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A gravel pile floating in space that might hit the Earth

flat rock near Osprey
Click for full image.

Come October the probe OSIRIS-REx will attempt a quick touchdown on the asteroid Bennu to grab some tiny particles, all smaller than 0.8 inches across.

Bennu is what scientists have described as a “rubble-pile” asteroid. They use that name because it is simply a conglomeration of a lot of rocks, pebbles, boulders, and stones, all jagged and of all sizes. The overall gravity has never been strong enough to squeeze them together, at least as far as we can see, and so they are piled up loosely across the asteroid’s surface wherever we look.

I think a better name for this asteroid would a floating gravel pile, since the material on it, as clearly shown in the image to the right (reduced and rotated to post here), more resembles the tailings one finds at a mine or quarry. This photo was taken by OSIRIS-REx on May 26, 2020 during its first dress rehearsal over its back-up touch-and-go sample grab site, Osprey. As the release caption notes,

The field of view is 12 ft (3.8 m). For reference, the bright rock [near] the tip of the boulder is 1 ft (0.3 m) across, which is about the size of a loaf of bread.

I have rotated the image 90 degrees so that east is up, because the full mosaic of the entire Osprey landing site, shown below, is oriented that way, and by rotating it to match it is easier to locate this image within it.

Osprey with flat rock inset indicated

The box to the upper right of Osprey shows this flat rock. As you can see, the OSIRIS-REx engineering team does not plan to put the spacecraft anywhere near this flat rock when they attempt their sample grab.

Nonetheless, this image illustrates the gravelly nature of Bennu, and the difficulties these engineers face in successfully grabbing a sample. They need particles smaller than three-quarters of an inch. This image helps give us some scale. As they note, the bright white rock is about a foot across. If you look in Osprey itself you will see numerous rocks of similar size. Even though it is an area of smaller pebbles and stones with more material in the tiny size range required for the sample grab, the site is still filled many rocks far larger.

Osprey landing site up close
Click for full resolution.

The photo to the right is taken from the full resolution mosaic of Osprey [a very large file], and focuses specifically on the Osprey back-up site.

Many of the rocks in this close-up can easily be seen in the mosaic above, and are clearly the same size as that white rock in the first image. Thus, Osprey is filled with many foot-sized pebbles, each one of which could seriously damage OSIRIS-REx should they jump from the surface during contact and hit the spacecraft.

The image however tells us a lot more about these gravel pile asteroids. They truly are like the tailings from mines, lots of stones barely held together by the asteroid’s truly tiny gravitational field.

Bennu is considered a potentially dangerous asteroid. Its orbit is such at there is a very tiny chance (less than 1 in 2,700) that it will hit the Earth late in the next century. What OSIRIS-REx has shown us, however, is that though the asteroid is 1,600 feet across with a mass of about 85 million tons, if it should cross paths with the Earth a large percentage of it, possibly almost all, will break apart and burn up in the atmosphere before hitting the ground.

At the same time, we know as yet little about the asteroid’s interior. While present data suggests the asteroid is 20 to 40 percent empty space, there still could be buried beneath its gravel pile surface much larger structurally sound pieces that could barrel their way through the atmosphere and smash into the ground.

To find out, we need to learn how to safely and accurately map its interior. Only then will we know if Bennu is truly a threat, or simply a vehicle for providing some future generation on Earth a truly spectacular fireworks show.


Conscious Choice cover

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  • David M. Cook

    Robert, wouldn‘t it cause a huge disturbance just by slamming into the air? Look at that Russian air-burst a few years ago; lots of damage, but the rock was tiny compared to Bennu.

  • David M. Cook: Your question is a good one, but I think the situation is different then the Chelyabinsk impact. In that case the asteroid was believed to have come from the asteroid belt (it was not a Near Earth asteroid) and thus was likely traveling much faster relative to the Earth than Bennu would be.

    Nonetheless, the size difference is significant. The atmospheric shock wave from Bennu could be a big concern. I just don’t have the math skills to calculate it.

  • Col Beausabre

    Wouldn’t Bennu have broken up before entering the atmosphere. We wouldn’t have a massive object but a bunch of small ones. BTW, Bennu illustrates how weak gravity is. it’s 85M tons and yet its gravitational field is so weak, it barely holds together. Maybe it should be described as a pile of rocks flying in very close formation instead of a discrete object

  • dave b

    Mass times earth escape velocity squared is a minimum estimate of energy released:
    85e6 Kg * (11 km/sec)^2 = 2.6e19 joule or about 6400 megaton TNT
    or 400000 Hiroshima (@ 15kT)
    or over 300 times the solar radiant energy on a square 1000 km on a side, for 1 minute (@ 1361 watt/m^2)

    Bad News

  • Shannon Love

    A disintegrating Bennu would be a significant threat to the space based infrastructure we will have by “late next century.”

  • Richard McEnroe

    Nature’s sawed-off shotgun, man!

  • dave b

    OOPS high by a factor of 2 (should have been 1/2*m*v^2 not m*v^2)
    only 3200 megaton TNT, 200000 Hiroshima, 150 times solar input)

  • GWB

    we know as yet little about the asteroid’s interior
    The interior is a dormant space probe, waiting for the right conditions to awaken and signal its forces to invade. The rubble surface is simply accumulated over its long presence in our system, waiting. Waiting….

    (Don’t you read science fiction? ;) )

    Col Beausabre
    June 20, 2020 at 1:59 am

    Wouldn’t Bennu have broken up before entering the atmosphere.
    Seems to me this would be a perfect candidate for tossing a missile at it and breaking it up long before it ever enters near-Earth space.

    Or for mining. Break it up by scooping up big loads of it and depositing them on the dark side of the moon. Use for building a base and extraction (if there’s anything valuable).

  • Larry

    Given that Bennu is a gravel pile loosely held together by its self gravity, the Roche Limit for it approaching Earth is going to be around 34,000 km. At which point the tidal shear will disassociate it. Some of the pieces might be big enough for something to reach the ground, but most will vaporize from hitting the atmosphere.

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