An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid’s surface composition.

The three broad composition classes of asteroids are C-, S-, and M-types.

The C-type (chondrite) asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system.

The S-types (“stony”) are made up of silicate materials and nickel-iron.

The M-types are metallic (nickel-iron). The asteroids’ compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.


Sampling the Gravel

Yesterday, four years after launching from Earth, NASA’s Osiris-Rex made a historic and brief landing on potentially hazardous asteroid Bennu, over 200 million miles away.

The spacecraft traveled all that way to perform a short touch-and-go maneuver with the goal of collecting a sample from the asteroid’s surface and is transporting it back to Earth for study.

It’s about the size of the Empire State Building

Bennu is classified as a B-type asteroid, which means it contains a lot of carbon in and along with its various minerals. Bennu’s carbon content creates a surface on the asteroid that reflects about four percent of the light that hits it — and that’s not a lot. For contrast, the solar system’s brightest planet, Venus, reflects around 65 percent of incoming sunlight, and Earth reflects about 30 percent. Bennu is a carbonaceous asteroid that hasn’t undergone drastic, composition-altering change, meaning that on and below its deeper-than-pitch-black surface are chemicals and rocks from the birth of the solar system.

There’s no guarantee Osiris-Rex has collected a significant sample. As the spacecraft approached and then spent two years orbiting and surveying Bennu, it became clear this tiny world is different than what scientists expected. The team hoped to find a number of sandy surfaces ideal for sampling, but it turns out Bennu is a rubble pile, with a rugged terrain strewn with boulders.

Should the mission gather up a sample, it will begin a long journey back to Earth, with a planned landing in the Utah desert in September, 2023.


Then there’s the mission to Psyche

The Psyche mission is a journey to a unique metal asteroid orbiting the Sun between Mars and Jupiter. Psyche appears to be the exposed nickel-iron core of an early planet, one of the building blocks of our solar system.

Deep within rocky, terrestrial planets – including Earth – scientists infer the presence of metallic cores, but these lie unreachably far below the planets’ rocky mantles and crusts. Because we cannot see or measure Earth’s core directly, Psyche offers a unique window into the violent history of collisions and accretion that created terrestrial planets. The launch is scheduled for 2022.


  1. Like many people, I have a few pieces of meteorites. They were probably asteroids one day. One is chunk of a meteorite that’s obviously iron and other metals. I have a few slices of a pallasite. Those polish to shiny white metal, with embedded stones of olivine, like peridot. All of them are just interesting pebbles to look at.

    When I saw the first pictures from Bennu, I wondered if it was in the size range where it just big enough to be held together by gravity, but not big enough to compress things and get a spherical shape. Sort of just a conglomeration of all the crap it went through in its orbit, that didn’t bounce hard enough to escape the little gravity. Looking at the surface pictures, it looks like a cosmic junk pile, just full of dirt, small rocks and other junk. Just like that idea.

    This is the first I’ve read about Psyche. That should be an interesting one to see.

    • I what a nuke at the center of Bennu would do to the gravel planetoid?

      Yes, the Psyche mission could end up being very exciting.

  2. It will be interesting to see if the dreams of sci-fi writers will come true.

    Back in the way-back-when, even Asimov wrote about going to the asteroids to mine water asteroids. Actually, putting rocket engines on a big ice asteroid and using it as a ship and as reaction mass to push the whole thing to Mars.

    Then some smart alec thinkers have postulated ‘bombing’ Mars with ice asteroids (hitting in unoccupied and boring areas) to stimulate atmosphere development.

  3. I cannot to comprehend the physics involved to orbit a probe around an asteroid.

    When magma is ejected into the Earth’s atmosphere, the falling bits will assume an aerodynamic form. My brother has a very good sample of such lava bombs. It is very stream-lined, looking like something designed at Wright-Pat wind tunnel.

    Question: In absence of an atmosphere, would ejecta (say, resulting from a strike on an asteroid) take a similar form? Given the ejecta would be at high temperature and subject to various strong forces.

    If such ejecta were to begin rotation, the form would tend to form a spiral (developed longitudinal axis) around the center of mass. Or so I think.

      • What if it was a water asteroid – 100% water. Close enough to a star that ice would not form? Could it happen? And one must assume that it would be a sphere.

  4. “The spacecraft traveled all that way to perform a short touch-and-go maneuver with the goal of collecting a sample…”

    For one brief instant, Osiris-Rex’s velocity relative to Bennu was zero. I would call that a full-stop landing.

  5. Ah, Cupid and Psyche! About time we had some Apuleius on this site.

    But yes, the asteroids are an issue. Fully expecting a strike on Nov 4.


  6. Asteroid – 2020 isn’t over yet…a hit from some space rock would be par for the course, better if it hit dead center on some Antifa/BLM “peaceful protest”. (Yes, not very Christian sounding, but at this point I give no quarter to anarchists.)

    • Since it won’t be back in 2020 — maybe? We really don’t need that in President Trump’s Second Term.

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