This area will cover relevant news of the threat to the planet from Near Earth Objects (NEOs) including concepts and designs for mitigation. All opinions are those of the author.

20 January 2010

Impact of Earth's Gravity on Nearby Asteroids

From MIT News Release...

For decades, astronomers have analyzed the impact that asteroids could have on Earth. New research by MIT Professor of Planetary Science Richard Binzel examines the opposite scenario: that Earth has considerable influence on asteroids — and from a distance much larger than previously thought. The finding helps answer an elusive, decades-long question about where most meteorites come from before they fall to Earth and also opens the door to a new field study of asteroid seismology.

By analyzing telescopic measurements of near-Earth asteroids (NEAs), or asteroids that come within 30 million miles of Earth, Binzel has determined that if an NEA travels within a certain range of Earth, roughly one-quarter of the distance between Earth and the moon, it can experience a “seismic shake” strong enough to bring fresh material called “regolith” to its surface. These rarely seen “fresh asteroids” have long interested astronomers because their spectral fingerprints, or how they reflect different wavelengths of light, match 80 percent of all meteorites that fall to Earth, according to a paper by Binzel appearing in the Jan. 21 issue of Nature. The paper suggests that Earth’s gravitational pull and tidal forces create these seismic tremors.

By hypothesizing about the cause of the fresh surfaces of some NEAs, Binzel and his colleagues have tried to solve a decades-long conundrum about why these fresh asteroids are not seen in the main asteroid belt, which is between Mars and Jupiter. They believe this is because the fresh surfaces are the result of a close encounter with Earth, which obviously wouldn’t be the case with an object in the main asteroid belt. Only those few objects that have ventured recently inside the moon’s orbital distance and have experienced a “fresh shake” match freshly fallen meteorites measured in the laboratory, Binzel said.

Clark Chapman, a planetary scientist at the Southwest Research Institute in Colorado, believes Binzel’s work is part of a “revolution in asteroid science” over the past five years that considers the possibility that something other than collisions can affect asteroid surfaces. 

How they did it: Binzel’s team used a large NASA telescope in Hawaii to collect information on NEAs, including a huge amount of spectral fingerprint data. Analyzing this data, the group examined where a sample of 95 NEAs had been during the past 500,000 years, tracing their orbits to see how close they’d come to Earth. They discovered that 75 NEAs in the sample had passed well inside the moon’s distance within the past 500,000 years, including all 20 fresh asteroids in the sample.

Binzel next determined that an asteroid traveling within a distance equal to 16 times the Earth’s radius (about one-quarter of the distance to the moon) appears to experience vibrations strong enough to create fresh surface material. He reached that figure based on his finding that about one-quarter of NEAs are fresh, as well as two known facts — that the space weathering process that ages regolith can happen in less than one million years, and that about one-quarter of NEAs come within 16 Earth radii in one million years.

Before now, people thought an asteroid had to come within one to two Earth radii to undergo significant physical change.

Next steps: Many details about the shaking process remain unknown, including what exactly it is about Earth that shakes the asteroids, and why this happens from a distance as far away as 16 Earth radii. What is certain is that the conditions depend on complex factors such as the velocity and duration of the encounter, the asteroid’s shape and the nature of the preexisting regolith. “The exact trigger distance depends on all those seismology factors that are the totally new and interesting area for cutting edge research,” Binzel said.

Further research might include computer simulations, ground observations and sending probes to look at the surfaces of asteroids. Binzel’s next steps will be to try to discover counterexamples to his findings or additional examples to support it. He may also investigate whether other planets like Venus or Mars affect asteroids that venture close to them.

Link: MIT News Release 

From Discovery News Article...

New research published in this week's Nature shows Earth's gravity triggers ground-shifting quakes on asteroids passing as far as about 30 million miles away.

The findings may not only help scientists deflect an Earth-bound asteroid, but also provide fresh insights into the connections between asteroids and meteorites.

Scientists made the discovery by comparing differences in light reflected off asteroids that have breezed by Earth with those that orbit farther away. Though they are made of the same materials, the asteroids that encounter Earth's gravity have fresh surfaces that are noticeably less weathered by the space environment.
Scientists believe the resurfacing is due to slow-falling landslides, triggered by tidal forces from Earth.

"Asteroids get sunburned out there by the light from the sun, the radiation from the sun," Dan Durda, a planetary scientist with the Southwest Research Institute in Boulder, Colo., told Discovery News.

Backtracking the orbits of 95 near-Earth asteroids, scientists determined that over the past 500,000 years, 75 of them had passed closer to Earth than the moon, which is about 239,000 miles (or about 385,000 kilometers) away. The 75 asteroids included 20 bodies with spectra of fresh surface materials.

