[meteorite-list] 2010 TK7: The First Earth Trojan Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 29 Jul 2011 15:52:02 -0700 (PDT)
Message-ID: <201107292252.p6TMq2Sp027451_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/news/news173.html

2010 TK7: The First Earth Trojan Asteroid
Paul Chodas & Don Yeomans
NASA/JPL Near-Earth Object Program Office
July 29, 2011

After years of searching, astronomers have finally found an Earth Trojan
asteroid, 2010 TK7. A team led by Martin Connors of Athabasca University
in Canada announced the discovery in the current issue of the journal
*Nature*. The asteroid was first detected last October by NASA's
Wide-field Infrared Explorer (WISE) mission, and follow-up observations
by Connors and his team confirmed the asteroid's Trojan classification.

A Trojan asteroid essentially shares its orbit with a planet and has an
almost identical orbital period. When viewed from the planet, the
asteroid appears to oscillate about one of the stable points in front
of, or behind, the planet. Thousands of Trojan asteroids are known to
share Jupiter's orbit, and others have been found sharing orbits with
Neptune and with Mars.

[Diagram 1]
The motion of 2010 TK7 in 2011 relative to Earth, looking
down from above the Solar System. Although Earth and asteroid both
actually orbit the Sun, the relative motion appears as a large loop. The
brighter portion of the trajectory is above the Earth's orbital plane.


[Diagram ]
The motion of 2010 TK7 in 2011 relative to the Earth, as
viewed from the Sun. The horizontal line is the Earth's orbit edge-on.

Asteroid 2010 TK7 remains on the leading side of the Earth as both go
around the Sun at almost precisely the same average rate. Because its
orbit is both quite eccentric and inclined to the Earth's orbit, the
asteroid appears to loop around an empty point in space, when viewed
from the Earth, taking one year to complete the cycle. The first diagram
shows the current annual cycle of the asteroid, viewed in a reference
frame rotating with Earth and looking down on the plane of the Earth's
orbit. The lighter portion of the loop is above the Earth's orbit, the
darker portion is below. The second diagram shows the same annual cycle
as viewed from the Sun.

[Diagram 3]
The annual cycle of 2010 TK7 relative to Earth slowly
drifts around the Earth's orbit, from its current position to its
maximum offset around 2209. It then reverses direction and heads back
towards its current position, which it reaches in about 395 years.

This annual cycle does not remain perfectly constant with respect to the
Earth's location: it slowly migrates along the Earth's orbit. The third
diagram shows the position of the annual cycle in selected future years,
changing from its current position just ahead of the Earth to its
farthest position from our planet in the year 2209. After that, the
cycle reverses its drift and slowly moves back towards the Earth. It
gets back to its current 2011 position around the year 2400, completing
a full period of what is called "libration." The net effect is that the
loop librates back and forth, from near the Earth to nearly on the
opposite side of the Sun, in an approximately 390-year cycle. Numerical
studies show that the loop will librate this way and remain on the
leading side of the Earth for at least the next several thousand years.
The librational motion effectively keeps the asteroid away from the
Earth over this period, so that a collision is not possible.
Received on Fri 29 Jul 2011 06:52:02 PM PDT