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The When and Where of Asteroid 101955 Bennu

You wouldn’t think the Yarkovsky effect would have any real significance on a half-kilometer wide pile of rubble like the asteroid 101955 Bennu. With a currently estimated mass somewhere between 60 and 80 billion kilograms, Bennu seems unlikely to receive much of a nudge from differences in heat on the object’s surface. But the people who specialize in these things say otherwise. Sunlight warms one side of the asteroid while the other experiences the cold of space. Rotation keeps the dark side radiating heat, accounting for a tiny thrust.

We call it the Yarkovsky effect after Ivan Osipovich Yarkovsky, a Polish engineer who came up with it in 1901, though if we want to give credit across the board, we might refer to the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. Here we honor, in addition to Yarkovsky, an American scientist, a Russian astronomer and a NASA aerospace engineer, all of whom played a role in our understanding of the phenomenon as it relates to asteroids.

Image: Ivan Osipovich Yarkovsky (1844-1902). Credit: Wikimedia Commons.

The YORP effect turns up in interesting places, such as the near-Earth asteroid 2000 PH5, whose rotation rate has been spun up about as fast as any asteroid known, an effect traced over a four-year period by a team led by Stephen Lowry at Queens University Belfast (citation below). When it comes to Bennu, where we now have OSIRIS-REx in active investigation, researchers have calculated that the effect has shifted its orbit about 284 meters per year toward the Sun since 1999. Remember that Bennu originally came our way from the main asteroid belt, a movement inward that was presumably assisted by the same YORP effect.

On a scale of billions of years, then, YORP can create serious movement within the Solar System. But one reason for having OSIRIS-REx on the case is that we need to learn more about how such effects work so we can make better predictions about the future position of asteroids. Will a given asteroid present problems, with a potential trajectory that could intersect with the Earth? The calculation is by no means easy. With YORP alone, so much depends on the nature of the object, and how it absorbs and releases heat. We’d better learn as much as we can about such objects, a need that plays a role in asteroid missions that also investigate the evolution of the Solar System and the ancient debris that circulates among the planets.

Image: This artist’s concept shows the Origins Spectral Interpretation Resource Identification Security – Regolith Explorer (OSIRIS-REx) spacecraft contacting the asteroid Bennu with the Touch-And-Go Sample Arm Mechanism or TAGSAM. The mission aims to return a sample of Bennu’s surface coating to Earth for study as well as return detailed information about the asteroid and its trajectory. Credit: NASA’s Goddard Space Flight Center.

If the YORP effect makes our orbital calculations problematic, so too do the gravitational forces imparted by the Sun, nearby planets and other asteroids. As this JPL news release points out, astronomers can predict the exact dates of the next four passes Bennu will make near our planet (defined here as within 7.5 million kilometers, or .05 AU). The years in question are 2054, 2060, 2080 and 2135. But things get increasingly tricky as we look further out. For each time Bennu comes near the Earth, our planet gives its trajectory another slight twitch.

If you’re trying to figure out where Bennu will be in coming decades, then, you have to take into account the increasingly fuzzy effects that occur with each pass by the Earth, so that by 2060, when another such passage is predicted, we can only say that the asteroid will pass the Earth at about twice the distance from Earth to the Moon. But it could pass any point in a 30 kilometer window of space. Keep magnifying these numbers with future orbits and you can see why firm predictions become so difficult.

By 2080, according to calculations performed by Steven Chesley at the Center for Near-Earth Object Studies (CNEOS) at JPL, the best window we can derive for Bennu’s passage is 14,000 kilometers wide. Switch ahead to 2135, a time when Bennu’s orbit is thought to take it closer than the Moon, and the flyby window reaches 160,000 kilometers. This is, by the way, a projection for the single near-Earth asteroid for which we have the best orbital assessment in our database.

We’ve been studying Bennu through optical, infrared and radio telescopes every six years since its discovery in 1999 to measure factors like shape, rotation rate and trajectory. Given all that, CNEOS can say that looking ahead over the next century, the asteroid has a 99.963 percent change of missing the Earth. That’s heartening, but it’s clear that tightening up our numbers will help. And we can do a lot by way of studying how the YORP effect nudges the asteroid.

