Particles As Big as Softballs Ejected From Asteroid Bennu Seem to Do the Impossible – Now Astrophysicists Know Why

Asteroid Bennu Particles

OSIRIS-REx observed small bits of material leaping off the surface of the asteroid Bennu on January 19, 2019. (Credit: NASA/Goddard/University of Arizona/Lockheed Martin)

In January 2019, NASA’s OSIRIS-REx spacecraft was orbiting the asteroid Bennu when the spacecraft’s cameras caught something unexpected: Thousands of tiny bits of material, some just the size of marbles, began to bounce off the surface of the asteroid—like a game of ping-pong in space. Since then, many more such particle ejection events have been observed at Bennu’s surface.

OSIRIS-REx is an unprecedented effort to investigate what makes up asteroids like Bennu and how they move through space. But, as those leaping particles show, the mission has already delivered a few surprises.

“We’ve been studying asteroids for a long time and no one had ever seen this phenomenon before—these little particles getting shot off of the surface,” said Daniel Scheeres, distinguished professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences. He leads the radio science team for OSIRIS-REx along with CU Boulder’s Jay McMahon.

Now, a series of new studies seeks to recreate and understand the observed particle ejection events, piecing together what happened and why. Scheeres and McMahon are focusing on one question in particular: How might the leaping particles change the long-term fate of Bennu and other asteroids like it?

In research published in the Journal of Geophysical Research: Planets, the duo and their colleagues report that such seemingly small occurrences may add up over time—perhaps even helping to give the asteroid its telltale shape, which is often compared to a spinning top.

OSIRIS REx Spacecraft Discovers Water on Asteroid Bennu

This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on December 2, 2018, by the OSIRIS-REx spacecraft from a range of 15 miles (24 km). Credits: NASA/Goddard/University of Arizona

“We want to know what that means for the bigger picture of how asteroids live their lives,” said McMahon, an assistant professor of aerospace engineering.

The University of Arizona leads science operations for OSIRIS-REx, which was built by the Colorado-based Lockheed Martin. NASA’s Goddard Space Flight Center in Maryland manages the overall mission.

Mass loss

McMahon added that the life of some asteroids can be pretty chaotic. One class of these bodies, which scientists call “active” asteroids, loses a significant amount of material on an ongoing basis. 

“They’re almost a cross between a comet and an asteroid,” McMahon said. “They’re losing mass, and it’s substantial enough that we can see it from Earth.”

Until recently, no one knew that the same thing could happen on a much smaller scale. But that’s precisely the case on Bennu. One hypothesis suggests that rapid shifts in temperature could be causing the surface of the asteroid to warp and crack, popping off small bits of material. Another study has contended that the ejections could be the result of small meteoroids smacking into Bennu. 

OSIRIS-REx Spacecraft Descending Towards Asteroid Bennu

This illustration shows NASA’s OSIRIS-REx spacecraft descending towards asteroid Bennu to collect a sample of the asteroid’s surface. Credit: NASA/Goddard/University of Arizona

Based on OSIRIS-REx’s observations, the particles ejected from Bennu can be as big as softballs and hit speeds of about 7 miles an hour. Even more surprising, McMahon said, a small number of these bits of debris seemed to do the impossible: They flew off the surface of Bennu, then orbited the asteroid for several days or longer. 

“That shouldn’t happen in typical orbital mechanics,” McMahon said.

Put differently, basic orbital calculations suggest that all of these particles should do one of two things: Jump off the surface and fall right back down or escape from Bennu’s gravity and never come back.

Close misses

To find out why some aren’t playing by the rules, McMahon and his colleagues used detailed computer models to track the trajectories of more than 17,000 test particles ejected from Bennu. They discovered a small subset of those seem to get an assist from an unlikely source: the sun.

McMahon explained that as these objects leap off the asteroid, they are exposed to heat and radiation coming from the sun and from Bennu itself—just a little bit, but enough to occasionally give them a slight boost in speed. With the right push, those particles can, essentially, fail at falling.

“The particle gets really close to the surface and just misses,” McMahon said. “If it can do that a few times then it can get into a situation where it can live in orbit for quite a while.”

OSIRIS REx Sample Bennu Surface

An artist’s depiction of OSIRIS-REx using its extendable arm to collect a sample of material from the surface of Bennu. Credit: NASA/Goddard/University of Arizona

In another study published in the same series, a team led by Scheeres and McMahon tried to figure out if ejection events might even influence Bennu’s own orbit around the sun—the answer is probably not.

The group did discover something else unusual: When particles eventually land on Bennu’s surface, many appear to disproportionately fall near its equator where the asteroid has a distinct bulge. As a result, these events could be reshaping the asteroid over thousands or millions of years by moving mass from its north and south to its middle.

The findings are a prelude to another major event in the life of Bennu. Next month, OSIRIS-REx will get closer to the asteroid than ever before. Once there, the spacecraft will use a retractable arm to grab a sample from the surface and bring it back home.

Scheeres and colleagues expect even more unexpected findings from an already surprising asteroid.

Reference: “Dynamical Evolution of Simulated Particles Ejected From Asteroid Bennu” by Jay W. McMahon, Daniel J. Scheeres, Steven R. Chesley, Andrew French, Daniel Brack, Davide Farnocchia, Yu Takahashi, Benjamin Rozitis, Pasquale Tricarico, Erwan Mazarico, Beau Bierhaus, Joshua P. Emery, Carl W. Hergenrother and Dante S. Lauretta, 18 May 2020, JGR Planets.
DOI: 10.1029/2019JE006229

Coauthors on the new study include researchers from the Jet Propulsion Laboratory, Planetary Science Institute, NASA Goddard Space Flight Center, Lockheed Martin, University of Arizona, The Open University and University of Tennessee. 

