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Trajectory Watch: Charting Asteroid Bennu’s Future Path

In addition to its sample return mission at asteroid Bennu, OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) has tightened our projections about the object’s future trajectory. Although the impact possibility on Earth through the year 2300 is on the order of 1 in 1750 (0.057%), it’s an object we want to keep an eye on, because in 2135 Bennu will make a close approach to Earth that could nudge its trajectory in ways that are difficult to anticipate.

OSIRIS-REx spent more than two years working near the 500-meter wide asteroid, studying its mass and composition while tracking its spin and orbital trajectory. In terms of the latter, even factors as tiny as the force the spacecraft exerted during its sample collection event in October of 2020, a mere touch-and-go, had to be considered (the study confirms that the effect was negligible). Far more significant is the Yarkovsky effect, which occurs as solar heating eases on the nightside during the asteroid’s rotation and is radiated away as infrared energy, generating thrust.

It’s the tiniest of effects at Bennu, says Steve Chesley (JPL), a co-investigator on the study that has just appeared in Icarus, but of course it builds over time and has consequences for the asteroid’s path:

“The Yarkovsky effect will act on all asteroids of all sizes, and while it has been measured for a small fraction of the asteroid population from afar, OSIRIS-REx gave us the first opportunity to measure it in detail as Bennu traveled around the Sun. The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant when determining Bennu’s future impact chances over the decades and centuries to come.”

Imagine how tricky it is to measure the cumulative effects of the Yarkovsky effect on a rotating object of uneven shape. Three close encounters in 1999, 2005 and 2011 were extensively tracked from the ground, but OSIRIS-REx data from the object itself have now allowed researchers to model an asteroid’s trajectory to the highest level of precision ever. Bennu’s future path is well known up to 2135. The question during the close encounter in that year will be whether it will pass through a dangerous ‘gravitational keyhole,’ an area where, as it responds to the effects of Earth’s gravity, the asteroid could become more likely to present an impact threat in the future. There is one ‘keyhole’ solution that would result in an Earth impact in 2182, with an impact probability of 1 in 2,700 (or about 0.037%).

In addition to the Yarkovsky factor, scientists have to consider the gravitational influence of the Sun and other objects in the Solar System including more than 300 asteroids that could have some effect. The solar wind streaming at variable rates outward from the Sun has to be considered as a source of pressure, and remember that OSIRIS-REx also discovered that Bennu was shedding rock particles, likely the result of thermal fracturing due to heating and cooling during rotation. These events along with the drag caused by interplanetary dust all affect the future trajectory.

Image: This mosaic of Bennu was created using observations made by NASA’s OSIRIS-REx spacecraft that was in close proximity to the asteroid for over two years. Credit: NASA/Goddard/University of Arizona.

The authors note that a close approach to Earth that will occur in 2037 will be the next opportunity to collect radar data and therefore gauge the accuracy of their work while improving the trajectory projections even more. In their conclusion they add this (and note that we are dealing with not one but numerous gravitational keyholes)::

…improved orbital knowledge allowed us to refine the impact hazard assessment, which we extended through 2300. The dense structure of keyholes on the B-plane of the 2135 encounter with Earth (Chesley et al., 2014) made it unlikely to avoid all possible pathways to impact. Still, the uncertainties for the 2135 encounter decreased by a factor of about 20, and so many of the most significant impacts found by Chesley et al. (2014) are now ruled out.

The fact that we have a sample return on the way adds another plus for OSIRIS-REx. Dante Lauretta (University of Arizona) is principal investigator for the mission:

“The spacecraft is now returning home, carrying a precious sample from this fascinating ancient object that will help us better understand not only the history of the solar system but also the role of sunlight in altering Bennu’s orbit since we will measure the asteroid’s thermal properties at unprecedented scales in laboratories on Earth.”

The opportunity given us by OSIRIS-REx to test models and calculate future trajectory probabilities shows what can be done with objects with even a remote chance of striking the planet. The trajectory changes coming in 2135’s close approach will be watched carefully by way of further understanding how to tighten such calculations. By then, in the highly unlikely event of a dangerous deflection toward impact, we can hope to have methods in place to force a further trajectory change of our own.

The paper is Farnocchia et al., “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” published online by Icarus 10 August 2011 (full text).


Comments on this entry are closed.

