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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.
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 2: The motion of 2010 TK7 in 2011 relative to the Earth, as viewed from the Sun.
Diagram 2: 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 drifts around the Earth's orbit, from its current position to its maximum offset around 2209.
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.

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