“There are a lot of factors that might affect the predictability of Bennu’s trajectory in the future, but most of them are relatively small,” says William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado, and a participating scientist on the OSIRIS-REx mission. “The one that’s most sizeable is Yarkvovsky.”

Optical images from OSIRIS-REx will help determine Bennu’s precise location and its exact orbital path as of now, giving us a read on how its trajectory is changing with time. With the spacecraft tracking Bennu over a two-year period, the variance from the projected trajectory will help to determine the size of the YORP effect’s changes. We’ll also learn a great deal about the amount of solar heat radiating from the asteroid from what type of surfaces, which will help us refine the YORP numbers, a huge help in tightening the trajectories of other asteroids.

OSIRIS-REx should eventually be able to tell us how craters and boulders change photon scattering and momentum transfer. Says Chesley:

“We know surface roughness is going to affect the Yarkovsky effect; we have models. But the models are speculative. No one has been able to test them.”

Refining models through on the spot observation is a major reason for doing OSIRIS-REx. When the mission is over, the team believes our projections of Bennu’s orbit will be 60 times better than what we now have. If only Ivan Osipovich Yarkovsky could be here to see this.

The paper on 2005 PH5 is Lowry et al., “Direct Detection of the Asteroidal YORP Effect,” Science Vol. 316, Issue 5822 (13 April 2007), pp. 272-274 (abstract).


{ 16 comments… add one }
  • Thomas R Mazanec December 10, 2018, 14:05

    (defined here as within 7.5 million kilometers, or 0.5 AU)

    Is it 75 million kilometers, or (as I suspect)) 0.05 AU?

    • Paul Gilster December 10, 2018, 15:01

      Right you are. I’ve fixed the typo: .05 AU.

  • ljk December 10, 2018, 16:26
  • Charley December 10, 2018, 16:32

    Finally, something that is interesting! Interesting in the sense that it isn’t about the amount of metal that you might find some distant star that we probably will never ever go to and highly unlikely will find another planet as good as our own.
    As for the topic of orbital calculations you manage to hit it right on the head, again, this is a topic that’s highly complex, which is what makes it so interesting, but predict mean future orbits is problematical at best, simply because of the fact that even the internal structure can over time perturb orbits in a way that is essentially unpredictable.
    Question to your readers out there; can anyone explain to me in a fashion that’s understandable-what does it mean when they talk about the concept of chaos theory? The idea behind chaos theory doesn’t seem to have any strong foundation in what I would call physical underpinnings. What I mean by that is when someone says the word chaos, they seem to imply that some small perturbation (undefined by what the word perturbation means) begins to affect something. You can call the something whether, orbits, stock market, etc. etc. but somehow or another this results in a affect which begins to become “chaotic”. They say the word chaotic and then they leave it there-seemingly unable to predict the future. But what does “chaotic” exactly mean? I don’t get it

    • Ron S. December 10, 2018, 23:10
    • Bruce D. Mayfield December 10, 2018, 23:45

      Chaos does indeed show up in many physical systems, most observably with fluid dynamics. Here’s a simple kitchen sink experiment where anyone can see it for themselves just by slowly opening a faucet valve. At some points the drips will be as regular as a clock, at others the exact timing and size of the drops will be unpredictable.

      Tiny changes can and do produce widely differing effects. That’s why weather forecasting is so challenging.

    • wdk December 11, 2018, 1:20

      A number of people might weigh in on this with more insight, but one thing that is worth considering is the idea of
      1. having slight changes/variations in a so-called initial condition such as a close passage nearby the Earth, and
      2. the extremely high sensitivity down range as a result.

      Some cases we don’t think of so much as chaotic, but definite branches. For example, you have a razor edge between skipping out of the atmosphere and entering. But for a passage by the Earth or another planet, you might have a narrow window to obtain a repeated passage or an aim on your next target ( e.g., another planet). And if you are outside of that, your trajectory’s outline will be extremely different – even though long term it does show certain patterns.