  • Mike Serfas August 13, 2021, 12:39

    I was just watching the lovely video linked from this article: https://www.universetoday.com/152167/osiris-rex-got-to-know-bennu-really-well-apparently-theres-now-a-1-in-1750-chance-that-itll-hit-earth-by-2300/ The quality of imaging of the asteroid is amazing. But it’s a pity that half the video caters to the fetish of mythological naming rather than discussing what else they found to mine besides the magnetite of the 100m boulder “Roc Saxum”. I would think that jettisoning thousands of tons of refined iron for Earthside pickup at the 2135 flyby should be a quite effective way to modify the course of the rest of the asteroid, but more varied bullion shouldn’t hurt (unless an ingot falls on your head…)

  • Ron S. August 13, 2021, 13:08

    “The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid”

    I really wish they would not try to be so clever in an attempt to make it more understandable. It can do the opposite, as it does here. Is it the weight of a grape on Bennu? On Earth? Haphazardly equating force and weight, without normalizing the two, invites confusion.

    • Mike Serfas August 14, 2021, 11:51

      I found Steven Chesley’s original preprint (pre-mission) at https://arxiv.org/abs/1402.5573 . It gives two models for the Yarkovsky effect, one of which is non-linear and doesn’t seem to give an acceleration. The other says the transverse acceleration is … -4.618E-14 au/d^2 . So first we gotta SI that: … x ( 149597870700 m / 1 au) x ( 1 d / 24 * 60 * 60 s)^2 [works out to x 20.04] = 9.254E-13 m/s^2. Multiply by the mass of Bennu (they use 7.8E+10 kg) for the force involved (7.2E-2 kg m / s^2). In 9.8 m/s^2 (Earth) gravity, that is the weight of 7.4 grams. Now I looked up someone who claimed to be weighing grapes and found them between 5 and 11 grams ( https://weighschool.com/how-much-do-grapes-weigh-conversions-calories/ ), while reference.com says 5 grams. I thought the transverse acceleration is time-averaged, yet I feel at most a grape and a half here. Did I foul up somewhere? So close…

  • charlie August 13, 2021, 13:50

    “Trajectory Watch: Charting Asteroid Bennu’s Future Path”

    Why not find Asteroid Bennu’s Future Path by attaching a radioisotope powered radio to the Asteroid when OSIRIS-REx spacecraft was in close proximity – to chart its path ???

  • Alexander Tolley August 13, 2021, 14:24

    The key number in the abstract is that the Yarkovsky Effect which is dominant results in a reduction of the semi-major axis by 284.6+/-0.2 m/yr, or about its own radius every year. IOW, without the influence of gravitational perturbations, it would take about 72 million years to collide with Earth, IF the orbit was not eccentric and not Earth-crossing.

    Without reading the details of the paper, I assume that the slow reduction in orbital radius increases the number of possible encounters with Earth over the long term until the aphelion reduces to less than 1 AU. All things being equal, that would take a little over 200 my.

    While not as large as the asteroid that caused the KT extinction, an impact would make for a “very bad day” for life on Earth, likely causing extinctions for some genera.

    If that impact could likely happen while our civilization is expanding into space, it would be imperative to deflect it in some way so that an impact is permanently avoided. Humans might be safe as a “multi-planet species” with colonies on the Moon and Mars, and in space, but our cultural heritage could be largely destroyed and our home biosphere damaged irreparably while our civilization exists, effectively destroying our home world as a haven to return to.

  • Geoffrey Hillend August 14, 2021, 15:36

    Alexander Tolley, we have to learn to survive here on Earth and take care of the biosphere because the gravity on the Moon and Mars is too low for healthy colonization and the human race would perish in space. It might be different one million years in the future. We could have interstellar FTL travel and possibly live in space indefinitely when our technology gets advanced enough with artificial gravity wave devices etc.

    • Alexander Tolley August 15, 2021, 10:36

      I am certainly not advocating that we leave Earth entirely, although that might give Earth a bit of a break from humanity. O’Neill-style space habitats are designed for Earth-like conditions for terrestrial life, including humans. At this point, we really don’t know how low a gravity humans can live in, or adapt to, although we will have genetic engineering to help adaptation, and there are various engineering solutions to compensate for reduced gravity.

      It has been suggested (first by E O Wilson, I think) that we need to decolonize half the land area of Earth to allow the biomes and ecosystems to recover. That means both continuing the trend to urbanization, but also ending much of the land use for agriculture and resource extraction. Reducing the population would be a good way to go, and some of the load for resources could be borne by extraction in space, rather than Earth.

      We don’t know what the future holds. Global civilization might collapse anytime in the next few millennia and be incapable of protecting the planet from extinction events due to asteroid impacts. Better to ensure that humans and ecosystems survive in extraterrestrial habitats as a safeguard. We won’t likely be able to save everything, although we might have a shot at saving a decent fraction of species if our technologies are sufficient. This will be very important if it turns out life is rare in the universe and we decide as a species to seed the universe with terrestrial life.

