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A Slow Motion Asteroid Breakup

The odd lightcurve of the star known as VVV-WIT-07, discussed here last Friday, reminds us that even as we start seeing such signatures, we are tuning up our ability to find others. It’s a point that bears repeating from the paper on this work:

…surveys like ours, apart of course from its irregular cadence, may perhaps not have found objects like WIT-VVV-07 more often primarily because they were not looking specifically for this kind of variability.

The authors go on to say that next generation surveys like LSST (Large Synoptic Survey Telescope), now under construction, as well as space-based assets like the upcoming WFIRST and PLATO missions, will likely pin down further instances of unusual light curves.

It’s a point worth making again when we pivot to today’s discussion, on the asteroid known as (6478) Gault, a 4 kilometer-wide object currently some 344 million kilometers from the Sun. What we see here are two dusty tails reminiscent of a comet that are streaming behind Gault, an apparently gentle release of material that tells us the asteroid is gradually coming apart.

“We didn’t have to go to Gault,” explained Olivier Hainaut of the European Southern Observatory in Germany, a member of the Gault observing team. “We just had to look at the image of the streamers, and we can see all of the dust grains well-sorted by size. All the large grains (about the size of sand particles) are close to the object and the smallest grains (about the size of flour grains) are the farthest away because they are being pushed fastest by pressure from sunlight.”

Image: This Hubble Space Telescope image reveals the gradual self-destruction of an asteroid, whose ejected dusty material has formed two long, thin, comet-like tails. The longer tail stretches more than 800,000 kilometers (500,000 miles) and is roughly 4,800 kilometers (3,000 miles) wide. The shorter tail is about a quarter as long. The streamers will eventually disperse into space. Credit: NASA, ESA, K. Meech and J. Kleyna (University of Hawaii), and O. Hainaut (European Southern Observatory).

At work here is apparently the same YORP (Yarkovsky–O’Keefe–Radzievskii–Paddack) effect that seems to be gradually spinning up asteroid Bennu as examined by the OSIRIS-REx mission. You would think a collision between asteroids could produce the same visual effect, but observations with the Hubble Space Telescope, which took the above image, have shown no signs of any debris other than what we see in the two ‘tails.’

Scientists believe these will begin to fade within months as the dust is gradually dispersed. Their narrow shape is an indication that they were released in short bursts, a series of puffs that probably did not last more than a few days. Perhaps, as with WIT-VVV-07, we are looking at a phenomenon we’re soon to see more of, for enhanced survey efforts like Pan-STARRS (the Panoramic Survey Telescope and Rapid Response System at Haleakala Observatory, Hawaii) and ATLAS (Asteroid Terrestrial-impact Last Alert System, at Haleakala and Mauna Loa), are likely to find far more active asteroids than we’ve seen thus far. Both operations saw the Gault debris, ATLAS in early 2019 and Pan-STARRS in subsequent analysis of archival data.

Especially intriguing is the two-hour rotation period measured for Gault, which is about the critical speed at which a ‘rubble pile’ asteroid begins to break up. Jan Kleyna, lead author of the paper on this work, calls it “…the best smoking-gun example of a fast rotator right at the two-hour limit.” As to the YORP effect, it occurs when an asteroid is unevenly heated by incident sunlight, producing infrared radiation that carries off not only heat but momentum. The scientists studying Gault believe it could have been spinning up for more than 100 million years.

From the paper:

…dust emission is strongly suggestive of a rotation-induced event due to the YORP effect, as the object is spun-up by re-radiation forces until the apparent surface gravity is zero, triggering disruption or landslide events… releasing near-zero-velocity debris that is swept away by radiation pressure. Sudden and brief landslides are in accord with the abrupt dust releases described in §4. Because of the large mass of material released, it is likely that these landslides were significant, and that the equatorial velocity of the object is very close to the liberation velocity, i.e., that the surface material is tenuously held to the surface, with a proclivity to rearrange itself.

So we’re learning more about YORP and its potential for changing trajectories and causing surface disruption on asteroids, with missions to such objects as well as ever more refined distant observation. Keeping an eye on Gault for possible future events should tell us more, and help us determine just how rare such activity on the roughly 800,000 asteroids between Mars and Jupiter is. Current estimates are that similar asteroid events will occur about once a year.

The paper is Kleyna et al., “The Sporadic Activity of (6478) Gault: A YORP-driven event?,” accepted at Astrophysical Journal Letters (preprint).


{ 4 comments… add one }
  • Charley April 1, 2019, 14:45

    This Yarkovsky–O’Keefe–Radzievskii–Paddack effect could have a significant beyond merely creating a previously unexplained dust tail behind these asteroids. If there is these facts occurring on a regular basis, there could be significant underestimation of how asteroids could change their stable orbits into becoming earth crossing impactors.
    It might be a wise thing to begin spectrographic analysis of these asteroids that make up the asteroid belt and tried to get an estimation if this uneven heating effect could ultimately destabilize their orbits sufficiently to make them potential earth crossing projectiles.

  • Patient Observer April 2, 2019, 22:27

    If the YORP effect is launching the particles at a velocity just slightly above the orbital velocity at the equatorial surface, it would seem that the particles would never acquire sufficient velocity to escape. One could imagine a very low orbiting ring of material; perhaps distorted by the irregular shape of the asteroid but nevertheless hanging around. I like the image of this slow motion ring cruising just above the surface like a lazy river of gravel.

    It is light pressure or electrostatic forces that allow the particles to eventually exceed the escape velocity of the asteroid?

    • Bruce D. Mayfield April 3, 2019, 10:13

      But for an asteroid that small the escape velocity would be next to nothing. Light pressure, as well as impacts from atoms in the solar wind is what is sweeping these particles clear. There would be no way a particle could orbit such a low mass in such a windy and luminous environment.

  • ljk April 18, 2019, 9:31

    Asteroids help scientists measure distant stars

    by Tracey Bryant, University of Delaware

    April 15, 2019

    Look up at the sky on a clear night, and you’ll see lots of stars. Sometimes they seem almost within reach or at least a short rocket ride. But the closest star to Earth—not counting our sun—is more than four light years away, at a distance of 25 trillion miles.

    There are more than 100 billion stars in our Milky Way Galaxy, and, while we have learned much about them, there are relatively few whose size has been directly measured because they are so far away. A star’s size is a key piece of information that unlocks many other mysteries about it. Several methods have been used to measure star sizes, yet each has its limitations.

    But now an international team, including researchers from the University of Delaware, has discovered a new way to determine the size of stars. Their method draws on the unique capabilities of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory in Arizona—and asteroids that pass by at just the right place and time.

    Using the technique, a collaboration of 23 universities and research institutes, led by Tarek Hassan of Deutsches Elektronen-Synchrotron (DESY) and Michael Daniel of the Smithsonian Astrophysical Observatory, has revealed the diameters of a giant star 2,674 light-years away, and a sun-like star at a distance of 700 light-years—the smallest star measured in the night sky to date. The research was reported on Monday, April 15 in the journal Nature Astronomy.

    Full article here:


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