Today’s post is part of a special series that first appeared on Planet Pailly. Every Thursday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s scientific term is:


Previously on Sciency Words, we examined carbonaceous (C-type) and siliceous (S-type) asteroids, the most common and second most common asteroid types, respectively. We now come to the third most common type. Unfortunately, this is where nomenclature gets complicated.

Astronomers currently use at least two different systems to identify and categorize asteroids: the Tholen classification system and the SMASS classification system. The two systems overlap in some ways and diverge in others. What Tholen calls an M-type asteroid is, in SMASS, mixed into a broader category called the X-group.

I’d guess that eventually one of these classification systems will “win.” Either that or a new system will replace them both, simplifying matters. In the meantime, an M-type asteroid by any other name would smell as sweet.

Characteristics of M-Type Asteroids

The M in M-type stands for metallic. It’s unlikely that an asteroid of pure (or nearly pure) metal could form by itself. Therefore, M-type asteroids are assumed to be the metal cores of larger asteroids or maybe even dwarf planets that, for one reason or another, broke apart.


A typical M-type asteroid could be worth billions upon billions of dollars.

Most M-types seem to be composites of iron and nickel with traces of other metals—including precious metals like rare earths and platinum group metals. You can expect to find these valuable metals in much higher quantities than you would on Earth.

If someone were to capture one of these asteroids and safely bring it back to Earth, that someone would become extremely rich.

Or maybe not…


The Problems with Asteroid Mining

Before you hop into your personal spacecraft and fly out to the asteroid belt hunting for M-types, a few notes of caution:

  • There’s no way to know the exact composition of an M-type asteroid based solely on observations from Earth. There’s no guarantee that you’ll find substantial amounts of valuable metals.
  • M-type asteroids can have wildly different orbits from each other, making some much harder to reach than others. Fuel costs will stack up rapidly, seriously cutting into your profits.
  • M-types are basically solid lumps of metal. You’ll need more than a pickaxe or jackhammer to crack them open. The difficulties associated with mining operations will also seriously cut into your profits.

And yet it’s hard to resist the lure of M-type asteroids. Businesses in existence today (not some far-off Sci-Fi tomorrow) are greedily eyeing M-types, trying to figure out how to go get one. That may soon become a driving force in human space exploration. Or the hunt for M-type asteroids could turn into a huge economic bust.

Just something science fiction writers might want to think about.

Book Recommendation

Asteroid Mining 101: Wealth for the New Space Economy by John S. Lewis. There are plenty of books and articles out there about asteroid mining, but this one takes a serious look at both the pros and cons of the idea. If this is a subject that interests you, Asteroid Mining 101 offers a well-balanced view of the future of the asteroid mining industry.

Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.

Article by James Pailly. Check out James’ blog for more great science articles.

  • Paulo R. Mendes

    Thanks for this awesome article. 🙂

  • Kirov

    Hopefully these will soon be the reason we develop space travel. Perhaps a base on Mars would be more economically feasible if it was used as a staging point for asteroid mining.

  • John H Reiher Jr

    The big issue with M-Type asteroids is that those precious metals may not be differentiated from each other. you’d have to smelt the asteroid and use some method to separate the metals.

    On Earth, we use chemicals to extract any of the platinum group metals into their pure forms. So you just can’t melt the asteroid and hope the good stuff bubbles to the top.

    Also, the amount of rare metals produced from your average M-Typer would crash the market for them, making them nearly worthless. Well, worthless as a commodity, not for what they would be used for in manufacturing.

  • Alex Heartnet

    I fail to see how the problems you listed with asteroid mining will stop an organized group of people if the payout is indeed in the billions. And the second asteroid will be cheaper then the first because you already know how to mine it – less R&D costs.