[meteorite-list] Orbital Estimates Pinpoint 20 Asteroids on Similar Paths to Rock That Exploded Near Chelyabinsk

[Ron Baalke]

http://www.nature.com/news/russian-meteor-may-have-gangmates-in-tow-1.13498

Russian meteor may have gangmates in tow

Orbital estimates pinpoint 20 asteroids on similar 
paths to rock that exploded near Chelyabinsk.

By Maggie McKee
Nature
02 August 2013

The house-sized rock that exploded spectacularly in the skies near Chelyabinsk, 
Russia, in February may have been a member of a gang of asteroids that 
still poses a threat to Earth, a new study says. The evidence is circumstantial, 
but future observations could help to settle the question.

On 15 February, an 11,000-tonne space rock slammed into the atmosphere 
above Russia, producing the most powerful impact since the Tunguska explosion 
in 1908 - which may also have been caused by an asteroid - and generating 
a shock wave that damaged buildings and injured more than 1,000 people. 
The 18-metre-wide object could not be seen as it approached the planet 
because it was obscured by the Sun's glare, but observations made while 
it was in the atmosphere have enabled several groups of researchers to 
estimate its orbit.

However, the estimates varied so much that there was no clear orbit that 
researchers could use to hunt for sibling asteroids on a similar path, 
say Carlos and Raul de la Fuente Marcos, orbital dynamicist brothers at 
the Complutense University of Madrid.

They decided to tackle the problem with brute computational force, running 
simulations of billions of possible orbits to find the ones most likely 
to have led to a collision. They then used the average of the ten best 
orbits to search a NASA asteroid catalogue for known objects on similar 
paths. They found about 20, ranging in size from 5 to 200 metres across, 
they report in an article to be published in Monthly Notices of the Royal 
Astronomical Society: Letters.

Broken family

The researchers propose that these rocks are pieces of a rubbly asteroid 
that came apart some time in the past 40,000 years. The break-up may have 
been triggered by stresses from temperature swings as the parent asteroid 
looped out past Mars and then back towards Venus on its travels around 
the Sun, says Carlos de la Fuente Marcos.

The rocky fragments could one day follow their planet-pummelling sibling 
to Earth, he says. "More objects with the same orbital signature may encounter 
our planet in the future."

But don't panic just yet. The researchers acknowledge that the gravitational 
pull of the planets might have affected the path of each asteroid in a 
slightly different way, and that those differences would tend to grow 
over time. So even if the orbits of these objects were very similar at 
first, they could change completely.

The authors also admit that the orbits of the asteroids in the proposed 
group are not known with certainty, meaning that the objects may not be 
related to each other or to the Chelyabinsk meteor.

The best sibling candidate, for instance, is a 200-metre-wide asteroid 
known as 2011 EO40, whose orbit was calculated from observations taken 
over just 34 days. That is too short a time to be able to pinpoint its 
position in the future, says Jon Giorgini, a senior analyst in the Solar 
System Dynamics Group at NASA's Jet Propulsion Laboratory in Pasadena, 
California. For that, researchers would need observations taken over the 
asteroid's two-year orbit, he says.

David Nesvorny, who studies asteroids at the Southwest Research Institute 
in Boulder, Colorado, also urges caution in drawing this particular family 
tree. "I think that the resemblance of orbits is coincidental," he says. 
"It is not obvious to me why [the Chelyabinsk meteor] cannot be a fragment 
that was produced by a collision in the main asteroid belt, and evolved 
to its impact orbit by a few planetary encounters."

The only way to prove a connection would be to collect samples from one 
of the proposed family members and compare them with meteorites collected 
from the Chelyabinsk region, says Carlos de la Fuente Marcos. However, 
a cheaper way to find out whether the suspected siblings have at least 
a passing resemblance could be to measure their composition from their 
light spectra.

Nature
doi:10.1038/nature.2013.13498

References

1. De la Fuente Marcos, C. & de la Fuente Marcos, R. preprint at http://arxiv.org/abs/1307.7918 
(2013).

2. Zuluaga, J. I., Ferrin, I. & Geens, S. preprint at http://arxiv.org/abs/1303.1796 
(2013).