. It was another galaxy.

If this ball were the Sun, the next nearest star would be in
New York. Photo: Greg Scheiderer.

Galaxies seem awfully far-flung to be cannibalizing each other, but Lurie explained that they’re actually relatively close together. He noted that if the Sun were a yellow ball a bit smaller than a pint beer glass (an apt analogy given the locale of the talk) our next nearest stellar neighbor would be in New York. However, if the disk of the Milky Way galaxy were represented by a frisbee, the next nearest major galaxy would be inside Bad Jimmy’s, a mere 20 feet away. In addition, between us and Andromeda are a number of dwarf galaxies. Astronomers have found streams of stars that are evidence that the Milky way has collided with one of them, the Sagitarius dwarf galaxy.

“That’s why I claim that our galaxy is actually a cannibal,” Lurie said. “It’s in the process of eating this galaxy. Gravitational tidal forces of the Milky Way are tearing the stars off of this dwarf galaxy and they’re being strewn out into space.”

Bigger fish

Lurie says that when it comes to cannibal galaxies there’s always someone bigger out there.
“The Andromeda galaxy is coming to get us,” he said. “It’s a little bit bigger than us, and we’re on a collision course.”

Not to worry. It won’t happen for another four billion years or so, and since individual stars are so spread out, the likelihood that two would collide is pretty small. Some stars could get flung out of the galaxy, but mostly the Milky Way and Andromeda will eventually coalesce into one big galaxy.

Mining asteroids

Matt Beasley of Planetary Resources
explained the best types of asteroids
for mining useful materials.
Photo: Greg Scheiderer.

Closer to home folks are thinking of mining nearby asteroids for the valuable materials they contain. Dr. Matthew Beasley, a senior engineer at Redmond-based Planetary Resources, gave a talk titled, “Resources on Asteroids: What’s There, How Much, and Why?”

Beasley noted that there are 872 known asteroids of about one-kilometer orbiting in near-Earth space, and perhaps as many as 20,000 smaller ones down to about 100 meters. That’s a lot of potential targets for asteroid mining.

Why go to the trouble?

“Asteroids are extremely rich in useful materials,” Beasley said.
There are three main types of asteroids. Beasley explained that the first ones Planetary Resources will target are C-type carbonaceous asteroids. These make up about 75 percent of all asteroids, but only about six percent of the known near-Earth asteroids. They’re hard to spot because they’re so dark in color, like a lump of black clay. C-type asteroids are around 20 percent water by mass, and that’s what makes them appealing. Water is handy for space explorers to drink, and it can be broken down into hydrogen and oxygen for spacecraft fuel.

“One 75-meter C-type asteroid full of water could have fueled all of the shuttle missions,” Beasley noted. It will cost a lot less to pick up water and fuel in space than it does to launch them into space from Earth.

The second target type of asteroid is the M-type, which is heavily metallic. M-type asteroids contain virtually no water, but are rich in metals such as nickel, iron, and platinum, and maybe some silicates.

“One 500-meter metallic contains more platinum than has ever been mined by humanity,” Beasley said, adding that all of the platinum on Earth probably got here through collisions with asteroids. Platinum-group metals are highly sought after for electronics and other manufacturing, and all of the metals could be useful for building things in space. As with the water, it’s a lot less expensive to find it out there than it is to take it with you.

A third common asteroid is the stony S-type. These contain no water, some metals, but basically are between 75 and 90 percent silicates.

“They’re a little light on volatiles and organics, lots of rock,” Beasley said, noting there’s little interest in this type of asteroid. “Basically, they’re fill dirt.”