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Creating Binary Asteroids

Photons streaming outward from the Sun can impart momentum, which is how a solar sail works. But even more subtle effects produced by the warming of irregular objects may have visible results. A new study of asteroid moons and how they form invokes the tongue-twister known as the Yarkovsky, O’Keefe, Radzievskii, Paddack effect, mercifully shortened to ‘YORP effect’ by those who study it. A body warmed by the Sun gives off infrared radiation, which carries momentum as well as heat. An asteroid’s spin can thus be speeded or slowed by sunlight.

Binary asteroid KW4

Add plenty of time and things get interesting. Start with the kind of asteroid that is little more than a pile of rocky rubble held together by gravity, then spin that rubble pile up slowly over a period of millions of years and eventually material will be slung off from the asteroid’s equator. Colliding materials of this nature can eventually coalesce into the satellite we see orbiting its parent, says Patrick Michel (Cote d’Azur Observatory, France), who goes on to note the implications for defending Earth against an incomng asteroid:

“Based on our findings, the YORP effect appears to be the key to the origin of a large fraction of observed binaries. The implications are that binary asteroids are preferentially formed from aggregate objects [rubble piles], which agrees with the idea that such asteroids are quite porous. The porous nature of these asteroids has strong implications for defensive strategies if faced with an impact risk to Earth from such objects, because the energy required to deflect an asteroid depends sensitively on its internal structure.”

Image: Three views of the binary asteroid KW4, a ‘rubble pile’ that may have spawned its own moon. Credit: NASA/JPL.

A binary impact is quite a thought, but the authors of the study point out that doublet craters formed by the nearly simultaneous impact of similar objects can be found on Earth as well as other planets. Learning how to counter such asteroids is going to push our technology to the limit, involving as it will missions to different types of asteroids to assess their makeup and figure out which methods for trajectory change are most likely to succeed.

If the YORP theory proves out, it will have solved an earlier conundrum. Small binary asteroids are not thought to have formed early in the history of the Solar System, so what brought them about later? You could create a model of collisions and planetary encounters to account for the binaries, but their sheer numbers make that model dubious. Fully fifteen percent of near-Earth and main belt asteroids (two dynamically different environments indeed) with diameters of less than ten kilometers are now believed to have satellites. The YORP model, which fits the test case of binary asteroid KW4, may well hold the key.

The paper is Walsh et al., “Rotational breakup as the origin of small binary asteroids,” Nature 454, (10 July 2008), pp. 188-191 (abstract)

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  • ljk August 6, 2008, 23:31

    Thermal inertia of main belt asteroids smaller than 100 km from IRAS data

    Authors: Marco Delbo (OCA), Paolo Tanga (OCA)

    (Submitted on 6 Aug 2008)

    Abstract: Recent works have shown that the thermal inertia of km-sized near-Earth asteroids (NEAs) is more than two orders of magnitude higher than that of main belt asteroids (MBAs) with sizes (diameters) between 200 and 1,000 km. This confirms the idea that large MBAs, over hundreds millions of years,have developed a fine and thick thermally insulating regolith layer, responsible for the low values of their thermal inertia, whereas km-sized asteroids, having collisional lifetimes of only some millions years, have less regolith, and consequently a larger surface thermal inertia.

    Because it is believed that regolith on asteroids forms as a result of impact processes, a better knowledge of asteroid thermal inertia values and its correlation with size, taxonomic type, and density can be used as an important constraintfor modeling of impact processes on asteroids. However, our knowledge of asteroids’ thermal inertia values is still based on few data points with NEAs covering the size range 0.1-20 km and MBAs that >100 km.

    Here, we use IRAS infrared measurements to estimate the thermal inertias of MBAs with diameters 100 km and known shapes and spin vector: filling an important size gap between the largest MBAs and the km-sized NEAs. An update to the inverse correlation between thermal inertia and diameter is presented.

    For some asteroids thermophysical modelling allowed us to discriminate between the two still possible spin vector solutions derived from optical lightcurve inversion. This is important for (720) Bohlinia: our preferred solution was predicted to be the correct one by Vokrouhlicky et al. (2003, Nature 425, 147) just on theoretical grounds.

    Comments: Planetary and Space Science (2008)

    Subjects: Astrophysics (astro-ph)

    DOI: 10.1016/j.pss.2008.06.015

    Cite as: arXiv:0808.0869v1 [astro-ph]

    Submission history

    From: Marco Delbo [view email] [via CCSD proxy]

    [v1] Wed, 6 Aug 2008 16:14:03 GMT (61kb)

    http://arxiv.org/abs/0808.0869

  • ljk August 13, 2008, 22:43

    MIT solves puzzle of meteorite-asteroid link

    New analysis makes it possible to ‘know our enemy’

    David Chandler, MIT News Office

    August 13, 2008

    For the last few years, astronomers have faced a puzzle: The vast majority of asteroids that come near the Earth are of a type that matches only a tiny fraction of the meteorites that most frequently hit our planet.

    Since meteorites are mostly pieces of asteroids, this discrepancy was hard to explain, but a team from MIT and other institutions has now found what it believes is the answer to the puzzle. The smaller rocks that most often fall to Earth, it seems, come straight in from the main asteroid belt out between Mars and Jupiter, rather than from the near-Earth asteroid (NEA) population.

    The puzzle gradually emerged from a long-term study of the properties of asteroids carried out by MIT professor of planetary science Richard Binzel and his students, along with postdoctoral researcher P. Vernazza, who is now with the European Space Agency, and A.T. Tokunaga, director of the University of Hawaii’s Institute of Astronomy.

    By studying the spectral signatures of near-Earth asteroids, they were able to compare them with spectra obtained on Earth from the thousands of meteorites that have been recovered from falls. But the more they looked, the more they found that most NEAs — about two-thirds of them — match a specific type of meteorites called LL chondrites, which only represent about 8 percent of meteorites. How could that be?

    “Why do we see a difference between the objects hitting the ground and the big objects whizzing by?” Binzel asks. “It’s been a headscratcher.” As the effect became gradually more and more noticeable as more asteroids were analyzed, “we finally had a big enough data set that the statistics demanded an answer. It could no longer be just a coincidence.”

    Full article here:

    http://web.mit.edu/newsoffice/2008/meteorites-0813.html

  • ljk August 10, 2009, 9:39

    August 7, 2009

    Near-Earth Object Has Two Moons

    Radar images have shown that a near-Earth object is actually a triple system; an asteroid with two small moons. NASA’s Goldstone Solar System Radar on June 12 and 14, 2009, revealed the new informaton about Asteroid 1994 CC. It came within 2.52 million kilometers (1.56 million miles) on June 10.

    Prior to the flyby, very little was known about this celestial body. 1994 CC is only the second triple system known in the near-Earth population. A team led by Marina Brozovic and Lance Benner, both scientists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., made the discovery.

    Full article and images here:

    http://www.universetoday.com/2009/08/07/near-earth-object-has-two-moons/