      • Gary Wilson August 16, 2021, 13:29

        Unless we want to be reduced to life in refugia in the near future (the year 2100 and after is looking very ominous to me) we will have to completely change our approach to living here on Earth. Only the blind and the willfully ignorant will not have noticed the changes to the climate in recent years. Heat waves of unprecedented size and intensity (we just had a 3 day period that killed 570 people in B.C.), droughts reducing arable land and food growing capacity, increasingly destructive storms including hurricanes and tornadoes, increasingly intense and frequent forest fires adding hundreds of millions of tons of CO2 to the atmosphere (an estimated 120 million extra tons from Canada alone this year) and on and on. We have left the moderate climate regime behind and entered the human induced and rapidly accelerating extreme climate of high atmospheric CO2. Speculating about leaving the Earth because we are making it uninhabitable would be funny if it wasn’t so tragic (definitely worthy of a Stanislav Lem novel). Without change the future will be bleak indeed.

        • Alex Tolley August 16, 2021, 22:44

          What is depressing is that with the negative impacts of climate change so very obvious to anyone, the resistance to change is quite strong. Nations are playing the same old games with climate as they did in the 1930s with the depression. There is still far too much kicking the can down teh road with imaginary net-zero targets that are clearly based on numbers that don’t add up. I am wondering whether industries that are hiding their carbon footprint by buying “offsets” will be forced to deal with the failure of those offsets as the planets heat up. Fossil fuel companies continue to pretend they are an important part of our energy future when in fact there needs to be a winding down as fast as possible and massive funding of renewables to compensate. COP 26 is already looking like it will be a failure unless some leading nations actually do something more than continue to make promises they cannot and will not keep.

          Years ago, James Lovelock thought we were f**ked due to our poor response. Little seems to have changed.

          “Escaping to space” is the same mentality as buying a survival bunker on an island and hoping to survive the maddened crowds with pitchforks as the world collapses.

  • Alex Tolley August 16, 2021, 11:03

    Grapes come in a variety of sizes from small “champagne” grapes to bloated, tasteless bags of water that are popular in grocery stores. [I have a grapevine in my backyard that produces pea-sized, sweet, white grapes.]

    Like Ron, I find some of these conversions to terrestrial experiences rather pointless and sometimes confusing, but no doubt they will continue to be used.

    • Mike Serfas August 16, 2021, 18:05

      I just found out a nickel is worth 5.000 grams. True, I also found out the metal is worth more than a nickel, suggesting this won’t be a hard-and-fast rule for long, but it MUST be better than using “grapes”.

  • Ljk August 16, 2021, 13:39

    A fascinating and frightening recreation of what happened when that big space rock hit Earth 65 million years ago…


    For those who still might think a cosmic impact wouldn’t be all that bad.

  • ljk February 1, 2022, 13:53


    [Submitted on 25 Jan 2022 (v1), last revised 27 Jan 2022 (this version, v2)]

    Don’t Forget To Look Up

    Philip Lubin, Alexander N. Cohen

    We discuss a hypothetical existential threat from a 10 km diameter comet discovered 6 months prior to impact. We show that an extension of our work on bolide fragmentation using an array of penetrators, but modified with small nuclear explosive devices (NED) in the penetrators, combined with soon-to-be-realized heavy lift launch assets with positive C3 such as NASA SLS or SpaceX Starship (with in-orbit refueling) is sufficient to mitigate this existential threat.

    A threat of this magnitude hitting the Earth at a closing speed of 40 km/s would have an impact energy of roughly 300 Teratons TNT, or about 40 thousand times larger than the current combined nuclear arsenal of the entire world.

    This is similar in energy to the KT extinction event that killed the dinosaurs some 66 million years ago. Such an event, if not mitigated, would be an existential threat to humanity.

    We show that mitigation is conceivable using existing technology, even with the short time scale of 6 months warning, but that the efficient coupling of the NED energy is critical.

    Comments: 15 pages, 8 figures

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Physics and Society (physics.soc-ph); Space Physics (physics.space-ph)

    Cite as: arXiv:2201.10663 [astro-ph.EP]
    (or arXiv:2201.10663v2 [astro-ph.EP] for this version)

    Submission history

    From: Philip Lubin Prof [view email]

    [v1] Tue, 25 Jan 2022 22:54:35 UTC (5,050 KB)
    [v2] Thu, 27 Jan 2022 20:23:57 UTC (5,050 KB)