Link: Article (Discovery News)

Nature Magazine (editor's summary)..

The 'ordinary chondrite problem' has been a factor in Solar System astronomy for three decades. It refers to the apparent anomaly that whereas about 80% of the meteorites falling to Earth are 'ordinary chondrites', they are rare among asteroids. The usual explanation is that 'space weathering' processes alter ordinary chondrite surfaces, producing reddened 'S-type' asteroids. A mystery remains, though, in the shape of a rare class of asteroids, the Q-types. These are found only near the Earth, and they do display 'fresh' spectral matches to ordinary chondrites. Now the combination of a new data set of 95 asteroid spectra with their detailed orbital histories shows that all Q-type asteroids have recently passed close to Earth at least within the lunar distance. Thus tidal stresses or seismic shaking during these encounters may have exposed new unweathered material on the surface. Intriguingly a test of this hypothesis may be at hand: 99942 Apophis, a potentially Earth-threatening asteroid currently displaying 'weathered' spectral colours, is due to pass within six orbital radii of Earth in 2029. It is predicted that it will experience a seismic 'fresh shake', which should expose new unreddened material on the surface. 

Link: Nature Magazine (Editor's Summary)

Asteroids: Stripped on passing by Earth
Nature 463, 305-306 (21 January 2010) | doi:10.1038/463305a; Published online 20 January 2010
Clark R. Chapman
Clark R. Chapman is in the Department of Space Studies, Southwest Research Institute, Boulder, Colorado 80302, USA.

Asteroids are weakly bound piles of rubble, and if one comes close to Earth, tides can cause the object to undergo landslides and structural rearrangement. The outcome of this encounter is a body with meteorite-like colours. We once thought that small asteroids were solid 'chips off the old block'. Instead, in a developing story, they are now known to be ephemeral piles of boulders in near-zero gravity, constantly evolving from delicate forces never before thought important.

Earth encounters as the origin of fresh surfaces on near-Earth asteroids
Nature 463, 331-334 (21 January 2010) | doi:10.1038/nature08709; Received 2 October 2009; Accepted 18 November 2009

Richard P. Binzel1,2, Alessandro Morbidelli3, Sihane Merouane4, Francesca E. DeMeo4, Mirel Birlan2, Pierre Vernazza5, Cristina A. Thomas6, Andrew S. Rivkin7, Schelte J. Bus8 & Alan T. Tokunaga8
  1. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  2. Institut de Mecanique Celeste et de Calcul des Ephemerides (IMCCE), Observatoire de Paris, Paris 75014, France
  3. Departement Cassiopee, UniversitĂ© de Nice Sophia-Antipolis, CNRS, Observatoire de la CĂ´te d’Azur, Nice 06304, France
  4. Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, Meudon 92195, France
  5. ESTEC/ESA, Noordwijk 2200 AG, Netherlands
  6. Department of Physics and Astronomy, Northern Arizona University, Flagstaff, Arizona 86011, USA
  7. Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA
  8. Institute for Astronomy, University of Hawaii, Hilo, Hawaii 96720, USA
Correspondence to: Richard P. Binzel1,2 Correspondence and requests for materials should be addressed to R.P.B. (Email:

Telescopic measurements of asteroids’ colours rarely match laboratory reflectance spectra of meteorites owing to a ‘space weathering’1, 2 process that rapidly3 reddens asteroid surfaces in less than 106 years. ‘Unweathered’ asteroids (those having spectra matching the most commonly falling ordinary chondrite meteorites), however, are seen among small bodies the orbits of which cross inside Mars and the Earth. Various explanations have been proposed for the origin of these fresh surface colours, ranging from collisions4 to planetary encounters5. Less reddened asteroids seem to cross most deeply into the terrestrial planet region, strengthening6 the evidence for the planetary-encounter theory5, but encounter details within 106 years remain to be shown. Here we report that asteroids displaying unweathered spectra (so-called ‘Q-types’7) have experienced orbital intersections closer than the Earth–Moon distance within the past 5 × 105 years. These Q-type asteroids are not currently found among asteroids showing no evidence of recent close planetary encounters. Our results substantiate previous work5: tidal stress8, strong enough to disturb and expose unweathered surface grains, is the most likely dominant short-term asteroid resurfacing process. Although the seismology details are yet to be worked out, the identification of rapid physical processes that can produce both fresh and weathered asteroid surfaces resolves the decades-long9 puzzle of the difference in colour of asteroids and meteorites.

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