15 Comments on "Particles As Big as Softballs Ejected From Asteroid Bennu Seem to Do the Impossible – Now Astrophysicists Know Why"

  1. Hmmm, i remember something similar happening on our moon
    Regolith particles spontaneously jump off the surface, but
    come back down shortly
    due to the moons stronger gravity
    These bodies are exceptionally dry, and
    theres very little atmos

    It turns out that
    due to the suns radiation and the solar wind,
    the particles acquire an electrostatic charge
    of the same polarity as the parent body and
    are repelled
    Thus, they launch off of the surface
    I suspect that this may also be happening on Bennu

  2. all this money they spend and my iphone can take a better picture. come back when you have a color picture that looks like something. how is this science in 2020? are you still using polaroids or something?

    • To duh,
      If this is some sort of attempt at sarcasm, you have failed.
      Surely you know (actually, it seems like you don’t know) that taking a picture from a couple dozen feet with the latest iPhone is totally different from a spacecraft taking pictures thousands of miles away with little to no ambient light available at its target.

      You can’t possibly be serious, ignoramus!

  3. Geoffrey Bailey | September 9, 2020 at 9:13 pm | Reply

    Perhaps the asteroid has a big magnet in it.

  4. Donald P Schaefer | September 10, 2020 at 6:36 am | Reply

    Dear “duh” The Osirus space craft was launched over 4 years ago, which means that it was designed and built at least a few years before that. Why don’t you try to take a picture with your “I” phone of something over 15 miles away and then transmit that image thru the debris of space 179,000,000 miles and see what kind of image you are left with. I think that this pic and these images are pretty incredible.

  5. i am curious why anything with mass can’t have some form of gravity?

    • Torbjörn Larsson | September 10, 2020 at 2:03 pm | Reply

      There is only one form of gravity – I think you mean some strength of gravity – and it has (at a very low strength): “Put differently, basic orbital calculations suggest that all of these particles should do one of two things: Jump off the surface and fall right back down or escape from Bennu’s gravity and never come back.”

  6. Bennu is reacting to changing voltage in the solar wind. Since it’s maintaining such a weak gravitational field, the particles aren’t having much difficulty jumping off the surface, but once they are immediately free of the surface, they’re going into a little orbit around Bennu, which is pretty much what a cometary coma is, anyway.
    Gravity does NOT rule in the stellar system–voltage differential does. Comets, asteroids, planets, and our star itself are all electrically charged bodies, and they behave accordingly.
    Seriously folks, it’s about time that people come to understand electrodynamics. It is fundamental, and foundational.
    Here’s a link to a video called “Symbols of an Alien Sky: The Electric Comet”. Don’t worry, it’s not about extraterrestrials or UFO’s…they’re not talking about ‘alien’ in that meaning.

    • Torbjörn Larsson | September 10, 2020 at 2:08 pm | Reply

      The solar wind has of course no (tangible, local) form of voltage. Gravity rules the solar system as it rules the universe [ ].

      The rest of the comment is just laughable superstition. Don’t worry, nobody watches such videos anyway. 👹

    • Torbjörn Larsson | September 10, 2020 at 2:20 pm | Reply

      In fact, it is relativity dissipation processes and not those of newtonian gravity – as one would naively think – that we have to thank our survival for [ ]. “The main surprise that comes from the numerical simulations of the recent years is that the probability for this catastrophic events to occur is relatively high, of the order of 1%, and thus not just a mathematical curiosity with extremely low probability values. At the same time, 99% of the trajectories will behave in a similar way as in the recent past millions of years, which is coherent with our common understanding that the Solar System has not much evolved in the past 4 Gyr. What is more surprising is that if we consider a pure Newtonian world, starting with the present initial conditions, the probability of collisions within 5 Gyr grows to 60%, which can thus be considered as an additional indirect confirmation of general relativity.”

      Gravity to the rescue! 😍

  7. To grab samples where no robot has grabbed samples before!

  8. How EXACTLY do they get ejected from the surface?
    “rapid shifts in temperature could be causing the surface of the asteroid to warp and crack, popping off small bits of material. ”
    is not enuf !

    • Torbjörn Larsson | September 10, 2020 at 2:12 pm | Reply

      The ejection mechanism seems (from the article, I wasn’t interested enough to read the paper) an open question: ” One hypothesis suggests that rapid shifts in temperature could be causing the surface of the asteroid to warp and crack, popping off small bits of material. Another study has contended that the ejections could be the result of small meteoroids smacking into Bennu.” Dunno how detailed those hypotheses are, such as asking for the stored and released amount of energy in the solar heating scenario.

      As they say, watch this space [science sites] for updates! (Meanwhile, I would place money on the well known impactor process. But that’s me.)

  9. Aren’t nearly all asteroids known to have gasses trapped within them? If so wouldn’t it be likely that these gasses are building pressure when submitted to “rapid shifts in temperature”

    • Torbjörn Larsson | September 10, 2020 at 2:16 pm | Reply

      I was initially thinking of that too! And there are examples of asteroids that literary behaves like the tail end of the comet distribution.

      But in general asteroids are pretty sun baked. Especially if they are Earth orbit crossing like Bennu in the video (which near distance at times figures, as they sent a probe there). So I dunno how likely that scenario is.

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