      Famous illustrations of chaotic motions are trajectory traces taken in various planes that surround two or more “attractors” giving a characteristic owl-eyes pattern.

      Another example that comes to mind: a binary system with some eccentricity such as two stars with unequal mass. Place a planet in the system in a circular orbit about one or the other ( possibly both) and it behaves regularly for a period of orbits. The orbits change due to the irregularities of elliptical motion of the two stars about their common center and then the planet’s motion goes unstable… Change the initial conditions a little. Is the new outcome similar to the previous? Well, it might go unstable after a while, but how, in any way, was the result similar to the previous? The succession of changed outcomes just might defy analytical prediction based on the initial condition changes of small regular increments. Yet if you look at a host of plots, some patterns appear. Which leads me to ask: When is something actually “random”?

  • Bruce D. Mayfield December 10, 2018, 19:22

    Earlier I had written an opinion that since Bennu is in a NEO it probably has fragments of Earth on it even from the great impact that formed our Moon, but I didn’t know then that this object has migrated inwards from the main asteroid belt due to this YORP effect. What is the estimated rate of its migration? Perhaps it has only collected a few pieces from much more recent impact events.

    • Karl Pestell December 11, 2018, 7:50

      284m a year (+/-1.5m).

      Ultimately 10% chance of Earth impact (p846 in pdf)
      Our results indicate that Bennu has a 48% chance of falling into the Sun. There is a10% probability that Bennu will be ejected out of the
      inner solar system, most likely after a close encounter with Jupiter. The highest impact probability for a planet is with Venus (26%), followed by the Earth (10%) and Mercury (3%).

      • Bruce D. Mayfield December 11, 2018, 14:54

        Thanks again Karl. 284 m/yr. is way more than I would have guessed.
        That means that this object wouldn’t be a good place to go hunting for Earth fossils I recon. Ah shucks.

        • Bruce D. Mayfield December 11, 2018, 18:12

          However, 284 m/yr x 65,000,000 is only about .12 AU, so dino fossils on Bennu just might still be within the realm of possibility. One can hope at least.

          • Karl Pestell December 12, 2018, 9:46

            Based on visual and near IR spectroscopy, Bennu is classified as a hydrated CI or CM chrondite, likely originating from the ‘Eulalia’ or ‘new Polana’ families of inner main belt asteroids, which are thought to have formed between 700Myr and 2000Myr ago. Bennu was ejected using the v6 secular resonance (Saturn increased its eccentricity until it became a Mars crosser and was then ejected inwards). We then get the YORP effects, but these are not constant; they increase with solar flux as Bennu drifts inwards and the induced torques reshape and resurface the asteroid which itself alters the YORP. Furthermore, simulations show that in the past Bennu is more than 80% likely to have spent some time in an orbit with a perihelion <0.8AU. Encounters with Earth or Venus would have caused further reshaping and torque changes.

            The current 284m rate was established from observations from 1999 (when it was discovered), but we can't do a linear interpolation too far back because it ceases to be deterministic and becomes a matter of statistical probability. We will learn a whole lot more when we actually get there!

      • Thomas R Mazanec December 11, 2018, 17:56

        10% hits Earth
        48% hits sun
        10% ejected
        26% hits Venus
        03% hits Mercury
        97% total
        So 3% chance of hitting one of the giant planets?

        • Karl Pestell December 12, 2018, 9:07

          The only additional figures given are 0.8% Mars and 0.2% Jupiter. I’d presume the big scenario not accounted for is that it doesn’t hit anything and continues its YORP spinning in the inner solar system.

          The figures come from 1000 simulations over 300 Myr of gravitational perturbations of all eight planets. This is because after the 2135 approach (when it will pass within the Moon’s orbit) the path ceases to be deterministic and probabilistic simulations have to be used.

  • Patient Observer December 10, 2018, 22:34

    There was an Arthur C Clarke story that mentioned, in passing, the placement of radio beacons on asteroids to allow better tracking. I wonder if placing such beacons on potentially earth impacting asteroids would help in precision tracking. Perhaps orbiting the asteroid could be easier than landing and deploying solar cells on an unknown surface. Or does optical tracking provide the same precision? Just a thought.

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